Thistlethwaite, J. R., Thompson, B. C., Gonzales, J. U., & Scheuermann, B. W. (2006). Excessive motor unit recruitment does not affect VO2 kinetics during subsequent heavy exercise. Medicine and Science in Sports and Exercise, 38(5), Supplement abstract 1517.

"At exercise intensities above the lactate threshold (LT), an additional increase in oxygen uptake (VO2) of delayed onset leads to a VO2 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". This investigation examined if the magnitude of the VO2 slow component is altered during heavy exercise when preceded by heavy warm-up exercise involving significantly different motor unit recruitment patterns. Males (N = 9) performed two bouts of heavy exercise, each bout lasting six minutes with six minutes of recovery. Surface EMG was recorded from the vastus lateralis and medialis to monitor motor unit recruitment. The sEMG signal was normalized relative to a maximum voluntary contraction obtained prior to exercise. During cycling-to-cycling trials (CYC1, CYC2) step transitions to a work rate corresponding to 50% of the difference between peak VO2 and lactate threshold (50% work rate difference) were performed. During knee extension-cycling 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 work rate corresponding to the 50% work rate difference.

EMG was higher during KE (enhanced intensity knee extensions) than in CYC1, CYC2, and CYC3. The gain for the primary response was not different between cycling trials. The amplitude of the VO2 slow component was lower in CYC2 and CYC3 than in CYC1.

Implication. Consistent with previous studies, heavy warm-up exercise does not effect the gain or the primary rise in VO2 during subsequent heavy exercise. The additional recruitment of presumably type II motor units during warm-up KE exercise did not alter 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.

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