Force-velocity relationship during isometric and isotonic fatiguing contractions

Abstract

Fatiguing contractions change the force-velocity relationship, but assessment of this relationship in fatigue has usually been obtained after isometric contractions. We studied fatigue caused by isometric or isotonic contractions, by assessment of the force-velocity relationship while the contractions maintaining fatigue were continued. This approach allowed determination of the force-velocity relationship during a steady condition of fatigue. We used the in situ rat medial gastrocnemius muscle, a physiologically relevant preparation. Intermittent (1/s) stimulation at 170 Hz for 100 ms resulted in decreased isometric force to ~35% of initial or decreased peak velocity of shortening in dynamic contractions to ~45% of initial. Dynamic contractions resulted in a transient initial increase in velocity, followed by a rapid decline until a reasonably steady level was maintained. Data were fit to the classic Hill equation for determination of the force-velocity relationship. Isometric and dynamic contractions resulted in similar decreases in maximal isometric force and peak power. Only V was different between the types of contraction ( P < 0.005) with greater decrease in V during isotonic contractions to 171.7 ± 7.3 mm/s than during isometric contractions to 208.8 mm/s. Curvature indicated by a/Po (constants from fit to Hill equation) changed from 0.45 ± 0.04 to 0.71 ± 0.11 during isometric contractions and from 0.51 ± 0.04 to 0.85 ± 0.18 during isotonic contractions. Recovery was incomplete 45 min after stopping the intermittent contractions. At this time, recovery of low-frequency isometric force was substantially less after isometric contractions, implicating force during intermittent contractions as a determining factor with this measure of fatigue. NEW & NOTEWORTHY The force-velocity relationship was captured while fatigue was maintained at a constant level during isometric and dynamic contractions. The curvature of the force-velocity relationship was less curved during fatigue than prefatigued, but within 45 min this recovered. Low-frequency fatigue persisted with greater depression of low-frequency force after isometric contractions, possibly because of higher force contractions during intermittent contractions.

Fatiguing stimulation increases curvature of the force-velocity relation in isolated fast-twitch and slow-twitch rat muscles

“…In contrast to our results, a recent study by Devrome and MacIntosh (2018) observed that the curvature was reduced during fatigue in rat fast-twitch gastrocnemius muscle, even though power was reduced by 70-75%. It is not clear what the reason for these contradictory findings is, although several differences between the study conditions could contribute.…”

“…It is not clear what the reason for these contradictory findings is, although several differences between the study conditions could contribute. Thus, Devrome and MacIntosh (2018) used an in situ set-up whereas we used isolated muscles and it is possible that the lack of blood flow to the muscles in our set-up might contribute to the increased curvature. Furthermore, Devrome and MacIntosh (2018) produced fatigue by consecutive contractions for 3 min, leading to a fatigue level that was kept constant as judged by contraction force or contraction velocity for 5 min by further contractions while the FV curve was measured.…”

“…In contrast, three studies reported that the curvature decreased during fatigue, and thereby attenuated the loss of power (Barclay, 1996;Curtin and Edman, 1994;Devrome and MacIntosh, 2018). One possible explanation for this discrepancy could be that the studies which observed an increase in curvature (Jones et al, 2006;Kristensen et al, 2017) were performed on muscles containing mainly slow-twitch fibers, whereas the other studies were performed on fast-twitch muscles/fibers (Barclay, 1996;Curtin and Edman, 1994;Devrome and MacIntosh, 2018).…”

“…Thus, it is well established that both V max and F max decrease during fatigue, contributing to the loss of maximal power (Jones et al, 2006). It has been suggested that an increased FV curvature during fatigue may exacerbate the loss of power (Jones, 2010), while a decrease in curvature will attenuate the loss of power (Devrome and MacIntosh, 2018). However, presently, there is no consensus about the association between fatigue and the curvature in the FV relationship.…”

PhysIOpathology of NEuromuscular function rElated to fatigue in chronic Renal disease in the elderly (PIONEER): study protocol

“…In CKD patients, this test demonstrates a close correlation with survival, nutritional status and quality of life [ 29 – 33 ]. Moreover, an isometric contraction is less traumatic than a dynamic contraction [ 34 ]. Finally, a handgrip is a simple routine gesture and does not usually cause pain.…”