Adaptations in the passive mechanical properties of skeletal muscle to altered patterns of use

The aim of this mini-review is to describe the present state of knowledge regarding the effects of chronic changes in the patterns of muscle use (defined as changes lasting >1 wk), including muscle stretching, strengthening, and others, on the passive mechanical properties of healthy human skeletal muscles. Various forms of muscle stretch training and some forms of strength training (especially eccentric training) are known to strongly impact the maximum elongation capacity of muscles in vivo (i.e., maximum joint range of motion), largely by increasing our ability to tolerate higher stretch loads. However, only small effects are observed in the passive stiffness of the muscle-tendon unit (MTU) or the muscle itself, although a reduction in muscle stiffness has been observed in the plantar flexors after both stretching and eccentric exercise interventions. No changes have yet been observed in viscoelastic properties such as the MTU stress-relaxation response, although a minimum of evidence indicates that hysteresis during passive stretch-relaxation cycles may be reduced by muscle stretching training. Importantly, data exist for relatively few muscle groups, and little is known about the effects of age and sex on the adaptive process of passive mechanical properties. Despite the significant research effort afforded to understanding the effects of altered physical activity patterns on the maximum range of motion at some joints, further information is needed before it will be possible to develop targeted physical activity interventions with the aim of evoking specific changes in passive mechanical properties in individuals or in specific muscles and muscle groups.