Effect of Japanese sitting style (seiza) on the center of foot pressure after standing

Seiza is one of the most commonly used sitting postures in various enrichment lessons of Japanese origin. It is reported that Seiza with large knee flexion produces harmful effects on the cartilage of knee joints and hemodynamics of the lower legs. This study aimed at examining the influence of Seiza on tissue oxygenation kinetics of the lower limbs, plantar somatic and cutaneous sensation, and the center of foot pressure (COP) sway using 10 young adults. COP sway was measured for 1 min just after sitting on a chair for 10 min (pre-test), after 30-min Seiza (post-test 1), and 5 min after Seiza (post-test 2). To evaluate the COP sway, we used 4 body sway factors; unit time sway factor (F1), front-back sway factor (F2), left-right sway factor (F3) and high frequency band power spectrum factor (F4). Physiological parameters (i.e., tissue oxygenation kinetics in the lower legs and sensation on the sole) were measured during 30-min Seiza (continuously on tissue oxygenation, and at 1 min intervals on sensation), and for 1 min just before each COP test (pre-test, post-test 1 and 2).Oxygenated hemoglobin/myoglobin (Hb/Mb) concentration decreased markedly and deoxygenated Hb/Mb concentration increased markedly, resulting in reaching a plateau state at around 7 min. Tissue Hb/Mb index changed little during Seiza. Proprioceptive perception thresholds increased rapidly about 17 min after Starting Seiza. Means of 3 COP sway factors of F1, F2 and F4 were significantly higher in post-test 1 than in pre-test and post-test 2. In conclusion, a marked decrease in tissue oxygen concentration of the lower legs within 4-5 min, and an increase of proprioceptive perception thresholds in the sole at about 17 min are induced by Seiza. Although wiggle and quick body sway in the antero-posterior axis increases markedly in an upright posture just after maintaining Seiza for 30 min, sway recovers after sitting on a chair for 5 min.     

Fatigue during sustained maximal voluntary contraction of different muscles in humans: dependence on fibre type and body posture

Nine healthy men, aged between 25 and 35 years, performed sustained maximal voluntary contractions (MVC) of foot plantar, foot dorsal, and finger flexor muscles. Contractions lasted 10 min and were followed by short test contractions at 30% MVC during recovery. Two positions of the working extremity high or low were established by different body postures (supine or sitting). Under these conditions, studies of force, integrated electromyogram (iEMG), blood pressure, and heart rate showed firstly that force decreased throughout the first few minutes of maximal contraction but reached a near steady-state value after 5 to 6 min. Secondly, force decay and steady-state level depended on muscle group and body position. When sitting (low leg), muscles with a high incidence of slow twitch fibres (plantar flexors) showed a slower force decay and a higher relative steady-state force than fast dorsal flexor muscles. When supine (high leg), plantar and dorsal flexor muscles reached about the same low level of relative steady-state force. Changes in iEMG, blood pressure, and heart rate did not differ in the two positions. Thirdly, during recovery, plantar flexor muscles showed higher iEMG values as well as higher values of blood pressure and heart rate when supine than when sitting. Recovery of dorsal flexor muscles was little affected by body posture. Fourthly, force development and recovery of predominantly fast finger flexor muscles were almost independent of arm position. It was concluded that muscle fibre composition was the main factor in determining endurance capacity. However, endurance was influenced by changes in the hydrostatic blood pressure component.(ABSTRACT TRUNCATED AT 250 WORDS)     

Low-frequency force steadiness practice in plantar flexor muscle reduces postural sway during quiet standing

The purpose of this study was to assess the effect of low-frequency force steadiness practice in the plantar flexor muscles on postural sway during quiet standing. Healthy young 21 men (21±1 yrs) were randomly assigned to a practice group (n=14) and a nonexercising control group (n=7). Practice groups were divided by frequency of practice: 7 participants practiced once a week, and the other 7 twice a week, for 4 weeks. Steadiness practice required practice group to 5 sets of 60-s contraction at levels corresponding to 10% and 20% maximal voluntary contraction (MVC) in the plantar flexor muscles. The 4-week-long practice period reduced the force fluctuations (assessed as the standard deviation (SD) of the outputted force during steady isometric plantar flexion) and postural sway (assessed as SD of the center of mass velocity during quiet standing). However, these practice effects were not significantly affected by the practice frequencies (1 vs. 2 sessions per week) examined in this study. Further, a linear regression analysis revealed the association between prepractice postural sway and the relative change in postural sway by the practice (r=-0.904) in the practice group. These results suggest that the steadiness practice in plantar flexor muscles improves postural stability during quiet standing, even though the practice is low-frequency (once a week) and low-intensity (within 20% MVC). These practice effects are dependent on prepractice postural stability. Further, the present results have provided the functional significance of force fluctuation in lower limb muscles.     

Force control of quadriceps muscle is bilaterally impaired in subacute stroke

We tested the hypothesis that force variability and error during maintenance of submaximal isometric knee extension are greater in subacute stroke patients than in controls and are related to motor impairments. Contralesional (more-affected) and ipsilesional (less-affected) legs of 33 stroke patients with sufficiently high motor abilities (62 ± 13 yr, 16 ± 2 days postinjury) and the dominant leg of 20 controls (62 ± 10 yr) were tested in sitting position. After peak knee extension torque [maximum voluntary contraction (MVC)] was established, subjects maintained 10, 20, 30, and 50% of MVC as steady and accurate as possible for 10 s by matching voluntary force to the target level displayed on a monitor. Coefficient of variation (CV) and root-mean-square error (RMSE) were used to quantify force variability and error, respectively. The MVC was significantly smaller in the more-affected than less-affected leg, and both were significantly lower than in controls. The CV was significantly larger in the more-affected than less-affected leg at 20 and 50% MVC, whereas both were significantly larger compared with controls across all force levels. Both more-affected and less-affected legs of patients showed significantly greater RMSE than controls at 30 and 50% MVC. The CV and RMSE were not related to the Fugl-Meyer motor score or to the Rivermead Mobility Index. The CV negatively correlated with MVC in controls but only in the less-affected leg of patients. It is concluded that isometric knee extension strength and force control are bilaterally impaired soon after stroke but more so in the more-affected leg. Future studies should examine possible mechanisms and the evolution of these changes.     

Difference in aftereffects following prolonged Achilles tendon vibration on muscle activity during maximal voluntary contraction among plantar flexor synergists.

It has been suggested that a suppression of maximal voluntary contraction (MVC) induced by prolonged vibration is due to an attenuation of Ia afferent activity. The purpose of the present study was to test the hypothesis that aftereffects following prolonged vibration on muscle activity during MVC differ among plantar flexor synergists owing to a supposed difference in muscle fiber composition. The plantar flexion MVC torque and surface electromyogram (EMG) of the medial head of gastrocnemius (MG), the lateral head of gastrocnemius (LG), and the soleus (Sol) were recorded in 13 subjects before and after prolonged vibration applied to the Achilles tendon at 100 Hz for 30 min. The maximal H reflexes and M waves were also determined from the three muscles, and the ratio between H reflexes and M waves (H/Mmax) was calculated before and after the vibration. The MVC torque was decreased by 16.6 +/- 3.7% after the vibration (P < 0.05; ANOVA). The H/Mmax also decreased for all three muscles, indicating that Ia afferent activity was successfully attenuated by the vibration in all plantar flexors. However, a reduction of EMG during MVC was observed only in MG (12.7 +/- 4.0%) and LG (11.4 +/- 3.9%) (P < 0.05; ANOVA), not in Sol (3.4 +/- 3.0%). These results demonstrated that prolonged vibration-induced MVC suppression was attributable mainly to the reduction of muscle activity in MG and LG, both of which have a larger proportion of fast-twitch muscle fibers than Sol. This finding suggests that Ia-afferent activity that reinforces the recruitment of high-threshold motor units is necessary to enhance force exertion during MVC.     

Effects of fatiguing isometric exercise upon achilles tendon reflex and plantar flexion reaction time components in man.

Effects of three different fatiguing local muscular exercises upon plantar flexion reaction time and achilles tendon reflex time have been studied in 24 normal males. The Exercise Conditions, each involving a series of 30 maximal voluntary isometric contractions (MVC) of the plantar flexors, differed by allowing either 5, 10 or 20 sec rest interval between each MVC. Decrements in strength ranged from 15% to 34% MVC. Trend analysis of the fatigue patterns revealed that a cubic orthogonal polynomial equation was sufficient to describe the profile of MVC decrement for all conditions. Following the fatiguing exercise, simple visual reaction time (plantar flexion), and its two components, premotor and motor time, failed to demonstrate any change from Pre Exercise Conditions. Achilles Tendon Reflex Times, however, demonstrated a marked augmentation, as manifest in reduced total reflex times, contraction times and half relaxation times. These results may suggest the differential fatigue of motor units employed in the three motor tasks, viz. MVC, voluntary reaction and achilles tendon reflex. A plausible explanation for the augmentation of the reflex contraction resides in the known potentiating effect of elevated intramuscular temperature. Alternatively, one might postulate a neurally mediated increase in gain of the stretch servomechanism. The possibility of both mechanisms being operative is not excluded.     

Repeatability of maximal voluntary force and of surface EMG variables during voluntary isometric contraction of quadriceps muscles in healthy subjects

The repeatability of initial values and rate of change of EMG signal mean spectral frequency (MNF), average rectified values (ARV), muscle fiber conduction velocity (CV) and maximal voluntary contraction (MVC) was investigated in the vastus medialis obliquus (VMO) and vastus lateralis (VL) muscles of both legs of nine healthy male subjects during voluntary, isometric contractions sustained for 50 s at 50% MVC. The values of MVC were recorded for both legs three times on each day and for three subsequent days, while the EMG signals have been recorded twice a day for three subsequent days. The degree of repeatability was investigated using the Fisher test based upon the ANalysis Of VAriance (ANOVA), the Standard Error of the Mean (SEM) and the Intraclass Correlation Coefficient (ICC).

Data collected showed a high level of repeatability of MVC measurement (normalized SEM from 1.1% to 6.4% of the mean). MNF and ARV initial values also showed a high level of repeatability (ICC>70% for all muscles and legs except right VMO). At 50% MVC level no relevant pattern of fatigue was observed for the VMO and VL muscles, suggesting that other portions of the quadriceps might have contributed to the generated effort. These observations seem to suggest that in the investigation of muscles belonging to a multi-muscular group at submaximal level, the more selective electrically elicited contractions should be preferred to voluntary contractions.     

Effects of room temperature and body position change on cerebral blood volume and center-of-foot pressure in healthy young adults

This study aimed to examine the effects of room temperature and body position changes on cerebral blood volume, blood pressure and center-of-foot pressure (COP). Cerebral oxygenation kinetics and blood pressure were measured by near infrared spectroscopy (NIRS) and volume-compensation, respectively, in 9 males and 9 females after rapid standing from sitting and supine positions in low (12 degrees C) or normal (22 degrees C) room temperatures. COP was also measured in a static standing posture for 90 s after rapid standing. The total hemoglobin (Hb) decreased just after standing. Blood pressure after standing at normal temperature tended to decrease immediately but at low temperature tended to decrease slightly and then to increase greatly. The decreasing ratio of total Hb and blood pressure upon standing from a supine position at normal room temperatures was the largest of any condition. Total Hb recovered to a fixed level approximately 25 sec after standing from a sitting position and approximately 35 sec after standing from a supine position. All COP parameters after standing tended to change markedly in the supine position compared to the sitting position, especially at normal temperatures. The COP parameters after standing in any condition were not significantly related to the decreasing ratio of total Hb but were related to the recovery time of total Hb after standing. In conclusion, decreasing ratios of total Hb and blood pressure after standing from a supine position at normal temperatures were large and may affect body sway.     

Adaptations in coactivation after isometric resistance training.

Twenty sedentary male university students were randomly assigned to an experimental or a control group. The experimental group trained the knee extensors of one leg by producing 30 isometric extension maximal voluntary contractions (MVC) per day, three times per week for 8 wk. After 8 wk of training, extensor MVC in the trained leg increased 32.8% (P less than 0.05), but there was no change in vastus lateralis maximal integrated electromyographic activity (IEMGmax). The most important finding was that the degree of hamstring coactivation during extension MVC decreased by approximately 20% (P less than 0.05) after the 1st wk of training. Less pronounced adaptations occurred in the untrained leg: extension MVC force increased 16.2% (P less than 0.05), hamstring coactivity decreased 13% (P less than 0.05) after 2 wk of training, and vastus lateralis IEMGmax was unchanged. The same measures in legs of the control group were not changed during the study. There were no changes in flexion MVC, biceps femoris IEMGmax, or the degree of quadriceps coactivity during flexion MVC in either leg of the control or experimental group. A reduction in hamstring coactivity in the trained and untrained legs indicates that these muscles provide less opposing force to the contracting quadriceps. We conclude that this small but significant decrease in hamstring coactivation that occurs during the early stages of training is a nonhypertrophic adaptation of the neuromuscular system in response to static resistance training of this type.     

Hyperthermia and central fatigue during prolonged exercise in humans.

The present study investigated the effects of hyperthermia on the contributions of central and peripheral factors to the development of neuromuscular fatigue. Fourteen men exercised at 60% maximal oxygen consumption on a cycle ergometer in hot (40 degrees C; hyperthermia) and thermoneutral (18 degrees C; control) environments. In hyperthermia, the core temperature increased throughout the exercise period and reached a peak value of 40.0 +/- 0.1 degrees C (mean +/- SE) at exhaustion after 50 +/- 3 min of exercise. In control, core temperature stabilized at approximately 38.0 +/- 0.1 degrees C, and exercise was maintained for 1 h without exhausting the subjects. Immediately after the cycle trials, subjects performed 2 min of sustained maximal voluntary contraction (MVC) either with the exercised legs (knee extension) or with a "nonexercised" muscle group (handgrip). The degree of voluntary activation during sustained maximal knee extensions was assessed by superimposing electrical stimulation (EL) to nervus femoralis. Voluntary knee extensor force was similar during the first 5 s of contraction in hyperthermia and control. Thereafter, force declined in both trials, but the reduction in maximal voluntary force was more pronounced in the hyperthermic trial, and, from 30 to 120 s, the force was significantly lower in hyperthermia compared with control. Calculation of the voluntary activation percentage (MVC/MVC + EL) revealed that the degree of central activation was significantly lower in hyperthermia (54 +/- 7%) compared with control (82 +/- 6%). In contrast, total force of the knee extensors (MVC + force from EL) was not different in the two trials. Force development during handgrip contraction followed the same pattern of response as was observed for the knee extensors. In conclusion, these data demonstrate that the ability to generate force during a prolonged MVC is attenuated with hyperthermia, and the impaired performance is associated with a reduction in the voluntary activation percentage.     

Muscle oxidative metabolism accelerates with mild acidosis during incremental intermittent isometric plantar flexion exercise

BACKGROUND: It has been thought that intramuscular ADP and phosphocreatine (PCr) concentrations are important regulators of mitochondorial respiration. There is a threshold work rate or metabolic rate for cellular acidosis, and the decrease in muscle PCr is accelerated with drop in pH during incremental exercise. We tested the hypothesis that increase in muscle oxygen consumption (o2mus) is accelerated with rapid decrease in PCr (concomitant increase in ADP) in muscles with drop in pH occurs during incremental plantar flexion exercise. METHODS: Five male subjects performed a repetitive intermittent isometric plantar flexion exercise (6-s contraction/4-s relaxation). Exercise intensity was raised every 1 min by 10% maximal voluntary contraction (MVC), starting at 10% MVC until exhaustion. The measurement site was at the medial head of the gastrocnemius muscle. Changes in muscle PCr, inorganic phosphate (Pi), ADP, and pH were measured by 31P-magnetic resonance spectroscopy. o2mus was determined from the rate of decrease in oxygenated hemoglobin and/or myoglobin using near-infrared continuous wave spectroscopy under transient arterial occlusion. Electromyogram (EMG) was also recorded. Pulmonary oxygen uptake (o2pul ) was measured by the breath-by-breath gas analysis. RESULTS: EMG amplitude increased as exercise intensity progressed. In contrast, muscle PCr, ADP, o2mus, and o2pul did not change appreciably below 40% MVC, whereas above 40% MVC muscle PCr decreased, and ADP, o2mus, and o2pul increased as exercise intensity progressed, and above 70% MVC, changes in muscle PCr, ADP, o2mus, and o2pul accelerated with the decrease in muscle pH (~6.78). The kinetics of muscle PCr, ADP, o2mus, and o2pul were similar, and there was a close correlation between each pair of parameters (r = 0.969~0.983, p < 0.001). CONCLUSION: With decrease in pH muscle oxidative metabolism accelerated and changes in intramuscular PCr and ADP accelerated during incremental intermittent isometric plantar flexion exercise. These results suggest that rapid changes in muscle PCr and/or ADP with mild acidosis stimulate accelerative muscle oxidative metabolism.     

Muscle weakness following sustained and rhythmic isometric contractions in man

The effects of sustained and rhythmically performed isometric contractions on electrically evoked twitch and tetanic force generation of the triceps surae have been investigated in 4 healthy male subjects. The isometric contractions were performed separately and on different occasions at 30%, 60% and 100% of the force of maximal voluntary contraction (MVC). The area under the maximal voluntary contraction (MVC) force/time curve during the rhythmic and sustained contractions was the same for each experiment. The results showed that following rhythmic isometric exercise there was a small decrease in low (10 and 20 Hz) and high (40 Hz) frequency tetanic tension which was associated with % MVC. However, there was no change in the 20/40 ratio of tetanic forces, MVC or the contraction times and force of the maximal twitch. In contrast, following sustained isometric exercise tetanic forces were markedly reduced, particularly at low frequencies of stimulation. The 20/40 ratio decreased and the induced muscle weakness was greater at 30% than 60% or 100% MVC. The performance of sustained isometric contractions also effected a decrease in contraction time of the twitch and MVC. The results are in accord with previous findings for dynamic work (Davies and White 1982), and show that if isometric exercise is performed rhythmically the effect on tetanic tensions is small and there is no evidence of a preferential loss of electrically evoked force at either high or low frequencies of stimulation following the contractions. For sustained contractions, however, the opposite is true, the ratio of 20/40 Hz forces is markedly reduced and following 30% sustained MVC there is a significant (p less than 0.05) change in the time to peak tension (TPT) of the maximal twitch.     

Muscle sympathetic nerve responses to static leg exercise.

Previous studies of muscle sympathetic nerve activity (MSNA) during static exercise have employed predominantly the arms. These studies have revealed striking increases in arm and leg MSNA during static handgrip (SHG) and postexercise circulatory arrest (PECA). The purpose of this study was to examine MSNA during static leg exercise (SLE) at intensities and duration commonly used during SHG followed by PECA. During 2 min of SLE (static knee extension) at 10% of maximal voluntary contraction (MVC; n = 18) in the sitting position, mean arterial pressure and heart rate increased significantly. Surprisingly, MSNA in the contralateral leg did not increase above control levels during SLE but rather decreased (23 +/- 5%; P < 0.05) during the 1st min of SLE at 10% MVC. We compared MSNA responses to SHG and SLE (n = 8) at 30% MVC. SHG and SLE elicited comparable increases (P < 0.05) in arterial pressure and heart rate, but SHG elicited significant increases in MSNA, whereas SLE did not. During PECA after SHG and SLE, mean arterial pressure remained significantly above control. However, MSNA was unchanged during PECA after SLE but was significantly greater than control during PECA after SHG. Because previous studies have indicated differences in MSNA responses to the arm and leg, we measured arm and leg MSNA simultaneously in six subjects during SLE at 20% MVC and PECA. During SLE and PECA, MSNA in the contralateral arm and leg did not differ significantly from each other.(ABSTRACT TRUNCATED AT 250 WORDS)     

Residual force enhancement following eccentric induced muscle damage

During lengthening of an activated skeletal muscle, the force maintained following the stretch is greater than the isometric force at the same muscle length. This is termed residual force enhancement (RFE), but it is unknown how muscle damage following repeated eccentric contractions affects RFE. Using the dorsiflexors, we hypothesised muscle damage will impair the force generating sarcomeric structures leading to a reduction in RFE. Following reference maximal voluntary isometric contractions (MVC) in 8 young men (26.5±2.8y) a stretch was performed at 30°/s over a 30° ankle excursion ending at the same muscle length as the reference MVCs (30° plantar flexion). Surface electromyography (EMG) of the tibialis anterior and soleus muscles was recorded during all tasks. The damage protocol involved 4 sets of 25 isokinetic (30°/s) lengthening contractions. The same measures were collected at baseline and immediately post lengthening contractions, and for up to 10min recovery. Following the lengthening contraction task, there was a 30.3±6.4% decrease in eccentric torque (P<0.05) and 36.2±9.7% decrease in MVC (P<0.05) compared to baseline. Voluntary activation using twitch interpolation and RMS EMG amplitude of the tibialis anterior remained near maximal without increased coactivation for MVC. Contrary to our hypothesis, RFE increased (～100-250%) following muscle damage (P<0.05). It appears stretch provided a mechanical strategy for enhanced muscle function compared to isometric actions succeeding damage. Thus, active force of cross-bridges is decreased because of impaired excitation-contraction coupling but force generated during stretch remains intact because force contribution from stretched sarcomeric structures is less impaired.     

Effect of active recovery on acute strength deficits induced by passive stretching

We herein examined whether immediate muscular activity (active recovery) after stretching decreased stretch-induced strength deficits in human muscles. Our within-subject study included 8 subjects who were used as their own controls. For each subject, both legs were subjected to the same warm-up and stretching treatments, and then one leg was exposed to active recovery (experimental treatment) while the other was allowed to recover passively (control). Unilateral maximal voluntary contraction (MVC) of knee extensors was measured at baseline, poststretching, and postrecovery to monitor strength evolution. Our results revealed that the MVC strength at the baseline time point for control (590.8 +/- 104.2) and treated (602.2 +/- 112.7) legs decreased poststretching by 8.0 and 8.9%, respectively, and further decreased postrecovery by 1.3 and 1.2%, respectively. Maximal voluntary contraction strength tests demonstrated very good reliability, having intraclass coefficients of correlation ranging from 0.92-0.98. Mixed analysis of variance showed that the stretching program yielded significantly increased flexibility (p < 0.01) and significantly decreased MVC (p < 0.001) in both legs. The over-time variability between legs was marginal (1%), and no significant between-leg differences were observed. Indeed, the improvement in strength restoration due to active vs. passive recovery was -0.5 +/- 15 N, which was significantly lower (p < 0.01; 1-tailed t-test) than the amount of strength inhibition (32.6 N), estimated as 60% of the overall strength deficit (54.3 +/- 29.7 N). These results confirm that significant strength is lost poststretching but fail to show greater improvement in strength following active vs. passive recovery. Collectively, the present findings indicate that, contrary to the belief of many coaches, muscular exercises during the poststretching period are unlikely to minimize stretch-induced strength deficits.     

The effect of six weeks endurance training on dynamic muscular control of the knee following fatiguing exercise

The aim of the study was to examine whether six weeks of endurance training minimizes the effects of fatigue on postural control during dynamic postural perturbations. Eighteen healthy volunteers were assigned to either a 6-week progressive endurance training program on a cycle ergometer or a control group. At week 0 and 7, dynamic exercise was performed on an ergometer until exhaustion and immediately after, the anterior–posterior centre of pressure (COP) sway was analyzed during full body perturbations. Maximal voluntary contractions (MVC) of the knee flexors and extensors, muscle fiber conduction velocity (MFCV) of the vastus lateralis and medialis during sustained isometric knee extension contractions, and power output were measured. Following the training protocol, maximum knee extensor and flexor force and power output increased significantly for the training group with no changes observed for the control group. Moreover, the reduction of MFCV due to fatigue changed for the training group only (from 8.6% to 3.4%). At baseline, the fatiguing exercise induced an increase in the centre of pressure sway during the perturbations in both groups (>10%). The fatiguing protocol also impaired postural control in the control group when measured at week 7. However, for the training group, sway was not altered after the fatiguing exercise when assessed at week 7. In summary, six weeks of endurance training delayed the onset of muscle fatigue and improved the ability to control balance in response to postural perturbations in the presence of muscle fatigue. Results implicate that endurance training should be included in any injury prevention program.     

Influence of high ambient temperatures on the physiological responses and body sway in healthy young adults after quickly standing

This study was aimed to compare the variations in cerebral oxygenation, blood pressure and center-of-foot pressure after standing from sitting and supine positions at normal (22 degrees C) and high (32 degrees C) room temperatures. Thirty young adults stood up from a resting posture (sitting or supine position) and kept the static standing posture for 90 sec. Meanwhile, their center-of-foot pressure (COP), blood pressure, and cerebral oxygenation kinetics were measured in continuity. The change of the frequency domain low-to-high frequency (LF/HF) ratio of the R-R interval before and after standing from a supine position was significantly higher than that from a sitting position under both temperature conditions. Blood pressure as well as total and oxygenated hemoglobin levels decreased immediately after standing up and the ratio of blood pressure change when moving from a supine position to standing at high room temperature was the largest as compared with the other conditions. Total hemoglobin (Hb) volume was found to temporarily decrease after standing and required 22-24 sec to recover when the subject started from the sitting position and 33-36 sec when the subject started from the supine position. Cerebral oxygenation kinetics tended to be larger under high, rather than normal, temperature conditions. All COP parameters after standing were significantly larger in the high temperature condition than in the normal temperature condition. Body sway after standing was larger in the high temperature condition than in the normal temperature condition and after standing from a supine position than from a sitting position. In conclusion, cerebral oxygenation kinetics and blood pressure measured after the subject moved to the standing position changed dramatically under high temperature conditions, and variations in this parameter may influence body sway.     

High-volume intermittent maximal intensity isometric exercise caused great stress,although central motor fatigue did not occur

To establish whether very high-volume, high-intensity isometric exercise causes stress to the body and how it affects peripheral and central fatigue. Nineteen physically active healthy male subjects (21.2 ± 1.7 years; height – 1.82 ± 0.41 m, body weight – 79.9 ± 4.5 kg; body mass index – 24.3 ± 2.1 kg/m2) volunteered to participate in this study. They participated in two experiments 3–5 days apart. Each experiment comprised six series of 60-s maximum voluntary contraction (MVC) force (knee extension) achieved as rapidly as possible. This very high-volume, high-intensity exercise (HVHIE) was performed at different quadriceps muscle lengths: short (SL) and long (LL). The MVC and the electrically stimulated contractile properties of the muscle were measured prior to HVHIE, immediately after and 3 min after each series, and at 3, 10, and 30 min after the end of HVHIE. We found that HVHIE caused high levels of stress (cortisol levels approximately doubled, heart rate and the root mean square successive difference of interval (RMSSD) decreased by about 75%); lactate increased to 8–11 mmol/L, voluntary and 100 Hz stimulation-induced force (recorded immediately after HVHIE) decreased by 55% at LL and 40% at SL. However, the central activation ratio during MVC did not change after either exercise. Isometric HVHIE performed using one leg caused high levels of stress (RMSSD decreased, cortisol increased after HVHIE equally at SL and LL; La increased more while exercising at LL) and the voluntary and electrostimulation-induced muscle force significantly decreased, but muscle central activation during MVC did not decrease.     

Central and peripheral contributions to fatigue in relation to level of activation during repeated maximal voluntary isometric plantar flexions.

This study aimed to investigate central and peripheral contributions to fatigue during repeated maximal voluntary isometric plantar flexions (MVCs). Changes in joint torque, level of activation (LOA), resting twitch amplitude (RT), electromyographic signals (EMG), and presynaptic inhibition of Ia afferents were investigated during 9 bouts of 10 MVCs. MVCs lasted for 2 s and were separated by 1 s. The interval between bouts was 10 s. Electrical stimulation was applied to the tibial nerve; at rest to evoke RTs, M waves, and two (1.5-s interval) H reflexes; with the soleus EMG at 30% of that during MVC to evoke M waves and two H reflexes; and during MVCs to measure LOA. Over the nine bouts, LOA decreased by 12.6% and RT by 16.2%. EMG root mean square during MVCs remained unchanged for the soleus and tibialis anterior muscles, but it decreased for medial gastrocnemius. Peripheral fatigue (decrease in RT) was positively correlated to LOA, whereas central fatigue (decrease in LOA) was not. Depression of both H reflexes suggests that presynaptic inhibition after the first bout was partly induced by homosynaptic postactivation depression of the Ia terminal. The H-reflex-to-M-wave ratio increased with fatigue in both passive and active states, with no change in the ratio of the second H reflex to the first, thereby indicating a decrease of presynaptic inhibition during fatigue. The results indicate that both central and peripheral mechanisms contributed to the fatigue observed during repeated MVCs and that the development of peripheral fatigue was influenced by the level of voluntary activation and initial plantar flexor torque.     

Effect of the time of day on central and peripheral fatigue during 2-min maximal voluntary contractions in persons with multiple sclerosis: gender differences

There is a lack of data on fatigue changes within 24 h among patients with multiple sclerosis. The purpose of this study was to evaluate the effect of time of day on central and peripheral fatigue during a continuous 2-min maximal voluntary contraction of the quadriceps muscle in women and men with multiple sclerosis (MS). We studied age-matched MS patients (range, 40–50 years). The inclusion criteria for patients were: a Kurtzke Expanded Disability Status score and a Fatigue Severity Scale score. We found a significant gender difference in central activation ratio (CAR) in the evening. At the end of the 2-min maximal voluntary contraction (MVC), the voluntary torque decreased by about 65% in men and women with MS in both the morning and evening. We also observed that, in women, CAR decreased markedly during the first 30 s in the evening test. The most interesting finding of our study is that central fatigue increased, whereas peripheral fatigue decreased markedly in the evening only in women. It remains unclear why women’s central fatigue is greater in the evening than in the morning.