Calcaneal tendon stiffness is not associated with dynamic time-dependent contractile output

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 RTD and rate of velocity development (RVD) in plantar flexor muscles. Young adult males (n = 13) and females (n = 2) performed prone isometric- and isotonic-mode maximal voluntary plantar flexion 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 with isometric RTD, but only for evoked contractions (RTD_0-50 ms: r = 0.54, p = 0.02, RTD_0-200 ms: 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 less so for isotonic contractions. The determinants of dynamic contractile rates are more complex and warrant further study.