UC Irvine

Skeletal muscles not only accelerate our bodies during movement, they also play a crucial role in deceleration. During decelerating tasks, such as landing and braking, muscles are stretched while actively producing force in order to dissipate mechanical energy. One negative consequence of energy dissipation is that active lengthening can cause muscle damage. Muscles are dynamic and complex- their ability to vary architecture and timing of activation allows for incredible variation in mechanical function. The presence of in-series tendons magnifies this diversity of motor functions by 1) decoupling the length changes of the MTU from the length changes of the fascicle and 2) providing geographical space for fibers to attach to beyond bone. In this dissertation, we look at muscle-tendon factors that may affect active lengthening on the level of the whole muscle-tendon unit. These factors include timing of activation, muscle-tendon kinetics and morphology. Using in situ and in vitro muscle preparations, we found that early timing of activation is an important risk management strategy, rate of fascicle stretch is driven by relaxation rate of muscle and that pennation of fascicles may provide a protective advantage during high impact eccentric contractions.