The job description of a strength and condidioning specialist will change with degrees of athlete preparation, injury prevention, and injury rehabilitation. For these requirements, the situational needs usually involve a blending of certain muscle actions—isometric, concentric, eccentric, or a coupling of an eccentric–concentric muscle action termed a stretch-shortening cycle (SSC). Understanding these muscle actions and how to program them into a resistance training protocol is an essential component of successful strength and conditioning practice.
Most human motion involves this eccentric–concentric coupling or SSC muscle action. It is the eccentric phase of this coupling that provides the focus of this article. This article discusses how we can modify the stress (load), strain (length or amplitude of movement), and velocity during the eccentric phase, to impose a variety of mechanical stimuli that have different adaptational and functional effects. Specifically, we will address the use of eccentric resistance training for (a) tendon injury rehabilitation via tendinous remodeling; (b) muscle injury prevention via shift in the optimum length of muscle; (c) supramaximal and/or accentuated eccentric loading (i.e., loads exceeding the 1 repetition maximum (1RM) and/or greater than the concentric load) for strength, performance, and hypertrophy; and (d) improved sports performance via SSC optimization. The physiologic rationale for each form of eccentric training and simple loading parameters are briefly described. This is by no means an exhaustive review of each of these areas but rather a brief introduction to the many faces of eccentric training.
ECCENTRIC LOADING FOR REHABILITATION OF ACHILLES TENDINOSIS
Achilles tendinosis etiology is a degenerative process because of overuse where there are no inflammatory cells, but changes of the collagen fiber structure result in the tendon's inability to adapt to changes in loading patterns (15,17,23). In addition to a loss of collagen fibers, there is also a loss of fiber cross-links, which results in a reduction in tendon strength (2). Furthermore, as tendons are collagenous structures with limited blood supplies, their slow metabolic rate leads to an impaired healing response (5). Many authors have considered achilles tendinosis as an overuse injury from repetitive loading, but others have observed the condition in sedentary populations (11,52). Depending on an individual's activity level, even short walks could be enough loading to cause overuse symptoms. Other suggested etiologic factors include aging, decreased blood supply, decreased flexibility, muscle imbalance, decreased concentric and eccentric calf strength, faulty musculoskeletal alignment, and training errors (2).
One form of treatment that has gained popularity in the rehabilitation of achilles tendinosis is eccentric strength training. Eccentric training models were based on the belief that tendon injuries could result from tensile loads exceeding the tendon's mechanical strength (24). If these loads were repeated, as in many activities and sports, then a symptomatic tendon condition could result. The eccentric training theory promoted the importance of structural adaptation of the symptomatic tendon, so it could cope with the increased repetitive loads and prevent injury (24). Early researchers believed that because most tendon injuries occurred during the eccentric phase of muscle work, eccentric exercise would be a viable treatment modality (75). Therefore, many earlier researchers recommended eccentric training programs to increase eccentric strength in the tendons during running and jumping (32). Currently, rehabilitation programs involving eccentric training using loads greater than body weight have provided positive results in the treatment of achilles tendinosis, with a decrease in pain and a higher percentage of patients returning to preinjury levels of physical activity (8,12,14,15,21,23,37,51,59).
There are many proposed reasons why eccentric strength training is an effective means of rehabilitation of achilles tendinosis. The most obvious is the increased eccentric strength of the calf muscles (2,6,52). Alfredson (1), and Alfredson and Lorentzon (2), a leading researcher in the area of achilles tendinosis, stated that the effects of loading induced hypertrophy, increased tensile strength, and the effect of stretching the muscle tendon unit were all positive adaptations from eccentric training. Tension produces electric potentials that help organize charged collagen fibrils (41). It has been proposed that eccentric strengthening can promote collagen production by activating mechanoreceptors in tenocytes (41). The improved strength and collagen production may be very important in breaking the tendinosis cycle. Lengthening of the muscle tendon unit results in less load on the tendon and better range of motion at the ankle joint (2,27,38,52).
Eccentric training is thought to alter the pain perceptions from the tendon (2). Eccentric strengthening regimens have resulted in decreased tendon volume and improved healing with collagen deposition and restoration of the matrix component through cell to cell communication (1,57,70). Also sensory neuropeptides (53) have been found in the achilles tendon after eccentric training, which (9) could stimulate an inflammatory response, increase blood flow, and thus promote tendon healing.
Despite the research interest in eccentric training, the exact loading parameters for eccentric strengthening of the achilles tendon remain far from clear. This is due principally to the research in this area suffering a variety of methodologic limitations. With this in mind and in an effort to provide some guidance as to the optimal loading parameters for eccentric rehabilitation for clinicians, and strength and conditioning specialists, a review of some of the eccentric loading parameters used in research for achilles tendinosis rehabilitation are detailed in Table 1.
Irrespective of the training programs used by the authors in Table 1 and the methodologic limitations of the studies, most studies reported positive functional outcomes. From the studies presented in Table 1, the following guidelines and suggestions are recommended for eccentric training in the rehabilitation of achilles tendinosis. A standing straight leg calf extension appeared to be the exercise of choice; however, further research is needed to determine if any benefit is gained from seated calf extensions (where gastrocnemius contribution is minimized) and other calf exercises. With regard to the exercises used in rehabilitation, the exercises appear easy to execute, can be performed at home, require minimal cost, and do not seem to suffer from major technical requirements or associated complications.
A minimum duration of 12 weeks is recommended for eccentric strengthening programs, given the research into the time course of tendon remodeling and regeneration. Three sets of 15 repetitions (reps), with increasing repetitions, appears the most common loading progression. However, these authors believe that it may be advantageous to decrease the repetitions and increase the load in keeping with traditional eccentric strength training guidelines. This contention is supported by the finding that moderate to heavy loads produced the most significant changes in visual analog scale (VAS) scores in those studies that used the VAS scale as the variable of interest, that is, indicator of pain (27,73). In terms of pain tolerance, it would seem that some pain is inevitable with eccentric training and regulating pain thresholds to no greater than 5/10 on the VAS scale would seem prudent. Another manner in which the loading could be progressed is via the use of different training modalities. This could involve progression from rubber tubing training, body weight, body weight plus load e.g., (backpack), and then the use of resisted strength training equipment whereby supramaximal loading can occur. Thereafter, fast eccentric loading through a full range of motion may be attempted, for example, jump training or drop jump training from progressive heights. It is difficult to disentangle an optimal frequency of training per week, but it would seem safe to perform these exercises daily and even twice daily given the frequency statistics observed in Table 1.
Adopting such exercises and loading parameters may prevent surgical intervention for sufferers of achilles tendinosis (4). Furthermore, for normal populations, standardized eccentric training programs could be used as preventative measures or as part of a conditioning program. It should be noted, however, that a great deal more research is needed in this area, particularly research that has an integrated clinical/physiotherapeutic and strength approach.
SHIFTING THE OPTIMUM LENGTH WITH ECCENTRIC EXERCISE
All muscles have an optimum length for producing peak tension. As the muscle continues to lengthen beyond its optimum length, tension levels decrease. This descending portion of the length–tension curve is thought to be the region of vulnerability in which muscle strain injuries occur. Many believe that athletes who produce peak tension at shorter than normal muscle lengths are more likely to suffer an acute muscle strain injury (15,18,50,62). Brockett et al. (14) explored this idea by measuring the optimum lengths in athletes who had previously injured their hamstrings. One leg served as the experimental leg (i.e., previously injured hamstring) and the other leg served as the control leg (i.e., noninjured hamstring). The previously injured hamstring produced peak tension at 12.7° less than the noninjured hamstring (i.e., shorter optimum length). It was also reported that the difference between eccentric and concentric hamstring strength was not different between legs. The authors concluded that the optimum length of peak tension was a greater risk factor for future muscle strain injuries than strength ratios.
It has been suggested that muscle strain injuries could be reduced if the optimum length is shifted to a longer length as is shown in Figure 1 (14,15). The only form of training that has been shown to consistently increase the optimum length of tension development has been eccentric exercise (15,30,62). The shift has been shown to occur in the elbow flexors, plantar flexors, knee flexors, and knee extensors (12,14,21,60,76). The magnitude of the shift depends on 3 variables: the load of eccentric exercise, the volume of eccentric exercise, and the length of the muscle during eccentric muscle actions. Shifts in optimum length have varied from 3.9° (76) to 18° (61) after eccentric exercise. The studies reporting the greatest shifts used protocols with either high volume or high load at long muscle lengths.
Since the exercise known as the “Nordic hamstring exercise,” shown in Figure 2, was detailed in 2001 by Brockett et al. (14), 5 studies have documented the effects of eccentric exercise on hamstring injuries in elite soccer, rugby, and Australian rules football players (7,10,18,30,62). However, none of these studies measured the optimum length of the hamstrings; thus, the shift in optimum length was not quantified nor was its significance investigated. Four of the studies used the Nordic hamstring exercise, and 1 study used the Yo-Yo hamstring curl exercise (10). The Yo-Yo exercise involves the athlete performing eccentric leg curls in the prone position. The Yo-Yo device is basically a flywheel that is accelerated during the concentric contraction and then decelerated during the eccentric muscle actions of the hamstrings. Similar to the Nordic hamstring exercise, the Yo-Yo leg curl is a bilateral open chain exercise.
Despite the recent interest in eccentric exercise for shifting the optimum length and reducing hamstring injury rates, loading patterns for athletic populations are yet to be developed. This is mostly because of the fact that there is currently no research that has reported the effects of both a shift in optimum length and injury rates in the same study. In addition, the majority of studies reporting a shift in optimum length have been acute muscle damage studies and not training studies. Currently, there are only 2 training studies in the literature. One of these was a pilot study (21) and the other did not use an athletic population (42).
All the studies in Table 2 reported either a shift in optimum length or a reduction in hamstring injury rates. The following guidelines are presented for shifting the optimum length to longer lengths: eccentric muscle actions should be performed at long muscle lengths; muscle contraction load should be moderate to high; the combinations of long muscle lengths/high load or long muscle length/high volume result in the greatest acute shifts; it is possible to maintain the shift for over 4 weeks; and muscle damage is not needed to induce a shift.
For reducing injury rates in chronic studies, the Nordic hamstring exercise appears to be the exercise of choice as observed in Table 2. However, there are a few limitations with the Nordic hamstring exercise as it is a bilateral and single joint exercise. Because hamstring injuries occur during unilateral and multi-joint movements, a more functional approach to exercise design is needed (19). Exercises should be developed that involve eccentric hip flexion as well as eccentric knee extension. Despite these limitations, each of the studies investigating the effects of eccentric exercise on hamstring injury rates has reported fewer injuries. Four of these studies used similar protocols of 2–4 sets of 6–12 repetitions. Only Gabbe et al. used a different protocol with very high volume (i.e., 12 sets of 6 repetitions). It should be noted that the compliance rate was very low in this study. Therefore, to prevent low compliance rates and to reduce injury rates, it is recommended that approximately 3 sets of approximately 8 repetitions be performed twice a week. Furthermore, it is recommended that it may be the best practice to include ground-based eccentric exercise training (19) at the beginning or completion of practice (20).
ACCENTUATED AND SUPRAMAXIMAL LOAD ECCENTRIC TRAINING
It has been shown that humans are able to recruit fewer motor units (with the same force development) during an eccentric muscle action than a concentric contraction at a given or absolute load. Therefore, the neural efficiency of eccentrics is greater, and it has been suggested to maximize neural activation and subsequent strength adaptation; during eccentric muscle actions, greater loads are required (40,45,64,83). It seems therefore that the eccentric portion of a traditional resistance exercise is underloaded even when the concentric portion is at maximum. Some research has suggested that subjects may be as much as 20–60% stronger eccentrically than concentrically (34). Given this information, a subject would be able to lower (or yield to) a weight much heavier than he/she can overcome concentrically, as much as 120–130% of the (concentric) 1RM (35,39,47). Logically, if the same subject is attempting to improve strength or hypertrophy using the overload principle (when a muscle increases in size and/or strength when forced to contract at circa maximal tension), then he/she would be able to increase the overload substantially by implementing supramaximal eccentric loads (SME) (16). Considering the potential performance enhancement possibilities of SME training, relatively little research has been devoted to exploring the possible benefits of SME training, and although some of the results have been positive, the benefits do not appear to be exclusive to eccentric training (22,74).
Some research has attempted to exploit these potential benefits using accentuated eccentric loads (AELs). AEL is similar to SME, in that the load used for the eccentric portion of the lift is greater than that which is used during the concentric. The fundamental difference between the 2 loading protocols is that the eccentric load used in AEL is not supramaximal.
The effects of SME or AEL on concentric strength are shown in some of the studies in Table 3. All studies compared an SME or AEL approach with a group who implemented a standard resistance training program (ST) where the concentric and eccentric load was the same. The greatest improvements using SME (110–120% 1RM) were seen by Brandenburg and Docherty (13) who reported a statistically significant (p < 0.05) increase in elbow extensor strength of 24% compared with 15% using ST, after 10 weeks of training 2–3 times per week. Kaminski et al. (39), while using an AEL protocol, reported similar findings: the AEL group significantly (p < 0.001) improved overall hamstring strength 29% and the ST group improved 19% after 6 weeks of training 2 times per week. More recently, Sheppard et al. (71) and Sheppard and Young (72) demonstrated that AEL benefited not only the bench throw exercise but also peak power in the countermovement jump.
Whether comparing an SME or an AEL approach with ST, all groups improved concentric strength although not always to the same degree as reported previously. Such was the case with Godard et al. (31) when they studied the effect of SME (120% 1RM) on quadriceps strength. They found that after 10 weeks of training 2 times per week, participants did not improve concentric strength in one group significantly more than the other. Brandenburg and Docherty (13) reported similar findings with the elbow flexors (SME increased 10% and ST increased 9%).
One of the issues with some of the current research is that despite an AEL or SME approach, eccentric strength is rarely assessed. Considering the well-documented principle of training specificity, eccentric strength has been shown to be best enhanced by eccentric-specific training (69). Hortobagyi et al. (36) recognized this limitation and addressed this in his study as well as any potential neurologic benefits to AEL training. As expected, eccentric strength gains for the AEL (27%) group were double that of the ST (11%) group; and any increase in concentric 1RM was accompanied by a directly proportional increase in electromyographic activity.
Neither SME nor AEL has demonstrated any advantage over ST with regard to hypertrophy. Ojasto and Häkkinen (58) subjected healthy men to a hypertrophy training protocol using AEL and found that it was not more favorable for hypertrophy when compared with ST. Other studies have shown that both AEL and SME present no clear benefit in increasing muscle cross-sectional area. Eccentric training at high speeds (180° per second) has been shown to be more effective for strength and hypertrophy than comparable concentric training (13,31,56,81). Higbie et al. (33) demonstrated that eccentric strength was best developed by eccentric-based training, whereas concentric strength was best for developing concentric strength.
It is very difficult to make conclusions as to the benefit of AEL or SME for sport performance enhancement. Based on the current research as shown in Table 3, there does appear to be certain advantages to an AEL or SME protocol for untrained populations, those requiring acute benefits from strength training, training specific muscle groups and for athletes required to perform at levels above lactate threshold or for specific muscle groups (13,36,39,80,81).
It should be noted that the research tabulated and discussed in this brief treatise is not without limitations. With the exception of Brandenburg and Docherty (13), all research used a nonathletic population. It is well known that untrained subjects respond differently to well-trained individuals. Furthermore, all studies, with the exception of the Yarrow et al. (80,81) and Ojasto and Häkkinen (58), chose single joint exercises such as knee extension, leg curls, and elbow flexion and extension. Although these exercises have merit in certain situations, increased performance in them is not indicative of potential for athletic performance, which requires multi-joint movement. Furthermore, the training protocols chosen for the studies, such as training every day only for 1 week or 1 set per day twice a week, are often inconsistent with that which are typically seen in sports performance–based training programs.
There is a clear need for future research. It is suggested that SME be used with multi-joint exercises such as the squat and dead lift on participants who are actively involved in competitive sport and/or resistance training. The program chosen needs to be volume adjusted to compensate for the increased eccentric load, and the set/rep/rest/frequency variables should be consistent with a sports performance–based program.
ECCENTRICS FOR STRETCH-SHORTENING CYCLE PERFORMANCE
When a muscle is required to overcome resistance, or contract concentrically, its ability to do so may be determined by whether that concentric contraction was preceded by an eccentric muscle action. Research has shown that concentric force production in isolation is relatively low compared with concentric contractions that are coupled with an initial eccentric muscle action (44). This pairing is termed the SSC as defined previously. The SSC may have large or small amounts of angular displacement of the relative joints, and it is composed of both voluntary and involuntary (stretch reflex [SR]) actions (46,68). For optimal SSC potentiation (i.e., a more forceful concentric contraction), a number of factors are thought critical:
- Preactivation of musculature before contact.
- Little or no coupling times (i.e., time between the termination of the eccentric phase and the onset of the concentric phase).
- Short-duration contractions.
- High eccentric muscle action velocities.
- Relatively small amplitude movements.
Dietz et al. (25) demonstrated that before ground contact in a drop jump, the extensor muscles were activated and it is this muscle action that created muscle stiffness that minimized the amount of lengthening taking place in the muscle itself, and most of the length change therefore took place in the tendon of the extensors upon contact with the ground (28). The force that is created upon ground contact produces energy, which is stored principally in the tendons and helps to create a more powerful concentric contraction.
Coupling times: Short contraction durations
Elasticity refers to the ability of an object to return to form after it has been altered, and elastic energy is the work done during this process (26). A number of tissues can store elastic energy during an SSC including the muscle's connective tissues (e.g., perimysium, epimysium, endomysium), structures in series with the muscle fibers (e.g., tendon, titin), and the contractile elements themselves. The latter occurs within the cross-bridges between filaments when the actual muscle lengthens without the “popping” of the actin–myosin cross-bridge. This energy stored in the various tissues, however, is finite in duration with a half-life of 0.85 second and a 55% decrease by 1.0 second; therefore, to make the most of the stored elastic energy, the coupling times need to be minimal and the SSC should last less than 0.25 milliseconds (66,78). That is, the force generated during the concentric phase will tend to be higher when the duration of the SSC is shorter. As the SSC duration lengthens, the benefits of stored energy dissipate (79).
High eccentric velocity: small amplitude movements
When preactivation occurs and when the athlete makes contact with the ground, a reflexive action results called an SR (54). The SR is a by-product of a signal sent by the spindles in the muscles to the central nervous system. Muscle spindles are receptors in the muscles, and they provide information about length and velocity of length change. In the drop jump, as the athlete makes contact with the ground, the muscle spindle senses the lengthening of the affected muscles (ankle plantar flexors and knee and hip extensors) and a signal is sent to the spinal cord via sensory motor neurons. A synapse occurs in the spinal cord, and excitatory messages are sent to the muscles via alpha motor neurons, which produce a concentric contraction in the muscles (to return the spindle to its initial length). The higher the velocity of the stretch, the greater potentiation of a reflexive forceful concentric contraction; this reflex is dependent on the level of motor neuron excitation and the amplitude of the movement, that is, small relative joint motion.
Research has shown that the optimal amount of energy stored during an SSC is largely determined by the amplitude of the relative joints. That is, some joint movement is necessary; however, too much angular displacement will decrease the number of actin and myosin cross-bridges interacting with each other and decrease reflex potentiation, which ultimately affects the storage and utilization of elastic energy within the muscle, thereby reducing its force production capability (63). Rack and Chu (63) demonstrated that drop jumps where the subjects maintained knee angles of less than 75° allowed them to keep their foot contact time to 416 ± 41 milliseconds, which created greater concentric force production than jumps where the subjects had knee angles greater than 85°. Longer contact times are indicative of larger stretch amplitudes of the relative muscles, and when muscles are stretched beyond a certain point, the resulting concentric contraction no longer benefits from the SR (49).
The training status of the athlete determines the optimal peak eccentric velocity or load of the exercise. If the athletes are untrained, fatigued, or both, then it will affect their ability to perform the SSC task appropriately. In untrained athletes, the peak eccentric velocity will be less to keep the amplitude of movement minimal. This athlete will have coupling times that are longer in duration and/or movement amplitudes that are too great, if eccentric velocity before contact is too high. That is, if the stretch load is too great and there is a large amplitude movement or the peak eccentric velocity is too great for the musculotendinous unit, the coupling time could be substantial and the benefits of the SR are therefore minimized or mitigated once more.
In summary, the stress, strain, and movement velocity associated with eccentric training can result in very different adaptational and functional outcomes, which are summarized in Table 4. As the strength and conditioning specialist gains a greater understanding of the eccentric phase and its defining characteristics, he or she will be able to systematically implement eccentric-based training for a variety of goals. For example, injuries may be rehabilitated or prevented by means of tendon remodeling and/or injury prevention by shifting the optimal length of a muscle to produce peak tension, respectively. Performance may also be enhanced with eccentrics by using accentuated and/or supramaximal loading as well as optimizing the SSC by ensuring high-velocity eccentric loading. Strength and conditioning for many athletes/sports will involve a blending of all these types of eccentric loading, and the art will be in ensuring that the effects sought with each type of loading are optimized. Therefore, careful within-session, short-term, and long-term planning is needed.
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