A metabolic basis for impaired muscle force production and neuromuscular compensation during sprint cycling

Abstract

For both different individuals and modes of locomotion, the external forces determining all-out sprinting performances fall predictably with effort duration from the burst maximums attained for 3 s to those that can be supported aerobically as trial durations extend to roughly 300 s. The common time course of this relationship suggests a metabolic basis for the decrements in the force applied to the environment. However, the mechanical and neuromuscular responses to impaired force production (i.e., muscle fatigue) are generally considered in relation to fractions of the maximum force available, or the maximum voluntary contraction (MVC). We hypothesized that these duration-dependent decrements in external force application result from a reliance on anaerobic metabolism for force production rather than the absolute force produced. We tested this idea by examining neuromuscular activity during two modes of sprint cycling with similar external force requirements but differing aerobic and anaerobic contributions to force production: one-and two-legged cycling. In agreement with previous studies, we found greater peak per leg aerobic metabolic rates [59% (Ϯ6 SD)] and pedal forces at V O2 peak [30% (Ϯ9)] during one-vs. two-legged cycling. We also determined downstroke pedal forces and neuromuscular activity by surface electromyography during 15 to 19 all-out constant load sprints lasting from 12 to 400 s for both modes of cycling. In support of our hypothesis, we found that the greater reliance on anaerobic metabolism for force production induced compensatory muscle recruitment at lower pedal forces during twovs. one-legged sprint cycling. We conclude that impaired muscle force production and compensatory neuromuscular activity during sprinting are triggered by a reliance on anaerobic metabolism for force production. motor control; aerobic and anaerobic contributions; performance HUMAN LOCOMOTOR PERFORMANCE depends directly upon the forces that the musculoskeletal system can generate and transmit to the environment. For example, runners and cyclists modulate their speed and power output primarily by altering the respective forces they apply to the ground (61) or pedals (29). The maximum external forces applied at top running speed or at peak cycling power output can be supported by skeletal muscle for only 3 s or less (41, 61). As the duration of all-out efforts become more prolonged, the external forces that the musculoskeletal system can provide become progressively reduced. As a result, performances decline from the 3-s maximum to a nearly sustainable level as the duration of all-out efforts extends to 300 s (10, 17, 30). The reductions in force production responsible for the well-characterized performanceduration relationship have been considered by some investigators to be a whole-body manifestation of muscle fatigue (9, 62). Given the rapidity with which the duration-dependent decrements in force production occur, shorter-duration, all-out efforts provide a potentially powerful approach for examining the time co...

Influence of duty cycle on the time course of muscle fatigue and the onset of neuromuscular compensation during exhaustive dynamic isolated limb exercise

“…8; see also Refs. 33 and 42) simplification that we have used conservatively here and previously (12,13,51). Nonetheless, the data indicate that the level of force production responsible for the transition from primarily aerobic metabolism to a net reliance on anaerobic sources of ATP resynthesis can differ markedly while completing the same muscular task.…”

“…8). In contrast, in trials with levels of force production that exceeded those eliciting the aerobic peaks, we measured progressive increases in EMG activity, as others have (12,15,41). In these trials, which necessarily require a net reliance on anaerobic metabolism, the compensatory behavior began at the outset of each exhaustive trial and continued contraction-by-contraction until the point of failure (Figs.…”

Complex Interplay Between Determinants of Pacing and Performance During 20-km Cycle Time Trials

“…It may therefore be that higher iEMG activity in the slower trials resulted from neuromuscular compensation for reduced force production capability in the skeletal muscles. Although the exact nature of any differences in peripheral physiological status at the beginning of the trials was unknown, this phenomenon has been demonstrated by Bundle et al, 30 who found compensatory neuromuscular activity resulted from increased reliance on anaerobic metabolism for force production. These authors also suggested that, regardless of specific physiological mechanisms, the principle of compensatory neuromuscular activity for impaired muscle contractile function seems likely to be a general response.…”

“…As superior performances were associated with a less conservative strategy, and RPE was similar throughout fastest and slowest trials then, in this study, it seems that affect may have been a more important regulator of pacing strategy than RPE. Rating of perceived exertion has previously been demonstrated to be influenced by acid-base status, 30,31 with lower pH being associated with higher RPE at a fixed workload. In the present study, and despite similar RPE values throughout exercise, postexercise pH was lower and blood lactate concentration was higher following fastest trials.…”

Exercise performance and neuromuscular activity at a fixed level of RPE following manipulation of peripheral physiological status

“…The authors proposed that the higher neuromuscular activity despite a lower power output in the slower trials was evidence for neuromuscular compensation for a sub-optimal peripheral physiological status. This concept of neuromuscular compensation had previously been described by Bundle et al (2006) who suggested that compensatory neuromuscular activity occurs in response to impaired muscle contractile function. Therefore, in the present study the finding of a higher EMG activity during the placebo trials may be explained by suggesting that due to a relatively inferior peripheral physiological status with regards to the demands of the exercise task, a greater degree of neuromuscular activity was required in order to produce the same absolute muscular work rate.…”