Central contributions to torque depression: an antagonist perspective

The history-dependent features of muscle force production: A challenge to the cross-bridge theory and their functional implications

The cross-bridge theory predicts that muscle force is determined by muscle length and the velocity of active muscle length changes. However, before the formulation of the cross-bridge theory, it had been observed that the isometric force at a given muscle length is enhanced or depressed depending on active muscle length changes before that given length is reached. These enhanced and depressed force states are termed residual force enhancement (rFE) and residual force depression (rFD), respectively, and together they are known as the history-dependent features of muscle force production. In this review, we introduce early attempts in explaining rFE and rFD before we discuss more recent research from the past 25 years which has contributed to a better understanding of the mechanisms underpinning rFE and rFD. Specifically, we discuss the increasing number of findings on rFE and rFD which challenge the cross-bridge theory and propose that the elastic element titin plays a role in explaining muscle history-dependence. Accordingly, new three-filament models of force production including titin seem to provide better insight into the mechanism of muscle contraction. Complementary to the mechanisms behind muscle history-dependence, we also show various implications for muscle history-dependence on in-vivo human muscle function such as during stretch-shortening cycles. We conclude that titin function needs to be better understood if a new three-filament muscle model which includes titin, is to be established. From an applied perspective, it remains to be elucidated how muscle history-dependence affects locomotion and motor control, and whether history-dependent features can be changed by training.

Eccentric exercise-induced muscle weakness amplifies the history dependence of force

INTRODUCTION

Following active lengthening or shortening contractions, isometric steady-state torque is increased (residual force enhancement; rFE) or decreased (residual force depression; rFD), respectively, compared to fixed-end isometric contractions at the same muscle length and level of activation. Though the mechanisms underlying this history dependence of force have been investigated extensively, little is known about the influence of exercise-induced muscle weakness on rFE and rFD.

PURPOSE

Assess rFE and rFD in the dorsiflexors at 20%, 60%, and 100% maximal voluntary torque (MVC) and activation matching, and electrically stimulated at 20% MVC, prior to, 1 h following, and 24 h following 150 maximal eccentric dorsiflexion contractions.

METHODS

Twenty-six participants (13 male, 24.7 ± 2.0y; 13 female, 22.5 ± 3.6y) were seated in a dynamometer with their right hip and knee angle set to 110° and 140°, respectively, with an ankle excursion set between 0° and 40° plantar flexion (PF). MVC torque, peak twitch torque, and prolonged low frequency force depression were used to assess eccentric exercise-induced neuromuscular impairments. History-dependent contractions consisted of a 1 s isometric (40°PF or 0°PF) phase, a 1 s shortening or lengthening phase (40°/s), and an 8 s isometric (0°PF or 40°PF) phase.

RESULTS

Following eccentric exercise; MVC torque was decreased, prolonged low frequency force depression was present, and both rFE and rFD increased for all maximal and submaximal conditions.

CONCLUSIONS

The history dependence of force during voluntary torque and activation matching, and electrically stimulated contractions is amplified following eccentric exercise. It appears that a weakened neuromuscular system amplifies the magnitude of the history-dependence of force.

The Effect of Shortening-induced Torque Depression on Fatigue-related Sex Differences

Residual torque depression (rTD) is the decrease in isometric (ISO) torque after active shortening of skeletal muscle compared with a purely ISO contraction performed at the same muscle length and level of activation. Performance fatigability is defined as any exercise-induced reduction in voluntary force or power, and females are typically more fatigue resistant than males at low-intensity ISO contractions.

PURPOSE

This study investigated performance fatigability in males and females during ISO contractions and ISO contractions after active shortening (rTD).

METHODS

Fourteen females (22 ± 2 yr) and 14 males (23 ± 2 yr) performed three baseline maximal voluntary contractions (MVCs) of the dorsiflexors. The MVCs were used to determine a 30% submaximal torque target, which participants matched as steadily as possible until task failure. The ISO fatigue task was performed at 10° plantar flexion. In the rTD session, the participants' ankle was rotated from 40° to 10° plantar flexion before performing the same fatigue task. MVCs were performed immediately after task failure, 30 s, and 1, 2, 3, 4, 5, 10, 20, and 30 min after task failure to track recovery.

RESULTS

The baseline MVC torque amplitude for males (32.1 ± 6.6 N·m) was 31% greater than that for females (22.3 ± 3.1 N·m; P < 0.001, ηp = 0.490). Females' time to task failure was 44% longer than that of males in the ISO state (P = 0.032, ηp = 0.164). However, there was no sex difference in the rTD state (P = 0.142).

CONCLUSION

It seems that the sex differences in fatigue resistance observed in a low-intensity ISO task are abolished in the ISO state after an active shortening contraction.

Inhibitory tendon-evoked reflex is attenuated in the torque-depressed isometric steady-state following active shortening

Residual torque depression (rTD) is the reduction in steady-state isometric torque following an active shortening contraction when compared with an isometric contraction at the same muscle length and activation level. We have shown that spinal excitability increases in the rTD state, yet the mechanisms remains unknown. Percutaneous electrical tendon stimulation was used to induce tendon-evoked inhibitory reflexes. We demonstrated that in the rTD state, reduced torque contributes to a reduction in inhibitory afferent feedback, which indicates that the history-dependent properties of muscle can alter spinal excitability and the voluntary control of submaximal contractions through changes in peripheral afferent feedback. Novelty Residual force depression is a basic property of skeletal muscle, which can influence spinal and supraspinal excitability via inhibitory reflex activity. Residual force depression alters the voluntary control of force.