Jack Cannon

Evidence for neuromuscular fatigue during high-intensity cycling in warm, humid conditions.

Abstract The purpose of this study was to examine and describe the neuromuscular changes associated with fatigue using a self-paced cycling protocol of 60-min duration, under warm, humid conditions. Eleven subjects [mean (SE) age 21.8 (0.8) years; height 174.9 (3.0) cm; body mass 74.8 (2.7) kg; maximum oxygen consumption 50.3 (1.8) ml · kg · min−1] performed one 60-min self-paced cycling time trial punctuated with six 1-min “all out” sprints at 10-min intervals, while 4 subjects repeated the trial for the purpose of determining reproducibility. Power output, integrated electromyographic signal (IEMG), and mean percentile frequency shifts (MPFS) were recorded at the mid-point of each sprint. There were no differences between trials for EMG variables, distance cycled, mean heart rate, and subjective rating of perceived exertion for the subjects who repeated the trial (n=4). The results from the repeated trials suggest that neuromuscular responses to self-paced cycling are reproducible between trials. The mean heart rate for the 11 subjects was 163.6 (0.71) beats · min−1. Values for power output and IEMG expressed as a percentage of that recorded for the initial sprint decreased during sprints 2–5, with normalised values being 94%, 91%, 87% and 87%, respectively, and 71%, 71%, 73%, and 77%, respectively. However, during the final sprint normalised power output and IEMG increased to 94% and 90% of initial values, respectively. MPFS displayed an increase with time; however, this was not significant (P=0.06). The main finding of this investigation is the ability of subjects to return power output to near initial values during the final of six maximal effort sprints that were included as part of a self-paced cycling protocol. This appears to be due to a combination of changes in neuromuscular recruitment, central or peripheral control systems, or the EMG signal itself. Further investigations in which changes in multiple physiological systems are assessed systematically are required so that the underlying mechanisms related to the development of fatigue during normal dynamic movements such as cycling can be more clearly delineated.

The effects of compression garments on recovery of muscle performance following high-intensity sprint and plyometric exercise

This study compared the effects of compression garments on recovery of evoked and voluntary performance following fatiguing exercise. Eleven participants performed 2 sessions separated by 7 days, with and without lower-body compression garments during and 24h post-exercise. Participants performed a 10-min exercise protocol of a 20-m sprint and 10 plyometric bounds every minute. Before, following, 2h and 24h post-exercise, evoked twitch properties of the knee extensors, peak concentric knee extension and flexion force were assessed, with blood samples drawn to measure lactate [La(-)], pH, creatine kinase (CK), aspartate transaminase (AST) and c-reactive protein (C-RP). Heart rate, exertion (RPE) and muscle soreness (MS) measures were obtained pre- and post-exercise. No differences (P=0.50-0.80) and small effect sizes (d<0.3) were present for 20-m sprint (3.59+/-0.22 vs. 3.59+/-0.18s) or bounding performance (17.13+/-1.4 vs. 17.21+/-1.7 m) in garment and control conditions. The decline and recovery in concentric force were not different (P=0.40) between conditions. Full recovery of voluntary performance was observed 2h post-exercise, however, evoked twitch properties remained suppressed 2h post-exercise in both conditions. No differences (P=0.40-0.80, d<0.3) were present between conditions for heart rate, RPE, [La(-)], pH, CK or C-RP. However, 24h post-exercise a smaller change (P=0.08; d=2.5) in AST (23.1+/-3.1 vs. 26.0+/-4.0) and reduced (P=0.01; d=1.1) MS (2.8+/-1.2 vs. 4.5+/-1.4) were present in the garments. In conclusion the effects of compression garments on voluntary performance and recovery were minimal; however, reduced levels of perceived MS were reported following recovery in the garments.

Exercise heat stress does not reduce central activation to non-exercised human skeletal muscle

In this study we measured the central activation ratio (CAR) of the leg extensors and the elbow flexor muscles before and after exhaustive exercise in the heat to determine whether exercise‐induced hyperthermia affects the CNS drive to exercised (leg extensors) and/or non‐exercised (forearm flexors) muscle groups. Thirteen subjects exercised at fixed intensities representative of a percentage of peak power output (PPO) for 10 min periods (50%, 40%, 60%, 50%) and then at 75% PPO until exhaustion in ambient conditions of 39.3 ± 0.8 °C and 60.0 ± 0.8% relative humidity. Before and immediately following exercise subjects performed a series of maximal voluntary contractions (MVCs) with the leg extensors (exercised muscles) and forearm flexors (non‐exercised muscles). The degree of voluntary activation during the sustained MVCs was assessed by superimposing electrical stimulation to the femoral nerve and the biceps brachii. Exercise to exhaustion increased the rectal temperature from 37.2 ± 0.2 to 38.8 ± 0.2 °C (P < 0.0001). The mean heart rate at the end of exercise to exhaustion was 192 ± 3 beats min−1. Leg extensor voluntary force was significantly reduced from 595 ± 143 to 509 ± 105 N following exercise‐induced hyperthermia but forearm flexor force was similar before and after exercise. The CAR of the leg extensors decreased from 94.2 ± 1.3% before exercise to 91.7 ± 1.5% (P < 0.02) following exercise‐induced hyperthermia. However, the CAR for the forearm flexors remained at similar levels before and after exercise. The data suggest that the central nervous system selectively reduces central activation to specific skeletal muscles as a consequence of exercise‐induced hyperthermia.

Comparative effects of resistance training on peak isometric torque, muscle hypertrophy, voluntary activation and surface EMG between young and elderly women

We compared the effect of a 10‐week resistance training program on peak isometric torque, muscle hypertrophy, voluntary activation and electromyogram signal amplitude (EMG) of the knee extensors between young and elderly women. Nine young women (YW; range 20–30 years) and eight elderly women (EW; 64–78 years) performed three sets of ten repetitions at 75% 1 repetition maximum for the bilateral leg extension and bilateral leg curl 3 days per week for 10 weeks. Peak isometric torque, EMG and voluntary activation were assessed before, during, and after the training period, while knee extensor lean muscle cross‐sectional area (LCSA) and lean muscle volume (LMV) were assessed before and after the training period only. Similar increases in peak isometric torque (16% and 18%), LCSA (13% and 12%), LMV (10% and 9%) and EMG (19% and 21%) were observed between YW and EW, respectively, at the completion of training (P<0·05), while the increase in voluntary activation in YW (1·9%) and EW (2·1%) was not significant (P>0·05). These findings provide evidence to indicate that participation in regular resistance exercise can have significant neuromuscular benefits in women independent of age. The lack of change in voluntary activation following resistance training in both age groups despite the increase in EMG may be related to differences between measurements in their ability to detect resistance training‐induced changes in motor unit activity. However, it is possible that neural adaptation did not occur and that the increase in EMG was due to peripheral adaptations.

Reduced voluntary activation of human skeletal muscle during shortening and lengthening contractions in whole body hyperthermia

This study examined the effect of whole body hyperthermia on the voluntary activation of exercised and non‐exercised skeletal muscle performing a series of lengthening and shortening contractions. Thirteen subjects exercised on a cycle ergometer at 60% of maximal oxygen consumption until voluntary exhaustion in ambient conditions of ∼40°C and 60% relative humidity. Before and immediately following the cycle protocol, subjects performed a series of 25 continuous isokinetic shortening and lengthening maximal voluntary contractions (MVCs) of the leg extensors and forearm flexors. Voluntary activation for shortening and lengthening contractions for the forearm and leg was assessed prior to and following the 25 MVCs by superimposing a paired electrical stimulus to the femoral nerve and the biceps brachii during additional MVCs. Exercise to exhaustion increased rectal temperature to 39.35 ± 0.50°C. Voluntary activation remained unchanged following the prehyperthermia endurance set of shortening and lengthening maximal contractions in both the forearm flexors and leg extensors. Similarly, voluntary activation remained at prehyperthermic levels for the single MVCs immediately following the cycle trial. However, by the time of completion of the posthyperthermia endurance contractions, voluntary activation had declined significantly by 5.87 ± 7.56 and 8.46 ± 9.26% in the shortening and lengthening phases, respectively, for the leg extensors but not for the forearm flexors. These results indicate that the central nervous system (CNS) reduces voluntary drive to skeletal muscle performing both shortening and lengthening contractions following exercise‐induced hyperthermia. The reductions in voluntary activation were only observed following a series of dynamic movements, indicating that the CNS allows for initial and brief ‘re‐activation’ of skeletal muscle following exercise‐induced hyperthermia.

Early-phase neuromuscular adaptations to high- and low-volume resistance training in untrained young and older women

Abstract We compared early-phase effects between high- and low-volume moderate-intensity resistance training on lean muscle volume, maximal bilateral leg extension strength, maximal isometric torque, normalized maximal bilateral leg extension strength, normalized maximal isometric torque, and muscle recruitment of the right knee extensors in previously untrained young (23.8 ± 3.7 years, range 20–30 years; n = 16) and older women (67.6 ± 6.3 years, range 60–78 years; n = 15). Participants performed either one set or three sets of 10 repetitions for the bilateral leg extension and bilateral leg curl at an intensity of 50–75% of maximal strength 3 days per week for 10 weeks. Main effects were observed over time for all variables (P < 0.05) with increases ranging from 7.1% to 27.8% and effect sizes (Cohens d) ranging from 0.45 to 1.38. No interactions between age and training volume over time were observed for any variable (P > 0.05). Our results provide a novel contribution to the literature demonstrating that additional neuromuscular adaptation during early-phase moderate-intensity resistance training in previously untrained young and older women may not be elicited through higher-volume training when training loads are matched provided that a minimal volume threshold is attained. These findings may have practical applications for the prescription of short-duration resistance training programmes to enhance muscle strength and achieve hypertrophic and non-hypertrophic adaptations in untrained women.

Cold‐water immersion decreases cerebral oxygenation but improves recovery after intermittent‐sprint exercise in the heat

This study examined the effects of post‐exercise cooling on recovery of neuromuscular, physiological, and cerebral hemodynamic responses after intermittent‐sprint exercise in the heat. Nine participants underwent three post‐exercise recovery trials, including a control (CONT), mixed‐method cooling (MIX), and cold‐water immersion (10 °C; CWI). Voluntary force and activation were assessed simultaneously with cerebral oxygenation (near‐infrared spectroscopy) pre‐ and post‐exercise, post‐intervention, and 1‐h and 24‐h post‐exercise. Measures of heart rate, core temperature, skin temperature, muscle damage, and inflammation were also collected. Both cooling interventions reduced heart rate, core, and skin temperature post‐intervention (P < 0.05). CWI hastened the recovery of voluntary force by 12.7 ± 11.7% (mean ± SD) and 16.3 ± 10.5% 1‐h post‐exercise compared to MIX and CONT, respectively (P < 0.01). Voluntary force remained elevated by 16.1 ± 20.5% 24‐h post‐exercise after CWI compared to CONT (P < 0.05). Central activation was increased post‐intervention and 1‐h post‐exercise with CWI compared to CONT (P < 0.05), without differences between conditions 24‐h post‐exercise (P > 0.05). CWI reduced cerebral oxygenation compared to MIX and CONT post‐intervention (P < 0.01). Furthermore, cooling interventions reduced cortisol 1‐h post‐exercise (P < 0.01), although only CWI blunted creatine kinase 24‐h post‐exercise compared to CONT (P < 0.05). Accordingly, improvements in neuromuscular recovery after post‐exercise cooling appear to be disassociated with cerebral oxygenation, rather reflecting reductions in thermoregulatory demands to sustain force production.

Effect of age on muscle activation and twitch properties during static and dynamic actions

In this investigation we examined age‐associated changes in peak torque, voluntary activation levels, and potentiated twitch properties of the knee extensors during isometric (ISO), shortening (SHO), and lengthening (LEN) actions in 18 young subjects (19–27 years) and 12 elderly subjects (64–77 years). Peak torque was lower for the elderly subjects under the ISO (−31%) and SHO (−28%) conditions (P < 0.05); however, the loss in LEN peak torque in the elderly was less marked (−17%) (P > 0.05). Voluntary activation levels within and between groups were not significantly different and ranged between 96.8% and 98.9% (P > 0.05). Peak twitch torque and some temporal twitch characteristics were altered with age (P < 0.05), but such changes were similar across all muscle actions (P > 0.05). These data suggest that the attenuated reduction in LEN muscle strength associated with age is probably not related to contraction‐specific changes in voluntary activation levels or potentiated twitch properties. Muscle Nerve, 2009

Effect of an acute bout of plyometric exercise on neuromuscular fatigue and recovery in recreational athletes

Drinkwater, EJ, Lane, T, and Cannon J. Effect of an acute bout of plyometric exercise on neuromuscular fatigue and recovery in recreational athletes. J Strength Cond Res 23(4): 1181-1186, 2009-Although plyometric training is widely used by sports coaches as a method of improving explosive power in athletes, many prescribe volumes in excess of the National Strength and Conditioning Association recommendations. The purpose of this study was to assess voluntary and evoked muscle characteristics to assess the neuromuscular impact of a high-volume bout of plyometric exercise that was non-exhaustive. Ten athletes who did not have plyometric training experience and were in their competitive season for club-level sport volunteered for the study. After at least 2 days without high-intensity activity, subjects were assessed on maximal twitch torque, time to peak torque, rate of twitch torque development, twitch half-relaxation time, rate of twitch relaxation, and voluntary activation by the interpolated twitch technique before, immediately after, and 2 hours after a high-volume plyometric training program (212 ground contacts). Data were analyzed by repeated-measures analysis of variance and described as mean ± SD and Cohen d. Statistically significant decrements appeared immediately after the training protocol in the total torque generated by maximal voluntary contractions (p < 0.05, d = −0.51) and twitch (p < 0.01, d = −0.92), rate of twitch torque development (p < 0.01, d = −0.77), and rate of relaxation (p < 0.01, d = −0.73). However, we did not observe any differences that remained statistically different after 2 hours. There were no significant differences observed at any time point in time to peak twitch, half-relaxation time, or voluntary activation. We conclude that high-volume plyometric training results primarily in peripheral fatigue that substantially impairs force and rate of force development. We recommend that coaches carefully monitor the volume of plyometric training sessions to avoid neuromuscular impairments that can result in suboptimal training.

Neuromuscular responses to hydration in moderate to warm ambient conditions during self-paced high intensity exercise

Objective To examine the neuromuscular responses to 60 minutes of self-paced high-intensity exercise punctuated with 6 × 1-minute “all-out” sprints at 10-minute intervals in moderate (19.8°C, SEM 0.3) and warm (33.2, SEM 0.1), humid (∼64% relative humidity) conditions with either complete hydration (CH) or without hydration (NF). Design Seven subjects (mean age 20.6 years (SE 1.1), mass 73.8 kg (SE 4.5), peak power 288 W (SE 11.3)) performed the time trial on four separate occasions, which were differentiated by ambient temperature and fluid ingestion. For each sprint interval, distance, power output and electromyographic (EMG) data from the rectus femoris and vastus lateralis muscles were recorded. Results The NF trials resulted in a reduction in body mass for the moderate and warm conditions of 1.7% and 2.1%, respectively. Final rectal temperatures were not different among conditions (∼38.7°C). Total body sweating was higher in the warm condition (19.1–21.3 ml/kg per hour) compared with the moderate condition (16.1–16.5 ml/kg per hour; p<0.05). Neither fluid ingestion nor ambient temperature altered total distance cycled for any of the trials (range 30.1–32.6 km). The normalised integrated EMG (as percentage of maximal voluntary contraction) when compared with the first sprint increased from sprint three for the rectus femoris muscle in both no and complete hydration but decreased for the vastus lateralis muscle. The mean percentile frequency shift increased for both the vastus lateralis and rectus femoris muscles in both no and complete hydration. Conclusions These results suggest that the integrity of the neuromuscular system is adjusted according to hydration status and ambient temperatures during intense self-paced cycling.