Plantar Flexor Dynamic Contractile Rates Are Not Dependent on Calcaneal Tendon Stiffness
Abstract
The ability to rapidly generate muscular torque and velocity is important in specialized activities and daily tasks of living. Tendon stiffness is one factor in the neuromuscular system that influences musculoskeletal torque transmission. Previous studies have reported weak-to-moderate correlations between tendon stiffness and rate of torque development (RTD). However, these correlations have been reported only for isometric contractions which may not be relevant to contractions involving joint rotation (i.e., dynamic). The purpose was to investigate the effect of calcaneal tendon stiffness on the dynamic rates of torque (RTD) and velocity (RVD) development in plantar flexor muscles. Young adult males (n=13) and females (n=2) performed prone isometric- and isotonic-mode plantar flexion maximal voluntary contractions (MVC). Ultrasound imaging was used to quantify tendon morphological characteristics to estimate Young's elastic modulus (YM). Maximal voluntary and electrically evoked (300 Hz) isometric- and isotonic-mode (at 10% and 40% MVC loads) contractions were evaluated for RTD and RVD through a 25° ankle joint range of motion. YM was correlated modestly with isometric RTD, but only for evoked contractions (RTD0–50 ms: r=0.54, p=0.02, RTD0–200ms: r=0.62, p=0.01). Conversely, YM was not correlated with dynamic RTD (voluntary: r=-0.07–0.41, p=0.06–0.40, evoked: r=-0.2–0.3, p=0.14–0.24) nor RVD (voluntary: r=-0.08–0.24, p=0.27–0.40, evoked: r=0.12– 0.3, p=0.14–0.34). These correlations would indicate that calcaneal tendon stiffness is an important factor for rapid isometric torque development, but a smaller factor for isotonic contractions. The determinants of dynamic contractile rates likely involve more factors than isometric contractions and warrant further study.