Excessive Motor Unit Recruitment Does Not Affect VO2 Kinetics During Subsequent Heavy Exercise

Abstract

At exercise intensities above the lactate threshold (LT), an additional increase in oxygen uptake (VO) of delayed onset leads to a VO that is higher than predicted from the VO2/work rate relationship for exercise below the LT. The mechanism underlying this additional O2 cost during heavy exercise (i.e. slow component, VO2 SC) is unclear. Previous studies suggest that the progressive recruitment of less-efficient type II muscle fibers may contribute to the VO2 SC. PURPOSE: To examine if the magnitude of the VO SC is altered during heavy exercise when preceded by heavy warm-up exercise involving significantly different motor unit recruitment patterns. METHODS: Nine males (26.6 ± 1.7 yrs, ±SE) performed two bouts of heavy exercise, each bout lasting 6 min with 6 min of recovery. Surface EMG was recorded from the vastus lateralis and medialis for monitoring motor unit recruitment. The sEMG signal was normalized relative to a maximum voluntary contraction obtained prior to exercise. During cyc ling-to-cyc ling trials (CYC1, CYC2) step transitions to a work rate (WR) corresponding to 50% of the difference between peak VO2 and LT (A50%) were performed. During knee extension-cyc ling trials (KE, CYC3), bilateral KE exercise was performed at an intensity that elicited twice the motor unit recruitment compared to CYC1 and followed by a single cycling transition to a WR corresponding to A50%. Pulmonary VO was measured breath-by-breath. VO was modeled using 3 exponentials for the determination of amplitudes (A2', A3') and time constants (τ2, τ3) of the primary response and VO2 SC. RESULTS: EMG was higher (p<0.05) during KE (37.6±2.7 %MVC) than CYC1 (20.8±1.9 %MVC), CYC2 (21.6±1.9 %MVC) and CYC3 (19.8±2.1 %MVC). No difference between ? or A ' was observed between CYC trials. The gain (as AVO / AWR) for the primary response was not different between CYC trials. The amplitude of the VO2 SC (A/) was lower (p<0.05) in CYC2 (197±40 ml/min) and CYC3 (163±17 ml/min) compared to CYC1 (325±42 ml/min); no difference in A3' was observed between CYC2 and CYC3. CONCLUSION: Consistent with previous studies, heavy warm-up exercise does not effect the gain or ? for the primary rise in VO during subsequent heavy exercise. The additional recruitment of presumably type II motor units during warm-up KE exercise did not alter A3' during subsequent heavy exercise differently than prior heavy cycling exercise suggesting that the additional recruitment of type II muscle fibers during warm-up exercise does not appreciably affect muscle metabolism during subsequent exercise. Funded by deArce Memorial Endowment Fund, The University of Toledo