This study tried to differentiate the consequences of chronic hypoxia on the electrophysiological and physiological properties and the histological characteristics of slow and fast muscles in rats. Animals inhaled a 10% O2 concentration for a 1-month period. Then, slow [soleus (SOL)] and fast [extensor digitorum longus (EDL)] muscles were analyzed in vitro by physiological and electrophysiological measurements and histological analyses. The results were compared to those obtained in corresponding muscles of an age-matched normoxic group. After exposure to hypoxia: (1) in SOL, there was a tendency to elevated Fmax, a significant increase in twitch force and tetanic frequency and a shortening of M-wave duration, and a reduced percentage of type I fibres, whereas the proportion of type IIa fibres doubled; (2) in EDL, Fmax and tetanic frequency were lowered, the muscle became less resistant to fatigue, and the proportion of type IId/x fibres was halved. Then, after 1 month of hypoxia, in the SOL muscle, both the contractile and histological properties resemble those of a fast muscle. By contrast, the EDL became slower, despite its histology was modestly affected. Reduced muscle use in hypoxia could explain the tendency for deteriorating adaptations in EDL, and the faster properties of SOL could result from hypoxia-induced inhibition of the growth-related fast-to-slow shift in muscle fibre types. 相似文献
Falls from heights resulting from a loss of balance are a major concern in the occupational setting. Previous studies have documented a deleterious effect of lower extremity fatigue on balance. The purpose of this study was to investigate the effect of lumbar extensor fatigue on balance during quiet standing. Additionally, the effects of fatigue rate on balance and balance recovery rate were assessed. Eight center-of-pressure-based measures of postural sway were collected from 13 participants, both before and after a protocol that fatigued the lumbar extensors to 60% of their unfatigued maximum voluntary exertion force. In addition, postural sway was measured for 30 min after the fatiguing protocol, at 5-min intervals, to quantify balance recovery rate during recovery from fatigue. Two different fatigue rates were achieved by fatiguing participants over either 10 min or 90 min. Results show an increase up to 58% in time-domain postural sway measures with lumbar extensor fatigue, but no change in frequency-domain measures. Fatigue rate did not affect the magnitude of these postural sway increases, nor did it affect the rate of balance recovery following fatigue. Statistical power for the latter result, however, was low. These results show that lumbar extensor fatigue increases postural sway and may contribute to fall-from-height accidents. 相似文献
The concept of the accumulated O2 deficit (AOD) assumes that the O2 deficit increases monotonically with increasing work rate (WR), to plateau at the maximum AOD, and is based on linear extrapolation of the relationship between measured steady-state oxygen uptake (O2) and WR for moderate exercise. However, for high WRs, the measuredO2 increases above that expected from such linear extrapolation, reflecting the superimposition of a "slow component" on the fundamentalO2 mono-exponential kinetics. We were therefore interested in determining the effect of theO2 slow component on the computed AOD. Ten subjects [31 (12) years] performed square-wave cycle ergometry of moderate (40%, 60%, 80% and 90%
), heavy (40%), very heavy (80%) and severe (110%O2peak) intensities for 10–15 min, where
is the estimated lactate threshold and is the WR difference between
andO2peak.O2 was determined breath-by-breath. Projected "steady-state"O2 values were determined from sub-
tests. The measuredO2 exceeded the projected value after ~3 min for both heavy and very heavy intensity exercise. This led to the AOD actually becoming negative. Thus, for heavy exercise, while the AOD was positive [0.63 (0.41) l] at 5 min, it was negative by 10 min [–0.61 (1.05) l], and more so by 15 min [–1.70 (1.64) l]. For the very heavy WRs, the AOD was [0.42 (0.67) l] by 5 min and reached –2.68 (2.09) l at exhaustion. For severe exercise, however, the AOD at exhaustion was positive in each case: +1.69 (0.39) l. We therefore conclude that the assumptions underlying the computation of the AOD are invalid for heavy and very heavy cycle ergometry (at least). Physiological inferences, such as the "anaerobic work capacity", are therefore prone to misinterpretation. 相似文献
The examination of the moment exerted by the hamstrings during maximum isokinetic knee extensor tests is useful when comparing
isokinetic strength and muscle activity patterns between children and adults. The purpose of this study was to examine the
effect of antagonist moment of the hamstrings on the isokinetic moment of the knee extensors in pubertal children and to determine
whether this effect is altered following a fatigue task. Eighteen healthy pubertal males [age 14.3 (0.5) years] performed
34 maximal isokinetic concentric efforts of the knee extensors at 60°·s−1. The average moment of force and electromyographic (aEMG) signal of vastus medialis (VM), vastus lateralis (VL) and biceps
femoris (BF) at 11–30°, 31–50°, 51–70° and 71–90° of knee flexion were calculated for each repetition. The hamstrings antagonist
moment was determined before and after the fatigue task by fitting the aEMG–moment relationship at different levels of muscle
effort using second-degree polynomials. The percentage contribution of the antagonist moment to the resultant joint moment
ranged from 7.1 % to 60.4 % throughout the range of motion, with the highest percentage observed close to full knee extension
(11–30°). The antagonist effect was significantly greater during concentric tests of the knee extensors compared to the corresponding
eccentric tests (p<0.05). Following the fatigue test, there was an overall decline of the resultant joint moment, but no changes in the predicted
hamstrings moment were observed. These results indicate that when testing maximal knee extensor isokinetic strength in pubertal
boys, activity of the hamstrings implies a reduction of the net extensor moment as compared to the isolated capacity of the
knee extensors. However, this antagonist effect is not altered following the performance of an isokinetic fatigue knee extension
task.
Electronic Publication 相似文献
The changes in muscle force associated with varying degrees of lower-limb ischaemia were investigated. Isometric torque production
by the triceps surae muscle was measured during a 5-min continuous train of 2-Hz electrical stimulation in six healthy young
adults under different thigh cuff occlusion pressures. The reproducibility of this protocol when performed under complete
ischaemia (tested five times over a 2-week period) was assessed as having a coefficient of variation (CV) for fatigue (end/initial
force) of [mean (SEM) 12 (1)%; n=5]. This compares favourably with that obtained for maximum voluntary contraction torque [CV 9 (1)%]. In six subjects, triceps
surae muscle fatigue was assessed under thigh cuff pressures of 0, 6.7 kPa (50 mmHg, venous occlusion) and 28 kPa (210 mmHg,
complete ischaemia), as well as two intermediate levels of occlusion that were established by cuff pressures of 13.4 (0.5)
and 20.3 (1.1) kPa [103 (4) and 152 (8) mmHg, respectively]. These corresponded to ankle-brachial pressure indices of 1.3
and 0.8, respectively when subjects were seated, or 0.8 and 0.36 when supine. With undisturbed lower-leg circulation, force
potentiated steadily over the 5 min of stimulation such that the final force was 135 (8)% of the initial value. With complete
ischaemia, force fell to 47 (2)% of the initial value. Stimulation under thigh occlusion pressures of 6.7, 13.4 and 20.3 kPa
elicited intermediate levels of reduction in force, graded according to the increasing restriction of perfusion. The results
show that low-force twitch contractions, which themselves do not occlude blood flow, are extremely sensitive to impaired perfusion
and may represent a viable alternative to established methods of muscle performance assessment in patients with blood flow
insufficiency.
Accepted: 5 November 1999 相似文献
It has been hypothesized that the ability of the neuromuscular system to co-contract muscles for joint stabilization may
be impaired during the development of fatigue. The purpose of this study was to examine muscle activation of the quadriceps
and hamstring muscles during a prolonged closed kinetic chain exercise, the forward lunge. Eight males and two females [mean
(SD) age 26.0 (2.3) years, height 177.2 (13.6) cm, body mass 82.8 (17.1) kg] with no prior knee pathology volunteered for
this study. Subjects performed repeated forward lunges onto their dominant leg at the cadence of one full lunge cycle every
2 s, until the point of volitional failure. Digital switches were positioned to record foot-strike and knee-strike of the
lunge leg at the midpoint of the lunge, as well as heel-strike upon return to stance. During the lunge performance, surface
electromyographic (EMG) signals of the vastus lateralis (VL), vastus medialis (VM), biceps femoris (BF), and semitendinosus
(ST) muscles of the supporting leg were measured. Heart rate was also monitored every 30 s during the performance. All EMG
data were full-wave rectified, partitioned into up and down phases, and integrated over the entire exercise period. The results
demonstrated a significant increase in activation of the VL, VM, and BF during performance of the forward lunge to volitional
failure (P < 0.05). No significant increase was shown for the ST. Heart rate increased significantly over the course of the lunge. These
findings suggest that activation of the VL, VM, and BF muscles occurs as a unit during performance of the forward lunge during
both concentric and eccentric lunge phases.
Accepted: 25 October 1999 相似文献
The long term success of total joint replacement can be limited by fatigue failure of the acrylic cement and the resulting disruption of the bone-cement interface. The incidence of such problems may be diminished by reduction of the fatigue notch factor in the cement, so that stress concentrations are avoided and the fatigue crack initiation time maximized. This study describes a method for numerical shape optimization whereby the finite element method is used to determine an optimal shape for the femoral stem of a hip prosthesis in order to minimize the fatigue notch factor in the cement layer and at interfaces with the bone and stem.
A two-dimensional model of the proximal end of a femur fitted with a total hip prosthesis was used which was equivalent to a simplified three-dimensional axisymmetric model. Software was developed to calculate the fatigue notch factor in the cement along the cement/stem and cement/bone interfaces and in the proximal bone. The fatigue notch factor in the cement at the cement/stem interface was then minimized using the ANSYS finite element program while constraining the fatigue notch factor at the cement/bone interface at or below its initial level and maintaining levels of stress in the proximal bone to prevent stress shielding. The results were compared with those from other optimization studies. 相似文献
Summary The effects of long-term fatigue upon maximal force and peak rate of tension development (PRTD) (dF/dt max) are studied in man (elbow flexors), in the rat (pseudo-isolated gastrocnemius muscle) and in the frog (isolated sartorius muscle). The muscles are fatigued by voluntary anisometric anisotonic contractions against an elastic resistance in man, and by maximal tetanic contractions in the frog and the rat. In man, the excitation level of the muscle is controlled by the integrated surface EMG of the biceps brachii. In the animals, the muscles are stimulated by a neurostimulator. The PRTD and the maximal isometric force are measured during fatigue tests.In man, frog and rat, the maximal voluntary isometric torque or the maximal force and the PRTD decrease initially more or less rapidly according to the power developed during the fatigue process, and then less rapidly. The relationship between PRTD and maximal force is linear in the animals and curvilinear in man.The variations of maximal force and PRTD are discussed in relation to the level of excitation of the muscles and of the composition in different motor units types and their spatio-temporal recruitment. From a biomechanical point of view, it seems necessary to study the behavior of the series elastic component during the evolution of long term fatigue. 相似文献
Summary Peak torque, work, mean power and electromyographic (EMG) activity were recorded for each of 150 repeated isokinetic maximal shoulder flexions (45°–90°) in 23 healthy females. From the EMG signals of trapezius, deltoid, infraspinatus and biceps brachii the mean power frequency and the signal amplitude were determined in real time. The mechanical output showed a steep decrease during the first 40 contractions, followed by a plateau maintained until the end. In all muscles, except the biceps brachii, significant decreases in mean power frequency occurred during the first 40 contractions, showing a tendency to stabilize around the same absolute frequency value. Signal amplitude increased in the trapezius, the deltoid and the infraspinatus, but was constant in the biceps brachii. For some individuals rather high EMG activity was recorded in the muscles during the time the arm was supposed to be passively extended to the starting position, and this was found to be associated with lower strength and endurance levels. Longitudinal analyses showed that the mean power frequencies correlated better than the signal amplitudes with the three mechanical variables. The results suggest that the initial steep decrease in mechanical performance and mean power frequency is caused by fatiguing of type 2 motor units. 相似文献