Medial gastrocnemius stiffness contributes to active ankle joint stiffness and ground contact time in trained track-and-field athletes.

PURPOSE: Minimizing ground contact time is important for maximizing stretch-shortening cycle (SSC) performance. However, intrinsic mechanical factors that enable short contact times remain unclear. This study examined whether ankle joint intrinsic stiffness and plantar flexion rate of torque development (RTD) are associated with rebound jump performance, and whether plantar flexor stiffness contributes to these joint-level properties. METHODS: Twenty-four highly trained male track-and-field athletes performed repetitive rebound jumps. Ankle joint intrinsic stiffness was assessed via short-range stretches during isometric plantar flexion at 0%, 20%, and 50% of maximal voluntary contraction (MVC). Medial gastrocnemius (MG) stiffness was measured at the same intensities using shear wave elastography. Plantar flexion RTD was obtained from explosive isometric contractions. RESULTS: Ground contact time was negatively correlated with active ankle joint stiffness at 50% MVC (r =  - 0.576), and this association remained significant after false discovery rate correction (P = 0.027). Ankle joint stiffness at 20% and 50% MVC was in turn positively correlated with MG muscle stiffness at the corresponding intensities even after false discovery rate correction (20% MVC: r = 0.598, P = 0.027; 50% MVC: r = 0.673, P = 0.009). Plantar flexion RTD was not correlated with any jump variable. CONCLUSION: Active intrinsic stiffness of the ankle joint, which is attributable to active MG stiffness, is associated with shorter ground contact time during rebound jumps in trained athletes. These findings suggest that MG stiffness is a mechanical determinant of efficient force transmission during brief ground contact.