Twitch potentiation after fatiguing exercise in man

Summary Twitch potentiation was studied in the human triceps surae complex before and after intermittent maximal voluntary contractions or electrical stimulation at 20 Hz. Both forms of exercise were conducted with intact circulation for a maximum of 10 min or with circulatory occlusion until force output declined 50%. The relative potentiation was determined when a control twitch was compared to a twitch obtained after 5 s of maximal voluntary plantar flexion. The unpotentiated twitch torque (PT) and potentiated twitch torque (PT^*) were reduced most severely after voluntary ischemic exercise (63.2% and 52.5% respectively, (P<0.001)). However, the relative potentiation (PT^*/PT) immediately after voluntary ischemic exercise increased to 1.65±0.18 from 1.22±0.13 at rest. Both PT and PT^* recovered quickly after exercise. At rest, twitch contraction time (CT) and one-half relaxation time (1/2 RT) in the unpotentiated twitch were longer than that of contraction (CT^*) and one-half relaxation time (1/2 RT^*) in the potentiated twitch. Following non-occluded exercise, CT, CT^*, 1/2 RT and 1/2 RT^* were shortened relative to rest. After ischemic exercise CT and CT^* were shortened although 1/2 RT and 1/2 RT^* increased relative to rest. Both CT^* and 1/2 RT^* quickly recovered to pre-exercise values by 5 min post-exercise. Ratios of potentiated/control twitch parameters were not altered after nonoccluded exercise, but were increased after ischemic exercise. These results suggest that the mechanisms of fatigue which depress voluntary torque and twitch and potentiated twitch torques, do not interfere with the extent of potentiation after fatiguing exercise.This study was supported by the Medical Research Council of Canada     

Incorporating voluntary unilateral knee flexion into balance corrections elicited by multi-directional perturbations to stance

Positive effects on lateral center of mass (CoM) shifts during balance recovery have been seen with voluntarily unilateral arm raising but not with voluntarily bilateral knee flexion. To determine whether unilateral voluntary knee movements can be effectively incorporated into balance corrections we perturbed the balance of 30 young healthy subjects using multi-directional rotations of the support surface while they simultaneously executed unilateral knee flexion. Combined pitch and roll rotations (7.5° and 60°/s) were presented randomly in six different directions. Subjects were tested in four stance conditions: balance perturbation only (PO); cued flexion of one knee only (KO); combined support surface rotation and cued (at rotation onset) flexion of the uphill knee, contralateral to tilt (CONT), or of the downhill knee, ipsilateral to tilt (IPS). Outcome measures were CoM motion and biomechanical and electromyography (EMG) responses of the legs, arms and trunk. Predicted measures (PO+KO) were compared with combined measures (CONT or IPS). Unilateral knee flexion of the uphill knee (CONT) provided considerable benefit in balance recovery. Subjects rotated their pelvis more to the uphill side than predicted. Downhill knee bending (IPS) also had a positive effect on CoM motion because of a greater than predicted simultaneous lateral shift of the pelvis uphill. KO leg muscle activity showed anticipatory postural activity (APA) with similar profiles to early balance correcting responses. Onsets of muscle responses and knee velocities were earlier for PO, CONT, and IPS compared to KO conditions. EMG response amplitudes for CONT and IPS conditions were generally not different from the PO condition and therefore smaller than predicted. Later stabilizing responses at 400 ms had activation amplitudes generally equal to those predicted from the PO+KO conditions. Our results suggest that because EMG patterns of anticipatory postural activity of voluntary unilateral knee flexion and early balance corrections have similar profiles, the CNS is easily able to incorporate voluntary activation associated with unilateral knee flexion into automatic postural responses. Furthermore, the effect on movement strategies appears to be non-linear. These findings may have important implications for the rehabilitation of balance deficits.     

肌内效贴对慢性踝关节不稳运动神经肌肉控制的影响

文题释义： 慢性踝关节不稳：定义为“外侧踝关节不稳定反复发作,外踝扭伤多次发生”,主要症状为疼痛、肿胀、打软腿及自觉踝关节功能下降。 运动神经肌肉控制：人体在运动过程中,对于自身和外界环境信息进行感觉传入,中枢整合后发出的运动反馈,是神经和肌肉骨骼系统之间相互作用的下意识行为,主要包括以下表现：本体感觉、肌肉活动、平衡功能以及运动生物力学。 背景：慢性踝关节不稳患者踝关节运动神经肌肉控制能力降低,影响其运动表现。肌内效贴在慢性踝关节不稳的防护治疗中已被广泛应用,然而目前肌内效贴对慢性踝关节不稳患者运动神经肌肉控制能力的相关研究有限,且异质性大,研究结果存在较大争议。 目的：阐述目前肌内效贴对慢性踝关节不稳患者运动神经肌肉控制影响的研究现状,为实际应用提供参考依据。 方法：检索2009年1 月至2019年7月期间在PubMed、Cochrane、WOS、CNKI数据库检索肌内效贴应用于慢性踝关节不稳的研究。中文检索词为“慢性踝关节不稳;慢性踝不稳;踝关节”和“肌内效贴;肌贴;贴扎”,英文检索词分别为“chronic ankle instability;ankle instability;ankle”AND“kinesio tap*; kinesiology tap*;kinaesthetic tap*;tap*”。 结果与结论：肌内效贴可能改善慢性踝关节不稳患者的踝关节本体感觉和跳跃类运动生物力学表现,但需要更多高质量研究进行深入探讨。当前并无足够证据支持肌内效贴可以改善慢性踝关节不稳患者的肌肉力量,肌肉激活特征及平衡功能。现有研究选取受试者异质性大,难以对不同运动种类和水平的慢性踝关节不稳患者进行综合归纳;不同研究纳入标准中的量表不同,研究在进行肌内效贴贴扎时选用的贴法、拉力、方向、持续时间等都存在明显差异,无统一标准;缺少高质量的Meta分析对其进行循证论证。 ORCID: 0000-0003-0738-2833(尹璐璐) 中国组织工程研究杂志出版内容重点：组织构建;骨细胞;软骨细胞;细胞培养;成纤维细胞;血管内皮细胞;骨质疏松;组织工程     

Validity and reliability of limits-of-stability testing: a comparison of 2 postural stability evaluation devices

Context:

A lack of published comparisons between measures from commercially available computerized posturography devices and the outcome measures used to define the limits of stability (LOS) makes meaningful interpretation of dynamic postural stability measures difficult.

Objectives:

To compare postural stability measures between and within devices to establish concurrent and construct validity and to determine test-retest reliability for LOS measures generated by the NeuroCom Smart Balance Master and the Biodex Balance System.

Design:

Cross-sectional study.

Setting:

Controlled research laboratory.

Patients or Other Participants:

A total of 23 healthy participants with no vestibular or visual disabilities or lower limb impairments.

Intervention(s):

The LOS were assessed during 2 laboratory test sessions 1 week apart.

Main Outcome Measure(s):

Three NeuroCom LOS variables (directional control, endpoint excursion, and movement velocity) and 2 Biodex LOS variables (directional control, test duration).

Results:

Test-retest reliability ranged from high to low across the 5 LOS measures (intraclass correlation coefficient [2,k] = 0.82 to 0.48). Pearson correlations revealed 4 significant relationships (P < .05) between and within the 2 computerized posturography devices (r = 0.42 to −0.65).

Conclusions:

Based on the wide range of intraclass correlation values we observed for the NeuroCom measures, clinicians and researchers alike should establish the reliability of LOS testing for their own clinics and laboratories. The low to moderate reliability outcomes observed for the Biodex measures were not of sufficient magnitude for us to recommend using the LOS measures from this system as the gold standard. The moderate Pearson interclass correlations we observed suggest that the Biodex and NeuroCom postural stability systems provided unique information. In this study of healthy participants, the concurrent and construct validity of the Biodex and NeuroCom LOS tests were not definitively established. We recommend that this study be repeated with a clinical population to further explore the matter.     

Transient electric changes immediately after surgical trauma

CONTEXT: Electric stimulation is frequently used to promote soft tissue healing, although we do not have a complete understanding of the tissue's electromagnetic properties. OBJECTIVE: To measure the transient electric changes in skin and muscle tissue immediately after trauma. DESIGN: 1-group time series. SETTING: Climate-controlled operating room in a public urban hospital. PATIENTS OR OTHER PARTICIPANTS: Eleven participants (8 females, 3 males) with a mean age of 65.18 +/- 11.36 years undergoing total hip arthroplasty. INTERVENTION(S): An incision approximately 10 cm distal to the posterior superior iliac spine extended distally over the greater trochanter and along the lateral limb. The incision was completed in 2 cuts: (1) skin and subcutaneous fat and (2) muscle tissue. MAIN OUTCOME MEASURE(S): Three measurement sessions were performed with an electrometer before and after a skin incision and after a muscle incision. Potential differences and current intensity were measured immediately after acute trauma to determine the transient electric changes associated with soft tissue injury. RESULTS: The electric potentials were significantly more negative after the skin incision (P = .036) and skin plus muscle incision (P = .008; preincision = 0.001 +/- 0.015 V, skin incision = -0.127 +/- 0.134 V, skin plus muscle incision = -0.192 +/- 0.153 V). Current intensity changed significantly after the skin plus muscle incision (P = .008; preincision = 0.046 +/- 0.112 pA, skin incision = -0.803 +/- 0.904 pA, skin plus muscle incision = -1.708 +/- 1.302 pA). CONCLUSIONS: Soft tissue trauma generated negative transient electric changes.     

Primary sensorimotor cortex activation with task-performance after fatiguing hand exercise

We have compared functional MRI signals in primary sensorimotor cortex (SM1) during a paced motor task of each hand before and after unimanual (right hand) fatiguing exercise. Our aims were to determine whether the degree of activation is different when a motor task is performed after a fatiguing exercise, and whether there are any differences in activation between movement of the fatigued and non-fatigued hands. There was a significant reduction in the number of voxels activated in SM1 in the hemisphere contralateral to movement of both the fatigued hand (38±5 pre-exercise versus 21±3 post-exercise; P<0.05) and the non-fatigued hand (32±4 pre-exercise vs 18±4 post-exercise; P<0.05). There was no significant difference in the magnitude of the functional magnetic resonance imaging signal before or after exercise, however, the variance increased significantly after exercise (6.0±0.5 pre-exercise vs 7.3±0.6 post-exercise; P<0.01). Reduced functional activation in SM1 may reflect increased variability in the activation rather than a reduction in activation of cortical motor networks after fatigue.     

Fluid control mechanisms after exercise dehydration

Summary Since the osmocontrol- (osmolality), the renin-angiotensin- (PRA), and the volume control- (central venous pressure, CVP) systems are involved in the maintainance of the salt-water balance, we investigated the pattern of these parameters in the recovery period after exercise dehydration in 13 well trained long-distance runners.On average, after exercise the athletes had lost 3.1% of their body weight (BW). After eating and drinking the BW was still 1.3% below control value, indicative of continuing deficits. Plasma osmolality increased, however, from an average value of 286–290 mosmol/kg after exercise as well as postprandially, but the change was not significant. PRA-Levels rose significantly from 0.167–0.599 ng/ml·h after exercise and decreased to 0.333 ng/ml·h postprandially.CVP was significantly altered after exercise (–3.5 cm H₂O) as well as postprandially (–2.4 cm H₂O).The results suggest that the salt-water balance is maintained by the interplay of all the three systems. In conflicting situations, however, as when intercompartmental water- and solute-shifts take place during the recovery period, the volume control system triggered off by the CVP is the dominant corrective response to the prevailing deficits.Supported by the Bundesinstitut für Sportwissenschaften, Köln     

Localized muscle pain causes prolonged recovery after fatiguing isometric contractions

The purpose of this study was to investigate the force and electromyographic (EMG) signal recorded from the muscles immediately after a sustained fatiguing contraction with or without muscle pain. Ten subjects performed sustained dorsi- and plantarflexions at two contraction levels (50 and 80% of maximum voluntary contraction) until exhaustion with or without muscle pain induced by injection of 6% hypertonic saline in one of the torque producing muscles. The muscle pain intensity was scored on a visual analogue scale (VAS, 0–10 cm). The root mean square (RMS) of the surface EMG signal from plantarflexors and dorsiflexors were estimated during maximum voluntary contractions (MVC) and ramp contractions before and after the fatiguing task at 0, 5, 10 and 15 min during the recovery phase. VAS scores immediately after the contractions with hypertonic saline (on average 3.2 ± 1.1 cm) progressively decreased during recovery and no pain was experienced 15 min after the contraction. After the painful contraction the RMS-EMG during MVC was on average decreased (23.4 ± 7.4%) compared to the non-painful condition both in muscles where pain was previously induced and in non-painful synergists. During recovery, the slope of the torque–EMG curve during ramp contraction was significantly decreased (28.4 ± 8.1%) after the painful contraction compared to the control contraction both for the muscle previously exposed to pain and also the other active synergists. The decreased EMG during recovery after painful contractions compared with control was not accompanied by significant reductions in force during MVC indicating a change in the strategy for motor unit recruitment. This study shows that localized muscle pain inhibits muscle activation and increases the effects of fatigue on EMG recovery curves both for painful and non-painful synergists probably by a central effect. These effects can modify the normal patterns of synergistic activation and can also generate overload problems in muscle pain patients if compensatory motor control strategies are applied.     

Effects of differently induced stretch loads on neuromuscular control in drop jump exercise

The neuromuscular characteristics of the triceps surae and vastus lateralis muscles and interactions between the pre-activation of these muscles and the muscle output itself during ground contact were investigated during various types of stretch-shortening cycle muscle loading. The loading of the muscles was effected by using three different types of drop jump exercise. These jumps allowed separate modifications of the loading of the leg extensor muscles by changing the velocity of the centre of gravity (CG) or by changing directly the body mass, which was also affected by changing artificially the acceleration of the CG. It was found that the eccentric peak angular velocity of the ankle joint was related to the various precontact and eccentric parameters in all the different types of jumping exercise. The correlations were higher for the gastrocnemius muscle (P < 0.001) than for the soleus muscle (n.s., P < 0.01). In all the experimental conditions, the pre-activation of the measured muscles started well before the impact of contact with the ground. However, the duration of the pre-activation phase depended on the type of stretch exercise. The results would suggest a clear interaction between the pre-activation of the muscles and that part of the muscle output which is effected by the segmental stretch reflex system. The control mechanism of the pre-activation itself appears to be multiple in character. It seems reasonable to assume that the pre-activation is preprogrammed, but which can, however, be modified by proprioceptive, vestibular and visual inputs. Thus, the possibility of conscious modification of the expected muscle load must be considered.     

Influence of cognitive load on the dynamics of neurophysiological adjustments during fatiguing exercise

We aimed to determine the neurophysiological mechanisms associated with reduced endurance performance during cognitive‐motor dual task at different levels of cognitive load, compared to a motor task alone. Eighteen healthy men performed isometric quadriceps contractions at 15% of maximal voluntary contraction (blocks of 170 s interspaced by neuromuscular evaluations) until exhaustion. This task was performed on three separate days: (a) in the absence of concomitant cognitive task, (b) with concomitant 1‐back task, and (c) with concomitant 2‐back task. Autonomic nervous system activity, perceived exertion, and cognitive performance were continuously monitored. Peripheral and central determinants of neuromuscular function were assessed at rest, between each block, and at task failure using femoral nerve stimulation. Endurance time was shorter during 2‐back (982 ± 545 s) and 1‐back (1,128 ± 592 s) conditions, compared with control (1,306 ± 836 s). Voluntary activation level was lower in 2‐back (87.1%; p < 0.001) and 1‐back (88.6%; p = 0.04) conditions compared to control (91.2%) at isotime (100% of the shortest test duration). Sympathetic activity showed a greater increase in 2‐back condition compared to control. Perceived muscular exertion was higher during 2‐back than during control. Cognitive performance decreased similarly with time during both cognitive‐motor dual task but was always lower during 2‐back condition. Motor performance is reduced when adding a concomitant demanding memory task to a prolonged isometric exercise. This can be explained by the interaction of various psychological and neurophysiological factors including higher perceived exertion, greater perturbations of autonomic nervous system activity, and cerebral impairments leading to earlier onset of central fatigue.     

Perception of finger forces within the hand after index finger fatiguing exercise

The effect of fatigue on finger force perception within a hand during ipsilateral finger force matching was examined. Thirteen subjects were instructed to match a reference force of an instructed finger using the same or different finger within the hand before and after index finger fatigue. Absolute reference force targets for the index or little finger were identical during pre- and post-fatigue sessions. Fatigue was induced by a 60-s sustained maximal voluntary contraction (MVC) of the index finger. Index finger MVC decreased approximately 29%, while there was a non-significant (about 5%) decrease in the little finger MVC. The results showed that: (1) the absolute reference and matching forces of the instructed fingers were not significantly changed after fatigue, while the total forces (sum of instructed and uninstructed finger forces) were increased after fatigue. (2) The relative forces (with respect to corresponding pre- and post-fatigue MVCs) of the index finger increased significantly in both reference and matching tasks, while the relative forces of the little finger remained unchanged after fatigue. (3) Matching errors remained unchanged after fatigue when the fatigued index finger produced the reference force, while the errors increased significantly when the fatigued index finger produced the matching force. (4) Enslaving (difference between total and instructed finger forces) increased significantly after fatigue, especially during force production by the fatigued index finger and when the little finger produced matching forces at higher force levels. (5) Enslaving significantly increased matching errors particularly after fatigue. Taken together, our results suggest that absolute finger forces within the hand are perceived within the CNS during ipsilateral finger force matching. Perception of absolute forces of the fatigued index finger is not altered after fatigue. The ability of the fatigued index finger to reproduce little finger forces is impaired to a certain degree, however. The impairment is likely to be attributable to altered afferent/efferent relationships of the fatigued index finger.     

Lower extremity control and dynamics during backward angular impulse generation in backward translating tasks

Observation of complex whole-body movements suggests that the nervous system coordinates multiple operational subsystems using some type of hierarchical control. When comparing two backward translating tasks performed with and without backward angular impulse, we have learned that task-specific modifications in trunk–leg coordination contribute to the regulation of total-body center of mass (CoM) position relative to the reaction force (RF). In this study, we hypothesized that task-specific differences in trunk–leg coordination would affect the control of the lower extremity joints during the impulse-generation phase of the tasks. Eight highly skilled performers executed a series of backward translating jumps with and without backward rotation (back somersault and back timer, respectively). Sagittal plane kinematics, RFs and electromyograms of lower extremity muscles were acquired during the take-off phase of both tasks. Lower extremity joint kinetics was calculated using inverse dynamics. The results indicate that between-task differences in the relative angles between the lower extremity segments and the net joint forces/RF contributed to significant reductions in knee-extensor net joint moments and increases in hip-extensor net joint moments during the push interval of the back somersault as compared to the back timer. Between-task differences in backward trunk angular velocity also contributed to the re-distribution of work done by the lower extremity net joint moments. Between-task differences in lower extremity joint kinetics were associated with synergistic activation of the bi-articular muscles crossing the knee and hip. These results indicated that task-specific control of CoM relative to the RF in order to regulate the backward angular-impulse-involved modification in the control and dynamics of the knee and hip joints. These results indicate that between-task differences in the control objectives at the total-body level (position of CoM relative to the RF) alters the control and dynamics of the multi-joint lower extremity subsystem.     

Reduced functional activation after fatiguing exercise is not confined to primary motor areas

We have previously shown that following a period of unimanual fatiguing exercise, there is a reduction in primary sensorimotor cortex (SM1) activation with movement of either the fatigued or the non-fatigued hand by Benwell et al. (Exp Brain Res 167:160–164, 2005). In the present study we have investigated whether this reduction is confined to motor areas or is more widespread. Functional imaging was performed before and after a 10-minute fatiguing exercise of the left hand (30% of maximum handgrip strength) in seven normal subjects (4 M, mean age 25 years). The activating task was a handgrip against a low resistance (1 kg) in response to a visual cue (chequerboard reversal every 2 ± 0.5 s). We compared activation in SM1, supplementary motor area (SMA), cerebellum (CB) and primary visual cortex (V1) before and after the fatiguing exercise. After exercise, contralateral SM1 activation was reduced by 33% (P < 0.05) compared to baseline for the fatigued hand and by 49% for the non-fatigued hand (P < 0.05). A similar pattern was seen for the bilateral SMA and ipsilateral CB following exercise (45 vs. 50% for SMA; 30 vs. 35% for CB; fatigued versus non-fatigued). Activation was also reduced in V1 but to a lesser extent than in motor areas (19 vs. 24%; fatigued versus non-fatigued). These results show that although the reduced functional activation during the recovery period after fatiguing exercise is more marked in motor areas, it also extends to non-motor areas such as the visual cortex, suggesting that there are more widespread changes in cerebral haemodynamic responses after fatigue.     

Different effects of fatiguing exercise on corticospinal and transcallosal excitability in human hand area motor cortex

Following forceful exercise that leads to muscle fatigue, the size of muscle evoked responses (MEPs) generated by transcranial magnetic stimulation (TMS) in the exercised muscle is depressed over a prolonged period. Strong evidence implicates intracortical mechanisms in this depression. As well as evoking MEPs in contralateral muscles, TMS also reduces MEPs evoked in ipsilateral muscles through interhemispheric inhibition mediated by a transcallosal pathway. Here we have sought to determine whether this effect is also depressed after exercise. Using two magnetic stimulators, the aftereffects of unilateral hand muscle exercise on the ability of TMS delivered to the hemisphere that generated the exercise were examined to i) generate MEPs in the exercised hand muscles, and ii) depress MEPs evoked by TMS pulses in contralateral (non-exercised) hand muscles. After exercise there was a significant reduction in the amplitudes of MEPs evoked by TMS in the exercised muscles (p<0.001). However, the same stimuli remained able to depress responses evoked by TMS to the contralateral hemisphere in the non-exercised muscles as effectively as before the exercise. We conclude that unlike the MEPs evoked by corticospinal output, interhemispheric inhibition evoked from the hemisphere that generated the exercise is not depressed after exercise. A similar differential effect on interhemispheric inhibition and corticospinal output has been reported recently for the effects of transcranial direct current (DC) stimulation of the motor cortex. Fatiguing exercise and transcranial DC stimulation may therefore engage similar intracortical mechanisms.     

Return of postural control to baseline after anaerobic and aerobic exercise protocols

CONTEXT: With regard to sideline concussion testing, the effect of fatigue associated with different types of exercise on postural control is unknown. OBJECTIVE: To evaluate the effects of fatigue on postural control in healthy college-aged athletes performing anaerobic and aerobic exercise protocols and to establish an immediate recovery time course from each exercise protocol for postural control measures to return to baseline status. DESIGN: Counterbalanced, repeated measures. SETTING: Research laboratory. PATIENTS OR OTHER PARTICIPANTS: Thirty-six collegiate athletes (18 males, 18 females; age = 19.00 +/- 1.01 years, height = 172.44 +/- 10.47 cm, mass = 69.72 +/- 12.84 kg). INTERVENTION(S): Participants completed 2 counterbalanced sessions within 7 days. Each session consisted of 1 exercise protocol followed by postexercise measures of postural control taken at 3-, 8-, 13-, and 18-minute time intervals. Baseline measures were established during the first session, before the specified exertion protocol was performed. MAIN OUTCOME MEASURE(S): Balance Error Scoring System (BESS) results, sway velocity, and elliptical sway area. RESULTS: We found a decrease in postural control after each exercise protocol for all dependent measures. An interaction was noted between exercise protocol and time for total BESS score (P = .002). For both exercise protocols, all measures of postural control returned to baseline within 13 minutes. CONCLUSIONS: Postural control was negatively affected after anaerobic and aerobic exercise protocols as measured by total BESS score, elliptical sway area, and sway velocity. The effect of exertion lasted up to 13 minutes after each exercise was completed. Certified athletic trainers and clinicians should be aware of these effects and their recovery time course when determining an appropriate time to administer sideline assessments of postural control after a suspected mild traumatic brain injury.     

肥胖儿童短时间静态平衡特征

目的 通过比较肥胖和非肥胖儿童6种站立情况下的压力中心(center of pressure,COP)偏移,研究肥胖儿童短时间静态平衡特征.方法 应用Footscan人体平衡系统获取47名肥胖儿童和50名非肥胖儿童分别双足、单足睁眼和闭眼站立10 s时COP摆动参数.结果 在双足闭眼站立时,肥胖儿童前后方向上的最大动摇...     

Effects of knee joint angles and fatigue on the neuromuscular control of vastus medialis oblique and vastus lateralis muscle in humans

The effects of different knee joint angles and fatigue on the neuromuscular control of the vastus medialis oblique (VMO) and vastus lateralis (VL) muscles were investigated in 17 (11 men, 6 women) young subjects. The electromyogram (EMG) activities and the force generation capacities were monitored before and after a fatigue protocol at three different knee joint angles, 90°, 150°, 175° of knee extension, on three occasions. In response to randomly triggered light signals, the subjects performed three isometric maximal voluntary contraction (IMVC) that lasted for 4 to 8 s. This was then followed by the fatigue protocol which consisted of six bursts of contractions fixed at 30 s on and 10 s off. Immediately after the exercise to fatigue, the subjects performed another three IMVC in response to the light signals. Repeated measures ANOVA were performed to examine the effects of fatigue at these three positions on the electromechanical delay (EMD), median frequency (f _med), peak force (F _peak) and root mean square (rms)-EMG:F _peak quotient of VMO and VL. The results revealed a significant effect of the three knee joint angles on the EMD before the fatigue (P < 0.05). The fatigue protocol induced a significant decrease in F _peak at all the three positions (P < 0.01). However, the fatigue induced a significant decrease of f _med at only 90° and 150° of knee extension (P < 0.01). This occurred in parallel with the lengthening of EMD at these two joint angles (P < 0.01 and P < 0.05). The effects of fatigue on the f _med and EMD were not significant between VMO and VL at all three angles. The insignificant difference in f _med and EMD between VMO and VL at the three knee positions before and after fatigue indicated that no preferential onset activation between VMO and VL had occurred. Accepted: 1 September 2000     

Ventilatory and circulatory responses at the onset of exercise after eccentric exercise

The purpose of this study was to clarify whether delayed onset muscle soreness (DOMS) and muscle damage after eccentric exercise (ECC) could affect the ventilatory and circulatory responses at the onset of exercise, and whether those effects would continue after the disappearance of DOMS. Ten males participated in this study. We measured ventilatory and circulatory responses at the onset of exercise, for the first 20 s, during knee extension–relaxation voluntary exercise (VOL) and passive movement (PAS), which was achieved by the experimenter alternatively pulling ropes connected to the subjects’ ankles for the same period and frequency as during VOL. VOL and PAS were performed before, 2 days after, and 7 days after ECC. The following results were found: (1) the gain of minute ventilation at the onset of VOL at 2 days after ECC was significantly larger than that of before ECC; (2) the gain of minute ventilation at 7 days after ECC during both VOL and PAS was also enhanced significantly as compared to that of before ECC; and (3) heart rate and blood pressure responses were unchanged throughout the experimental period. In conclusion, ventilatory response at the onset of exercise is augmented during DOMS and EIMD after ECC and the enhanced ventilatory response continued after the disappearance of DOMS. It is suggested that enhanced ventilatory response during exercise after ECC is attributed to at least the changes in neural factors and that the mechanisms inducing these augmented ventilatory responses should be different during the period after ECC.