Neuromuscular recovery from severe- and extreme-intensity exercise in men and women

Maximal voluntary contraction force (MVC), potentiated twitch force (Qpot), and voluntary activation (%VA) recover to baseline within 90s following extreme-intensity exercise. However, methodological limitations masked important recovery kinetics. We hypothesized reductions in MVC, Qpot, and %VA at task failure following extreme-intensity exercise would be less than following severe-intensity exercise, and Qpot and MVC following extreme-intensity exercise would show significant recovery within 120s but remain depressed following severe-intensity exercise. Twelve subjects (6 men) completed two severe-intensity (40, 50%MVC) and two extreme-intensity (70, 80%MVC) isometric knee-extension exercise bouts to task failure (Tlim). Neuromuscular function was measured at baseline, Tlim, and through 150s of recovery. Each intensity significantly reduced MVC and Qpot compared to baseline. MVC was greater at T¬lim (p<0.01) and at 150s of recovery (p=0.004) following exercise at 80%MVC compared to severe-intensity exercise. Partial recovery of MVC and Qpot were detected within 150s following Tlim for each exercise intensity; Qpot recovered to baseline values within 150s of recovery following exercise at 80%MVC. No differences in %VA were detected pre- to post-exercise or across recovery for any intensity. Although further analysis showed sex-specific differences in MVC and Qpot, future studies should closely examine sex-dependent responses to extreme-intensity exercise. It is clear, however, that these data reinforce that mechanisms limiting exercise tolerance during extreme-intensity exercise recover quickly. NOVELTY: •Severe- and extreme-intensity exercise cause independent responses in fatigue accumulation and the subsequent recovery time courses. •Recovery of MVC and Qpot occurs much faster following extreme-intensity exercise in both men and women.

Heterogeneity of human adaptations to bed rest and hypoxia: a retrospective analysis within the skeletal muscle oxidative function

This retrospective study was designed to analyse the interindividual variability in the responses of different variables characterizing the skeletal muscle oxidative function to normoxic (N-BR) and hypoxic (H-BR) bed rests, and to a hypoxic ambulatory confinement (H-AMB) of 10 and 21 days. We also assessed whether and how the addition of hypoxia to bed rest might influence the heterogeneity of the responses. In vivo measurements of O2 uptake and muscle fractional O2 extraction were carried out during an incremental one-leg knee-extension exercise. Mitochondrial respiration was assessed in permeabilized muscle fibers. A total of 17 subjects were included in this analysis. This analysis revealed a similar variability among subjects in the alterations induced by N-BR and H-BR both in peak O2 uptake (SD: 4.1 and 3.3% after 10 days; 4.5 and 8.1% after 21 days, respectively) and peak muscle fractional O2 extraction (SD: 5.9 and 7.3% after 10 days; 6.5 and 7.3% after 21 days), independently from the duration of the exposure. The individual changes measured in these variables were significantly related (r=0.66, P=0.004 after N-BR; r=0.61, P=0.009 after H-BR). Mitochondrial respiration showed a large variability of response after both N-BR (SD: 25.0 and 15.7% after 10 and 21 days) and H-BR (SD: 13.0 and 19.8% after 10 and 21 days), no correlation was found between N-BR and H-BR changes. When added to bed rest, hypoxia altered the individual adaptations within the mitochondria but not those intrinsic to the muscle oxidative function in vivo, both after short and medium-term exposures.

Comparison of quadriceps muscle activation in exercises with different duration of concentric and eccentric contractions

Aim: The objective of the present study was to compare the amplitude of the electromyographic (EMG) signal of the quadriceps muscle portions vastus medialis (VM), vastus lateralis (VL) and rectus femoris (RF) and the activation ratio (VM/VL, VM/RF and VL/RF) in protocols with different durations of concentric and eccentric muscular actions. Material and method: Twelve female volunteers performed the knee extensor exercise with two different protocols [1s for concentric muscle action and 5s for eccentric muscle action (1:5); 5s of concentric muscle action and 1s of eccentric muscle action (5:1)] and 3 sets of 6 repetitions, 180s of pause between each sets and a intensity of 50% of 1RM. The root mean square of the amplitude of the normalized EMG signal was calculated for each repetition in each series. Results: it was observed an increase in the activation of the VM and VL portions in equivalent repetitions of each series and for the VL portion, the 1: 5 protocol provided greater activation compared to the other protocol. No differences were found for muscles activation ratios VM/RF and VL/RF, being that for the VM/VL ratio there was only change at one repetition. Conclusion: The results suggest that the portions of the quadriceps muscle may present different EMG responses in similar protocols, but this fact may not interfere in the synergism between them. The reduced degrees of freedom of the knee extension exercise and the characteristics of the protocols adopted may be the elements that contributed to the limited alterations that occurred in the present study.

­Heart Failure with Preserved Ejection Fraction Diminishes Peripheral Hemodynamics and Accelerates Exercise-Induced Neuromuscular Fatigue

This study investigated the impact of heart failure with a preserved ejection fraction (HFpEF) on neuromuscular fatigue and peripheral hemodynamics during small muscle mass exercise not limited by cardiac output. Eight HFpEF patients (ejection-fraction: 61±2%, NYHA II-III) and eight healthy-controls performed dynamic single-leg knee-extension exercise [80% peak workload] to task-failure and maximal intermittent isometric quadriceps contractions (8×15-s, 20-s rest). Controls repeated knee-extension at the same absolute workload as the HFpEF. During knee-extension, leg blood flow was quantified using Doppler ultrasound. Pre- to post-exercise changes in quadriceps twitch-torque (ΔQtw, peripheral fatigue), voluntary-activation (ΔVA, central fatigue), and corticospinal excitability were quantified. At the same relative intensity, HFpEF (24±5W) and controls (42±6W) had a similar time to task-failure (~10min), ΔQtw (~50%), and ΔVA (~6%) (P>0.3). This resulted in a greater exercise-induced change in neuromuscular function per unit work in HFpEF, which was significantly correlated with a slower leg blood flow response time (r=0.77). Knee-extension exercise at the same absolute workload resulted in a ~40% lower leg blood flow and greater ΔQtw (56±15 vs 11±10%) and ΔVA (5±3 vs 0±2%) in HFpEF than controls (P<0.05). Corticospinal excitability remained unaltered during exercise in both groups. Finally, despite a similar ΔVA, ΔQtw was larger in HFpEF compared to controls during isometric exercise (-49±9 vs -23±2%, P<0.05). In conclusion, HFpEF are characterized by a greater susceptibility to neuromuscular fatigue during exercise not limited by cardiac output. The patients' compromised peripheral hemodynamic response to exercise likely accounts, at least partly, for the attenuated fatigue resistance in this population.