Prolonged loss of force and power following fatiguing contractions in rat soleus muscles. Is low-frequency fatigue an issue during dynamic contractions?

Low-frequency fatigue (LFF) is defined by a relatively larger deficit in isometric force elicited by low-frequency electrical stimulation compared with high-frequency stimulation. However, the effects of LFF on power during dynamic contractions elicited at low and high frequencies have not been thoroughly characterized. In the current study, rat soleus muscles underwent fatiguing either concentric, eccentric, or isometric contractions. Before and 1 h after the fatiguing contractions, a series of brief isometric and dynamic contractions elicited at 20 and 80 Hz stimulation to establish force-velocity relationships. Maximal force (Fmax), velocity (Vmax), and power (Pmax) were assessed for each frequency. Sarcoplasmic reticulum (SR) Ca2+ release and reuptake rates were assessed pre- and postfatigue. Prolonged fatigue was observed as a loss of Fmax and Pmax in muscles fatigued by concentric or eccentric, but not by isometric contractions. When quantified as a decrease in the ratio between 20 Hz and 80 Hz contractile output, LFF was more pronounced for isometric force than for power (-21% vs. -16% for concentrically fatigued muscles, P = 0.003; 29 vs. 13% for eccentrically fatigued muscles, P < 0.001). No changes in SR Ca2+ release or reuptake rates were observed. We conclude that LFF is less pronounced when expressed in terms of power deficits than when expressed in terms of force deficits, and that LFF, therefore, likely affects performance to a lesser degree during fast concentric contractions than during static or slow contractions.

Is curvature of the force-velocity relationship affected by oxygen availability? Evidence from studies in ex vivo and in situ rat muscles.

The power of shortening contractions in skeletal muscle is determined by the force-velocity relationship. Fatigue has been reported to either increase or decrease the force-velocity curvature depending on experimental circumstances. These discrepant findings may be related to experimental differences in oxygen availability. We therefore investigated how the curvature of the force-velocity relationship in soleus and gastrocnemius rat muscles is affected during fatigue, in both an ex vivo setup without an intact blood perfusion and in an in situ setup with an intact blood perfusion. Furthermore, we investigated the effect of reduced oxygen concentrations and reduced diffusion distance on the curvature of the force-velocity relationship in ex vivo muscles, where muscle oxygen uptake relies on diffusion from the incubation medium. Muscles were electrically stimulated to perform repeated shortening contractions and force-velocity curves were determined in rested and fatigued conditions. The curvature increased during fatigue in the soleus muscles (both in situ and ex vivo), and decreased for the gastrocnemius muscles (in situ) or remained unchanged (ex vivo). Furthermore, under ex vivo conditions, neither reduced oxygen concentrations nor reduced diffusion distance conferred any substantial effect on the force-velocity curvature. In contrast, reduced oxygen availability and increased diffusion distance did increase the loss of maximal power during fatigue, mainly due to additional decreases in isometric force. We conclude that oxygen availability does not influence the fatigue-induced changes in force-velocity curvature. Rather, the observed variable fatigue profiles with regard to changes in curvature seem to be linked to the muscle fiber-type composition.