Time Course of Performance Fatigability during Exercise below, at, and above the Critical Intensity in Females and Males.

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.

Comparing muscle V̇o2 from near-infrared spectroscopy desaturation rate to pulmonary V̇o2 during cycling below, at, and above the maximal lactate steady state.

Muscle oxygen uptake (V̇o2m) evaluated from changes in the near-infrared spectroscopy oxygen desaturation slope during a 5-s arterial blood flow occlusion has been proposed as an estimation of the actual V̇o2m. However, its correspondence with pulmonary oxygen uptake (V̇o2p) during exercise remains unknown. This study investigated the V̇o2m and V̇o2p relationship in females and males in response to prolonged constant-load cycling exercise at different intensities. Eighteen participants (8 females) visited the laboratory on six occasions: 1) ramp incremental test; 2 and 3) 30-min constant power output (constant-PO) exercise bout to determine the maximal lactate steady state (MLSS); 4-6) constant-PO exercise bouts to task failure at i) 15% below MLSS (MLSS-15%); ii) MLSS; and iii) 15% above MLSS (MLSS+15%). V̇o2m was estimated at baseline, at min 5, min 10, min 20, min 30, and at task failure. V̇o2p was continuously recorded during the constant-PO bouts. V̇o2p and V̇o2m significantly increased from min 5 to min 30 in MLSS condition (all P values <0.05) and from min 5 to min 10 in MLSS+15% condition (all P < 0.05). V̇o2p and V̇o2m were correlated (r2adj range of 0.70-0.98, all P < 0.001) among exercise intensities in both females and males. In addition, both variables were also correlated when expressed as percent (r2adj range of 0.52-0.77, all P < 0.001). V̇o2p and V̇o2m responses were similar when exercising below, at, and above the MLSS independently of sex. Most importantly, V̇o2p and V̇o2m were correlated regardless the exercise intensity and sex of the participants.NEW & NOTEWORTHY It was unknown if muscle oxygen uptake (V̇o2m) derived from near-infrared spectroscopy oxygen desaturation slope during 5-s blood flow occlusion, was 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) these observations were consistent between females and males.

The effects of exercise intensity and duration on the relationship between the slow component of V̇O2 and peripheral fatigue.

AIM: If the development of the oxygen uptake slow component (V̇O2SC ) 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 (V̇O2 ) was continuously recorded during the constant-PO bouts and V̇O2SC was characterized based on each individual V̇O2 kinetics during moderate transitions. RESULTS: The development of V̇O2SC and peripheral fatigue were correlated across time (r2 adj range of 0.64-0.80) and amongst each exercise intensity (r2 adj range of 0.26-0.30) (all P < .001). Also, TTF was correlated with V̇O2SC and neuromuscular fatigue parameters (r2 adj range of 0.52-0.82, all P < .001). CONCLUSION: The V̇O2SC and peripheral fatigue development are correlated throughout the exercise in a time- and intensity-dependent manner, suggesting that the V̇O2SC may depend on muscle fatigue even if the mechanisms of reduced contractile function are different amongst intensities.

Slight power output manipulations around the maximal lactate steady state have a similar impact on fatigue in females and males.

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 task failure, TTF) is unknown. This study compared NMF and TTF in females and males in response to exercise performed at MLSS, 10 W below (MLSS-10) and above (MLSS+10). Twenty participants (9 females) performed three 30-min constant-PO exercise bouts followed (1-min delay) by a TTF 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 TTF bouts. 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, relative to baseline, in IMVC and TwPt compared to males after constant-PO bouts (all P < 0.05), this difference being not dependent on intensity. TTF was negatively impacted by increasing the PO in the constant-PO (P < 0.001), with no differences in end-exercise NMF (P > 0.05). Slight manipulations 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.NEW & NOTEWORTHY It is unknown whether minimum changes in power output (PO) below and above the maximal lactate steady state (MLSS) affect neuromuscular fatigue (NMF) development in females and males. The present data showed that a decrease or increase of 10 W in PO in relation to MLSS elicited lower and greater impairments in contractile function, respectively. Even though females had less of an overall decline in NMF than males, similar exercise intensity-dependent response occurred independently of sex.