Effectiveness of Eccentric Exercise for Flexibility

Introduction

For a long time, stretching has been the option to increase muscle flexibility in a varied population. In neurological conditions, stretching was used to avoid muscle contractures, in athletes it was thought to decrease post-exercise muscle soreness and even in physiotherapy it was a well-known method to increase flexibility around several joints. However, the research train always keeps going and it showed that stretching did not necessarily lead to improvements in athletic performance, decreases in all-cause or overuse injuries, and even in those with neurological injury, stretching was not able to prevent or treat contractures. It thus seems that stretching is losing its place in rehabilitation or athletic training. In more recent years more attention has been given to eccentric exercise. As this form of exercise allows the muscle to lengthen while contracting, it was thought that this may be a better strategy. In 2012 already, O’Sullivan et al. reported that eccentric exercise could improve flexibility at the hip, knee, or ankle, but this study’s conclusions were based upon unpooled data so the magnitude of the effect remained unknown. When looking at the biceps femoris, eccentric exercise was able to cause a large increase in muscle fascicle length. Therefore it was thought that similar findings could be obtained in other body regions. This systematic review updated the previous study and also incorporated a meta-analysis to look at the effectiveness of eccentric exercise on flexibility measures.

Methods

A systematic review updating and extending the search of O’Sullivan in 2012 to the upper limb was conducted. Randomized controlled trials that compared the effect of eccentric exercise on flexibility measures of the upper or lower limb could be included. The primary outcome included flexibility, measured through either joint range of motion or measures of muscle fascicle length. Secondary outcomes included the effectiveness of eccentric exercise on flexibility measures in separate analyses of the joint ROM or muscle fascicle length.

Results

This systematic review included 35 comparisons coming from 27 trials in the meta-analysis. In total, 911 participants were included. So, what is the effectiveness of eccentric exercise on flexibility measures? In this sample, eccentric exercise was able to improve joint flexibility with a medium effect size (g=0.54 (0.34-0.74)) but when a trial with a large effect size was excluded in a sensitivity analysis, the effect was borderline medium in size (g=0.49 (0.34-0.64)). The overall quality was downgraded to moderate or low due to bias and inconsistency.

Effectiveness of eccentric exercise on flexibility measures — From: Diong et al., Musculoskelt Sci Pract (2022)

Secondary analyses revealed that the same was true when flexibility was measured in joint ROM or in muscle fascicle length separately (for joint ROM: Hedges’ g=0.52 (0.31-0.74) and for muscle fascicle length: g=0.57 (0.28-0.87)). Considering the joint ROM, the evidence was of high quality, whereas for muscle fascicle length the evidence was downgraded to low due to the risk of bias and inconsistency.

Subgroup analyses revealed that the moderate effect size was seen in improvements in the lower limb (g=0.57 (0.34-0.79)), but not in the upper limb (g=0.37 (0-0.74)). When compared to other interventions (g=0.66 (0.28-1.03)) or no intervention (g=0.48 (0.28-0.69)), eccentric exercise was able to cause similar effects.

Questions and thoughts

Only a minority were lost to follow-up (82) and few side effects were reported, making this exercise approach seem reasonable. However, important to note was that not every study reported side effects. In fact, 6 did and the side effects were reports of delayed onset muscle soreness, ranging from light to mild. The average duration of the interventions was 8 weeks and a median of 24 (range 12-33) sessions were held. Trials of less than 4 weeks were excluded as the authors reasoned that these studies were mostly based on post-exercise muscle damage. Whether or not, it seems reasonable to exclude those short-term studies as we may expect that the adaptations required to increase flexibility may take a couple of weeks.

The heterogeneity in the results reflects differences between the included trials. The authors give the example of several studies finding large positive effects after comparing 2-3x/w heavy eccentric loading against concentric, whereas another study found negative effects from applying eccentric loads twice daily. It thus seems that there may exist an ‘optimal dosage’, but this was beyond this study’s scope.

For the upper limb, an attempt was made to synthesize findings, but only 4 articles could be included. However, since the effect is at the border of significance, there may be some reasons to believe that future studies including more upper limb trials, may be able to find a potential benefit of eccentric exercise training.

Talk nerdy to me

Four trials were excluded from the meta-analysis because the findings came from the same participants in different publications. Only once the outcomes from these data were included, and the exclusion of these studies seems justified. Findings were characterized by moderate variability as trials were heterogeneous in nature. To account for this, random effects models were applied, so the depicted effects are conservative in nature, and not likely to be overestimated. Interestingly, the secondary outcomes looked at separate analyses of joint flexibility through either muscle ROM or muscle fascicle length indicating that eccentric exercise was equally effective. This increases confidence in the outcomes as two different outcomes measures combined, also independently state the same effect.

The included trials were of moderate quality. Ten studies scored below 5 on the PEDro scale. It would have been interesting to know whether there would be a difference in outcomes between trials of low quality compared to those of high quality. When interpreting the results, it should be acknowledged that the lower limits of the confidence intervals indicate small Hedges’ g values, therefore it is important not to overestimate the effects as a significant part of the individual study’s results will lie in that lower limit of the confidence interval.

Take home messages

What should we take away from these results? This meta-analysis confirms what was already speculated, that eccentric exercise was able to cause real improvements in flexibility. Especially in the lower limb and when compared to other interventions (including concentric exercise, general exercise) or compared to doing nothing (which sounds reasonable). To compare eccentric exercise against stretching was beyond the scope of this study, but as the efficacy of stretching has been questioned more and more lately, the main focus to achieve these improvements should lie on eccentric exercise.