Purpose: Following a shortening or lengthening muscle contraction, the torque produced in the isometric steady state is distinctly lower (residual torque depression; rTD) or higher (residual torque enhancement; rTE), respectively, compared to a purely isometric contraction at the same final muscle length and level of activation. This is referred to as the history dependence of force. When matching a given torque level, there is greater muscle activation (electromyography; EMG) following shortening and less activation following lengthening. Owing to these differences in neuromuscular activation, it is unclear whether perception of effort is altered by the history dependence of force. Methods: Experiment 1 tested whether perception of effort differed between the rTD and rTE state when torque was matched. Experiment 2 tested whether perception of effort differed between the rTD and rTE state when EMG was matched. Finally, experiment 3 tested whether EMG differed between the rTD and rTE state when perception of effort was matched. Results: When torque was matched, both EMG and perception of effort were higher in the rTD compared to rTE state. When EMG was matched, torque was lower in the rTD compared to rTE state while perception of effort did not differ between the two states. When perception of effort was matched, torque was lower in the rTD compared to rTE state and EMG did not differ between the two states. Conclusion: The combined results from these experiments indicate that the history dependence of force alters ones perception of effort, dependent on the level of motor command.

Purpose: Following a shortening or lengthening muscle contraction, the torque produced in the isometric steady state is distinctly lower (residual torque depression; rTD) or higher (residual torque enhancement; rTE), respectively, compared to a purely isometric contraction at the same final muscle length and level of activation. This is referred to as the history dependence of force.When matching a given torque level, there is greater muscle activation (electromyography; EMG) following shortening and less activation following lengthening. Owing to these differences in neuromuscular activation, it is unclear whether perception of effort is altered by the history dependence of force. Methods: Experiment 1 tested whether perception of effort differed between the rTD and rTE state when torque was matched. Experiment 2 tested whether perception of effort differed between the rTD and rTE state when EMG was matched. Finally, experiment 3 tested whether EMG differed between the rTD and rTE state when perception of effort was matched.Results: When torque was matched, both EMG and perception of effort were higher in the rTD compared to rTE state. When EMG was matched, torque was lower in the rTD compared to rTE state while perception of effort did not differ between the two states. When perception of effort was matched, torque was lower in the rTD compared to rTE state and EMG did not differ between the two states. Conclusion: The combined results from these experiments indicate that the history dependence of force alters one's perception of effort, dependent on the level of motor command.

Improved Ca 2+ sensitivity has been suggested as a mechanism behind enhancements in muscle mechanical function following eccentric training. However, little is known regarding the effects of eccentric training on single muscle fiber Ca 2+ sensitivity. Adult male Sprague–Dawley rats (sacrificial age ~18 weeks; mass = 400.1 ± 34.8 g) were assigned to an eccentric training ( n = 5) or sedentary control group ( n = 6). Eccentric training consisted of 4 weeks of weighted downhill running 3×/week at a 15° decline and 16 m/min for 35 min per day in 5‐min bouts. After sacrifice, vastus intermedius single muscle fibers were dissected, chemically permeabilized, and stored until testing. Fibers ( n = 63) were isolated, and standard Ca 2+ sensitivity, force, rate of force redevelopment ( k tr ), and active instantaneous stiffness tests were performed using [Ca 2+ ] ranging from 7.0 to 4.5. Following all mechanical testing, fiber type was determined using SDS‐PAGE. There was no difference in pCa 50 (i.e., [Ca 2+ ] needed to elicit half of maximal force) between groups or between fiber types. However, when comparing normalized force across pCa values, fibers from the control group produced greater forces than fibers from the trained group at lower Ca 2+ concentrations ( p < 0.05), and this was most evident for Type I fibers ( p = 0.002). Type II fibers produced faster ( p < 0.001) k tr than Type I fibers, but there were no differences in absolute force, normalized force, or other measures of mechanical function between fibers from the trained and control groups. These findings indicate that eccentric training does not appear to improve single muscle fiber Ca 2+ sensitivity.

Skeletal muscle function is impaired in menopause and exercise may mitigate this decline. We used the VCD model of menopause to investigate the effects of gradual ovarian failure on skeletal muscle contractile function and whether high intensity interval training (HIIT) can mitigate impairments. Sexually mature female CD-1 mice were assigned to one of three groups: control (n=5), VCD-sedentary (n=5), or VCD-training (n=5). Following ovarian failure, the VCD-training group underwent 8 weeks of uphill HIIT. Mice were sacrificed 8 weeks after ovarian failure, representing late menopause. Single muscle fibres from the soleus (SOL) and extensor digitorum longus (EDL) muscles were dissected, chemically permeabilized, and mechanically tested. Single muscle fibres were maximally activated (pCa 4.5) then isotonic load clamps were performed to calculate force-velocity-power curves. Absolute force and peak power were 31% and 32% lower in VCD-sedentary fibres compared to control fibres, respectively, in both SOL and EDL muscles. Despite reductions in absolute force and therefore lighter relative loads imposed during the isotonic contractions in VCD-sedentary fibres, there were no concomitant increases in contractile velocity. HIIT was partially effective at mitigating power loss (22% higher peak power in VCD-training compared to VCD-sedentary), but only in fast-type SOL fibres. These findings indicate that ovarian failure impairs dynamic contractile function likely through a combination of lower force-generating capacity and slower shortening velocity, and that HIIT may be insufficient to completely counteract the deleterious effects of menopause at the cellular level.