Timing of muscle activation of the lower limbs can be modulated to maintain a constant pedaling cadence.

This study investigated changes in muscle activity when subjects are asked to maintain a constant cadence during an unloaded condition. Eleven subjects pedaled for five loaded conditions (220 W, 190 W, 160 W, 130 W, 100 W) and one unloaded condition at 80 rpm. Electromyographic (EMG) activity of six lower limb muscles, pedal forces and oxygen consumption were calculated for every condition. Muscle activity was defined by timing (EMG onset and offset) and level (integrated values of EMGrms calculated between EMG onset and EMG offset) of activation, while horizontal and vertical impulses were computed to characterize pedal forces. Muscle activity, pedal forces and oxygen consumption variables measured during the unloaded condition were compared with those extrapolated to 0 W from the loaded conditions, assuming a linear relationship. The muscle activity was changed during unloaded condition: EMG onset and/or offset of rectus femoris, biceps femoris, vastus medialis, and gluteus maximus muscles were delayed (p<0.05); iEMGrms values of rectus femoris, biceps femoris, gastrocnemius medialis and tibialis anterior muscles were higher than those extrapolated to 0 W (p<0.05). Vertical impulse over the extension phase was lower (p<0.05) while backward horizontal impulse was higher (p<0.05) during unloaded condition than those extrapolated to 0 W. Oxygen consumptions were higher during unloaded condition than extrapolated to 0 W (750+/-147 vs. 529+/-297 mLO(2) x min(-1); p<0.05). Timing of activation of rectus femoris and biceps femoris was dramatically modified to optimize pedal forces and maintain a constant cadence, while systematic changes in the activation level of the bi-articular muscles induced a relative increase in metabolic expenditure when pedaling during an unloaded condition.