Prior heavy knee extension exercise does not affect $\dot{V}\hbox{O}_{2}$ kinetics during subsequent heavy cycling exercise

Abstract

This study examined the magnitude of the oxygen uptake slow component $(\dot{V}\hbox{O}_{2}\; \hbox{SC})$ during heavy exercise when preceded by heavy knee extension (KE) exercise. Nine males (26.6 ± 1.7 years,  ±SE) performed repeated bouts of heavy exercise, each lasting 6 min with 6 min of recovery. Cycling–cycling trials (CYC₁, CYC₂) involved step transitions to a workrate corresponding to 50% of the difference between peak $\dot{V}\hbox{O}_{2}$ and the lactate threshold (Δ 50%). During bilateral KE-cycling trails (KE, CYC₃), KE was performed at an intensity requiring twofold greater muscle activation relative to CYC₁ followed by a cycling transition to Δ 50%. $\dot{V}\hbox{O}_{2}$ was measured breath-by-breath and was modeled using three exponentials to determinate the amplitudes (A ₂′, A ₃′) and time constants (τ ₂, τ ₃) of the primary phase and $\dot{V}\hbox{O}_{2}$ SC. Electromyography (EMG) recorded from the vastus lateralis and medialis was averaged and reported relative to maximal voluntary contraction (%MVC). EMG was higher (p < 0.05) during KE (37.6 ± 8.1 %MVC) than CYC₁ (20.8 ± 1.9 %MVC), CYC₂ (21.6 ± 5.7 %MVC) and CYC₃ (19.8 ± 6.3 %MVC). The amplitude of the $\dot{V}\hbox{O}_{2}$ SC was lower (p < 0.05) in CYC₂ (197 ± 120 ml min⁻¹) and CYC₃ (163 ± 51 ml min⁻¹) compared to CYC₁ (325 ± 126 ml min⁻¹). No difference in $\dot{V}\hbox{O}_{2}$ SC was observed between CYC₂ and CYC₃. Although the activation of additional motor units during KE exercise reduced the amplitude of the $\dot{V}\hbox{O}_{2}$ SC, the decrease was similar to that observed following heavy cycling exercise. Thus, the activation of motor units in excess of those required for the activity does not alter the$\dot{V}\hbox{O}_{2}$ response during a subsequent bout of exercise.