The mechanisms underpinning the slow component of [Formula: see text] in humans

PURPOSE

When exercising above the lactic threshold (LT), the slow component of oxygen uptake ([Formula: see text]) appears, mainly ascribed to the progressive recruitment of Type II fibers. However, also the progressive decay of the economy of contraction may contribute to it. We investigated oxygen uptake ([Formula: see text]) during isometric contractions clamping torque (T) or muscular activation to quantify the contributions of the two mechanisms.

METHODS

We assessed for 7 min T of the leg extensors, net oxygen uptake ([Formula: see text]) and root mean square (RMS) from vastus lateralis (VL) in 11 volunteers (21 ± 2 yy; 1.73 ± 0.11 m; 67 ± 14 kg) during cyclic isometric contractions (contraction/relaxation 5 s/5 s): (i) at 65% of maximal voluntary contraction (MVC) (FB-Torque) and; (ii) keeping the level of RMS equal to that at 65% of MVC (FB-EMG).

RESULTS

[Formula: see text] after the third minute in FB-Torque increased with time ([Formula: see text] = 94 × t + 564; R2 = 0.99; P = 0.001), but not during FB-EMG. [Formula: see text]/T increased only during FB-Torque ([Formula: see text]/T = 1.10 × t + 0.57; R2 = 0.99; P = 0.001). RMS was larger in FB-Torque than in FB-EMG and significantly increased in the first three minutes of exercise to stabilize till the end of the trial, indicating that the pool of recruited MUs remained constant despite [Formula: see text].

CONCLUSION

The analysis of the RMS, [Formula: see text] and T during FB-Torque suggests that the intrinsic mechanism attributable to the decay of contraction efficiency was responsible for an increase of [Formula: see text] equal to 18% of the total [Formula: see text].

Priming exercise accelerates oxygen uptake kinetics during high-intensity cycle exercise in middle-aged individuals with type 2 diabetes

Background: The primary phase time constant of pulmonary oxygen uptake kinetics (V · O 2 τ p) during submaximal efforts is longer in middle-aged people with type 2 diabetes (T2D), partly due to limitations in oxygen supply to active muscles. This study examined if a high-intensity "priming" exercise (PE) would speedV · O 2 τ p during a subsequent high-intensity cycling exercise in T2D due to enhanced oxygen delivery. Methods: Eleven (4 women) middle-aged individuals with type 2 diabetes and 11 (4 women) non-diabetic controls completed four separate cycling bouts each starting at an 'unloaded' baseline of 10 W and transitioning to a high-intensity constant-load. Two of the four cycling bouts were preceded by priming exercise. The dynamics of pulmonaryV · O 2 and muscle deoxygenation (i.e. deoxygenated haemoglobin and myoglobin concentration [HHb + Mb]), were calculated from breath-by-breath and near-infrared spectroscopy data at the vastus lateralis, respectively. Results: At baselineV · O 2 τ p, was slower (p < 0.001) in the type 2 diabetes group (48 ± 6 s) compared to the control group (34 ± 2 s) but priming exercise significantly reducedV · O 2 τ p (p < 0.001) in type 2 diabetes (32 ± 6 s) so that post priming exercise it was not different compared with controls (34 ± 3 s). Priming exercise reduced the amplitude of theV · O 2 slow component (As) in both groups (type 2 diabetes: 0.26 ± 0.11 to 0.16 ± 0.07 L/min; control: 0.33 ± 0.13 to 0.25 ± 0.14 L/min, p < 0.001), while [HHb + Mb] kinetics remained unchanged. Conclusion: These results suggest that in middle-aged men and women with T2D, PE speedsV · O 2 τ p likely by a better matching of O2 delivery to utilisation and reduces theV · O 2 As during a subsequent high-intensity exercise.

Improvement of Performance and Reduction of Fatigue With Low-Level Laser Therapy in Competitive Cyclists

Evidence indicates that low-level laser therapy (LLLT) minimizes fatigue effects on muscle performance. However, the ideal LLLT dosage to improve athletes'performance during sports activities such as cycling is still unclear. Therefore, the goal of this study was to investigate the effects of different LLLT dosages on cyclists'performance in time-to-exhaustion tests. In addition, the effects of LLLT on the frequency content of the EMG signals to assess fatigue mechanisms were examined. Twenty male competitive cyclists participated in a crossover, randomized, double-blind, placebo-controlled trial. They performed an incremental cycling test to exhaustion (on day 1) followed by 4 time-to-exhaustion tests (on days 2-5) at their individual maximal power output. Before each time-to-exhaustion test, different dosages of LLLT (135, 270, and 405 J/thigh, respectively) or placebo were applied at the quadriceps muscle bilaterally. Power output and muscle activation from both lower limbs were recorded throughout the tests. Increased performance in time-to-exhaustion tests was observed with the LLLT-135 J (∼22 s; P < .01), LLLT-270 J (∼13 s; P = .03), and LLLT-405 J (∼13 s; P = .02) compared to placebo (149 ± 23 s). Although LLLT-270 J and LLLT-405 J did not show significant differences in muscle activation compared with placebo, LLLT-135 J led to an increased high-frequency content compared with placebo in both limbs at the end of the exhaustion test (P ≤ .03). In conclusion, LLLT increased time to exhaustion in competitive cyclists, suggesting this intervention as a possible nonpharmacological ergogenic agent in cycling. Among the different dosages, LLLT-135 J seems to promote the best effects.

The slow component of pulmonary O2 uptake accompanies peripheral muscle fatigue during high-intensity exercise

During constant-power output (PO) exercise above lactate threshold (LT), pulmonary O2 uptake (V̇o2p) features a developing slow component (V̇o2pSC). This progressive increase in O2 cost of exercise is suggested to be related to the effects of muscle fatigue development. We hypothesized that peripheral muscle fatigue as assessed by contractile impairment would be associated with the V̇o2pSC. Eleven healthy men were recruited to perform four constant-PO tests at an intensity corresponding to ∼Δ60 (very heavy, VH) where Δ is 60% of the difference between LT and peak V̇o2p. The VH exercise was completed for each of 3, 8, 13, and 18 min (i.e., VH3, VH8, VH13, VH18) with each preceded by 3 min of cycling at 20 W. Peripheral muscle fatigue was assessed via pre- vs. postexercise measurements of quadriceps torque in response to brief trains of electrical stimulation delivered at low (10 Hz) and high (50 Hz) frequencies. During exercise, breath-by-breath V̇o2p was measured by mass spectrometry and volume turbine. The magnitude of V̇o2pSC increased ( P < 0.05) from 224 ± 81 ml/min at VH3 to 520 ± 119, 625 ± 134, and 678 ± 156 ml/min at VH8, VH13, and VH18, respectively. The ratio of the low-to-high frequency (10/50 Hz) response was reduced ( P < 0.05) at VH3 (−12 ± 9%) and further reduced ( P < 0.05) at VH8 (−25 ± 11%), VH13 (−42 ± 19%), and VH18 (−46 ± 16%), mirroring the temporal pattern of V̇o2pSC development. The reduction in 10/50 Hz ratio was correlated ( P < 0.001, r2 = 0.69) with V̇o2pSC amplitude. The temporal and quantitative association of decrements in muscle torque production and V̇o2pSC suggest a common physiological mechanism between skeletal muscle fatigue and loss of muscle efficiency.

Oxygen uptake kinetics

Muscular exercise requires transitions to and from metabolic rates often exceeding an order of magnitude above resting and places prodigious demands on the oxidative machinery and O2-transport pathway. The science of kinetics seeks to characterize the dynamic profiles of the respiratory, cardiovascular, and muscular systems and their integration to resolve the essential control mechanisms of muscle energetics and oxidative function: a goal not feasible using the steady-state response. Essential features of the O2 uptake (VO2) kinetics response are highly conserved across the animal kingdom. For a given metabolic demand, fast VO2 kinetics mandates a smaller O2 deficit, less substrate-level phosphorylation and high exercise tolerance. By the same token, slow VO2 kinetics incurs a high O2 deficit, presents a greater challenge to homeostasis and presages poor exercise tolerance. Compelling evidence supports that, in healthy individuals walking, running, or cycling upright, VO2 kinetics control resides within the exercising muscle(s) and is therefore not dependent upon, or limited by, upstream O2-transport systems. However, disease, aging, and other imposed constraints may redistribute VO2 kinetics control more proximally within the O2-transport system. Greater understanding of VO2 kinetics control and, in particular, its relation to the plasticity of the O2-transport/utilization system is considered important for improving the human condition, not just in athletic populations, but crucially for patients suffering from pathologically slowed VO2 kinetics as well as the burgeoning elderly population.

Muscle fiber recruitment and the slow component of O2 uptake: constant work rate vs. all-out sprint exercise

The slow component of pulmonary O(2) uptake (Vo(2)) during constant work rate (CWR) high-intensity exercise has been attributed to the progressive recruitment of (type II) muscle fibers. We tested the following hypotheses: 1) the Vo(2) slow component gain would be greater in a 3-min all-out cycle test than in a work-matched CWR test, and 2) the all-out test would be associated with a progressive decline, and the CWR test with a progressive increase, in muscle activation, as estimated from the electromyogram (EMG) of the vastus lateralis muscle. Eight men (aged 21-39 yr) completed a ramp incremental test, a 3-min all-out test, and a work- and time-matched CWR test to exhaustion. The maximum Vo(2) attained in an initial ramp incremental test (3.97 ± 0.83 l/min) was reached in both experimental tests (3.99 ± 0.84 and 4.03 ± 0.76 l/min for all-out and CWR, respectively). The Vo(2) slow component was greater (P < 0.05) in the all-out test (1.21 ± 0.31 l/min, 4.2 ± 2.2 ml·min(-1)·W(-1)) than in the CWR test (0.59 ± 0.22 l/min, 1.70 ± 0.5 ml·min(-1)·W(-1)). The integrated EMG declined by 26% (P < 0.001) during the all-out test and increased by 60% (P < 0.05) during the CWR test from the first 30 s to the last 30 s of exercise. The considerable reduction in muscle efficiency in the all-out test in the face of a progressively falling integrated EMG indicates that progressive fiber recruitment is not requisite for development of the Vo(2) slow component during voluntary exercise in humans.

The O2 cost of the tension-time integral in isolated single myocytes during fatigue

One proposed explanation for the Vo(2) slow component is that lower-threshold motor units may fatigue and develop little or no tension but continue to use O(2), thereby resulting in a dissociation of cellular respiration from force generation. The present study used intact isolated single myocytes with differing fatigue resistance profiles to investigate the relationship between fatigue, tension development, and aerobic metabolism. Single Xenopus skeletal muscle myofibers were allocated to a fast-fatiguing (FF) or a slow-fatiguing (SF) group, based on the contraction frequency required to elicit a fall in tension to 60% of peak. Phosphorescence quenching of a porphyrin compound was used to determine Delta intracellular Po(2) (Pi(O(2)); a proxy for Vo(2)), and developed isometric tension was monitored to allow calculation of the time-integrated tension (TxT). Although peak DeltaPi(O(2)) was not different between groups (P = 0.36), peak tension was lower (P < 0.05) in SF vs. FF (1.97 +/- 0. 17 V vs. 2. 73 +/- 0.30 V, respectively) and time to 60% of peak tension was significantly longer in SF vs. FF (242 +/- 10 s vs. 203 +/- 10 s, respectively). Before fatigue, both DeltaPi(O(2)) and TxT rose proportionally with contraction frequency in SF and FF, resulting in DeltaPi(O(2))/TxT being identical between groups. At fatigue, TxT fell dramatically in both groups, but DeltaPi(O(2)) decreased proportionately only in the FF group, resulting in an increase in DeltaPi(O(2))/TxT in the SF group relative to the prefatigue condition. These data show that more fatigue-resistant fibers maintain aerobic metabolism as they fatigue, resulting in an increased O(2) cost of contractions that could contribute to the Vo(2) slow component seen in whole body exercise.

Effects of ammonium chloride ingestion on phosphocreatine metabolism during moderate- and heavy-intensity plantar-flexion exercise

This study examined the effects of NH(4)Cl ingestion on phosphocreatine (PCr) metabolism during 9 min of moderate- (MOD) and heavy- (HVY) intensity constant-load isotonic plantar-flexion exercise. Healthy young adult male subjects (n = 8) completed both a control (CON) and NH(4)Cl ingestion (ACID) trial. Phosphorus-31 magnetic resonance spectroscopy was used to monitor changes in intracellular pH (pHi), [Pi], [PCr], and [ATP]. During the Middle (3-6 min) and Late (6-9 min) stages of HVY, ACID was associated with a higher (P < 0.05) intracellular hydrogen-ion concentration ([H(+)]i) [Middle: 246 (SD 36) vs. 202 (SD 36) mmol/l]; [Late: 236 (SD 35) vs. 200 (SD 39) mmol/l]. In addition, ACID was associated with a lower (P < 0.05) [PCr] relative to CON during the Early (0-3 min) [18.1 (SD 5.1) vs. 20.4 (SD 5.4) mmol/l] and Middle stages [14.1 (SD 5.4) vs. 16.7 (SD 6.0) mmol/l] of HVY. The amplitude of the primary component of PCr breakdown during the transition to HVY was greater in ACID than CON [14.5 (SD 5.8 vs. 11.3 (SD 4.8) mmol/l], however, the PCr slow component (continued slow decline in [PCr]) showed no difference (P > 0.05). The time constant for PCr breakdown (tauPCr) was greater in HVY than MOD for both conditions [58 (SD 22) vs. 28 (SD 15) s ACID; 51 (SD 20) vs. 29 (SD 14) s CON] (P < 0.05). In summary, ACID increased PCr breakdown during the transition from MOD to HVY, but did not increase the magnitude of the PCr slow component.

Relationship between oxygen uptake slow component and surface EMG during heavy exercise in humans: influence of pedal rate

The aim of this study was to test the hypothesis that extreme pedal rates contributed to the slow component of oxygen uptake (VO(2) SC) in association with changes in surface electromyographic (sEMG) during heavy-cycle exercise. Eight male trained cyclists performed two square-wave transitions at 50 and 110 rpm at a work rate that would elicit a VO(2) corresponding to 50% of the difference between peak VO(2) and the ventilatory threshold. Pulmonary gas exchange was measured breath-by-breath and sEMG was obtained from the vastus lateralis and medialis muscles. Integrated EMG flow (QiEMG) and mean power frequency (MPF) were computed. The relative amplitude of the VO(2) SC was significantly higher during the 110-rpm bout (556+/-186 ml min(-1), P<0.05) with compared to the 50-rpm bout (372+/-227 ml min(-1)). QiEMG values increased throughout exercise only during the 110-rpm bout and were associated with the greater amplitude of the VO(2) SC observed for this condition (P<0.05). MPF values remained relatively constant whatever the cycle bout. These findings indicated a VO(2) SC at the two pedal rates but the association with sEMG responses was observed only at high pedal rate. Possible changes in motor units recruitment pattern, muscle energy turnover and muscle temperature have been suggested to explain the different VO(2) SC to heavy pedal rate bouts.

Prior moderate and heavy exercise accelerate oxygen uptake and cardiac output kinetics in endurance athletes

Cardiorespiratory interactions at the onset of dynamic cycling exercise are modified by warm-up exercises. We tested the hypotheses that oxygen uptake (Vo(2)) and cardiac output (Q) kinetics would be accelerated at the onset of heavy and moderate cycling exercise by warm-up. Nine male endurance athletes (peak Vo(2): 60.5 +/- 3.2 ml.min(-1).kg(-1)) performed multiple rides of two different 36-min cycling protocols, involving 6-min bouts at moderate and heavy intensities. Breath-by-breath Vo(2) and beat-by-beat stroke volume (SV) and Q, estimated by Modelflow from the finger pulse, were measured simultaneously with kinetics quantified from the phase II time constant (tau(2)). One novel finding was that both moderate (M) and heavy (H) warm-up bouts accelerated phase II Vo(2) kinetics during a subsequent bout of heavy exercise (tau(2): after M = 22.5 +/- 2.7 s, after H = 22.1 +/- 2.9 vs. 26.2 +/- 3.2 s; P < 0.01). Q kinetics in heavy exercise were accelerated by both warm-up intensities (tau(2): M = 22.0 +/- 4.1 s, H = 23.8 +/- 5.6 s vs. 27.4 +/- 7.2 s; P < 0.05). During moderate exercise, prior heavy-intensity warm-up (one or two bouts) accelerated Vo(2) kinetics and elevated Q at exercise onset, with no changes in Q kinetics. A second novel finding was a significant overshoot in the estimate of SV from Modelflow in the first minutes of each moderate and heavy exercise bout. These findings suggest that the acceleration of Vo(2) kinetics during heavy exercise was enabled by the acceleration of Q kinetics, and that rapid increases in Q at the onset of moderate and heavy exercise might result, in part, from an overshoot of SV.

Influence of priming exercise on pulmonary O2 uptake kinetics during transitions to high-intensity exercise from an elevated baseline

It has been suggested that the slower O2 uptake (V̇o2) kinetics observed when exercise is initiated from an elevated baseline metabolic rate are linked to an impairment of muscle O2 delivery. We hypothesized that “priming” exercise would significantly reduce the phase II time constant (τ) during subsequent severe-intensity cycle exercise initiated from an elevated baseline metabolic rate. Seven healthy men completed exercise transitions to 70% of the difference between gas exchange threshold (GET) and peak V̇o2 from a moderate-intensity baseline (90% GET) on three occasions in each of the “unprimed” and “primed” conditions. Pulmonary gas exchange, heart rate, and the electromyogram of m. vastus lateralis were measured during all tests. The phase II V̇o2 kinetics were slower when severe exercise was initiated from a baseline of moderate exercise compared with unloaded pedaling (mean ± SD τ, 42 ± 15 vs. 33 ± 8 s; P < 0.05), but were not accelerated by priming exercise (42 ± 17 s; P > 0.05). The amplitude of the V̇o2 slow component and the change in electromyogram from minutes 2 to 6 were both significantly reduced following priming exercise (V̇o2 slow component: from 0.47 ± 0.09 to 0.27 ± 0.13 l/min; change in integrated electromyogram between 2 and 6 min: from 51 ± 35 to 26±43% of baseline; P < 0.05 for both comparisons). These results indicate that the slower phase II V̇o2 kinetics observed during transitions to severe exercise from an elevated baseline are not altered by priming exercise, but that the reduced V̇o2 slow component may be linked to changes in muscle fiber activation.