Muscle fiber recruitment and the slow component of O2 uptake: constant work rate vs. all-out sprint exercise

Am J Physiol Regul Integr Comp Physiol. 2011 Mar;300(3):R700-7. doi: 10.1152/ajpregu.00761.2010. Epub 2010 Dec 15.


The slow component of pulmonary O(2) uptake (Vo(2)) during constant work rate (CWR) high-intensity exercise has been attributed to the progressive recruitment of (type II) muscle fibers. We tested the following hypotheses: 1) the Vo(2) slow component gain would be greater in a 3-min all-out cycle test than in a work-matched CWR test, and 2) the all-out test would be associated with a progressive decline, and the CWR test with a progressive increase, in muscle activation, as estimated from the electromyogram (EMG) of the vastus lateralis muscle. Eight men (aged 21-39 yr) completed a ramp incremental test, a 3-min all-out test, and a work- and time-matched CWR test to exhaustion. The maximum Vo(2) attained in an initial ramp incremental test (3.97 ± 0.83 l/min) was reached in both experimental tests (3.99 ± 0.84 and 4.03 ± 0.76 l/min for all-out and CWR, respectively). The Vo(2) slow component was greater (P < 0.05) in the all-out test (1.21 ± 0.31 l/min, 4.2 ± 2.2 ml·min(-1)·W(-1)) than in the CWR test (0.59 ± 0.22 l/min, 1.70 ± 0.5 ml·min(-1)·W(-1)). The integrated EMG declined by 26% (P < 0.001) during the all-out test and increased by 60% (P < 0.05) during the CWR test from the first 30 s to the last 30 s of exercise. The considerable reduction in muscle efficiency in the all-out test in the face of a progressively falling integrated EMG indicates that progressive fiber recruitment is not requisite for development of the Vo(2) slow component during voluntary exercise in humans.

Publication types

  • Comparative Study

MeSH terms

  • Adult
  • Bicycling
  • Electromyography
  • Exercise Test
  • Exercise*
  • Humans
  • Kinetics
  • Male
  • Muscle Contraction*
  • Muscle Fatigue
  • Muscle Fibers, Skeletal / metabolism*
  • Muscle Strength
  • Oxygen Consumption*
  • Pulmonary Gas Exchange
  • Quadriceps Muscle / metabolism*
  • Young Adult