Role of exercise duration on metabolic adaptations in working muscle to short-term moderate-to-heavy aerobic-based cycle training

Effect of short-term, high-intensity exercise training on human skeletal muscle citrate synthase maximal activity: single versus multiple bouts per session

The legs of 9 men (age 21 ± 2 years, 45 ± 4 mL/(kg·min)) were randomly assigned to complete 6 sessions of high-intensity exercise training, involving either one or four 5-min bouts of counterweighted, single-leg cycling. Needle biopsies from vastus lateralis revealed that citrate synthase maximal activity increased after training in the 4-bout group (p = 0.035) but not the 1-bout group (p = 0.10), with a significant difference between groups post-training (13%, p = 0.021). Novelty Short-term training using brief intense exercise requires multiple bouts per session to increase mitochondrial content in human skeletal muscle.

Physiological adaptations to interval training and the role of exercise intensity

Interval exercise typically involves repeated bouts of relatively intense exercise interspersed by short periods of recovery. A common classification scheme subdivides this method into high-intensity interval training (HIIT; 'near maximal' efforts) and sprint interval training (SIT; 'supramaximal' efforts). Both forms of interval training induce the classic physiological adaptations characteristic of moderate-intensity continuous training (MICT) such as increased aerobic capacity (V̇O2 max ) and mitochondrial content. This brief review considers the role of exercise intensity in mediating physiological adaptations to training, with a focus on the capacity for aerobic energy metabolism. With respect to skeletal muscle adaptations, cellular stress and the resultant metabolic signals for mitochondrial biogenesis depend largely on exercise intensity, with limited work suggesting that increases in mitochondrial content are superior after HIIT compared to MICT, at least when matched-work comparisons are made within the same individual. It is well established that SIT increases mitochondrial content to a similar extent to MICT despite a reduced exercise volume. At the whole-body level, V̇O2 max is generally increased more by HIIT than MICT for a given training volume, whereas SIT and MICT similarly improve V̇O2 max despite differences in training volume. There is less evidence available regarding the role of exercise intensity in mediating changes in skeletal muscle capillary density, maximum stroke volume and cardiac output, and blood volume. Furthermore, the interactions between intensity and duration and frequency have not been thoroughly explored. While interval training is clearly a potent stimulus for physiological remodelling in humans, the integrative response to this type of exercise warrants further attention, especially in comparison to traditional endurance training.

Superior mitochondrial adaptations in human skeletal muscle after interval compared to continuous single-leg cycling matched for total work

KEY POINTS

A classic unresolved issue in human integrative physiology involves the role of exercise intensity, duration and volume in regulating skeletal muscle adaptations to training. We employed counterweighted single-leg cycling as a unique within-subject model to investigate the role of exercise intensity in promoting training-induced increases in skeletal muscle mitochondrial content. Six sessions of high-intensity interval training performed over 2 weeks elicited greater increases in citrate synthase maximal activity and mitochondrial respiration compared to moderate-intensity continuous training matched for total work and session duration. These data suggest that exercise intensity, and/or the pattern of contraction, is an important determinant of exercise-induced skeletal muscle remodelling in humans.

ABSTRACT

We employed counterweighted single-leg cycling as a unique model to investigate the role of exercise intensity in human skeletal muscle remodelling. Ten young active men performed unilateral graded-exercise tests to measure single-leg V̇O2, peak and peak power (W_peak ). Each leg was randomly assigned to complete six sessions of high-intensity interval training (HIIT) [4 × (5 min at 65% W_peak and 2.5 min at 20% W_peak )] or moderate-intensity continuous training (MICT) (30 min at 50% W_peak ), which were performed 10 min apart on each day, in an alternating order. The work performed per session was matched for MICT (143 ± 8.4 kJ) and HIIT (144 ± 8.5 kJ, P > 0.05). Post-training, citrate synthase (CS) maximal activity (10.2 ± 0.8 vs. 8.4 ± 0.9 mmol kg protein^-1  min^-1 ) and mass-specific [pmol O₂ •(s•mg wet weight)^-1 ] oxidative phosphorylation capacities (complex I: 23.4 ± 3.2 vs. 17.1 ± 2.8; complexes I and II: 58.2 ± 7.5 vs. 42.2 ± 5.3) were greater in HIIT relative to MICT (interaction effects, P < 0.05); however, mitochondrial function [i.e. pmol O₂ •(s•CS maximal activity)^-1 ] measured under various conditions was unaffected by training (P > 0.05). In whole muscle, the protein content of COXIV (24%), NDUFA9 (11%) and mitofusin 2 (MFN2) (16%) increased similarly across groups (training effects, P < 0.05). Cytochrome c oxidase subunit IV (COXIV) and NADH:ubiquinone oxidoreductase subunit A9 (NDUFA9) were more abundant in type I than type II fibres (P < 0.05) but training did not increase the content of COXIV, NDUFA9 or MFN2 in either fibre type (P > 0.05). Single-leg V̇O2, peak was also unaffected by training (P > 0.05). In summary, single-leg cycling performed in an interval compared to a continuous manner elicited superior mitochondrial adaptations in human skeletal muscle despite equal total work.

Maximal power output during incremental cycling test is dependent on the curvature constant of the power–time relationship

The aim of this study was to investigate whether the maximal power output (Pmax) during an incremental test was dependent on the curvature constant (W′) of the power–time relationship. Thirty healthy male subjects (maximal oxygen uptake = 3.58 ± 0.40 L·min−1) performed a ramp incremental cycling test to determine the maximal oxygen uptake and Pmax, and 4 constant work rate tests to exhaustion to estimate 2 parameters from the modeling of the power–time relationship (i.e., critical power (CP) and W′). Afterwards, the participants were ranked according to their magnitude of W′. The median third was excluded to form a high W′ group (HIGH, n = 10), and a low W′ group (LOW, n = 10). Maximal oxygen uptake (3.84 ± 0.50 vs. 3.49 ± 0.37 L·min−1) and CP (213 ± 22 vs. 200 ± 29 W) were not significantly different between HIGH and LOW, respectively. However, Pmax was significantly greater for the HIGH (337 ± 23 W) than for the LOW (299 ± 40 W). Thus, in physically active individuals with similar aerobic parameters, W′ influences the Pmax during incremental testing.

Effects of 2 weeks of low-intensity cycle training with different pedaling rates on the work rate at lactate threshold

PURPOSE

This study examined (1) the effects of a single bout of exercise at different pedaling rates on physiological responses, pedal force, and muscle oxygenation, and (2) the effects of 2 weeks of training with different pedaling rates on work rate at lactate threshold (WorkLT).

METHODS

Sixteen healthy men participated in the study. An incremental exercise test involving pedaling a cycling ergometer at 50 rpm was conducted to assess maximal oxygen consumption and WorkLT. The participants performed constant workload, submaximal exercise tests at WorkLT intensity with three different pedaling rates (35, 50, and 75 rpm). Oxygen consumption ([Formula: see text]O2), blood pressure, heart rate (HR), blood lactate, and pedal force were measured and oxy-hemoglobin/myoglobin concentration (OxyHb/Mb) at vastus lateralis was monitored by near-infrared spectroscopy during exercise. The participants were then randomly assigned to cycling exercise training at WorkLT in either the low or high frequency pedaling rate (LFTr, 35 rpm or HFTr, 75 rpm) group. Each 60-min training session was performed five times/week.

RESULTS

Despite maintaining the same work rate, [Formula: see text]O2 and HR were significantly lower at 35 than 75 rpm. Conversely, integrated pedal force was significantly higher at 35 than 75 rpm. Peripheral OxyHb/Mb was significantly lower at 35 than 75 rpm. After 2 weeks of training, WorkLT normalized to body mass significantly increased in the LFTr, but not the HFTr group.

CONCLUSIONS

Pedaling rate and the corresponding pedal force and peripheral oxygenation during cycling exercise influence the effect of training at LT on WorkLT.