Stretching Human Muscles Makes Them Stronger

Research on residual force enhancement and muscle stretching and contraction observed under physiological conditions.
Dilson E. Rassier - Department of Kinesiology and Physical Education, McGill University

Outline of the Study
This review article explores changes in muscle fiber, specifically examining a phenomenon known as residual force enhancement. The primary focus is on a study conducted by Pinniger and Cresswell, which sought to explore the relationship between laboratory research on residual force enhancement and muscle stretching and contraction observed under physiological conditions.

The object of study for the authors is the large leg muscles of healthy subjects. Specifically, joint torque around the ankle, indicative of muscular force, was measured. The results were compared with residual force enhancement generated via electrical stimulation during plantar flexion. Residual force enhancement was shown in physiological conditions to be a fundamental characteristic of contracting skeletal muscle. While residual force enhancement following electrical stimulation appears to produce similar results to those obtained in isolated preparations, the relation of this effect to that produced by voluntary activation requires further study.

What the Study Was Trying to Prove
Steady-state isometric force after active stretching of a muscle has been frequently shown to be greater than the steady-state isometric force resulting from purely isometric contraction at the same length. This property of skeletal muscle is referred to as residual force enhancement, though the precise mechanisms responsible have remained a topic of debate. Residual force enhancement is a feature observed following active stretching of both skeletal muscles and single fibers. The study has attempted to link existing findings, such as those derived from myofibrils, single fibers, and isolated muscles with the voluntarily muscle contraction occurring in everyday situations. The article stresses the challenges inherent in such studies, one of which is the variance of voluntary muscle activation among different individuals.

Summary of Results
Measurements of residual force enhancement were made with the large leg muscles of healthy individuals Sub-maximal plantar flexion and dorsiflexion was maintained by allowing the study subjects to view their electromyography signals monitored from the soleus or tibialis anterior muscles. Muscle stretching was achieved by changing the angle of the ankle within a normal range of motion. Force enhancements of 7% and 12% were observed for plantar flexion and dorsifelxion, respectively. Residual force enhancement produced by electrical stimulation was comparable, increasing by 13%. The mechanism of residual force enhancement, the authors believe, involves an increased stiffness in titin molecules, rather than the formation of cross-bridges or unstable sarcomeres, as often proposed elsewhere.

Conclusions
Pinniger and Cresswell's results suggest that residual force enhancement may influence muscular system performance during everyday activities. Such enhancement appears to be a fundamental characteristic of human skeletal muscles contracting in physiological situations. Fiber type may well play a role in the observed phenomena, given that greater residual force enhancement was seen in the tibialis anterior compared with the soleus muscle. These results open the pathway for additional studies aimed at exploiting force enhancement in functional tasks. The effects of physical training, fatigue, muscle atrophy, and muscular disease on force enhancement are cited as profitable areas for future research. Pinniger and Cresswell's work represents the first clinical approximation of regular muscle activity during residual force enhancement.

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Credits, Miscellaneous

This study appeared in the Journal of Applied Physiology 102: 5-6, 2007

References