Treadmill running: how long before soft-tissue vibrations reach a steady state?

The aim of this study was to determine whether a steady state was reached over time in soft-tissue vibrations (STV) when running with different midsole hardness. Kinematics, kinetics and electromyography changes were also investigated in an attempt to be as comprehensive as possible. Forty-five physically active adults (17 females) with experience in treadmill running performed an 8.5 minutes trial at a self-selected speed with soft, medium and hard midsoles. STV of gastrocnemius muscle, kinetics, kinematics and electromyographic activity of gastrocnemius muscle and tibialis anterior were recorded for 30 seconds at minutes 0, 2, 4, 6 and 8. No time effects were found for STV, passive peak, loading rate and leg muscle activity. No significant footwear x time interactions were observed, suggesting that familiarisation is independent of midsole hardness. The fastest adjustments were an increase of foot inversion and active peak within the first 4 minutes respectively before reaching a steady state. Step frequency and vertical stiffness decreased and plateaued at minute 6. Duty factor and contact time increased whereas flight time decreased during the entire running trial. Results indicate that a familiarisation to midsole hardness is not needed for STV reliable measurements. Depending on the other biomechanical parameters assessed, we recommend at least a 6-minute running trial and a longer running trial for contact and flight times assessment.