Effect of vertical loading on energy cost and kinematics of running in trained male subjects

This investigation examined, in a group of 10 trained male runners, the effect of vertical loading during level treadmill running at a velocity of 5 m/s. The net energy cost of running (C-r), the external work of the center of mass of the body (W-ext; both expressed in J . kg(-1) . m(-1)), and the eccentric-to-concentric ratio (Ecc/Con) of integrated electromyographic activity for the vastus lateralis (VL) and gastrocnemius lateralis muscles were measured. It was observed that W-ext and Ecc/Con for the VL could explain a large part of the interindividual variations in C-r. This result reinforces the hypothesis that Ecc/Con could be a good index of effectiveness in the stretch-shortening cycle. When the subjects ran with a vertical load of 9.3% of their body mass, C-r and W-ext were significantly reduced (P < 0.01 and P < 0.05, respectively), whereas Ecc/Con for the VL and gastrocnemius lateralis remained unchanged. The variations in C-r and W-ext due to vertical loading were significantly correlated (r = 0.75; P < 0.01). It was then concluded that the significant improvement of C-r observed with the added load was mainly due to the fact that W-ext was significantly decreased.