Derek Kay

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.

Different neuromuscular recruitment patterns during eccentric, concentric and isometric contractions

Aim. The purpose of this study was to determine the neuromuscular fatigue profiles during 100 s isometric (ISO), concentric (CON), and eccentric (ECC) activity.Methods. Twelve subjects (age 25.1+/-3.7 years, mass 70.1+/-8.2 kg, mean+/-SD) performed ISO, CON and ECC maximal voluntary contractions and 100 s endurance trials on an isokinetic dynamometer. Raw EMG data were recorded throughout each trial from the rectus femoris of the right limb. Corresponding data for integrated electromyography (IEMG), percentile frequency shifts (MPFS) and peak torque output were divided into five 5 s epochs and subsequently normalised with the first epoch being the reference point, in order to assess changes over time.Results. There were no significant differences between ECC, CON and ISO peak torque output (211+/-63 vs 169+/-41 vs 177+/-61 Nm; ECC, CON, ISO) and IEMG activity (280+/-143 vs 305+/-146 vs 287+/-143 mV; ECC, CON, ISO) during maximal contractions. Serial reductions in torque output were greatest in ISO in which torque output during the final epoch was 31+/-13% of initial values, similar to the final torque values in CON (58+/-15%), but significantly less than ECC (108.6+/-38.6%; P<0. 001) values. In CON and ECC, IEMG was maintained (95+/-27% and 93+/-21%; CON and ECC), whereas IEMG for ISO decreased to 38+/-13% of initial values. The greatest reduction in MPFS occurred in CON (69+/-10%) compared to ISO (78+/-9%; P<0.05) and ECC (93+/-6%; P<0.001).Conclusion. These data demonstrate distinct neuromuscular fatigue profiles for the different types of muscle contraction. Whereas eccentric activity was largely fatigue resistant, isometric and concentric contractions displayed different neuromuscular fatigue profiles.

Fluid ingestion and exercise hyperthermia: Implications for performance, thermoregulation, metabolism and the development of fatigue

The development of fatigue during exercise and the subsequent onset of exhaustion occur earlier in the heat than in cooler environments. The underlying mechanisms responsible for the premature development of fatigue in the heat have yet to be clearly identified. However, the proposed mechanisms include metabolic, cardiovascular and central nervous system perturbations, together with an elevated core temperature. Fluid ingestion is one of three strategies that have been shown to be successful in enhancing the performance of endurance exercise in the heat, with the other interventions being precooling and acclimatization. However, like the development of fatigue in the heat, the mechanisms by which fluid ingestion allows for improved exercise performance remain unclear. We propose that fluid ingestion enhances exercise performance in the heat by increasing the heat storage capacity of the body. We suggest that the thermoregulatory, metabolic and cardiovascular alterations that occur as a result of this increased heat storage capacity contribute to performance enhancement when fluid is ingested during exercise heat stress.

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.

Influence of the amyloid precursor protein locus on dementia in Down syndrome.

Background: The amyloid precursor protein (APP) locus on chromosome 21 influences the development of Alzheimer disease. Method: The authors investigated the relationship between a tetranucleotide repeat on intron 7 of the APP gene and the age at onset of dementia in Down syndrome (DS). Results: There was a 13-year difference in the age at onset of dementia in DS associated with the number of tetranucleotide repeat alleles in APP. Conclusion: APP is an important locus predicting the age at onset of dementia in people with Down syndrome.

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.

Reproducibility and changes in twitch properties associated with age and resistance training in young and elderly women

We compared knee extensor twitch contractile properties (TP) between nine young women (YW, 20–30 years) and 10 elderly women (63–78 years) and examined changes associated with resistance training in addition to measurement reproducibility. Data were obtained on two occasions 3 weeks apart after which subjects performed bilateral leg extension and bilateral leg curl exercises 3 days/week for 10 weeks. TP demonstrated moderate to good reproducibility in both age groups with Pearsons r and the intra‐class correlation coefficient ranging from 0.67 to 0.85 (P<0.05) and the technical error of the measurement ranging from 4.2% to 7.8%. Pre‐training, peak twitch torque, rate of torque development, and the rate of relaxation were 24–32% greater for the YW than EW (P<0.05). Time to peak torque, half‐relaxation time, and contraction duration were not significantly different between age groups. Post‐training, changes of 2.6–6.1% were observed in TP; however, these changes were not significant in either group. These data suggest the presence of an age‐associated slowing in the rate of muscle contraction. Furthermore, the lack of change in TP in both groups suggests resistance training failed to alter contractile function. However, these findings are discussed in relation to measurement reproducibility and the meaningfulness of the data obtained.

Normalized lengthening peak torque is associated with temporal twitch characteristics in elderly women but not young women

Aim:  To determine if greater normalized torque during maximal effort lengthening actions in elderly women compared with young women is related to age‐associated adjustments in neural activation and/or contractile function.