Torque, power and muscle activation of eccentric and concentric isokinetic cycling.

This study aimed to establish the effect of cycling mode and cadence on torque, external power output, and lower limb muscle activation during maximal, recumbent, isokinetic cycling. After familiarisation, twelve healthy males completed 6 × 10 s of maximal eccentric (ECC) and concentric (CON) cycling at 20, 40, 60, 80, 100, and 120 rpm with five minutes recovery. Vastus lateralis, medial gastrocnemius, rectus femoris, and biceps femoris surface electromyography was recorded throughout. As cadence increased, peak torque linearly decreased during ECC (350-248 N·m) and CON (239-117 N·m) and peak power increased in a parabolic manner. Crank angle at peak torque increased with cadence in CON (+13°) and decreased in ECC (-9.0°). At all cadences, peak torque (mean +129 N·m, range 111-143 N·m), and power (mean +871 W, range 181-1406 W), were greater during ECC compared to CON. For all recorded muscles the crank angle at peak muscle activation was greater during ECC compared to CON. This difference increased with cadence in all muscles except the vastus lateralis. Additionally, peak vastus laterallis and biceps femoris activation was greater during CON compared to ECC. Eccentric cycling offers a greater mechanical stimulus compared to concentric cycling but the effect of cadence is similar between modalities. Markers of technique (muscle activation, crank angle at peak activation and torque) were different between eccentric and concentric cycling and respond differently to changes in cadence. Such data should be considered when comparing between, and selecting cadences for, recumbent, isokinetic, eccentric and concentric cycling.

The Effect of Cognitive-Task Type and Walking Speed on Dual-Task Gait in Healthy Adults.

In a number of studies in which a dual-task gait paradigm was used, researchers reported a relationship between cognitive function and gait. However, it is not clear to what extent these effects are dependent on the type of cognitive and walking tasks used in the dual-task paradigm. This study examined whether stride-time variability (STV) and trunk range of motion (RoM) are affected by the type of cognitive task and walking speed used during dual-task gait. Participants walked at both their preferred walking speed and at 25% of their preferred walking speed and performed a serial subtraction and a working memory task at both speeds. Although both tasks significantly reduced STV at both walking speeds, there was no difference between the two tasks. Trunk RoM was affected by the walking speed and type of cognitive task used during dual-task gait: Mediolateral trunk RoM was increased at the slow walking speed, and anterior-posterior trunk RoM was higher only when performing the serial subtraction task at the slow walking speed. The reduction of STV, regardless of cognitive-task type, suggests that healthy adults may redirect cognitive processes away from gait toward cognitive-task performance during dual-task gait.

Effect of Practical Precooling on Neuromuscular Function and 5-km Time-Trial Performance in Hot, Humid Conditions Among Well-Trained Male Runners.

This study investigated whether torso and thigh precooling during a warm-up effects neuromuscular function and 5-km time-trial performance in hot, humid conditions. Eight well-trained male runners completed 3 randomized time-trials in 32.2 ± 0.8° C and 48.6 ± 6.7% relative humidity. A 30-minute warm-up was completed with no cooling (Control), precooling by an ice vest (Vest), or ice packs covering the thighs (Packs). Before the warm-up and after the time-trial, supramaximal femoral nerve stimulation was delivered during and following maximal isometric contractions. Core and skin temperature, heart rate, and perceptual ratings were recorded before and during the warm-up and time-trial. Overall performance time was improved in Packs compared with Control (1,407 ± 80 seconds vs. 1,492 ± 88 seconds; p ≤ 0.05) but not in Vest (1,444 ± 71 seconds; p > 0.05). In Packs, a higher exercise intensity (p ≤ 0.05) and less cumulative time (p < 0.01) were evident during the last kilometer compared with Control. Maximum voluntary force, voluntary activation, muscle contractility, and membrane excitability were not different after exercise or between conditions. Ten minutes after the warm-up, skin temperature was lower in Vest and Packs compared with Control (p < 0.01). Thermal strain and body heat content change was lower in Vest and Packs, respectively (p ≤ 0.05). Findings indicate that torso and thigh precooling during a warm-up reduces thermoregulatory strain. However, thigh opposed to torso precooling provides greater performance improvements. Neuromuscular function did not aid performance, indicating that transient changes in afferent feedback and muscle recruitment may enhance endurance trial performance.

The effect of transcranial direct current stimulation on task processing and prioritisation during dual-task gait.

The relationship between cognition and gait is often explored using a dual-task gait paradigm, which represents the ability to divide cognitive resources during walking. Recent evidence has suggested that the prefrontal cortex is involved in the allocation of cognitive resources during dual-task gait, though its precise role is unclear. Here, we used anodal and cathodal transcranial direct current stimulation (tDCS) to probe the role of the prefrontal cortex in the control of stride time variability (STV), trunk RoM and cognitive task performance during dual-task gait. As task difficulty has been shown to mediate the dual-task cost, we also manipulated walking speed to see whether the effects of tDCS on dual-task gait were influenced by walking difficulty. Ten adults performed a serial subtraction task when walking at either preferred walking speed or 25 % of preferred walking speed, before and after receiving tDCS of the left prefrontal cortex. Anodal tDCS reduced STV and the dual-task cost on STV and improved cognitive task performance. Cathodal tDCS increased STV and appeared to increase the dual-task cost on STV, but did not affect cognitive task performance. There was no effect of tDCS on trunk RoM, and the effects of tDCS were not mediated by walking speed. The effect of dual-task gait on stride time variability and cognitive task performance was altered by the application of tDCS, and these effects were polarity dependent. These results highlight the role of the prefrontal cortex in biasing task performance during dual-task gait and indicate that tDCS may be a useful tool for examining the role of the cortex in the control of dual-task gait.

The influence of hot humid and hot dry environments on intermittent-sprint exercise performance.

PURPOSE: To examine the effect of a hot humid (HH) compared with a hot dry (HD) environment, matched for heat stress, on intermittent-sprint performance. In comparison with HD, HH environments compromise evaporative heat loss and decrease exercise tolerance. It was hypothesized that HH would produce greater physiological strain and reduce intermittent-sprint exercise performance compared with HD. METHOD: Eleven male team-sport players completed the cycling intermittent-sprint protocol (CISP) in 3 conditions, temperate (TEMP; 21.2°C ± 1.3°C, 48.6% ± 8.4% relative humidity [rh]), HH (33.7°C ± 0.5°C, 78.2% ± 2.3% rh), and HD (40.2°C ± 0.2°C, 33.1% ± 4.9% rh), with both heat conditions matched for heat stress. RESULTS: All participants completed the CISP in TEMP, but 3 failed to completed the full protocol of 20 sprints in HH and HD. Peak power output declined in all conditions (P < .05) but was not different between any condition (sprints 1-14 [N = 11]: HH 1073 ± 150 W, HD 1104 ± 127 W, TEMP, 1074± 134; sprints 15-20 [N = 8]: HH 954 ± 114 W, HD 997 ± 115 W, TEMP 993 ± 94; P > .05). Physiological strain was not significantly different in HH compared with HD, but HH was higher than TEMP (P < .05). CONCLUSION: Intermittent-sprint exercise performance of 40 min duration is impaired, but it is not different in HH and HD environments matched for heat stress despite evidence of a trend toward greater physiological strain in an HH environment.

Peak power output provides the most reliable measure of performance in prolonged intermittent-sprint cycling.

The aims of this study were to determine the reliability of an intermittent-sprint cycling protocol and to determine the efficacy of one practice session on main trials. Eleven men, moderately trained team-sport athletes, completed three visits to the laboratory involving a graded-exercise test and practice session and two trials of a cycling intermittent-sprint Protocol separated by three days. Data for practice and main trials were analysed using typical error of measurement, intra-class correlation and least-products regression to determine reliability. Typical error of measurement (expressed as a coefficient of variation) and intra-class correlation for peak power output from all 20 sprints for trial 1 and trial 2 were 2.9 ± 12.8% (95% confidence interval: 2.0-5.0%) and 0.96 (95% confidence interval: 0.85-0.99), respectively. Typical errors of measurement and intra-class correlation for mean power output for all 20 sprints for trials 1 and 2 were 4.2 ± 11.9% (95% confidence interval: 2.9-7.4%) and 0.90 (95% confidence interval: 0.66-0.97), respectively. The results suggest that peak power output provides a more reliable measure than mean power output. The Cycling Intermittent-Sprint Protocol provides reliable measures of intermittent-sprint performance.

Supraspinal fatigue after normoxic and hypoxic exercise in humans.

Inadequate cerebral O2 availability has been proposed to be an important contributing factor to the development of central fatigue during strenuous exercise. Here we tested the hypothesis that supraspinal processes of fatigue would be increased after locomotor exercise in acute hypoxia compared to normoxia, and that such change would be related to reductions in cerebral O2 delivery and tissue oxygenation. Nine endurance-trained cyclists completed three constant-load cycling exercise trials at ∼80% of maximal work rate: (1) to the limit of tolerance in acute hypoxia; (2) for the same duration but in normoxia (control); and (3) to the limit of tolerance in normoxia. Throughout each trial, prefrontal cortex tissue oxygenation and middle cerebral artery blood velocity (MCAV) were assessed using near-infrared spectroscopy and trans-cranial Doppler sonography, respectively. Cerebral O2 delivery was calculated as the product of arterial O2 content and MCAV. Before and immediately after each trial, twitch responses to supramaximal femoral nerve stimulation and transcranial magnetic stimulation were obtained to assess neuromuscular and cortical function, respectively. Exercise time was reduced by 54%in hypoxia compared to normoxia (3.6 ± 1.3 vs. 8.1 ± 2.9 min; P<0.001). Cerebral O2 delivery,cerebral oxygenation and maximum O2 uptake were reduced whereas muscle electromyographic activity was increased in hypoxia compared to control (P <0.05).Maximum voluntary force and potentiated quadriceps twitch force were decreased below baseline after exercise in each trial;the decreases were greater in hypoxia compared to control (P<0.001), but were not different in the exhaustive trials (P>0.05). Cortical voluntary activation was also decreased after exercise in all trials, but the decline in hypoxia (Δ18%) was greater than in the normoxic trials (Δ5-9%)(P <0.05). The reductions in cortical voluntary activation were paralleled by reductions in cerebral O2 delivery. The results suggest that curtailment of exercise performance in acute severe hypoxia is due, in part, to failure of drive from the motor cortex, possibly as a consequence of diminished O2 availability in the brain.

Cerebrovascular and corticomotor function during progressive passive hyperthermia in humans.

The present study examined the integrative effects of passive heating on cerebral perfusion and alterations in central motor drive. Eight participants underwent passive hyperthermia [0.5°C increments in core temperature (Tc) from normothermia (37 ± 0.3°C) to their limit of thermal tolerance (T-LIM; 39.0 ± 0.4°C)]. Blood flow velocity in the middle cerebral artery (CBFv) and respiratory responses were measured continuously. Arterial blood gases and blood pressure were obtained intermittently. At baseline and each Tc level, supramaximal femoral nerve stimulation and transcranial magnetic stimulation (TMS) were performed to assess neuromuscular and cortical function, respectively. At T-LIM, measures were (in a randomized order) also made during a period of breathing 5% CO(2) gas to restore eucapnia (+5% CO(2)). Mean heating time was 179 ± 51 min, with each 0.5°C increment in Tc taking 40 ± 10 min. CBFv was reduced by ∼20% below baseline from +0.5°C until T-LIM. Maximal voluntary contraction (MVC) of the knee extensors was decreased at T-LIM (-9 ± 10%; P < 0.05), and cortical voluntary activation (VA), assessed by TMS, was decreased at +1.5°C and T-LIM by 11 ± 8 and 22 ± 23%, respectively (P < 0.05). Corticospinal excitability (measured as the EMG response produced by TMS) was unaltered. Reductions in cortical VA were related to changes in ventilation (Ve; R(2) = 0.76; P < 0.05) and partial pressure of end-tidal CO(2) (Pet(CO(2)); R(2) = 0.63; P < 0.05) and to changes in CBFv (R(2) = 0.61; P = 0.067). Interestingly, although CBFv was not fully restored, MVC and cortical VA were restored towards baseline values during inhalation of 5% CO(2). These results indicate that descending voluntary drive becomes progressively impaired as Tc is increased, presumably due, in part, to reductions in CBFv and to hyperthermia-induced hyperventilation and subsequent hypocapnia.

Effect of graded hypoxia on supraspinal contributions to fatigue with unilateral knee-extensor contractions.

Supraspinal fatigue, defined as an exercise-induced decline in force caused by suboptimal output from the motor cortex, accounts for over one-quarter of the force loss after fatiguing contractions of the knee extensors in normoxia. We tested the hypothesis that the relative contribution of supraspinal fatigue would be elevated with increasing severities of acute hypoxia. On separate days, 11 healthy men performed sets of intermittent, isometric, quadriceps contractions at 60% maximal voluntary contraction to task failure in normoxia (inspired O(2) fraction/arterial O(2) saturation = 0.21/98%), mild hypoxia (0.16/93%), moderate hypoxia (0.13/85%), and severe hypoxia (0.10/74%). Electrical stimulation of the femoral nerve was performed to assess neuromuscular transmission and contractile properties of muscle fibers. Transcranial magnetic stimulation was delivered to the motor cortex to quantify corticospinal excitability and voluntary activation. After 10 min of breathing the test gas, neuromuscular function and cortical voluntary activation prefatigue were unaffected in any condition. The fatigue protocol resulted in ∼ 30% declines in maximal voluntary contraction force in all conditions, despite differences in time-to-task failure (24.7 min in normoxia vs. 15.9 min in severe hypoxia, P < 0.05). Potentiated quadriceps twitch force declined in all conditions, but the decline in severe hypoxia was less than that in normoxia (P < 0.05). Cortical voluntary activation also declined in all conditions, but the deficit in severe hypoxia exceeded that in normoxia (P < 0.05). The additional central fatigue in severe hypoxia was not due to altered corticospinal excitability, as electromyographic responses to transcranial magnetic stimulation were unchanged. Results indicate that peripheral mechanisms of fatigue contribute relatively more to the reduction in force-generating capacity of the knee extensors following submaximal intermittent isometric contractions in normoxia and mild to moderate hypoxia, whereas supraspinal fatigue plays a greater role in severe hypoxia.

Muscle contractile function and neural control after repetitive endurance cycling.

PURPOSE: To examine alterations in muscle contractile properties, cortical excitability, and voluntary activation as a consequence of 20 d of repetitive endurance cycling within a 22-d period. METHODS: Eight well-trained male cyclists completed 20 prolonged cycling stages interspersed by two rest days (days 9 and 17), which replicated the 2007 Tour de France route and schedule. Isometric knee extensor torque and EMG responses of the vastus lateralis in response to percutaneous electrical stimulation and transcranial magnetic stimulation were measured before, on days 9 and 17, and 2 d after completion of Tour de France. Postexercise measurements on days 9 and 17 were taken >18 h after cessation of the previous exercise bout. RESULTS: Maximal voluntary contraction of the knee extensors decreased by 20 +/- 10% (P < 0.01) during Tour de France but recovered after 2 d of rest. Peripherally evoked M-wave and potentiated twitch responses were also significantly decreased during Tour de France, up to 31 +/- 21% and 22 +/- 18%, respectively (P < 0.05), but returned to baseline values after 2 d of recovery. Voluntary activation was reduced to 75 +/- 8% (P < 0.05) during Tour de France and remained significantly depressed (79 +/- 7%, P < 0.05) after completion. The amplitude of motor evoked potentials was decreased by 44 +/- 28% (P < 0.01) on day 9 and remained significantly depressed during the remainder of, and after, Tour de France. CONCLUSIONS: A reduction in knee extensor strength, which occurs after repetitive prolonged cycling exercise, is a result of both central and peripheral processes. Reduced sarcolemmal excitability and impairment of contractile mechanisms exists even after 18 h of recovery. An enduring reduction in corticomotor output persists even after 2 d of rest.

Fatigue during maximal sprint cycling: unique role of cumulative contraction cycles.

UNLABELLED: Maximal cycling power has been reported to decrease more rapidly when performed with increased pedaling rates. Increasing pedaling rate imposes two constraints on the neuromuscular system: 1) decreased time for muscle excitation and relaxation and 2) increased muscle shortening velocity. Using two crank lengths allows the effects of time and shortening velocity to be evaluated separately. PURPOSES: We conducted this investigation to determine whether the time available for excitation and relaxation or the muscle shortening velocity was mainly responsible for the increased rate of fatigue previously observed with increased pedaling rates and to evaluate the influence of other possible fatiguing constraints. METHODS: Seven trained cyclists performed 30-s maximal isokinetic cycling trials using two crank lengths: 120 and 220 mm. Pedaling rate was optimized for maximum power for each crank length: 135 rpm for the 120-mm cranks (1.7 m x s(-1) pedal speed) and 109 rpm for the 220-mm cranks (2.5 m x s(-1) pedal speed). Power was recorded with an SRM power meter. RESULTS: Crank length did not affect peak power: 999 +/- 276 W for the 120-mm crank versus 1001 +/- 289 W for the 220-mm crank. Fatigue index was greater (58.6% +/- 3.7% vs 52.4% +/- 4.8%, P < 0.01), and total work was less (20.0 +/- 1.8 vs 21.4 +/- 2.0 kJ, P < 0.01) with the higher pedaling rate-shorter crank condition. Regression analyses indicated that the power for the two conditions was most highly related to cumulative work (r2 = 0.94) and to cumulative cycles (r2 = 0.99). CONCLUSIONS: These results support previous findings and confirm that pedaling rate, rather than pedal speed, was the main factor influencing fatigue. Our novel result was that power decreased by a similar increment with each crank revolution for the two conditions, indicating that each maximal muscular contraction induced a similar amount of fatigue.

Time course of neuromuscular changes during running in well-trained subjects.

PURPOSE: Prolonged exercise reduces the capacity of the neuromuscular system to produce force, which is known as fatigue. The purpose of this study was to examine the time course of neural and contractile processes during a 20-km running bout. METHODS: Eight experienced runners (mean T SD: age = 31 T 6 yr, VO2max = 60.1 T 2.2 mL x kg(-1) x min(-1)) completed an all-out self-paced 20-km treadmill run. Isometric knee extensor torque and EMG responses of the vastus lateralis (VL) in response to percutaneous electrical stimulation and voluntary contraction were measured before and after 5, 10, 15, and 20 km of exercise. RESULTS: Participant's RPE, measured using the Borg 6-20 scale, increased steadily throughout the run to a value of 18 T 1 at exercise termination. Maximal voluntary contraction (MVC) of the knee extensors only decreased during the final 5 km of running, with a 15% +/- 12% (P = 0.02) decrease at 20 km. Vastus lateralis EMG during an MVC was reduced after 15 km (-18% +/- 21%, P = or <0.01) and 20 km (-20% +/- 22%, P = 0.03). A significant correlation (r = 0.71, P = 0.048) was observed between the final reduction in MVC and the maximal EMG. Voluntary activation, estimated by the twitch interpolation technique, decreased by 13% +/- 6% at 20 km (P = or < 0.01), and this was significantly correlated (r = 0.70, P = 0.049) with MVC loss. There were no significant changes in the amplitude of the electrically evoked muscle action potential (M-wave) or potentiated twitch during or after the 20-km run. CONCLUSIONS: A reduction in knee extensor MVC only occurs during the final 5 km of a 20-km self-paced run. Impaired voluntary activation and neural drive but not contractile processes are responsible for this decreased strength.

Effects of dynamic and static stretching on vertical jump performance and electromyographic activity.

The results of previous research have demonstrated that static stretching (SS) can reduce muscular performance and that dynamic stretching (DS) can enhance muscular performance. The purpose of this study was to assess the effects of SS and DS on vertical jump (VJ) performance and electromyographic (EMG) activity of the m. vastus medialis. Eleven healthy men (age 21 +/- 2 years) took part in 3 conditions (no stretching [NS], SS, and DS), on separate occasions in a randomized, crossover design. During each condition, measurements of VJ height and EMG activity during the VJ were recorded. A repeated-measures analysis of variance and post hoc analysis indicated that VJ height was significantly less (4.19 +/- 4.47%) after SS than NS (p < 0.05) and significantly greater (9.44 +/- 4.25%) in DS than SS (p < 0.05). There was significantly greater EMG amplitude in the DS compared with the SS (p < 0.05). The results demonstrated that SS has a negative influence on VJ performance, whereas DS has a positive impact. Increased VJ performance after DS may be attributed to postactivation potentiation, whereas the reduction in VJ performance after SS may be attributable to neurological impairment and a possible alteration in the viscoelastic properties of the muscular tendon unit (MTU). This investigation provides some physiological basis for the inclusion of DS and exclusion of SS in preparation for activities requiring jumping performance.

Influence of acute inspiratory loading upon diaphragm motor-evoked potentials in healthy humans.

Acute prior activity of the inspiratory muscles can enhance inspiratory muscle strength and reduce effort perception during subsequent inspiratory efforts. However, the mechanisms subserving these changes are poorly understood. Responses to magnetic stimulation in 10 subjects were studied after an acute bout of nonfatiguing inspiratory muscle loading (IML), corresponding to 40% of subjects' initial maximal inspiratory pressure (MIP), and after an acute bout of nonloaded, forced inspiration (NLF). Motor-evoked potentials elicited by cortical stimulation (MEP(c)) and by phrenic nerve stimulation (MEP(p)) were recorded transcutaneously from the diaphragm before, immediately after, and 15 min after two sets of 30 inspiratory efforts, at rest and during an MIP effort. After IML, MIP increased to 113 +/- 3% (SE) of baseline and diaphragm MEP(p) (during MIP) significantly increased (129 +/- 10% of baseline). Diaphragmatic MEP(c) (during MIP), expressed as a percentage of maximal MEP(p), decreased after IML (from 29 +/- 9% to 20 +/- 6%; P = 0.017) and after NLF (from 43 +/- 5% to 31 +/- 5%; P = 0.032). Observations from the biceps brachi demonstrated that changes after IML and NLF were specific to the inspiratory muscle, since no significant changes were observed in biceps force generation or in MEP(p) or MEP(c) amplitudes. These data indicate that after IML increased global inspiratory strength is accompanied by increased peripheral excitability and by a dampening of corticospinal excitability of the diaphragm.

Corticomotor excitability contributes to neuromuscular fatigue following marathon running in man.

It is unknown whether changes in corticomotor excitability follow prolonged exercise in healthy humans. Furthermore, the role of supraspinal fatigue in decrements of force production and voluntary activation following prolonged exercise has not been established. This study investigated peripheral and central fatigue after a marathon (42.2 km) on a treadmill. Isometric ankle dorsiflexion force and electromyographic responses of the tibialis anterior in response to magnetic stimulation of the peroneal nerve (PNMS) and the motor cortex (TMS) were measured before, immediately after, 4 and 24 h post-marathon (MAR) in nine volunteers (mean +/- s.d. completion time, 208 +/- 22 min). Maximal voluntary contraction decreased by 18 +/- 7% immediately after MAR (P = 0.009) and remained significantly decreased after 4 h. The amplitude of the evoked response to TMS, but not to PNMS, was depressed immediately post-MAR by 57 +/- 25% (P = 0.04). Potentiated resting twitch force was reduced in response to both TMS and PNMS post-MAR (71 +/- 8 and 35 +/- 2% decrease, P = 0.035 and 0.037, respectively), and voluntary activation was reduced to 61.9 +/- 18% immediately post-MAR (P < 0.05). All measures had returned to baseline values after 24 h. These results suggest that fatigue was attributable to both a disturbance of the contractile apparatus within the muscle and submaximal output from the motor cortex.