Rapid activation of glycogen synthase and protein phosphatase in human skeletal muscle after isometric contraction requires an intact circulation

The effects of isometric contraction (66% of maximal force) and recovery on glycogen synthase fractional activity (GSF) in human skeletal muscle have been studied. Biopsies were taken from the quadriceps femoris muscle at rest, at fatigue and 5 min postexercise on two occasions: after one of the contractions, the circulation to the thigh was occluded during the 5 min recovery (OCC), and after the other contraction, the circulation was intact (control, CON). During CON, GSF decreased from (mean ± SE) 0.34±0.05 at rest to 0.24±0.02 at fatigue and then increased to 0.74±0.04 at 5 min postexercise; corresponding values for OCC were 0.37±0.04, 0.25±0.04 and 0.48±0.05 (P<0.001 vs. CON for 5 min postexercise only). Compared with the value at fatigue, protein phosphatase activity (PP) increased by 79±16% during CON recovery (P<0.01), whereas no change was observed during OCC recovery. Uridine diphosphate glucose increased by approximately 2.5-fold at fatigue, remained elevated during OCC recovery, but reverted to the preexercise level during CON recovery (P<0.001 vs. OCC recovery). Glucose 6-P increased approximately 5-fold at fatigue and was higher at 5 min postexercise in OCC vs. CON recovery (8.6±1.5 vs. 4.1±0.9 mmol/kg dry wt; P<0.01). It is concluded that the rapid increase in GSF after intense exercise with an intact circulation may be at least partly attributed to an increase in the specific activity of PP. The increase in GSF during recovery in OCC may be at least partly attributed to the high glucose 6-P content in vivo, which enhances the substrate suitability of GS for PP. Thus, separate mechanisms exist for the activation of PP and GS during recovery from intense short term exercise.     

Passive hyperthermia reduces voluntary activation and isometric force production

It has been suggested that a critically high body core temperature may impair central neuromuscular activation and cause fatigue. We investigated the effects of passive hyperthermia on maximal isometric force production (MVC) and voluntary activation (VA) to determine the relative roles of skin (T_sk) and body core temperature (T_c) on these factors. Twenty-two males [O_2max=64.2 (8.9) ml kg^–1 min^–1, body fat=8.2 (3.9)%] were seated in a knee-extension myograph, then passively heated from 37.4 to 39.4°C rectal temperature (T_re) and then cooled back to 37.4^oC using a liquid conditioning garment. Voluntary strength and VA (interpolated twitch) were examined during an isometric 10-s MVC at 0.5°C intervals during both heating and cooling. Passive heating to a T_c of 39.4^oC reduced VA by 11 (11)% and MVC by 13 (18)% (P<0.05), but rapid skin cooling, with a concomitant reduction in cardiovascular strain [percentage heart rate reserve decreased from 64 (11)% to 29 (11)%] and psychophysical strain did not restore either of these measures to baseline. Only when cooling lowered T_c back to normal did VA and MVC return to baseline (P<0.05). We conclude that an elevated T_c reduces VA during isometric MVC, and neither T_sk nor cardiovascular or psychophysical strain modulates this response. Results are given as mean (SD) unless otherwise stated.     

Motor unit recruitment and rate coding in response to fatiguing shoulder abductions and subsequent recovery

The purpose of the present study was to investigate motor unit (MU) recruitment and firing rate, and the MU action potential (MUAP) characteristics of the human supraspinatus muscle during prolonged static contraction and subsequent recovery. Eight female subjects sustained a 30° shoulder abduction, requiring 11–12% of maximal voluntary contraction (MVC), for 30 min. At 10 and 30 min into the recovery period, the shoulder abduction was repeated for 1 min. The rating of perceived exertion for the shoulder region increased to “close to exhaustion” during the prolonged contraction, and the surface electromyography (EMG) recorded from the deltoid and trapezius muscles showed signs of local muscle fatigue. From the supraspinatus muscle, a total of 23,830 MU firings from 265 MUs were identified using needle electrodes. Of the identified MUs, 95% were continuously active during the 8-s recordings, indicating a low degree of MU rotation. The mean (range) MU firing rate was 11.2 (5.7–14.5) Hz, indicating the relative force contribution of individual MUs to be larger than the overall mean shoulder muscle load. The average MU firing rate remained stable throughout the prolonged abduction, although firing rate variability increased in response to fatigue. The average concentric MUAP amplitude increased by 38% from the beginning (0–6 min) to the end (24–29 min) of the contraction period, indicating recruitment of larger MUs in response to fatigue. In contrast, after 10 min of recovery the average MU amplitude was smaller than seen initially in the prolonged contraction, but not different after 30 min, while the MU firing rate was higher during both tests. In conclusion, MU recruitment plays a significant role during fatigue, whereas rate coding has a major priority during recovery. Furthermore, a low degree of MU rotation in combination with a high relative load at the MU level may imply a risk of overloading certain MUs during prolonged contractions. Accepted: 6 June 2000     

A method for assessing muscle fatigue during sprint exercise in humans using a friction-loaded cycle ergometer

This study investigated the mechanical changes induced by muscle fatigue caused by repeated sprints and determined whether a friction-loaded cycle ergometer has any advantages for assessing muscle fatigue. Nine subjects performed 15 sprints, each of 5 s with a 25-s rest, on a friction-loaded cycle ergometer. The averaged force, power and velocity of each push-off were calculated. Maximal power decreased by 17.9%, with a concomittent slowing of muscle contraction, but without any change in the maximal force. These results demonstrated that repeated sprints slow down muscle contraction, leading to a fall in maximal power without any loss of force. This would suggest that fast twitch fibres are selectively fatigued by repeated sprints. However, the ergometer used in the present study made it difficult to evaluate the relative influences of contraction velocity and sprinting time. This was certainly the most important limitation. On the other hand, it showed the advantage of measuring instantaneous power and total work dissipated in the environment simultaneously. It also permitted a force-velocity relationship to be obtained from a single sprint and this relationship is known to be closely related to the muscle fibre composition. Accepted: 5 March 1998     

抗疲劳1号对游泳大鼠血液、肌肉和脑中ATP含量及血乳酸含量的影响

目的：观察抗疲劳１号（ＡＦ－１）对游泳运动大鼠血液ＡＴＰ和乳酸，以及脑和肌肉组织中ＡＴＰ含量的影响。方法：用生物发光法测定６组不同游泳强度大鼠血液和组织中ＡＴＰ含量；血乳酸含量变化用乳酸自动分析仪监测。结果：给药组大鼠游泳１０ｍｉｎ，血中ＡＴＰ含量明显高于对照组；血中乳酸含量明显低于对照组（Ｐ＜００１）。在游泳力竭鼠中，给药组血ＡＴＰ含量也较对照组高；而血乳酸含量较对照组低（Ｐ＜００５，Ｐ＜００１）。给药大鼠中，游泳１０ｍｉｎ组肌肉ＡＴＰ含量明显高于对照组（Ｐ＜００１）；游泳力竭组仍可保持在对照组水平（游泳时间不同）。给药鼠游泳１０ｍｉｎ和力竭两组脑组织中ＡＴＰ含量与未给药动物无明显差异，但未游泳组明显高于对照组。未游泳给药鼠肌肉中ＡＴＰ含量也明显高于对照组（Ｐ＜００５，Ｐ＜００１）。结论：抗疲劳１号具有抗疲劳功用，可能通过增加组织ＡＴＰ的生成和贮存，促进能量代谢，减少乳酸堆积，进而增加运动强度及运动时间     

Effects of prolonged cycle ergometer exercise on maximal muscle power and oxygen uptake in humans

Summary The mechanical power (W_tot, W·kg^–1) developed during ten revolutions of all-out periods of cycle ergometer exercise (4–9 s) was measured every 5–6 min in six subjects from rest or from a baseline of constant aerobic exercise [50%–80% of maximal oxygen uptake (VO_2max)] of 20–40 min duration. The oxygen uptake [VO₂ (W·kg^–1, 1 ml O₂ = 20.9 J)] and venous blood lactate concentration ([la]_b, mM) were also measured every 15 s and 2 min, respectively. During the first all-out period, W_tot decreased linearly with the intensity of the priming exercise (W_tot = 11.9–0.25·VO₂). After the first all-out period (i greater than 5–6 min), and if the exercise intensity was less than 60% VO_2max, W_tot, VO₂ and [la]_b remained constant until the end of the exercise. For exercise intensities greater than 60% VO_2max, VO₂ and [la]_b showed continuous upward drifts and W_tot continued decreasing. Under these conditions, the rate of decrease of W_tot was linearly related to the rate of increase of V [(d W_tot/dt) (W·kg^–1·s^–1) = 5.0·10^–5 –0.20·(d VO₂/dt) (W·kg^–1·s^–1)] and this was linearly related to the rate of increase of [la]_b [(d VO₂/dt) (W·kg^–1·s^–1) = 2.310^–4 + 5.910^–5·(d [la]_b/dt) (mM·s^–1)]. These findings would suggest that the decrease of W_tot during the first all-out period was due to the decay of phosphocreatine concentration in the exercising muscles occurring at the onset of exercise and the slow drifts of VO₂ (upwards) and of W_tot (downwards) during intense exercise at constant W_tot could be attributed to the continuous accumulation of lactate in the blood (and in the working muscles).     

Effect of Fatiguing Exercise on Longitudinal Bone Strain as Related to Stress Fracture in Humans

Muscular fatigue in the training athlete or military recruit has been hypothesized to cause increased bone strain that may contribute to the development of a stress fracture. Under normal circumstances, muscles exert a protective effect by contracting to reduce bending strains on cortical bone surfaces. In vivo strain studies in dogs show that muscle fatigue following strenuous exercise elevates bone strain and changes strain distribution. However, a similar experiment has yet to be performed in humans. The purpose of this work was to test the hypothesis in humans that strenuous fatiguing exercise causes an elevation in bone strain. It was also hypothesized that this elevation is greater in younger people than in older people due to the decline in muscle strength and endurance that normally occurs with age. To test these hypotheses, strain in the tibiae of seven human volunteers was measured during walking before and after a period of fatiguing exercise. Neither hypothesis was sustained. Post-hoc analysis of the strain data suggests that strain rate increases after fatigue with a greater increase in younger as opposed to older persons. Although not conclusive, this suggests that it is strain rate, rather than strain magnitude, that may be causal for stress fracture. © 1998 Biomedical Engineering Society. PAC98: 8745Dr, 8745Bp, 0180+b     

The effect of caffeine ingestion on physical performance after prolonged exercise

Summary The purpose of this study was to determine the effect of caffeine ingestion on physical performance after prolonged endurance exercise. Twenty three trained male volunteers participated in a 40-km march and were divided into two groups, matched for caffeine clearance rate and aerobic capacity. The experimental group ingested, prior to the march, a caffeinated drink at a dose of 5 mg·kg⁻¹ body mass and at the 3rd and 5th h of marching an additional drink at a dose of 2.5 mg·kg⁻¹ body mass. The control group ingested a drink of equal volume at the same times. Upon termination of the march each subject performed a cycle ergometer test at an intensity of 90% maximal oxygen consumption. Time to exhaustion and rate of perceived exertion (RPE) were recorded. Blood samples were drawn predrink, at the 3rd and 5th h of marching and immediately after the cycle ergometer test, and were analysed for caffeine, free fatty acids (FFA), lactate and glucose levels. Plasma FFA levels increased during the march (p<0.05), with no significant difference between groups. Lactate levels increased in the experimental group (p<0.05), with no significant change in the control group. Glucose levels did not change significantly in either group. After the cycle ergometer test, lactate levels were significantly higher in the experimental, as compared to the control group (3.77±0.33 vs 2.52±0.35 mmol·l⁻¹, respectively). There was no significant difference between treatments in the time to exhaustion on the cycle ergometer, but RPE was different (p<0.05). Under the conditions of this study, the results do not indicate caffeine ingestion as an ergogenic aid which will postpone exhaustion following prolonged endurance exercise. This work was presented, in part, at the Canadian Association of Sports Sciences Annual Meeting, October 1987, Lake Louise, Alberta, Canada     

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     

Effects of marathon running on running economy and kinematics

The present study was designed to investigate interactions between running economy and mechanics before, during, and after an individually run marathon. Seven experienced triathletes performed a 5-min submaximal running test on a treadmill at an individual constant marathon speed. Heart rate was monitored and the expired respiratory gas was analyzed. Blood samples were drawn to analyze serum creatine kinase activity (S-CK), skeletal troponin I (sTnI), and blood lactate (B-La). A video analysis was performed (200 frames · s⁻¹) to investigate running mechanics. A kinematic arm was used to determine the external work of each subject. The results of the present study demonstrate that after the marathon, a standardized 5-min submaximal running test resulted in an increase in oxygen consumption, ventilation, and heart rate (P < 0.05), with a simultaneous decrease in the oxygen difference (%) between inspired and expired air, and respiratory exchange ratio (P < 0.05). B-La did not change during the marathon, while sTnI and S-CK values increased (P < 0.05), peaking 2 h and 2 days after the marathon, respectively. With regard to the running kinematics, a minor increase in stride frequency and a similar decrease in stride length were observed (P < 0.01). These results demonstrate clearly that weakened running economy cannot be explained by changes in running mechanics. Therefore, it is suggested that the increased physiological loading is due to several mechanisms: increased utilization of fat as an energy substrate, increased demands of body temperature regulation, and possible muscle damage. Accepted: 20 March 2000