Resistance training (RT) can be performed with 3 distinct types of muscle actions: concentric, eccentric, and isometric. Concentric actions occur when a muscle produces force, as it shortens; eccentric actions occur when a muscle produces force, as it forcibly elongates; and isometric actions occur when a muscle produces force without a change in its length (24). Of these actions, it has been postulated that eccentric actions are the most important from a hypertrophy standpoint. This hypothesis is supported by findings that eccentric actions elicit more rapid elevations in muscle protein synthesis and greater increases in intracellular anabolic signaling and gene expression versus concentric actions (6,11,19,29). However, acute studies cannot necessarily be generalizable to long-term adaptations. Results from longitudinal studies examining muscle actions and hypertrophy adaptations are somewhat equivocal on the matter. Although some studies show that eccentric actions promote greater muscle growth compared with concentric and isometric contractions (8,13,17,20), others have failed to show significant hypertrophic differences between conditions (9,11,18).
A recent meta-analysis by Schoenfeld et al. (28) sought to provide clarity on this topic. Pooled analysis of 15 studies directly comparing hypertrophic changes between different dynamic muscle actions showed a greater magnitude of effect for eccentric versus concentric RT, but results did not reach statistical significance. When quantified on a percentage basis, the average gain from eccentric actions corresponded to 10.0% compared with 6.8% for concentric actions, a finding that can be deemed potentially meaningful from a practical standpoint. A potential confounding issue with these findings is that most studies matched the number of repetitions as opposed to total work. Given that strength capacity for eccentric actions exceeds that of concentric actions by as much as 50% (1), it cannot be ruled out that growth-related advantages favoring eccentric exercise resulted from the greater loads used during training. Moreover, many of the studies were performed using isokinetic dynamometry, and thus, the results cannot necessarily be generalized to isoinertial RT commonly performed in gym settings.
It has been proposed that an increase in muscle damage may at least partially account for the superior hypertrophic effects of eccentric actions (26). Although myodamage is seen with the performance of concentric and isometric training (4,15), eccentric exercise causes the greatest damage to muscle tissue (7), purportedly because of heightened forces distributed among fewer active fibers (24). Muscle damage is considered to be 1 of the 3 primary mechanisms of exercise-induced muscle hypertrophy (25). Although the factors responsible for this phenomenon remain to be elucidated, current theory postulates that structural microtears to contractile elements and the extracellular matrix initiates anabolic processes to reinforce these tissues and thereby safeguard them from future injury (2). Intriguingly, there is some evidence that concentric and eccentric actions elicit regional-specific adaptive hypertrophic responses in the vastus lateralis, with greater distal growth from eccentric actions and greater growth in the midportion of the muscle from concentric actions (11). The reasons for these findings have yet to be elucidated, but it has been speculated that they may result from regional muscle damage along the length of the fiber that, in turn, causes nonuniform alterations in muscle activation (16).
The potential hypertrophic benefits to eccentric training raise the possibility that muscular growth could be enhanced by supplementing RT with eccentric overload training (23). Such a strategy involves performing isolated eccentric actions with a load greater than the concentric 1 repetition maximum (RM) or performing concentric actions at a given load, and increasing the load over the concentric 1RM on the eccentric part of the lift. Hypothetically, the greater magnitude of load used in this type of training may enhance the growth response by heightening stimulation of type II fibers and increasing structural perturbations to the working musculature. In addition, there may be beneficial effects on regional hypertrophic adaptations, particularly in the quadriceps femoris. Some studies also report that eccentric overload training might increase metabolite accumulation, specifically blood lactate (32). Metabolite accumulation has also been hypothesized to be an important factor contributing to muscle hypertrophy (27).
Recently, Fisher et al. (10) directly investigated the effects of coupling isoinertial eccentric overload training with traditional RT on measures of lean mass. A cohort of young, resistance-trained men and women were randomly assigned to 1 of the 3 different groups: (a) a traditional training group that performed 8–12RM of combined concentric and eccentric actions twice per week; (b) an eccentric training group that performed 1 traditional training session and 1 eccentric-only session per week; or (c) an eccentric-only group that performed 2 weekly eccentric training sessions. Eccentric training was performed at a load equating to 105% of the subject's 1RM, allowing muscular failure to occur in ∼6 repetitions. At the conclusion of the 10-week study period, no significant differences in fat-free mass changes were found between conditions. Although these findings suggest that there is no advantage to eccentric overload training, it should be noted that measures of fat-free mass were obtained by air displacement plethysmography, which includes all nonfat components of body composition and therefore is not specific to changes in muscle mass.
Brandenburg and Docherty (3) compared a group training at a load of 75% of 1RM for 4 sets of 10 repetitions with a group that used the same load for the concentric part of the lift but increased the load to ∼115% of the concentric 1RM for the eccentric portion. To equate for volume load, the latter group performed 3 sets of 10 repetitions. The sample consisted of 18 young trained men, and training was performed using the preacher curl and supine elbow extension exercises. Magnetic resonance imaging (MRI) was used for the assessment of hypertrophy, which is considered to be the “gold standard” measurement tool (21). Surprisingly, after 9 weeks of the training intervention, no pre-to-post changes in muscle cross-sectional area occurred in both groups. Of importance is the fact that 2 participants in the eccentric overload group dropped out of the study because of forearm pain, which might have been caused by this type of training.
Walker et al (31) compared the effects of accentuated eccentric loading (concentric load + 40%) with traditional RT methods. The eccentric load was added using weight releasers for the leg press, whereas for the leg extension, the researchers manually had to add and remove weight plates. Results showed that both groups increased quadriceps cross-sectional area from pre-to-post intervention, but no significant differences were noted between groups.
Based on these findings, it seems that added eccentric loading does not contribute to additional muscle hypertrophy. However, both Brandenburg and Docherty (3) and Walker et al. (31) used in vivo measures of hypertrophy (MRI and ultrasound, respectively) which do not detect changes at the muscle fiber level. Friedmann-Bette et al. (14) also reported that similar increases in muscle hypertrophy (as assessed by MRI) occurred between the traditional RT and the eccentric overload group. However, in addition to MRI, an in vitro method of hypertrophy (muscle biopsy sampling) was also performed. Muscle biopsy data showed that only the group that trained with added eccentric load experienced a significant increase in the cross-sectional area of type IIX muscle fibers. In both groups, no significant changes were noted for type I and type IIA muscle fibers. Such findings support the notion that eccentric overload training induces preferential type IIX muscle fiber hypertrophy.
There is also evidence that eccentric actions may promote different structural remodeling compared with concentric actions. Specifically, research indicates that eccentric training increases the addition of sarcomeres in series (11,22). This can favorably alter the length-tension relationship, resulting in a greater maximum velocity of shortening of muscle fibers (12). The extent to which eccentric overload training can promote in series hypertrophy remains to be determined, but the potential for eliciting such adaptations provides a rationale for its inclusion in RT programs.
Despite a sound hypothetical rationale, there is insufficient direct research to draw causality as to whether eccentric overload training enhances muscle hypertrophy when combined with traditional RT. From a practical standpoint, it is important to highlight that in some cases, eccentric overload training requires special equipment (i.e., weight releasers) or assistance from a coach/personal trainer/training partner. In that regard, this type of training is somewhat more demanding than traditional RT. There is some evidence that eccentric overload training might enhance strength (such as maximal isokinetic concentric torque), power, and speed performance (5,30,31). As such, eccentric overload training might be of interest to athletes in various sports. However, the same advantage for muscle hypertrophy is not corroborated in the current body of evidence. Some studies (32) reported that rate of perceived exertion is greater when using eccentric overload training, which should be taken into account when working with RT-naive individuals. The limited body of evidence suggests that with eccentric overload training, a potential preferential growth of type IIX muscle fibers and an increased number of sarcomeres in series occur; however, this remains preliminary because of the scarcity of evidence on this topic. There is also evidence that eccentric actions elicit selective distal hypertrophy in the quadriceps that conceivably would result in better symmetrical muscle development under overload conditions. There is no evidence that eccentric overload is an inferior method compared with traditional training. Thus, given the sound rationale for the strategy, it warrants experimentation in hypertrophy-oriented RT program design until research shows otherwise.
There are numerous ways to integrate eccentric overload training in the context of a carefully structured routine. That said, the supramaximal loads and damaging nature of the strategy merits a conservative approach. Initially, adding 1 or 2 eccentric overload sets per muscle group at ∼120% of concentric 1RM is a feasible way to potentially enhance muscular development while minimizing the potential for undue complications. This approach can then be modified based on individual response over time by manipulating the volume, intensity, and/or frequency of implementation.
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