Neuromuscular fatigue (NMF) and exercise performance are affected by exercise intensity and sex differences. However, whether slight changes in power output (PO) below and above the maximal lactate steady-state (MLSS) impact NMF and subsequent performance (time to exhaustion, TTE) is unknown. Purpose: This study compared NMF and TTE in females and males in response to exercise performed at MLSS, 10 W below (MLSS-10) and above (MLSS+10). Methods: Twenty participants (9 females) performed three 30-min constant-PO exercise bouts followed (1 min delay) by a TTE at 80% of the peak-PO. NMF was characterized by isometric maximal voluntary contractions (IMVC) and femoral nerve electrical stimulation of knee extensors [e.g. peak torque of potentiated high-frequency (Db100) and single twitch (TwPt)] before and immediately after the constant-PO and TTE bouts. Results: IMVC declined less after MLSS-10 (-18±10%) compared to MLSS (-26±14%) and MLSS+10 (-31±11%) (all p<0.05), and the Db100 decline was greater after MLSS+10 (-24±14%) compared to the other intensities (MLSS-10: -15±9%; MLSS: -18±11%) (all p<0.05). Females showed smaller reductions in IMVC and TwPt compared to males after constant-PO bouts (all p<0.05), this difference being not dependant on intensity. TTE was negatively impacted by increasing the PO in the constant-PO (p<0.001), with no differences in end-exercise NMF (p>0.05). Conclusion: Slight changes in PO around MLSS elicited great changes in the reduction of maximal voluntary force and impairments in contractile function. Although NMF was lower in females compared to males, the changes in PO around the MLSS impacted both sexes similarly.

It is unknown if muscle oxygen uptake (V̇o2m) derived from near-infrared spectroscopy oxygen desaturation slope during 5-s blood flow occlusion, is similar to the pulmonary oxygen uptake (V̇o2p) across time and among different exercise intensities. We showed that 1) V̇o2m and V̇o2p had similar responses across time and among intensities below, at, and above the maximal lactate steady state; 2) both variables were strongly correlated; and 3) there was no difference between females and males.

Aim: If the development of the oxygen uptake slow component (VȮ 2SC ) and muscle fatigue are related, these variables should remain coupled in a time-and intensity-dependent manner. Methods: 16 participants (7 females) visited the laboratory on 7 separate occasions: (1) three 6-minutes moderate-intensity cycling exercise bouts proceeded by a ramp incremental test; (2-3) 30-minutes constant power output (PO) exercise bout to determine the maximal lactate steady state (MLSS); (4-7) constant-PO exercise bouts to task failure (TTF), pseudorandomized order, at (i) 15% below the PO at MLSS; (ii) 10 W below MLSS; (iii) MLSS; (iv) 10 W above MLSS (first intensity and randomized order thereafter). Neuromuscular fatigue was characterized by isometric maximal voluntary contractions and femoral nerve electrical stimulation of knee extensors to measure peripheral fatigue at baseline, at min 5, 10, 20, 30 and TTF. Pulmonary oxygen uptake (VȮ 2 ) was continuously recorded during the constant-PO bouts and VȮ 2SC was characterized based on each individual VȮ 2 kinetics during moderate transitions. Results:The development of VȮ 2SC and peripheral fatigue were correlated across time (r 2 adj range of 0.64-0.80) and amongst each exercise intensity (r 2 adj range of 0.26-0.30) (all P < .001). Also, TTF was correlated with VȮ 2SC and neuromuscular fatigue parameters (r 2 adj range of 0.52-0.82, all P < .001). Conclusion:The VȮ 2SC and peripheral fatigue development are correlated throughout the exercise in a time-and intensity-dependent manner, suggesting that the VȮ 2SC may depend on muscle fatigue even if the mechanisms of reduced contractile function are different amongst intensities.

Purpose: This study aimed to investigate the time course and amplitude of performance fatigability during cycling at intensities around the maximal lactate steady state (MLSS) until task failure (TTF). Methods: Ten females and 11 males were evaluated in eight visits: 1) ramp incremental test; 2-3) 30-min constant power output (PO) cycling for MLSS determination; and 4-8) cycling to TTF at PO relative to the MLSS of (i) −15%, (ii) −10 W, (iii) at MLSS, and (iv) +10 W, and (v) +15%. Performance fatigability was characterized by femoral nerve electrical stimulation of knee extensors at baseline; minutes 5, 10, 20, and 30; and TTF. Oxygen uptake, blood lactate concentration, muscle oxygen saturation, and perceived exertion were evaluated. Results: Approximately 75% of the total performance fatigability occurred within 5 min of exercise, independently of exercise intensity, followed by a further change at minute 30. Contractile function declined more in males than females (all P < 0.05). At task failure, exercise duration declined from MLSS −15% to MLSS +15% (all P < 0.05), accompanied by a greater rate of decline after MLSS +15% and MLSS +10 compared with MLSS, MLSS −10 , and MLSS −15% for voluntary activation (−0.005 and −0.003 vs −0.002, −0.001 and −0.001%•min −1 , respectively) and contractile function (potentiated single twitch force, −0.013 and −0.009 vs −0.006, −0.004 and −0.004%•min −1 , respectively). Conclusions: Whereas the time course of performance fatigability responses was similar regardless of exercise intensity and sex, the total amplitude and rate of change were affected by the distinct metabolic disturbances around the MLSS, leading to different performance fatigability etiologies at task failure.