Reports of the length dependence of fatigue are greatly exaggerated

Abstract

Relative force depression associated with muscle fatigue is reported to be greater when assessed at short vs. long muscle lengths. This appears to be due to a rightward shift in the force-length relationship. This rightward shift may be caused by stretch of in-series structures, making sarcomere lengths shorter at any given muscle length. Submaximal force-length relationships (twitch, double pulse, 50 Hz) were evaluated before and after repetitive contractions (50 Hz, 300 ms, 1/s) in an in situ preparation of the rat medial gastrocnemius muscle. In some experiments, fascicle lengths were measured with sonomicrometry. Before repetitive stimulation, fascicle lengths were 11.3 +/- 0.8, 12.8 +/- 0.9, and 14.4 +/- 1.2 mm at lengths corresponding to -3.6, 0, and 3.6 mm where 0 is a reference length that corresponds with maximal active force for double-pulse stimulation. After repetitive stimulation, there was no change in fascicle lengths; these lengths were 11.4 +/- 0.8, 12.6 +/- 0.9, and 14.2 +/- 1.2 mm. The length dependence of fatigue was, therefore, not due to a stretch of in-series structures. Interestingly, the rightward shift that was evident when active force was calculated in the traditional way (subtraction of the passive force measured before contraction) was not seen when active force was calculated by subtracting the passive force that was associated with the fascicle length reached at the peak of the contraction. This calculation is based on the assumption that passive force decreases as the fascicles shorten during a fixed-end contraction. This alternative calculation revealed similar postfatigue absolute active force depression at all lengths. In relative terms, a length dependence of fatigue was still evident, but this was greatly diminished compared with that observed when active force was calculated with the traditional method.

Low-frequency fatigue at maximal and submaximal muscle contractions

“…However, it has been shown that the apparent rightward shift in the force-length relationship is similar after isometric contractions and after eccentric contractions (9) and it has been suggested that this shift may be an artifact of the manner in which active force is calculated (10). Furthermore, it has been demonstrated that fascicle length is not affected by a series of fatiguing isometric contractions (11).…”

Potentiation of isometric and isotonic contractions during high-frequency stimulation

“…Phosphorylation of the light chains should also be greater with a higher number of stimulations, as this is a Ca 2+ dependent process. Fatigue and potentiation are also both length dependent [16,22], but in these experiments, potentiation clearly has the greater impact at short lengths.…”

Impact of length during repetitive contractions on fatigue in rat skeletal muscle

“…Fascicle lengths change half as much as whole muscle length [16] during passive length adjustments, so the range of lengths tested corresponds to RL±11% of fascicle length. Twitch, double-pulse, and 50 Hz (300 ms) contractions were recorded at each test length, similar to a previous study [19]. When testing lengths longer than RL, the muscle was held at the stretched length only long enough to obtain the contraction and was returned to RL for the rest period.…”

“…After 45 min, submaximal active force-length relationships for twitch, double-pulse, and 50-Hz contractions were recorded again in each series. This recovery period would be expected to permit metabolic recovery in the muscle, but low-frequency fatigue is known to persist [13,19]. Waiting for this period would permit evaluation of contractile amplitude at several lengths without detectable impact of continuing recovery affecting these measurements.…”

“…Interestingly, Aljure and Borrero [1] showed that the rate of fatigue resulting from repetitive stimulation at a given length depended on whether the consequence was measured at the length where the stimulation was conducted or at a common reference length. This observation relates to the fact that after a bout of repetitive contractions, there can be a different effect on active force measured at a short length and at a long length [10,19]. This phenomenon is referred to as the length dependence of fatigue.…”