Eccentric contraction response of stimulated skeletal muscle fascicle at the various strain rates and stimulation timing

Abstract

Muscle injuries are the most common sports injuries in eccentric contraction. There are many factors that could influence the severity of muscle injuries, including strain, strain rate and stimulation. This study evaluated the interaction of these factors on the biomechanical properties of the muscle–tendon bundle and their role in injuries. A Hopkinson bar system, an MTS machine and an electrical pulse generator were utilized to collect eccentric contraction response data of over 150 frog muscle–tendon samples at strain rates ranging from 0.01 to 300 s⁻¹. The results have shown that the maximum eccentric stress has increased and peaked at the strain rate of about 150 s⁻¹. That peak value has then maintained at the following strain rates. In contrast, Young’s modulus reduced as the strain rate changed from 50 to 300 s⁻¹. That trend was in contrast to unstimulated muscle bundles. In addition, strain rate has significantly influenced stimulated tendon–muscle bundle fracture. Samples tend to rupture at a minor strain of about 3.5% with strain rates over 200 s⁻¹. Because of the increasing stiffness of the muscle area at high strain rates, increased strain in the tendon region resulted in frequent injuries in the tendon area. On the other hand, a maximum stress reduction was detected when the muscle bundles were stimulated at muscle strain greater than 0.2. The results showed that improper timing of stimulation could increase muscle injury. The study shows that the stimulation and strain rate dramatically impact muscle–tendon properties and the risk of injury.