Apparently coined by Verkhoshansky and Siff (169), eccentric quasi-isometric (EQI) contractions, also known as yielding, holding, or eccentric isometrics (50,71,136), have many variations and proposed applications. However, for this review, EQIs will be defined as “holding a position until isometric failure and maximally resisting the subsequent eccentric phase.” Theoretically, the prolonged quasi-isometric and eccentric component enables a large accumulation of mechanical tension and metabolic stress that would contribute to improvements in work capacity, muscle size, and connective tissue health. Although traditional high-intensity isometric contractions and eccentric muscle actions are commonly used by practitioners, with well-established value in the modern scientific literature (38,44,45,79,90,120), EQIs remain relatively unexplored. Therefore, this review aims to synthesize and critically analyze relevant research and subsequently develop a rationale for the value of EQI training and highlight potential areas of future research.
Defining Eccentric Quasi-Isometric Training
Before the EQI contraction, a submaximal (being hereafter relative to 1 repetition maximum [1RM]) eccentric contraction where the muscle-tendon unit undergoes an active lengthening is performed. Once the prescribed joint position is met, the trainee shifts to yielding isometric muscle action and attempts to hold the position for as long as possible. The final phase occurs as fatigue accumulates, and an eccentric contraction commences while the trainee attempts to resist muscle lengthening maximally. Some practitioners contend that this second lengthening phase places additional stretch and strain on the musculotendinous system similar to supramaximal eccentric training (112,142). Practitioners have recommended a wide range of loads, with the goal of holding the quasi-isometric contraction for 5–90 seconds (112,142). Consistent with traditional resistance training, greater intensities and shorter contraction durations are recommended for strength and power athletes, whereas lower loads and longer contractions may be advantageous for oxidative or rehabilitative purposes (112,142). Anecdotally, increased muscle thickness, improved range of motion (ROM), altered force-angle relationships, and improved tendon health have been reported after EQI training (112,142). Although quasi-isometric muscle actions have been used to describe sport-specific and stabilizing positions in sailing, speed-skating, cycling, and sprinting gait (24,99,150,152,166–169), there are no published empirical data on EQIs, and much if the related literature uses animal models.
Literature Search Methodology
An electronic search for relevant literature was conducted using MEDLINE, SPORTDiscus, PubMed, and CINAHL databases from inception to May 2019. Key terms were searched for within the article title, abstract, and keywords using conjunctions “OR” and “AND” with truncation “*.” Combinations of the following Boolean phrases comprised the search terms: isometric, static, eccentric, contraction, occlusion, blood flow restriction, hypertrophy, strength, power, endurance, muscle, fiber, cross-sectional area, tendon, fascicle, pennation, and neuromuscular. Reference lists and books were also utilized. The Sports Performance Research Institute New Zealand at the Auckland University of Technology approved this brief review.
Inclusion and Exclusion Criteria
Studies were included in the review based on the following criteria: (a) full text available in English and (b) peer-reviewed journal publications or doctoral dissertations. Studies were excluded if they (a) were conference papers/posters/presentations.
Percent change and Cohen's d effect sizes (ES) were calculated wherever possible to indicate the magnitude of the practical effect. Effect sizes were interpreted using the following criteria: trivial <0.2, small 0.2–0.49, moderate 0.5–0.79, and large >0.8 (47). All reported ES and percentage changes are pre-post within-group, unless otherwise stated.
Eccentric Quasi-Isometrics and Morphological Adaptations
Eccentric quasi-isometric training seems to be a valuable tool for targeting specific musculotendinous morphological adaptations such as increased muscle thickness and fascicle length, and tendon stiffness and elasticity. Functional morphology refers to the structure and function of organisms and their specific structural features. Although morphology affects function in all tissues, this review will focus on the musculoskeletal system, which is often broken down into the 3-component model of force transmission (Figure 1) (66). The 3 component model provides insight into the determinants of force production and transmission—the contractile element (CE), series elastic component (SEC), and parallel elastic component (PEC) (69,70,104,126). The PEC, synonymous with the extracellular matrix, includes the elastic tissues surrounding the myofibrils (the endo, peri, and epimysium) as well as the sarcolemma and fascia. These tissues are believedblieved to contribute to sensations of pressure and, although yet to be fully quantified, may play a meaningful role in force transmission between joints and body segments (69,70,104). The SEC encompasses the spring-like tissues in series with actin and myosin, the tendon, and aponeurosis being most obvious. Controversy exists regarding the exact function of the titin myofilament, which seems to play a role in both active and passive force transmission (41,64,68). For example, titin was originally believed to be somewhat innate and only contributes to passive tension in a fully stretched sarcomere (68). However, contemporary research has demonstrated that titin is activated by calcium ions and adenosine triphosphate, contributing to active force transmission (41,64,95). Finally, the CE consists of the myofibril, and more specifically, the myofilaments of actin and myosin.
Muscle Length and Joint Angle
Typically prescribed at long muscle length (LML) and often held through full ROM, EQIs fulfill the scientifically based criteria of mechanical stretch and tension for improving muscular hypertrophy and function. Produced by force generation and stretch, mechanical tension is effective in promoting muscular hypertrophy regardless of contraction type (15,55,137). In animal models, prolonged mechanical tension has been shown to produce dramatic increases in muscle size. For instance, extreme increases in muscle mass (318%), muscle length (51%), mean fiber thickness (39%), and fiber number (82%) were reported after loaded stretching of avian wings over 28 days (9). Similarly, Tabary et al. (155) reported that cat soleus muscles immobilized in a lengthened position had 20% more serial sarcomeres, whereas a shortened soleus group had 40% fewer sarcomeres in series than normal muscle, respectively (155). An increase in muscle hypertrophy of up to 30%, with an increase of up to 250% RNA content in 4 days, was observed after electrically induced overload in stretched rabbit tibialis anterior muscles (55). The effect of mechanical tension on skeletal hypertrophy was examined by Ashida et al. (15) using electrically induced contractions in mice. Peak torque and torque-time integrals were highly correlated with increased muscle mass and mechanistic target of rapamycin (mTOR) regulating p7S6k phosphorylation in isometric contractions and eccentric muscle actions (15). Thus, animal models suggest that loaded stretching may provide a unique stimulus for inducing gene transcription and muscular hypertrophy (55).
Recently, loaded stretch training with human subjects has grown in popularity (10,63,148). For example, after 6 weeks of loaded (20–45% of maximal voluntary contraction [MVC]) stretching for 5, 3-minute sessions per week, fascicle length (25%), ROM (14.9%), and muscle thickness (5.6%) significantly increased, whereas the pennation angle of the lateral gastrocnemius significantly decreased (7.1%) (148). However, no change (p = 0.94, ES = 0.08) in maximal voluntary isometric contraction (MVIC) or voluntary activation (p < 0.05, ES = 0.13) was present (148), despite several cross-sectional investigations supporting the relationship between muscle architecture and performance (3,7,23,89,118,163). Yet, the causal relationship between alterations in muscle architecture and muscular strength has become a hot topic in the contemporary literature (36,117). In additionIn addition, the concept of constant-torque versus constant-angle stretching has been recently examined (10,63). For example, Herda et al. (63) examined the short-term effects of acute knee flexor stretching at a constant angle or under constant torque where the muscle was initially held at a point of mild discomfort followed by additional muscle-tendon unit lengthening through “muscle creep” and stretch-induced analgesia occurred. Although both groups experienced similar improvements in passive ROM and passive torque, only the constant-torque treatment resulted in decreased muscle-tendon–unit stiffness (p < 0.001) (63). Unfortunately, Herda et al. (63) did not report any performance measures, a trend that is common in stretching research (10). From these results, it seems that in young men, loaded stretching can provide sufficient stimulus to affect musculotendinous architecture, viscoelastic properties, and likely, acute pain thresholds (10,63,148). Because variants of loaded stretching use extended periods at or near end ROM, the results of the aforementioned research lend credence to the hypothesis that EQI training may be a valuable training methodology for improving acute and chronic flexibility and musculotendinous function. However, there is a dearth of stretch research elucidating the ideal stretching intensity and the efficacy of loaded stretching to improve muscular or athletic performance (10).
Although eccentric muscle actions have the highest potential for muscular force production, isometric muscle actions are the only contraction type that has no ROM-dependent endpoint. Isometric training is also easily implemented as simply flexing (cocontracting the agonists and antagonists of a limb) can increase muscle size and strength in active men (100,175); however, the value of co-contraction training in a well-trained population has yet to be elucidated. In addition, isometric contractions enable training at specific joint angles and, therefore, muscle-tendon lengths. Although strength improvements are joint-angle–specific (97), increases in muscular hypertrophy, which is larger after full ROM and LML training (106), transfer to all joint angles (5,87,115,116). McMahon et al. (106) compared the effects of dynamic resistance training executed with full or partial ROM. The full ROM group experienced significantly greater improvements in the distal anatomical cross-sectional area (59 vs. 16%), fascicle length (23 vs. 10%), and isometric force at all 7 (30–90° of flexion) measured knee joint angles (11–30% vs. −1 to 6%) (106) when compared with the partial ROM group. Although isometric contractions resulted in less muscle damage and less dramatic muscular-tendinous adaptations compared with maximal eccentrics, MVICs at LML increased markers of acute muscle damage and soreness relative to MVICs at short muscle length (SML) despite lower torque outputs (6). Isometric training at LML produces greater hypertrophy, force production at different joint angles, and dynamic performance benefits compared with training at SML after long-term trials (5,17,87,115,116,160). In a recent systematic review into the effects of isometric training variations, Oranchuk et al. (120) determined that isometric training at LML produced greater increases in muscular hypertrophy than volume-equated SML training, (0.86–1.69%·wk−1, ES·wk−1 = 0.03–0.09; and 0.08–0.83%·wk−1, ES·wk−1 = −0.003 to 0.07, respectively) (120) likely due to increased mechanical tension throughout all tissues involved in force transmission (Figure 2).
The larger architectural and functional adaptations after LML training might be due, at least in part, to the greater degree of fascicle stretch, which results in increased muscle damage and sarcomere compliance (6,27) demonstrated by acute optimal angle shifts toward longer muscle lengths. Although more dramatic after eccentric muscle actions, these angle shifts have also been observed after concentric contractions at long fascicle lengths (58). For example, Guex et al. (59) examined the effect of 3 weeks of maximal eccentric knee flexions at either LML or SML on fascicle length and optimal angle. Although fascicle length increased in both groups (SML, 4.9%, ES = 0.57: and LML; 9.3%, ES = 0.89), the SML group only experienced a shift in the optimal concentric angle (8.8°), whereas the LML group experienced optimal angle shifts in both concentric contractions and eccentric muscle actions (17.3 and 10.7°, respectively) (59). There is evidence to support the principle that mechanical tension can increase muscle volume and that isometric training at LML leads to greater hypertrophy and a shift in the optimal angle.
Contraction Intensity and Duration
Cumulative tension and total workload are the key determinants of hypertrophic adaptation, regardless of contraction type (110). Moore et al. (110) found that changes in torque and muscle thickness were not significantly different between load-matched concentric and eccentric resistance training groups, despite the eccentric group requiring 40% fewer contractions to match training load. Morphological adaptations to isometric resistance training are similar between work-matched high- and low-intensity training (120). Although much of the literature recommends high-load over low-load resistance training for strength development (102), many periodization models emphasize muscular hypertrophy and general muscular endurance early in a macro and mesocycle (34,169). Accordingly, EQI training emphasizing time under tension with the application of practitioner-recommended intensities of 30–80% of 1RM may be a useful training method to alter muscle size.
Total time under tension, acute hypoxia, and metabolic stress are mechanisms that contribute to morphological adaptations (25,49,51,120,123,140,157,158). Several studies have reported significant reductions in oxygen availability from submaximal isometric contractions at 30–50% of MVC (4,150). In addition, blood flow does not seem to decrease linearly with intensity (107). Isometric contractions at 60% of MVIC result in greater short-term blood flow restriction relative to 30 and 100% MVIC, as the moderate intensity contraction could be sustained for a significantly greater duration than 100%, whereas the tension created by the 30% contraction was not enough to reduce blood flow and metabolite clearance (107). These occlusive effects have several potential effects, including increased metabolite build-up and postcontraction blood flow, both of which stimulate muscular hypertrophy (98). Several studies have examined the impact of blood flow restriction on hormones and hypertrophic markers in humans (49,123,157,158). Fujita et al. (49) examined the metabolic and hormonal effects of blood flow restriction during low-intensity resistance training and found 46% greater mTOR-regulated muscle protein synthesis through significantly greater S6K1 phosphorylation markers compared with the exercise-only group. Gentil et al. (51) also found that both isometric contractions and vascular occlusion resulted in greater blood lactate responses, which can increase muscle cell myogenesis, satellite cell activation, and phosphorylation of mTOR and P70SK (113). In addition, acute ischemia combined with low-intensity muscular contraction can significantly increase growth hormone, IGF-1, and mechano–growth factor production (42), which are physiological responses to decreased muscle and blood pH (123,157,158). Occlusion may also help to bypass the size principle by reducing the amount of oxygen available for the oxidative type-1 motor units, resulting in preferential recruitment of fast-twitch fibers at relatively low intensities (111). Long-term morphological adaptations to blood flow restriction training include increased muscle thickness and function in a variety of training circumstances (98,159,172).
Although sparse, a few studies have examined the effect of blood flow and metabolites during isometric training (37,140). de Ruiter et al. (37) examined the oxygen consumption characteristics of isometric contractions at several knee angles. Isometric contractions at LML (60 and 90°) consumed significantly greater quantities of blood oxygen compared with SML contractions (30°) at 10, 30, and 50% of MVC (37). These findings may, in part, explain why long-term isometric training at LML has a greater effect on muscle thickness and strength, compared with SML training at least in “healthy,” or “recreationally active” subjects (5,17,87,97,115,116,120). Schott et al. (140) compared the metabolic response and adaptations with short- (4 sets of 10 × 3-second contractions) or long- (4 contractions of 30 seconds) duration isometric contractions at 70% MVC. Although blood flow was not measured, the long-contraction limbs experienced greater changes in metabolites and larger decreases in pH (140). Muscle thickness also significantly increased in the upper (10.1%) and lower (11.1%) portions of the quadriceps in the long-contraction, but not in the short-contraction limb (140). Although blood flow restriction has many benefits in older and injured populations, it does not seem to offer any additional adaptations in healthy well-trained athletes (141). Furthermore, although low-intensity single-joint isometric contractions have been found to result in blood flow restriction, the effects of multijoint isometric and quasi-isometric contractions have yet to be examined.
Exercise-Induced Muscle Damage
Although exercise-induced muscle damage is not needed to promote muscular hypertrophy (137), emerging research suggests that exercise-induced muscle damage may play some role in morphological adaptations (138). When exposed to a novel stimulus, acute myofibril microtrauma occurs as an abundance of Ca2+ enters and remains in the myofibril (35). Eccentric muscle actions typically result in a greater degree of acute trauma as evidenced by elevated serum creatine kinase, myoglobin, and skeletal troponin-1 levels, and delayed-onset muscle soreness (32). These markers typically coincide with a temporary reduction in muscle force and power (32). Although detrimental to short-term performance, exercise-induced muscle damage is associated with changes in a variety of chemokines that attract inflammatory cells, which influence muscle hypertrophy remodeling associated with phagocytosis, free radical production, and circulating cytokines and growth factors (80). In addition, a novel delayed-onset muscle soreness inducing a stimulus may lead to increased sarcoplasmic reticulum reuptake of Ca2+ by altering t-tubule structure (35) and increasing the concentrations of proteins such as calsequestrin (20) and dysferlin (77). These proteins function to promote debris clearance and increased concentrations of IGF-1, fibroblast growth factor, nerve growth factor, and interleukin-6, which increase satellite cell proliferation (16,18) and rates of protein and collagen synthesis (76). Although acute increases in myofibril protein synthesis do not necessarily correlate with long-term hypertrophy (108), these increased synthesis rates theoretically result in thicker, stronger tissues that are less susceptible to future damage (45).
The repeated bout effect refers to the substantial reduction in muscle damage from subsequent training (105). Although this is most commonly observed after eccentric exercise (45,105), the protective effects have also been found to occur after isometric exercise (2), especially at LML (6,31). Isometric training at LML results in greater delayed-onset muscle soreness and acute performance decrements (6) as well as chronic adaptations, compared with isometrics at SML (5,87,115,116,120,160). Likewise, greater exercise-induced muscle damage and delayed-onset muscle soreness are reported after maximal-effort high-velocity (210°·s−1) isokinetic eccentric muscle actions when compared with an equal volume bout at low velocity (30°·s−1) (29). Because a greater number of high- vs. low-velocity eccentric muscle actions are needed to equalize volume, the difference in muscle damage and soreness is likely due to increasing the total number of sarcomere bonds and “popping” sarcomeres, which increase Z-disk streaming and subsequent inflammation (39,146). Similarly, 8 weeks of maximal high-velocity (180°·s−1) eccentric training resulted in greater hypertrophic adaptations when compared with maximal low-velocity (30°·s−1) training (44). Conversely, submaximal (70% 1RM) slow-velocity (∼3 seconds) eccentric muscle actions during the barbell bench press have been found to stimulate higher blood lactate and recombinant human growth hormone, by promoting a hypoxic environment (28). Although EQIs may lead to substantial levels of local fatigue due to a potential lack of blood flow and high metabolite levels, it is unlikely that the low-velocity eccentric component would produce exercise-induced muscle damage (29).
Series Elastic Component
Tendon, the primary tissue of the SEC, can undergo morphological and functional adaptations through inactivity, injury, sporting activities, and resistance training (14,86,101,127). Tendon and other connective tissues which comprise specifically aligned collagen fibers have significant resistance to mechanical strain (101). Optimal performance requires the efficient transfer of force from muscle to bone (101,114), necessitating transmission by a tendon that is sufficiently stiff to minimize electromechanical delay while avoiding rupture (101). Properly executed dynamic, eccentric, and isometric training can improve tendon structure and function (11,12,81,82,85,86,90,101,127).
A single study has directly investigated the effect of joint angle on tendon morphology by comparing volume-equated isometric knee flexion training at LML (100°) or SML (50°) (87). Although both SML (10%, ES = 0.82) and LML (11%, ES = 1.06) groups improved quadriceps volume, only LML training resulted in significant tendon stiffness improvements (50.9%, ES = 1.22) (87). Although the sparse results of the preceding studies expose a gap in the existing literature, they tend to support holding prolonged isometric contractions at LML with near maximal loads if tendon structural adaptations are paramount.
The titin myofilament, although believed to be a secondary structure to a tendon in the SEC, has several important functions and is likely partly responsible for the residual force enhancement after an active stretch (46,65,124,143–145,147). Titin adds stability, stiffness, and passive and active force transmission at LMLs (64,125) and is a likely factor in injury prevention. Several studies have found titin to regulate muscle force and length in mechanically lengthened fibers (95,125). Baumert et al. (19) examined the relationship between force production, delayed-onset muscle soreness, and genotyping related to titin stiffness (19). Subjects with the allele linked to greater titin stiffness (TRIM63 A-allele) had greater MVICs (35%, ES = 1.42, p = 0.006) and recovered more quickly (ES = 1.14, p = 0.022) compared with the other subjects (TRIM63 G-allele) (19). Titin protein fragments have been found in the urine of healthy young men after bouts of a dynamic calf-raise exercise and were strongly correlated with traditional markers of exercise-induced muscle damage (75). Although the eccentric muscle action after a fatiguing isometric with EQIs is unlikely to produce significant muscle damage due to low velocities (29), it is plausible that titin may be activated. Although occurring at a range of joint angles (143), residual force enhancement magnitude is greater at LMLs (147), suggesting that LML training may preferentially use titin (65). Thus, it may be prudent to examine the effects of quasi-isometric holds in the lengthened position on markers of breakdown and expression of titin.
Movement Velocity and Muscle Action
The SEC seems to be affected differently by movement velocity. The impact of movement velocity on titin is difficult to determine because many questions remain regarding the myofilament contributions to phenomena such as residual force enhancement (74,145). Although studies have observed the breakdown of titin after resistance training movements, which tend to be relatively slow when compared with activities such as sprinting or jumping (75), there are conflicting data regarding the velocity of stretch and residual force enhancement. Although most residual force enhancement examinations use eccentric angular velocities between 30 and 60°·s−1 (46,124,144,147), Lee and Herzog (94) compared stretch angular velocities of 10, 20, and 60°·s−1. Although eccentric force during the stretch increased with velocity, there was no significant difference in proceeding isometric force between the 3 protocols (94). However, the aforementioned research is intriguing as the effect of velocity on titin is unknown due to several confounding variables, including different neuromuscular strategies and contributions from the CE and PEC (43,122).
The relationship between velocity, residual force enhancement, and titin is not yet determined; however, the effect of movement velocity on the tendon holds greater clarity. Acutely, it seems that isometric contractions provide superior analgesic effects compared with dynamic resistance exercise (128,130,161). Rio et al. (128) examined patellar tendon pain during a decline squat exercise in 6 male volleyball players with tendonitis. The pain was evaluated before and after performing either slow isotonic leg extensions for 4 sets at an 8RM load or 5 sets of 45s isometric knee extensions at 70% of MVIC (128). Although both the isometric (−97%, ES = 3.6) and dynamic (−40%, ES = 0.67) groups significantly reduced pain acutely, pain reduction only remained significant at the 45-minute mark after isometric exercise (128). Loaded between 30 and 80% of 1RM (136,169) and maintained for similar periods as Rio et al.'s (128–130), the zero to low-velocity EQI contractions may have the potential to reduce tendon pain, despite recent controversy (57).
Long-term changes in tendon morphology seem minimal in healthy, mature human tissue (79,101). However, injured tendinous tissue can undergo dramatic adaptations (79,136). Tendon adaptation is independent of contraction type, so long as a minimum mechanical load threshold is reached (61,62,70), which likely explains why traditional exercises with an eccentric emphasis have been found to be superior to dynamic contractions in tendon rehabilitation (81,90,101). However, movement velocity is critical because healthy tendon fibers will “spare” the damaged tissue by transmitting a greater portion of the load when high velocities are used (13,101). Conversely, damaged tendon tissue can undergo sufficient loading during slow contractions (13,101). For example, Kongsgaard et al. (81) compared 12 weeks of single-leg decline squats with an eccentric emphasis, bilateral heavy and slow (3-second eccentric and concentric phases) resistance training, and corticosteroid injections. Although both resistance training groups experienced significant improvements in several measures of performance and architectural and physiological markers, the heavy and slow resistance training group reported greater satisfaction of clinical outcomes (70%) compared with the eccentric (42%) group (81). The researchers theorized that the decreased tendon pain, tendon collagen content, and voluntary force production were due to the greater intensity-induced mechanical overload throughout the training period (81). These data demonstrate that tendon adaptation can be achieved through relatively slow movement velocities and that maximal or supramaximal eccentric exercise is not necessarily required. Because EQI contractions are slow and submaximal, they may be a viable tool for treating diseased tendonous tissues.
Contraction Intensity and Duration
Contraction intensity, duration, and type have different effects on tendon properties. Kubo et al. (84) compared the effects of 12 weeks of isometric and plyometric training on muscle and tendon stiffness. Active muscle stiffness at 30, 50, and 70% of MVIC only increased significantly after plyometric training (38.1–69.6%, ES = 1.35–2.57 vs. 12.4–23.6%, ES = 0.46–0.75), whereas ballistic and ramp tendon stiffness increased exclusively after isometric training (23.7–42.1%, ES = 0.92–1.21) (84). Likewise, Burgess et al. (26) compared the effects of isometric and plyometric training on the plantar flexors. Although no statistically significant difference between groups was present (p < 0.05), the isometric group experienced very large increases (61.6%, ES = 4.91) in tendon stiffness when compared with the plyometric group (29.4%, ES = 1.44) (26). Interestingly, no significant differences between the isometric and plyometric groups were apparent for concentric-only jump height (64.3%, ES = 2.87 vs. 58.6%, ES = 2.85) or the rate of force development (28.1%, ES = 1.89 vs. 14.6%, ES = 1.38); however, no measures of stretch-shortening cycle function were included (26). These findings demonstrate that although isometric contractions are effective in improving tendon stiffness (thereby reducing electromechanical delay) and improving tendon health (14,128), improvements in stretch-shortening cycle performance likely require specific training to increase ultrasonically assessed elasticity (86–88), suggesting that isometric contractions are an effective addition to traditional resistance training.
In regards to contraction intensity, Kongsgaard et al. (82) examined the effect of a 12-week, work-equated dynamic isotonic leg extension training program using either “heavy” (70% 1RM) or light loads. The “heavy” group experienced thickening of the distal (4%, p < 0.05) and proximal (6%, p < 0.05) patella tendon, whereas the light group only saw significant proximal hypertrophy (7%, p < 0.05) (82). In addition, tendon stiffness significantly improved after “heavy” resistance training (14.6%, ES = 1.37), whereas the light load group experienced a nonsignificant decrease (−9.18%, ES = 0.83) (82). Similarly, Arampatizis et al. (11,12) compared 14-week training programs consisting of volume-equated isometric plantar flexion at low (∼55% MVIC) or high (∼90% MVIC) intensities. Only the high-intensity training groups improved Achilles tendon cross-sectional area and stiffness (17.1–36%, ES = 0.82–1.57, p < 0.05 vs. −5.2 to 7.9%, ES = 0.26–0.37, p > 0.05) (11,12). Furthermore, tendon elasticity only increased after low-intensity training (14–16.1%, ES = 0.56–0.84, p > 0.05 vs. −1.4 to 3.9%, ES = 0.06–0.20, p > 0.05) (11,12). Although the aforementioned studies investigated different tendons and used different training intensities, both point to the superiority of high- over low-intensity contractions when an improvement in tendon stiffness is desired. However, unlikely to directly improve plyometric performance, high-intensity isometric training may be a valuable tool in improving tendon thickness and stiffness, which may decrease injury rates and improve performance when included as a supplement to traditional resistance training (26,84,86,88).
Parallel Elastic Component
The effect of resistance training on the PEC and extracellular matrix is lacking due to the methodological challenge of separating connective tissue from intrafibrillar elements to evaluate their relative contributions to force transmission (131). Subjective measures such as pain and ROM are limited in utility because they contain confounding variables and often manifest gradually (131). However, we do know that the PEC is comprised primarily of collagen fibers (126) and that adding collagen around the myofibrils leads to an increase in stiffness and transmission of force to the passive structures of the extracellular matrix (52). Thus, it is postulated that the increase in extracellular matrix stiffness is a contributing factor to more energy-efficient eccentric muscle actions (149).
Several studies have examined resistance training and collagen formation in healthy humans (73,79,81,82,90,101). In situ investigations by Mass et al. (103) and Gomez et al. (56) have reported that damaged tendons and ligaments healing under tension had higher collagen contents compared with passively healing controls. It is understood that damaged tendons experience more efficient healing when factors including transforming growth-factor ß1, platelet-derived growth factor, and IGF-1 are elevated (96). Therefore, resistance training, which places a tissue under tension, increases hormonal and molecular signaling factors, providing optimal extracellular matrix maintenance in the elderly (149). In addition, resistance training can cause exercise-induced muscle damage and increase local inflammation (32,149). Muscle damage after unaccustomed loading has been observed to acutely increase collagen synthesis and extracellular matrix remodeling (72,156), whereas chronic resistance training has resulted in increased intramuscular collagen (73). Interestingly, eccentric and concentric contractions seem equally proficient for increasing collagen synthesis when total work is equated (109). However, eccentric muscle actions enable greater force production or greater work performed at the same load (39) and, therefore, lead to greater adaptation when total sets and repetitions are equal (67).
In summary, EQI training seems to offer a time- and energy-efficient means of triggering morphological adaptations in all primary components of force transmission. Therefore, EQI training should be implemented when increasing muscle size and improving tendon, and other connective tissue health is of utmost importance.
Eccentric Quasi-Isometric Contractions and Neurological Qualities
Eccentric quasi-isometric training could be expected to improve muscle function at low, but not high, velocities. Although a few acute studies are examining and describing EQI exercise on musculotendinous (24,99,150,152) and neuromuscular adaptations (4,166), the lack of any long-term investigations makes any definitive conclusions problematic. However, there is a significant amount of research examining fatiguing contractions (107,135), yielding isometrics (4,50,71,132–134,136), slow-tempo resistance training (159,170–172,174), and joint angle (37,115,116,135,154), which allow conjecture and identify areas for future research.
Contraction intent is an important factor to consider when evaluating the effect of resistance training (33). Although the intent of the trainee during EQIs is to maintain a movement velocity of zero, once isometric failure occurs at low velocity, lengthening follows despite maximal effort due to accumulated fatigue (112,142). Although a variety of isometric training and exercise methods have been described (50,71,136), most experiments have used maximal contractions against an immovable object. Although maximal isometrics serve as a valuable and highly reliable means of evaluating neuromuscular function (40,119), results from these studies are difficult to apply to EQI exercise. Recently, researchers have demonstrated that “yielding” (resisting an external force) isometrics, with the intent of preventing eccentric muscle action, creates different fatigue and neuromuscular characteristics compared with “pushing” (exerting force against an immovable object) isometrics (50,71,132–134,136). Hunter et al. (71) compared time with task failure and neuromuscular function when maintaining a constant force (pushing) of 15% of MVIC or by supporting an equivalent inertial load while maintaining a constant joint angle (holding). Pushing resulted in significantly greater time to failure (1,402 ± 728 seconds) than holding (702 ± 582 seconds) (71). Similarly, Schaefer and Bittmann (136) examined pushing and holding isometric actions at 80% of MVIC and found that subjects could maintain the target force for twice as long when pushing (41 ± 24 vs. 19 ± 8 seconds). Hunter et al. (71), Schaefer and Bittmann (136), and other investigators (50,132–134) have also demonstrated that agonist activation at failure is greater when pushing, whereas coactivation of antagonist and synergist muscles is greater when holding (50,132–134,136). Although the increased coactivation during position tasks is a likely cause of the decreased endurance time (50,71,133,136), it is plausible that position task training may lead to superior joint stabilization and thus carry value in rehabilitative settings (132). In addition, several activities and sporting actions involve bracing to avoid dynamic muscle action (93,167). Therefore, although pushing isometrics likely allow for greater morphological adaptations, due to larger forces and time under tension, training with the intent to maintain specific positions instead of exerting force against an immovable object may provide improved carry over to specific tasks that involve maintaining specific joint angles or postures due to the similarity of neural characteristics (93,168).
Ballistic and ramp contractions are additional means of distinguishing movement intent (120). When comparing the result of several isometric training studies directly comparing contraction intents, Oranchuk et al. (120) determined that training with ballistic intent resulted in constantly greater improvements in muscular activation (3/4 studies) (1.04–10.5%·wk−1, ES = 0.02–0.31 vs. 1.64–5.53%·wk−1, ES·wk−1 = 0.03–0.20) and rapid (0–150 ms) force production (3/3 studies) (1.2–13.4%·wk−1, ES·wk−1 = 0.05–0.61 vs. 1.01–8.13%·wk−1, ES·wk−1 = 0.06–0.22). Furthermore, Behm and Sale (21) compared the effects of isometric contractions performed with ballistic intent and high angular velocity (240°·s−1) concentric contractions. Both concentric and isometric training lead to similar (all p < 0.01) improvements in peak isometric force, rate of force development and relaxation, and peak torque at 14.9, 29.8, 59.6, 88.8, 173, and 240°·s−1 (21). These results highlight the importance of contraction intent, and not necessarily movement velocity, on neurological qualities and performance alterations. Although comparing the above results with EQI training is difficult, given that EQIs are nonballistic, it is reasonable to suggest that they would be unlikely to improve explosive neuromuscular performance. Thus, a progressive resistance training program to improve explosive performance would avoid incorporating EQIs in late training cycles; they may be best positioned early in a periodized plan, likely as an adjunct to traditional resistance training.
Although research on isometric contraction intensity is emerging, the only long-term training investigations examining neurological adaptations use traditional, pushing isometrics. Investigations directly comparing isometric training intensity have determined that little difference in morphological or performance adaptations exist if the total volume is equated (120). Although little evidence exists regarding different isometric contraction intensities on neurological adaptations, the wealth of data on dynamic contractions may provide insight. High-load dynamic training has been found to increase coordination (162) and reduce neuromuscular inhibition (1), which is valuable when optimal performance is desired (33). In addition, a significant portion of the existing literature has determined that high-intensity dynamic resistance training is superior for improving neuromuscular function and sports performance when compared with lower-intensity methods (33).
Motor-unit activation and muscle inhibition are strongly affected by the joint angle (114,135,154). The strain-sensing organelles of the Golgi tendon organ and muscle spindles undergo different levels of stimulation at varying muscle-tendon lengths (114,135). For example, Suter and Herzog (154) examined muscle inhibition and joint angle by comparing voluntary force and force produced by superimposed femoral nerve stimulation at 15, 30, 45, 60, and 90° of knee flexion. Although muscle inhibition was present at all assessed joint angles, the largest superimposed twitches were present at LMLs (154). Greater muscular stretch, patellofemoral pressure, and ligament strain at knee angles between 45 and 60° of flexion are theorized to underpin the greater degree of muscle inhibition (154); however, these observations are not necessarily applicable for all joints or movements to differing tendon structural properties, fascicle lengths, and cocontraction dynamics (8,22,60,78,92,173). Although muscle inhibition is necessary for extreme situations, improving muscular activity is important when returning to activity or when optimizing performance (33,114).
Advantages of LML isometric training for improving muscle size and force production throughout a full ROM exist (5,17,87,115,116,120,160). Interestingly, studies investigating the effect of restricted ROM resistance training have determined that limiting dynamic contractions to LML does not result in meaningful changes in the length-tension relationship (164,165). Although EQI contractions use a full ROM, they are inherently low velocity. Therefore, EQI training should be implemented early in a yearly training plan to improve morphology and improve position-specific functions.
Applications to Performance and Rehabilitation
Performance in sport is dependent on a variety of physical qualities. As such, training methodologies have differing utility and value depending on the type of sport, proximity to competition, individual training age, and a multitude of additional factors. With few exceptions (93,168,169), quasi-isometric and EQI contractions have not been widely used in training plans. However, although no direct investigations on EQI contraction or training exist, relevant research (e.g., isometric, eccentric, time under tension, and blood flow restriction) suggests that EQIs may have a place in intelligently designed programs. The theoretical potential of EQIs in relation to dynamic (eccentric and concentric), eccentric-only, and isometric resistance training is summarized in Table 1 (based on Ref. 153).
A systematic increase in the exposure to the total volume that a muscle or muscle-group undergoes is a common means of improving muscular endurance (83,169). Training with EQIs may have the potential to provide a unique stimulus for promoting muscular endurance, as a primary aim is to increase the amount of time that the prescribed position is maintained. In addition, although a high volume of submaximal dynamic contractions is commonly used to improve muscular endurance, the constant muscular tension present in isometric and quasi-isometric contractions can alter blood flow and muscle oxygenation (4,150). Although far from conclusive, this mild and temporary alteration in oxygenation may lead to alterations in aerobic and anaerobic enzymes and significant, yet temporary, increases in several anabolic signaling factors (111,158). Furthermore, muscular endurance training may lead to adaptations to the t-tubule structure and increase Ca2+ reuptake (35), therefore offering a protective effect from delayed-onset muscle soreness and short-term performance decrements that may occur from future high-load training (30,31).
Eccentric quasi-isometric training may also offer a novel sport-specific training stimulus to athletes who undergo regular, sustained quasi-isometric contractions. Although actual sports participation offers the greatest level of sport-specific adaptation, using quasi-isometric or EQI contractions in a controlled environment, such as a weightroom, offers the ability to apply focused overload. For example, a speed skater wishing to increase lower-body muscular endurance in a skating-specific ROM, via morphological adaptations, may wish to experiment with quasi-isometric or EQI training by using a leg press (Figure 3).
Muscle mass is highly related to strength (53) and is, therefore, an important factor in sports performance. Although heavy loading, including supramaximal eccentric training, offers a strong stimulus, total work and training volume are the most important determinants of hypertrophic adaptation (91,110,137,139). Although moderate resistance training allows for a time-efficient means of accumulating volume, EQIs may be superior in specific circumstances. Depending on the intensity, initial joint angle, and other factors, EQI contractions can expose a muscle group to substantial total load in a relatively short period. Eccentric quasi-isometric contractions also offer a likely advantage over dynamic training when it comes to accumulating volume as shortening contractions are less energetically and mechanically efficient (110). Although a nonlinear, inverse relationship exists between intensity and time under tension (83,169), exclusion of the less efficient concentric phase allows for higher intensities for throughout a set duration or more work at the same intensities (38,110). Therefore, a single EQI contraction would likely impart greater time under tension than a similarly loaded set of dynamic contractions when both are taken to failure. Likewise, similar set durations could be met with a greater external load applied to an EQI contraction compared with a dynamic alternative. In addition, EQIs are likely to reduce muscle oxygenation and metabolite clearance (4,150), which may lead to preferential recruitment of type-II muscle fibers with increased capacity to increase cross-sectional area and force production, and signal anabolic hormones known to contribute to the hypertrophic response (98,140,158).
Strength and Power
A variety of morphological and neurological factors including muscle size, muscle fiber type, and motor-unit recruitment characteristics determine strength and power (7,33,34,38,43,48,93,114,153,163,172,175). Although EQI contractions may be a viable tool for improving total hypertrophy, an abundance of evidence supports the use of high-velocity contractions and maximal to supramaximal loads for preferentially targeting type-II muscle fibers (48,121). From a neurological perspective, the ability to express maximal force and power is contingent on several factors. Although isometric and eccentric resistance training can lead to the neurological and neuromuscular adaptations of rate coding, agonist, antagonist, and synergist activation and coactivation, the adaptations aforementioned are highly specific (1). As EQI contractions are inherently submaximal and intentionally low velocity, it is likely that direct carry over to high threshold activities would be minimal. However, the slow, relatively high accumulated loading synonymous with EQI contractions may potentially lead to improved rates of collagen synthesis and stiffness of the SEC and PEC (81,90,101). There is reason to believe that these morphological adaptations may improve force transmission by decreasing the electromechanical delay, therefore improving the rate of force development and stretch-shortening cycle function (52,101). Quasi-isometric and EQI contractions are postulated to build position-specific strength and potentially reduce injury risk (167,168). Verkhoshansky and Siff (169) described weightlifters using EQI training to strengthen key positions in their weightlifting pulls. For example, a weightlifter who struggles to maintain an ideal position throughout the “first pull” (151) may wish to experiment with EQI contractions (Figure 4).
Injuries to any of the 3 components of force transmission require mechanical overload at some point in the rehabilitation process (101). Isometric and quasi-isometric exercises are already commonplace in the initial phases of muscular and tendon rehabilitation protocols as they enable tight control over ROM and intensity (54,83,128–130,161). Sustained submaximal isometric contractions avoid large peak forces and acutely reduce tendon pain, potentially allowing for periods of pain-free dynamic exercise (129,130,161). Furthermore, although progressive mechanical tension is crucial (79,81), slow movement velocities should be prescribed to stimulate damaged fibers (13). Therefore, the combined static and lengthening phases of EQI contractions may provide an analgesic effect while stimulating connective tissue reformation. In the case of serious injuries, such as bone fractures or severe connective tissue strains, patients may undergo a period of full or partial immobilization. These periods of immobilization often result in significant muscle atrophy and fascicle shortening (155). Eccentric quasi-isometric exercise may offer a submaximal means of improving tendon morphology, work capacity, muscle thickness, and neuromuscular function while returning fascicles to a normal length (101). Eccentric quasi-isometric contractions can be performed with a wide range of loads and can be easily implemented through a specific ROM. For example, a patient may experiment with EQI contractions by performing an EQI elbow flexion, with the torso inclined, until the elbow reaches the end ROM (Figure 4). At this point, a second EQI with a focus on the shoulder flexors can be initiated to impart further mechanical loading, metabolic stress to the target tissues (Figure 5). Although currently highly speculative, a hypothetical training plan including EQIs for an athlete recovering from patellar tendonitis is provided in Table 2.
Owing to the lack of any long-term investigations regarding EQI or quasi-isometric resistance training, limitations are abundant in this review. Like many methods of resistance training, EQIs can be applied with an endless combination of variables including intensity, contraction duration, repetitions, sets, rest periods, frequencies, and exercise selection. Any adjustment to the aforementioned parameters will alter the resemblance of EQI to traditional methods. Similarly, much is left to be determined regarding established training methods such as isometrics. For example, although the characteristics of “pushing” and “holding” isometric contractions differ (50,71,132–134,136), there is a paucity of research examining long-term consequences to such altered loading parameters. Researchers and practitioners need to progress the knowledge and understanding of the acute and short-term neuromuscular, biomechanical, and metabolic effects of quasi-isometric and EQI contractions (50,71,132–134,136). Furthermore, long-term investigations are needed to compare the potential structural and functional adaptations to established training methods.
It is common for “novel” training methods to precede evidence-based practice. Although there are limited data on long-term adaptations, short-term investigations, anecdotal evidence, and relevant scientific knowledge make a strong case for the investigation of EQI loading and training. Based on the existing literature, the value of EQIs seems to relate most strongly to triggering morphological rather that neuromuscular adaptations and is likely best applied early in a periodized training plan, distal to high threshold neuromuscular work. Quasi-isometric and EQI training may also hold value in prehabilitation and rehabilitation contexts to modify muscle-tendon structures, provide analgesic effects, and closely match functional movements from a neurological perspective. Finally, EQIs seem to have the potential to provide an efficient means of increasing total load and volumes in specific positions.
Examination of EQI muscle actions and training is required due to the complete lack of direct empirical evidence investigating this area. As such, several foci for potential research exist. From an acute standpoint, EQI contractions may involve unique neuromuscular activation and muscular contraction dynamics that would be worthy of investigation. Researchers may also wish to compare the short-term effects of EQI exercise with volume-equated modalities such as dynamic or isokinetic contractions on neuromuscular fatigue, delayed-onset muscle soreness, or the repeated bout effect. Furthermore, long-term training studies are required to determine optimal loading parameters and exercise selection, as well as whether adaptation is population-specific.
D.J. Oranchuk is supported by the Auckland University of Technology's Vice Chancellors Doctoral Scholarship.
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