Relationship between Achilles Tendon Stiffness and Ground Contact Time during Drop Jumps

: The purpose of this study was to investigate the relationship between Achilles tendon stiffness and ground contact time (GCT) during drop jumps. The property of "springiness" and a short GCT during the movement is required in several types of sports. Therefore, a stiff tendon might be advantageous due to the quick force transmission from the muscle to the bone. Hence, a secondary aim was to relate Achilles tendon stiffness with squat jump (SJ) and counter movement jump (CMJ) performance, respectively. Nineteen physically active healthy males (mean ± SD: 26.7 ± 3.9 years, 1.77 ± 0.07 m, 76.5 ± 6.7 kg) participated in this study. Subjects were asked to perform squat jumps and counter movement jumps to determine jump height, and drop jumps were undertaken on a force plate to determine GCT. We then simultaneously measured isometric maximum voluntary contraction (MVIC) of the plantar flexors with a dynamometer and recorded the elongation of the tendon with ultrasound; hence, we could calculate tendon stiffness. The results show a correlation between GCT and Achilles tendon stiffness (r = -0.50) and MVIC (r = -0.48), respectively. Achilles tendon stiffness was not significantly correlated with squat jump and counter movement jump height, respectively. According to the results, we can confirm the main hypothesis that a stiff Achilles tendon tends to result in a shorter GCT during drop jumps. However, Achilles tendon stiffness does not appear to be a key determinant in jumping performance.