Muscle fiber recruitment and the slow component of O2uptake: constant work rate vs. all-out sprint exercise

Abstract

The slow component of pulmonary O 2 uptake (V̇o 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 V̇o 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 V̇o 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 V̇o 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 V̇o 2 slow component during voluntary exercise in humans.