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

Abstract

Sundberg CW, Bundle MW. Influence of duty cycle on the time course of muscle fatigue and the onset of neuromuscular compensation during exhaustive dynamic isolated limb exercise. Am J Physiol Regul Integr Comp Physiol 309: R51-R61, 2015. First published April 15, 2015 doi:10.1152/ajpregu.00356.2014.-We investigated the influence of altered muscle duty cycle on the performance decrements and neuromuscular responses occurring during constant-load, fatiguing bouts of knee extension exercise. We experimentally altered the durations of the muscularly inactive portion of the limb movement cycle and hypothesized that greater relative durations of inactivity within the same movement task would 1) reduce the rates and extent of muscle performance loss and 2) increase the forces necessary to trigger muscle fatigue. In each condition (duty cycle ϭ 0.6 and 0.3), male subjects [age ϭ 25.9 Ϯ 2.0 yr (SE); mass ϭ 85.4 Ϯ 2.6 kg], completed 9 -11 exhaustive bouts of two-legged knee extension exercise, at force outputs that elicited failure between 4 and 290 s. The novel duty cycle manipulation produced two primary results; first, we observed twofold differences in both the extent of muscle performance lost (DC 0.6 ϭ 761 Ϯ 35 N vs. DC0.3 ϭ 366 Ϯ 49 N) and the time course of performance loss. For example, exhaustive trials at the midpoint of these force ranges differed in duration by more than 30 s (t 0.6 ϭ 36 Ϯ 2.6 vs. t0.3 ϭ 67 Ϯ 4.3 s). Second, both the minimum forces necessary to exceed the peak aerobic capacity and initiate a reliance on anaerobic metabolism, and the forces necessary to elicit compensatory increases in electromyogram activity were 300% greater in the lower vs. higher duty cycle condition. These results indicate that the fatigue-induced compensatory behavior to recruit additional motor units is triggered by a reliance on anaerobic metabolism for ATP resynthesis and is independent of the absolute level or fraction of the maximum force produced by the muscle. muscle fatigue; neuromuscular compensation; performance-duration relationship IT HAS LONG BEEN RECOGNIZED that there is a duration-dependent relationship between the levels of muscular force and mechanical power that can be maintained in exercise performed until failure. Between exhaustive efforts that elicit failure within seconds to several minutes, the losses in muscular force and power output are exponential; however, beyond this initial period of rapid decrements, similar levels of muscular performance can be sustained for hours (19,27, 31,40,45). For example, the current human record running speed (29) is 185% faster than the speed that can be sustained for an 8-min run, yet extending the duration from several minutes to 2 h results in only an additional 8% loss in running speed (32). Thus, within the endurance portion of this relationship, performance levels are essentially sustainable, a recognition that has long been attributed to the reliance on aerobic metabolism for the ATP resynthesis necessary to support the muscular contraction. In contrast,...

Bioenergetic basis for the increased fatigability with ageing

“…This approach resulted in an average duty cycle (i.e. the ratio of the duration of muscle force application to the entire duration between contractions) (Sundberg & Bundle, ) of 17 ± 3% and 27 ± 5% in the young and old adults, respectively. The higher duty cycle in the old compared to young adults ( P < 0.001; = 0.65) was a result of the longer contractile durations (young = 0.33 ± 0.05 s, old = 0.57 ± 0.12 s; P < 0.001; = 0.64) from the slower velocities and not a result of differences in the duration between the start of each contraction (young = 2.01 ± 0.01 s, old = 2.07 ± 0.11 s; P = 0.166; = 0.14).…”

Sex Differences in Mechanisms of Recovery after Isometric and Dynamic Fatiguing Tasks

“…The peak angular acceleration was calculated during the concentric phase of each MVCC as the maximal value of the first derivative of the angular velocity curve. The work-to-rest ratio (duty cycle) was calculated as: (active contraction time) · (active contraction time + relaxation time) −1 (34). The variables from the dynamic fatiguing task are presented as the average from five consecutive contractions during the task, at baseline (contractions 1–5) or the end of the fatiguing task (contractions 116–120).…”

Rates of performance loss and neuromuscular activity in men and women during cycling: evidence for a common metabolic basis of muscle fatigue

“…In contrast, when tasks are performed at relatively low-intensities, muscle fatigue develops more slowly, and the performance of the task can be sustained for considerably longer durations. This fundamental property of the neuromuscular system is known as the performance-duration relationship, and for dynamic exercises, such as cycling, running and knee extensions, can be accurately defined by an exponential function with three physiologically based components (14,62,67,68). The first two components set the upper and lower limits of an individual's performance-duration relationship and are, respectively, 1) the maximum force or power that can be generated for 3 s or less (F max and P max ) and 2) the upper level of force or power that can be supported primarily by aerobic metabolism (F aer and P aer ).…”

“…Studies on both isolated limb and whole body dynamic exercises have found that this time constant is similar between individuals despite considerable differences in their absolute performances (14,62,67,68). The consistency of this finding across different modes of exercise and experimental conditions suggests that the exponential loss in muscle performance has a common metabolically based mechanism determined by the extent that ATP is resynthesized by anaerobic metabolism (14,62,67,68). However, this conclusion comes from studies conducted almost exclusively on men and to date has assumed that the same rate-limiting mechanisms determine the performance-duration relationship in women.…”

“…This limitation is due, in large part, to inadequacies in the techniques currently available to accurately quantify the amount of anaerobic energy flux, especially during whole body dynamic exercise. To circumvent this concern, we had men and women perform multiple constant-load bouts of cycling exercise (i.e., 11-14 bouts to failure between P max and P aer for each individual) at a range of intensities that spanned across each individual's nonsustainable performance range and that are known to require a net reliance on anaerobic metabolism (14,62,67,68).…”