Spring behavior of the lower extremity and muscle activities of the vastus lateralis and medial gastrocnemius in response to increased running speed in highly trained distance runners.

Previous studies have examined the effects of increasing running speed on stiffness-related mechanics and muscle activation; however, the relationships between changes in vertical stiffness (kvert) and leg stiffness (kleg) and corresponding lower-limb muscle activity, particularly in highly trained distance runners, remain unclear. This study examined the relationships between changes in kvert and kleg and muscle activities of the vastus lateralis (VL) and medial gastrocnemius (MG) with increasing running speed. Eighteen male distance runners (5000-m personal best time: 14'20″31 ± 16″01) ran at 15.0, 17.4, and 19.8 km·h-1 on a motorized treadmill while surface electromyography (EMG) was recorded. Motion in the sagittal plane was captured using a high-speed camera to calculate the kvert and kleg. With increasing running speed, kvert increased, whereas kleg remained unchanged; the VL EMG was unchanged, whereas the MG EMG increased. The EMG amplitude of the VL during the pre-touchdown phase was significantly correlated with kvert (rrm = 0.452) and kleg (rrm = 0.445) within individuals, whereas that during the braking phase was not. Significant intra-individual correlations were observed between kvert and the EMG amplitude of the MG during the pre-touchdown (rrm = 0.894) and braking phases (rrm = 0.721). However, no significant correlations were observed between kleg and the EMG amplitude of the MG. These results suggest that pre-activation of the VL and MG, as well as MG activation during the braking phase, is associated with the spring behavior of the lower extremity in highly trained distance runners with increasing running speed.