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Eccentric-Training Controversies, Resolved
Len Kravitz, PhD

In exercise and sport, muscles provide both positive and negative external work. The positive work is a result of concentric muscle actions where there is a shortening of muscle. With eccentric actions, the force applied to the muscle exceeds the force produced by the muscle, resulting in a lengthening of the muscle. In exercise and sport, eccentric muscle actions usually decelerate a movement. For example, during an eccentric action the muscle often works like a shock absorber, such as in downhill running. In this column, results of several investigative studies, on controversial topics in eccentric training, are reviewed.

Eccentric Training Modestly Better than Concentric Training for Increasing Muscle Size
In a systematic review and meta-analysis, Schoenfeld et. al. (2017) investigated the long-standing controversy whether there is a difference in hypertrophic effects of eccentric versus concentric exercise. Fifteen studies met the following criteria for inclusion in this systematic review: All studies were at least 6-week experimental investigations, with healthy participants not exhibiting any injury, that compared concentric and eccentric actions without the use of external implements (i.e., blood pressure cuffs). Hypertrophy changes were measured using biopsy, imaging (e.g., MRI, computerized tomography, or ultrasound), bioelectrical impedance (i.e., small electrical current) or densitometry (i.e., underwater weighing).

Eccentric muscle actions resulted in a greater effect size compared with concentric actions, but results did not rise to statistical significance. In other words, the meta-analysis found that eccentric training produced slightly greater increases in hypertrophy compared with concentric training. The researchers concluded that eccentric and concentric actions are both very effective and important to include in hypertrophy-oriented resistance training programs.

Neuromuscular Adaptations to Work-Matched Maximal Eccentric versus Concentric Training
Almost all studies that have compared eccentric versus concentric training have used a fixed number of repetitions and sets in the training programs. However, Maeo et al. (2018) cite research that clearly shows that higher (>20%) torque can be produced during eccentric versus concentric contractions. The authors submit that in previous research the positive results (for eccentric training) may be due to the eccentric training groups doing greater work. Therefore, the purpose of Maeo's study was to examine the neuromuscular adaptations after maximal eccentric versus concentric training when MATCHED for total work. Twelve male volunteers completed maximal knee extension work on one leg (ECCENTRIC LEG) versus the other leg (CONCENTRIC LEG) on 2 sessions per week for 10 weeks. The primary result of this study was that even when total work was MATCHED, eccentric training was more effective in inducing hypertrophy than concentric training. Interestingly, significant changes in hypertrophy were observed in the eccentric training leg as early as 4 weeks. Neurological adaptations were also greater in the eccentric leg versus the concentric leg.

Are There Sex-Related Differences in the Acute Response of Males and Females to Maximal Eccentric Training?
Morawetz et al. (2020) explain that the acute responses of unaccustomed eccentric exercise leads to strength loss, muscle soreness, muscle damage, and higher concentrations of creatine kinase (a muscle enzyme that is elevated when there is damage to muscle tissue) in the blood. The researchers note there is no accepted evidence that women and men display similar acute effects after eccentric exercise. Therefore, the aim of this systematic review (studies published since 2018) was to evaluate research examining if sex-related differences, to acute maximal eccentric exercise in healthy men and women (18 to 70 years; no athletes or rehab patients), exist in terms of delayed onset soreness, creatine kinase levels post-exercise and eccentric muscle strength. Of the original 272 studies in the primary search, 23 studies met complete inclusion criteria for this investigation. Here are some of the interesting findings of this research.

1) When torque (the rotational equivalent of linear force) is normalized for muscle cross-sectional area, body mass or fat-free mas there is no statistically significant difference between women and men.

2) Immediately following maximal eccentric exercise, women exhibit a slightly greater loss in muscular strength in the upper and lower extremities.

3) For most trials, men have significantly higher creatine kinase levels compared to women.

4) There is no significant difference in muscle soreness post-exercise between the sexes.

From a practical application summary, Morawetz et al. suggest that one relevant suggestion from this review is that since women tend to have a slightly greater loss of strength, post maximal eccentric training, personal trainers and coaches should provide slightly longer recovery time after an eccentric training session. In regards to the higher creatine kinase levels observed in men, the researchers hypothesize the following. Estrogen is known to enhance cell membrane stability, which may reduce the leakage of creatine kinase from skeletal muscle into blood. Therefore, estrogen seems to have a protective role against muscle damage.

How Hard Should You Train for the Best Results with Eccentric Training?
In a quest to develop stronger skeletal muscles, there is a great deal of interest in the type of eccentric training utilized in a training program for athletes and exercise enthusiasts. However, the majority of study's investigating eccentric-training models have limited applications, because these studies often focus on highly specific physiological adaptations using a single muscle group on some specialized piece of equipment. These studies do not explain the use of eccentric resistance training as it applies to the use in a standard gym setting. Therefore, Schroder and colleagues (2004) investigated the musculoskeletal adaptations of a total body, multiple-muscle group, eccentric progressive resistance training protocol using free weights and standard strength-training machines in 37 young women volunteers (24 ys; who had done no resistance training within the 6 months prior to the study).

In this 16-week study, participants were randomly assigned to one of three groups: high-intensity eccentric training at 125% intensity (HRT) of 1RM values, 75% intensity (LRT) of 1RM values or a non-exercise control group. In 2 training sessions each week participants completed 6 exercises: the seated chest press, lat pull down, standing biceps curl, standing triceps extension, seated single-leg extension, and seated double leg curl. All exercises consisted of only eccentric muscle action with no concentric component. This was achieved by having the participant lower the weight against gravity (for 4 seconds) while the trainer lifted the weight through the concentric component of the exercise.

For each exercise the LRT group completed 3 sets of 10 repetitions at an intensity that was 75% of their concentric 1RM for each exercise. The HRT group completed 3 sets of 6 repetitions at an intensity that was 125% of their concentric 1RM. The participants in the LRT group performed 10 repetitions, as opposed to the 6 repetitions performed by the HRT group, to guarantee that the training volume performed was the same for both groups. Training volume (TV) is calculated the following manner: intensity x sets x repetitions. Therefore, the TV for the exercise groups was the following .75 x 3 x 10 = 22.5 for the LRT and 1.25 x 3 x 6 = 22.5 for the HRT.

The surprising results of this study demonstrated the LRT (75% of a concentric 1RM) was as effective as HRT (125% of a concentric 1RM). Muscular strength in the 6 exercises increased similarly in both groups with 20% to 40% increases in maximal strength (after 16 weeks of training). For Fitness Pros, this finding has eye-catching applications. Eccentric training with 75% 1RM loads allows clients to physically complete the concentric movement without assistance. Personal trainers can empower clients to incorporate this highly effective eccentric training method in their daily workouts, when the trainer is not present. One note of thoughtfulness. This study was conducted with 24yr old untrained women. Fitness pros may need to modify accordingly for other exercise populations.

In personal training, fitness pros are continuously searching for effective training strategies to stimulate physical adaptive changes for their clients. Eccentric exercise has the unique potential to tremendously overload the muscular system to maximize maximal muscle strength, power and hypertrophy. In view of these promising results, inclusion of eccentric training is encouraged as a viable evidence-based strategy to further lead clients to goal-oriented successes in training.

Side Bar 1. What is the Potential 'Untapped' Future Population for Eccentric Exercise Therapy?
In a reflective opinion article, Mitchell et al (2017) indicate that critically ill patient survivors commonly experience disability because of loss of muscle mass, compromised muscle function, and the consequent loss of strength. Loss of muscle mass and weakness are major predictors of mortality in older people and patients with conditions such as chronic obstructive pulmonary disease. The researchers argue that future advances in managing these critically ill patients should include therapies that minimize the decrement in skeletal muscle function. They conclude the theoretical benefits of exercise regimens designed around eccentric contractions for the critically ill patient should be considered in future studies of physical rehabilitation and exercise therapy.

Len Kravitz, PhD, CSCS, is the program coordinator of exercise science at the University of New Mexico, where he received the Outstanding Teacher of the Year and Presidential Award of Distinction.

Maeo, S., Shan, X., Otsuga, S. et al. (2018). Neuromuscular Adaptations to Work-matched Maximal Eccentric versus Concentric Training. Medicine and Science in Sports and Exercise. 50(8), 1629-1640.
Mitchell, W.K., Taivassalo, T., Narici, M.V., et al. (2017). Eccentric exercise and the critically ill patient. Frontiers in Physiology, Februay 2017, Volume 8, Article 120, doi: 10.3389/fphys.2017.00120
Morawetz, D., Blank, C., Koller, A. et al. (2020). Sex-related differences after a single bout of maximal eccentric exercise in response to acute effects: A systematic review and meta-analysis. Journal of Strength and Conditioning Research. 34(9), 2697-2707.
Schoenfeld, B.J., Ogborn, D.I., Vigotsky, A.D., et al. (2017). Hypertrophic efects of concentric vs. eccentric muscle actions. A systematic review and meta-analysis. Journal of Strength and Conditioning Research, 31(9), 2599-2608.
Schroeder, E.T., Hawkins, S.A., Jaque, S.V. (2004). Musculoskeletal adaptations to 16 weeks of eccentric progressive resistance training in young women, Journal of Strength and Conditioning Research, 8(2), 227-235.