Relationship between motor corticospinal excitability and ventilatory response during intense exercise.

PURPOSE: Effort sense has been suggested to be involved in the hyperventilatory response during intense exercise (IE). However, the mechanism by which effort sense induces an increase in ventilation during IE has not been fully elucidated. The aim of this study was to determine the relationship between effort-mediated ventilatory response and corticospinal excitability of lower limb muscle during IE. METHODS: Eight subjects performed 3 min of cycling exercise at 75-85 % of maximum workload twice (IE1st and IE2nd). IE2nd was performed after 60 min of resting recovery following 45 min of submaximal cycling exercise at the workload corresponding to ventilatory threshold. Vastus lateralis muscle response to transcranial magnetic stimulation of the motor cortex (motor evoked potentials, MEPs), effort sense of legs (ESL, Borg 0-10 scale), and ventilatory response were measured during the two IEs. RESULTS: The slope of ventilation (l/min) against CO2 output (l/min) during IE2nd (28.0 ± 5.6) was significantly greater than that (25.1 ± 5.5) during IE1st. Mean ESL during IE was significantly higher in IE2nd (5.25 ± 0.89) than in IE1st (4.67 ± 0.62). Mean MEP (normalized to maximal M-wave) during IE was significantly lower in IE2nd (66 ± 22 %) than in IE1st (77 ± 24 %). The difference in mean ESL between the two IEs was significantly (p < 0.05, r = -0.82) correlated with the difference in mean MEP between the two IEs. CONCLUSIONS: The findings suggest that effort-mediated hyperventilatory response to IE may be associated with a decrease in corticospinal excitability of exercising muscle.

Voluntary breathing increases corticospinal excitability of lower limb muscle during isometric contraction.

The aim of the present study was to determine the effects of voluntary breathing on corticospinal excitability of a leg muscle during isometric contraction. Seven subjects performed 5-s isometric knee extension at the intensity of 10% of maximal voluntary contraction (10% MVC). During the 10% MVC, the subjects were instructed to breath normally (NORM) or to inhale (IN) or exhale (OUT) once as fast as possible. Motor-evoked potentials (MEPs) induced by transcranialmagnetic stimulation in the right vastus lateralis (VL) during the 10% MVC were recorded and compared during the three breathing tasks. MEPs in IN and OUT were significantly higher than that in NORM. Effort sense of breathing was significantly higher in IN and OUT than in NORM. There was a significant positive correlation between MEP and effort sense of breathing. These results suggest that activation of the breathing-associated cortical areas with voluntary breathing is involved in the increase in corticospinal excitability of the VL during isometric contraction.

Effects of humoral factors on ventilation kinetics during recovery after impulse-like exercise.

To clarify the ventilatory kinetics during recovery after impulse-like exercise, subjects performed one impulse-like exercise test (one-impulse) and a five-times repeated impulse-like exercises test (five-impulse). Duration and intensity of the impulse-like exercise were 20 sec and 400 watts (80 rpm), respectively. Although blood pH during recovery (until 10 min) was significantly lower in the five-impulse test than in the one-impulse test, ventilation (.VE) in the two tests was similar except during the first 30 sec of recovery, in which it was higher in the five-impulse test. In one-impulse, blood CO2 pressure (PCO2) was significantly increased at 1 min during recovery and then returned to the pre-exercise level at 5 min during recovery. In the five-impulse test, PCO2 at 1 min during recovery was similar to the pre-exercise level, and then it decreased to a level lower than the pre-exercise level at 5 min during recovery. Accordingly, PCO2 during recovery (until 30 min) was significantly lower in the five-impulse than in one-impulse test..VE and pH during recovery showed a curvilinear relationship, and at the same pH, ventilation was higher in the one-impulse test. These results suggest that ventilatory kinetics during recovery after impulse-like exercise is attributed partly to pH, but the stimulatory effect of lower pH is diminished by the inhibitory effect of lower PCO2.

Relationship between effort sense and ventilatory response to intense exercise performed with reduced muscle glycogen.

The purpose of the present study was to examine the effects of muscle glycogen reduction on surface electromyogram (EMG) activity and effort sense and ventilatory responses to intense exercise (IE). Eight subjects performed an IE test in which IE [100-105% of peak O(2) uptake ([Formula: see text]), 2 min] was repeated three times (IE(1st), IE(2nd) and IE(3rd)) at 100-120-min intervals. Each interval consisted of 20-min passive recovery, 40-min submaximal exercise at ventilatory threshold intensity (51.5 ± 2.7% of [Formula: see text]), and a further resting recovery for 40-60 min. Blood pH during IE and subsequent 20-min recovery was significantly higher in the IE(3rd) than in the IE(1st) (P < 0.05). Effort sense of legs during IE was significantly higher in the IE(3rd) than in the IE(1st) and IE(2nd). Integrated EMG (IEMG) measured in the vastus lateralis during IE was significantly lower in the IE(3rd) than in the IE(1st). In contrast, mean power frequency of the EMG was significantly higher in the IE(2nd) and the IE(3rd) than in the IE(1st). Ventilation ([Formula: see text]) in the IE(3rd) was significantly higher than that in the IE(1st) during IE and the first 60 s after the end of IE. These results suggest that ventilatory response to IE is independent of metabolic acidosis and at least partly associated with effort sense elicited by recruitment of type II fibers.

Ventilatory response to moderate incremental exercise performed 24 h after resistance exercise with concentric and eccentric contractions.

In order to test our hypothesis that muscle condition has an effect on the cognition of self-motion and consequently on the ventilatory response during exercise, six healthy subjects performed a moderate incremental exercise test (IET) on a cycle ergometer under two conditions [resistance exercise condition (REC) and control condition (CC)]. In the REC, resistance exercise (30 incline leg presses) was conducted during two sessions scheduled at 48 and then 24 h prior to the IET. For the CC, the subjects were instructed to refrain from participating in strenuous exercise for a period of 2 days prior to the IET. In the IET, the workload was increased from 78 to 118 watts in steps of 8 watts every 3 min. Although the ventilatory response during the IET was significantly higher in the REC than in the CC, there were no significant differences in cognitive indexes (RPE and awareness of change in workload) between the two conditions. In addition, the magnitude of muscle soreness was significantly higher in the REC than in the CC. However, the level of soreness in the REC was very low, and there were no significant differences in blood lactate concentration and integrated EMG between the two conditions. These results suggest that a change in peripheral neural reflex is the primary cause of increased ventilatory response to moderate exercise after resistance exercise, although the role of a cognitive element cannot be absolutely excluded.

Effects of sodium bicarbonate ingestion on EMG, effort sense and ventilatory response during intense exercise and subsequent active recovery.

To determine whether post-exercise ventilation is related to decrease in blood pH and also whether post-exercise ventilation, associated or not with decreased blood pH, involves an increase in central motor command during exercise, we examined the effects of NaHCO(3) ingestion on the ventilatory response ([Formula: see text]E), integrated electromyogram (iEMG) and effort sense of legs (ESL) during intense exercise (IE) and subsequent active recovery. Subjects performed two IE tests (105-110% of maximal work rate, 2 min) after ingestion of NaHCO(3) or CaCO(3). Subjects performed light load exercise (20 W) before and after IE for 6 min and 30 min, respectively. Although there was a significant difference in blood pH between the two conditions during and after IE, [Formula: see text]E, iEMG and ESL were similar. iEMG returned to the pre-IE level immediately after the end of IE, while ESL showed slow recovery. [Formula: see text]E decreased rapidly until about 50 s after the end of IE (fast phase) and then showed a slow recovery kinetics (slow phase). The ventilatory responses during the fast phase and during the slow phase were correlated with ESL at the end of IE and from 3 min after the end of IE, respectively. Moreover, there was no significant difference in the slopes and intercepts of regression lines between [Formula: see text]E and ESL under the two conditions in both phases. These results suggest that the ventilatory response after IE is associated with effort sense indirectly-elicited by central motor command, but the effort sense-mediated response is not affected by blood pH.

Effect of blood volume in resting muscle on heart rate upward drift during moderately prolonged exercise.

The aim of this study was to determine whether the increase in blood volume in resting muscle during moderately prolonged exercise is related to heart rate (HR) upward drift. Eight healthy men completed both arm-cranking moderately prolonged exercise (APE) and leg-pedaling moderately prolonged exercise (LPE) for 30 min. Exercise intensity was 120 bpm of HR that was determined by ramp incremental exercise. During both APE and LPE, HR significantly increased from 3 to 30 min (from 108±9.3 to 119±12 bpm and from 112±8.9 to 122±11 bpm, respectively). However, there was no significant difference between HR in APE and that in LPE. Oxygen uptake was maintained throughout the two exercises. Skin blood flow, deep temperature, and total Hb (blood volume) in resting muscle continuously increased for 30 min of exercise during both APE and LPE. During both APE and LPE, there was a significant positive correlation between total Hb and deep temperature in all subjects. Moreover, there was a significant positive correlation between HR and total Hb (in seven out of eight subjects) during LPE. However, during APE, there was no positive correlation between HR and total Hb (r=0.391). These findings suggest that an increase of blood pooling in resting muscle could be proposed as one of the mechanisms underlying HR upward drift during moderately prolonged exercise.

Effects of awareness of change in load on ventilatory response during moderate exercise.

This study was designed to determine whether awareness of change in load alters ventilatory response during moderate exercise. Subjects performed two incremental exercise protocols on a cycle ergometer. The load was increased from 1.0 to 1.5kp in steps of 0.1kp every 3min. Subjects were provided true information about the load in the control protocol and untrue information that the load would remain constant in the deception protocol. Slope of ventilation against CO2 output was significantly lower in the deception protocol than control protocol. Integrated EMG (iEMG) and ratings of perceived exertion (RPE) were similar between the two protocols, but awareness of change in load was significantly attenuated by the deception protocol. However, there was no temporal coincidence between awareness and actual change in load. These results suggest that ventilatory response during moderate exercise depends not so much on RPE but mainly on awareness or attention that is closely connected to information detection.

Examination of oxygen uptake kinetics in decremental load exercise by a numerical computation model.

The purpose of this study was to establish a numerical computation model for estimation of oxygen uptake (V(.)O2) kinetics in decremental load exercise (DLE) starting from a work rate (WR) above the ventilatory threshold (>VT). In the model, WR in DLE were separated into several steps (constant load exercise, CLE) of which the durations increased step by step. V(.)O2 kinetics in each step was estimated using an exponential equation, and the sum of VO2 values from all steps at a given time was regarded as simulated V(.)O2 in DLE. In the model, the time constants were set symmetrically in a step VT at onset and offset (tau(off)) of exercise. As a result, simulated V(.)O2 qualitatively, but not quantitatively, approximated measured V(.)O2. Consequently, we incorporated a new model in which a step >VT was subdivided into several parts. Although there was a slight difference quantitatively, the interval of subdivision of 3.0 min and tau(off) of 2.8 min allowed for qualitative approximation. The numerical computation model adopted in this study is useful for estimation of V(.)O2 kinetics during DLE starting from high intensity (>VT).

Effect of oral administration of sodium bicarbonate on surface EMG activity during repeated cycling sprints.

The purpose of this study was to determine the effect of oral administration of sodium bicarbonate (NaHCO3) on surface electromyogram (SEMG) activity from the vastus lateralis (VL) during repeated cycling sprints (RCS). Subjects performed two RCS tests (ten 10-s sprints) interspersed with both 30-s and 360-s recovery periods 1 h after oral administration of either NaHCO3 (RCSAlk) or CaCO3 (RCSPla) in a random counterbalanced order. Recovery periods of 360 s were set before the 5th and 9th sprints. The rate of decrease in plasma HCO3- concentration during RCS was significantly greater in RCSAlk than in RCSPla, but the rates of decline in blood pH during the two RCS tests were similar. There was no difference between change in plasma lactate concentration in RCSAlk and that in RCSPla. Performance during RCSAlk was similar to that during RCSPla. There were no differences in oxygen uptake immediately before each cycling sprint (preVO2) and in SEMG activity between RCSAlk and RCSPla. In conclusion, oral administration of NaHCO3 did not affect SEMG activity from the VL. This suggests that the muscle recruitment strategy during RCS is not determined by only intramuscular pH.

A 350-S recovery period does not necessarily allow complete recovery of peak power output during repeated cycling sprints.

The aim of this study was to determine whether a 350-s recovery period allows recovery of peak power output (PPO) to its initial value under the condition of a blood lactate (La) concentration higher than 10 mmol.L-1 during repeated cycling sprints (RCS). RCS (10x10-s cycling sprints) were performed under two conditions. Under one condition, the recovery period of RCS was fixed at 35 s (RCS35), and under the other condition, a 350-s recovery period was set before the 5th and 9th sets, and a 35-s recovery period was set before the other sets (RCScomb). In RCScomb, PPO in the 5th set recovered to that in the 1st set, but PPO in the 9th set did not. Under both conditions, blood La concentration progressively increased and reached approximately 14 mmol.L-1 at the end of the RCS. In RCScomb, VO2 immediately before the 5th set was not significantly different from that immediately before the 9th set. Mean power frequency (MPF) values estimated by a surface electromyogram from the vastus lateralis in the 5th and 9th sets were significantly higher in RCScomb than in RCS35. In conclusion, a 350-s recovery period does not allow recovery of PPO to its initial value under the condition of a blood La concentration of 14 mmol.L-1 during RCS.

Effect of blood lactate concentration and the level of oxygen uptake immediately before a cycling sprint on neuromuscular activation during repeated cycling sprints.

The purpose of this study was to determine whether neuromuscular activation is affected by blood lactate concentration (La) and the level of oxygen uptake immediately before a cycling sprint (preVO(2)). The tests consisted of ten repeated cycling sprints for 10 sec with 35-sec (RCS(35)) and 350-sec recovery periods (RCS(350)). Peak power output (PPO) was not significantly changed despite an increase in La concentration up to 12 mmol/L in RCS(350). Mean power frequency (MPF) of the power spectrum calculated from a surface electromyogram on the vastus lateralis showed a significantly higher level in RCS(350). In RCS(35), preVO(2) level and La were higher than those in RCS(350) in the initial stage of the RCS and in the last half of the RCS, respectively. Thus, neuromuscular activation during exercise with maximal effort is affected by blood lactate concentration and the level of oxygen uptake immediately before exercise, suggesting a cyclic system between muscle recruitment pattern and muscle metabolites.

Kinetics of oxygen uptake during arm cranking with the legs inactive or exercising at moderate intensities.

The purpose of this study was to compare the kinetics of oxygen uptake (VO(2)) during arm cranking with the legs inactive or exercising. Each subject (n = 8) performed three exercise protocols: 6-min arm cranking at an intensity of 60% of peak oxygen uptake (VO(2peak), AC(60)) and 6-min combined arm cranking and leg cycling in which AC(60) was added to on-going leg cycling at an intensity of 20% or 40% of VO(2peak) (LC(20) and LC(40): AC(60)LC(20) and AC(60)LC(40), respectively). After the onset of arm cranking, VO(2) tended to increase until the end of arm cranking in all of the three exercise modes. The amplitudes of this increase in VO(2) were 0.98 (0.18), 0.93 (0.16) and 0.84 (0.12) l.min(-1) during AC(60), AC(60)LC(20) and AC(60)LC(40), respectively, and there were significant differences between values for each exercise. The data are presented as means and standard deviations. There were no significant differences in the effective VO(2) time constant, partial O(2) deficit, and the difference between the values of VO(2) measured at 3 and 6 min in the three exercise modes. The present results indicate that the amplitude of the increase in VO(2) is reduced during arm cranking with the legs exercising, that this reduction becomes greater with increases in the intensity of leg cycling, and that the rate of increase in VO(2) is not affected by the additional muscle mass of the legs exercising below moderate intensities. The decrease in the amplitude of increase in VO(2) might be caused by reduction in oxygen supply to the exercising arms due to large muscle mass and/or overlaps of activity of stabilizing muscles during combined arm and leg exercise.

Kinetics of oxygenation in inactive forearm muscle during ramp leg cycling.

This study was carried out to determine whether hemodynamics in inactive forearm muscle during ramp leg cycling is affected from the ventilatory threshold (VT) and respiratory compensation point (RCP), at which the rate of increase in ventilation (VE) against power output begins to increase abruptly. Change in hemodynamics was evaluated by change in oxygenation index (difference between concentrations of oxygenated hemoglobin and deoxygenated hemoglobin, HbD) measured using near-infrared spectrometry (NIRS). Each subject (n=9) performed 4-min constant-work-rate leg cycling and subsequent ramp leg cycling at an increasing rate of 10 watts.min(-1) in power output. The work rates at VT, RCP and peak oxygen uptake (VO(2 peak)) were 107 +/- 11, 172 +/- 21 and 206 +/- 20 watts, respectively. The rates of increase in VE between 10-watt leg cycling, VT, RCP and VO(2 peak) were 0.19 +/- 0.03, 0.44 +/- 0.07 and 1.32 +/- 0.47 l.min(-1).watts(-1), respectively. In one subject, HbD started to decrease during ramp exercise from the VT, and the rate of decrease increased at a high intensity of exercise. In eight subjects, although no decrease in HbD from the VT was observed, HbD showed a sudden drop at a high intensity of exercise. The work rate at which HbD began to decrease at a high intensity of exercise was 174 +/- 23 watts. This work rate was not significantly different from that at the RCP and was significantly correlated with that at the RCP (r=0.72, P<0.05). The results suggest that the abrupt increase in VE from the RCP affects hemodynamics, resulting in a decrease in HbD in inactive forearm muscle.

Relationship in simulation between oxygen deficit and oxygen uptake in decrement-load exercise starting from low exercise intensity.

The purpose of the present study was to determine by simulation whether oxygen deficit kinetics in decrement-load exercise (DLE) starting from a low exercise intensity is related to the oxygen uptake (Vo(2)) kinetics. In this simulation, work rate in DLE was separated into steps that were regarded as constant-load exercises (CLEs). It was assumed that Vo(2) kinetics behaved exponentially at the onset and offset of each CLE, respectively. Vo(2) at the onset of CLEs increases at the same time and becomes a recovery phase step-by-step corresponding to the decrement of work rate. The sum of Vo(2) values at the onset of CLEs at a given time (nt-Vo(2)) corresponds to Vo(2) excluding oxygen debt in DLE. The sum of Vo(2) values at the offset of CLEs at a given time (dt-Vo(2)) corresponds to Vo(2) related to oxygen debt in DLE. The total of net- and dt-Vo(2) values is equivalent to Vo(2) actually observed in DLE (gs-Vo(2)). As the oxygen requirement level is a steady-state value of Vo(2) in CLE, the oxygen deficit level can be obtained by subtracting Vo(2) at the onset of CLE from the steady-state value. The oxygen deficit level at a given time was added in all CLEs. This is oxygen deficit per unit time (df-Vo(2)). Oxygen debt and oxygen deficit were calculated by integrating df-Vo(2) and dt-Vo(2) from the start of exercise to a given time, respectively. Gs-Vo(2) increased, reached a peak, and decreased linearly until the end of the DLE. Oxygen deficit increased rapidly and showed a steady state. Oxygen debt increased linearly after a time lapse. The difference between oxygen deficit and oxygen debt changed like gs-Vo(2) kinetics. Therefore, it is concluded that if we consider the repayment of oxygen debt in the oxygen deficit in DLE, the kinetics of the oxygen deficit becomes similar to gs-Vo(2) kinetics in the simulation.

Approximation equation for oxygen uptake kinetics in decrement-load exercise starting from low exercise intensity.

The purpose of the present study was to determine the degree of fitting an approximation equation for oxygen uptake (Vo(2)) in decrement-load exercise (DLE). Work rate was started from 120 watts and was decreased by a rate of 15 watts per min. The initial work rate of DLE corresponded to 72+/-10% of the work rate at anaerobic threshold determined in incremental-load exercise (ILE). Vo(2) in DLE increased rapidly, reached a peak, and decreased linearly until the end of the exercise. Vo(2) in DLE was higher than that in ILE at the same work rate except in the early periods in ILE and DLE. This difference ranged from 300 to 400 ml/min. This difference is a result of repayment of oxygen debt in DLE and from the oxygen deficit induced by the delay of response of Vo(2) in ILE. As work rate in DLE can be obtained by the difference between work rates in constant-load exercise (CLE) and ILE, we postulated that the approximation equation for Vo(2) kinetics in DLE could be expressed by a combination of approximation equations in CLE and in ILE. When time delay was taken into consideration in this equation, the fitting of data obtained by using the equation was better than that of data obtained by using the equation without a parameter of time delay. The degree of fitting ranged from 94 to 98% (r(2)). Thus, it seems that Vo(2) including oxygen debt in DLE can be approximated by the equation used in this study.