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.