In sports, like team handball, fatigue has been associated with an increased risk of anterior cruciate ligament (ACL) injury. While effects of fatigue on muscle function are commonly assessed during maximal isometric voluntary contraction (MVC), such measurements may not relate to the muscle function during match play. The purpose of this study was to investigate the effect of muscle fatigue induced by a simulated handball match on neuromuscular strategy during a functional sidecutting movement, associated with the incidence of ACL injury. Fourteen female team handball players were tested for neuromuscular activity [electromyography (EMG)] during a sidecutting maneuver on a force plate, pre and post a simulated handball match. MVC was obtained during maximal isometric quadriceps and hamstring contraction. The simulated handball match consisted of exercises mimicking handball match activity. Whereas the simulated handball match induced a decrease in MVC strength for both the quadriceps and hamstring muscles (P
The purpose of this study was to evaluate conditions for conducting a 30 s Wingate test such as load selection, and the method of starting the test (stationary or flying start). Nine male and four female athletes volunteered to be tested on four laboratory visits. Tests were performed on a modified Monark cycle ergometer (Varberg, Sweden) equipped with force transducers on the friction belt and an optical encoder for velocity measurement. Power was calculated with the moment of inertia (I) of the flywheel taken into consideration. One laboratory visit was used to determine individualized optimal resistance conditions. The other three visits were for performance of one of three Wingate tests: a flying start with 0.834 N x kg(-1) [85 g x kg(-1) body weight (BW)] resistance (FLY-0.8); a stationary start with 0.834 N x kg(-1) BW resistance (ST-0.8), or a stationary start with optimal resistance (ST-OPT). FLY-0.8 gave a lower (P
This study compared the effect of repetitive work in thermoneutral and cold conditions on forearm muscle electromyogram (EMG) and fatigue. We hypothesize that cold and repetitive work together cause higher EMG activity and fatigue than repetitive work only, thus creating a higher risk for overuse injuries. Eight men performed six 20-min work bouts at 25 degrees C (W-25) and at 5 degrees C while exposed to systemic (C-5) and local cooling (LC-5). The work was wrist flexion-extension exercise at 10% maximal voluntary contraction. The EMG activity of the forearm flexors and extensors was higher during C-5 (31 and 30%, respectively) and LC-5 (25 and 28%, respectively) than during W-25 (P
The effect of high altitude (HA) on exercise-induced diaphragm fatigue in normal subjects was examined. Eight normal subjects completed an incremental exercise test at sea level (SL) and at 3,325 m. Before (baseline), during, and after exercise (recovery), maximal transdiaphragm pressure (Pdi,sniff), breathing pattern, and diaphragmatic effort (PTPdi) were measured. Arterialized blood lactate was measured at baseline and during recovery. At maximal exercise (WRmax) Pdi,sniff fell to 72% and 61% of baseline at SL and HA respectively, recovering to baseline in 60 min at SL, and >60 min at HA. At the 5th min of recovery, circulating lactate was six-fold and seven-fold baseline at SL and HA, respectively. The time course of circulating lactate recovery was as for Pdi,sniff. At WRmax PTPdi was 80.74+/-9.87 kPa.s(-1) at SL and 64.13+/-8.21 kPa.s(-1) at HA. HA WRmax compared to isowork rate, SL data showed a lower Pdi,sniff (8.90+/-0.68 versus 11.24+/-0.59 kPa) and higher minute ventilation (117+/-11 versus 91+/-13 L.min(-1)), PTPdi being equal. To conclude, in normal subjects hypoxia-related effects, and not an increase in diaphragm work, hastens exercise-induced diaphragm fatigue and delays its recovery at high altitude compared to sea level.
INTRODUCTION: We investigated surgeon-modified factors in the oculocardiac reflex (OCR) including repeating tension after return to baseline pulse (fatigue) or modifying the tension place on the extraocular muscle (EOM). METHODS: A control group of 842 adult and pediatric strabismus cases underwent controlled, 200 gram, 10-second square wave tension on at least two EOM. The effect of TENSION was studied in 90 random cases by placing a tension of 50 grams on one of the later muscles and a tension of 400 grams on another subsequent muscle. The effect of FATIGUE was studied in 30 cases when the initial tension elicited a profound bradycardia. The heart rate was allowed to return to within 96% of pre-tension levels, and the same muscle had a second identical tension. RESULTS: For the 842 control cases, aged 12 +/- 16 (s.d.) years, initial 200 gram OCR was -16.9 +/- 19% and the second muscle 15 +/- 7 minutes later was significantly less percent change -14.6+/- 17%, paired t=3.9, p= .0001. TENSION: OCR was a similar reduction of 16-18% for 200 gram and 400 gram tension but had a modest reduction of 10% for 50 gram EOM tension. FATIGUE: For cases with profound initial OCR, waiting 3 minutes produced only a 17% reduction in OCR severity. CONCLUSION: A surgeon can reduce oculocardiac reflex by exerting very gentle 50 gram tension on the EOM or waiting for return to baseline heart rate before fatiguing the reflex. These reductions are of less clinical value than pharmacologic intervention.
The influence of NO on the efficiency of oxygen usage by a skeletal muscle under fatigue of dog's gastrocnemius muscle was investigated. In control experiments was shown, that 10 short-term (30") electrical stimulation (8 Hz, 5 ms, 20 V) with 5" interval resulted in significant reduction of the muscle contraction force (more than 40%) and increased considerably oxygen cost of muscle gastrocnemius work (more than 130%) compared to the initial parameters. The registered depression of the muscle contraction force testified to development of gastrocnemius muscle fatigue, accompanied by mitochondrial factor (MF) appearance in blood from femoralis vein, which, as shown by us earlier, is a marker of the mPTP opening. Injection of L-NMMA, a NOS inhibitor (2.7 mg/kg, i.a.) resulted in pronounced fall (more than 1.5 times) of the initial force parameters, in comparison with the control experiments. Under these conditions the magnitude of oxygen cost of gastrocnemius muscle work exceeded control parameters considerably. The development of gastrocnemius muscle fatigue under L-NMMA action was accompanied, as well as in the control condition by the mPTP opening. The preliminary injection of sodium nitroprusside, a NO donor (0.2 mg/kg, i.v.) prevented a fall of muscle contractions force and considerable inhibition of oxygen usage efficiency by gastrocnemius muscle under conditions similar to control. Furthemore, gastrocnemius muscle fatigue was not developed, and MF concentration in blood from femoralis vein was much lower, than in the control experiments, that testified to absence of the mPTP opening. Apparently, preliminary short-term (30") electrical stimulation (8 Hz, 5 ms, 20 V) with 2' interval, created the precondition effect and raised the level of authentic NO. Under these conditions, as well as under preliminary injection of the NO donor, we did not register the marked inhibition of oxygen usage efficiency and development of gastrocnemius muscle fatigue. At the same time, MF in blood from v. femoralis was practically absent, that testify to absence of the mPTP opening. Thus, NO in physiological concentration by inhibition of mPTP opening, can prevent decrease of oxygen usage efficiency and development of the working skeletal muscle fatigue.
Responses of gastrocnemius-soleus (G-S) motoneurones to stretches of the homonymous muscles were recorded intracellularly in decerebrate cats before, during and after fatiguing stimulation (FST) of G-S muscles. Ventral roots (VR) L7 and S1 were cut, and FST was applied to VR S1, a single FST session including 4 to 5 repetitions of 12-s periods of regular 40 s(-1) stimulation. Muscle stretches consisted of several phases of slow sinusoidal shortening-lengthening cycles and intermediate constant lengths. The maximal stretch of the muscles was 8.8 mm above the rest length. Effects of FST on excitatory postsynaptic potentials (EPSPs) and spikes evoked by the muscle stretches were studied in 12 motoneurones from ten experiments. Stretch-evoked EPSPs and firing were predominantly suppressed after FST, with the exception of a post-contraction increase of the first EPSP after FST, which was most likely due to after-effects in the activity of muscle spindle afferents. The post-fatigue suppression of EPSPs and spike activity was followed by restoration within 60-100 s. Additional bouts of FST augmented the intensity of post-fatigue suppression of EPSPs, with the spike activity sometimes disappearing completely. FST itself elicited EPSPs at latencies suggesting activation of muscle spindle group Ia afferents via stimulation of beta-fibres. The suppression of the stretch-evoked responses most likely resulted from fatigue-evoked activity of group III and IV muscle afferents. Presynaptic inhibition could be one of the mechanisms involved, but homosynaptic depression in the FST-activated group Ia afferents may also have contributed.
Effects of low- and high-frequency fatigue were studied on muscle dynamics in isometric conditions of the cat gastrocnemius. Fatiguing sessions consisted of 25-28 repetitions of the standard tests that included an 18-s interval of continuous frequency-modulated stimulation preceded and followed by single stimuli evoking twitch contractions. The rate of the continuous part was changed in accordance with a symmetrical double-trapezoidal signal, including three successive phases of constant rate at 10, 40 and 10s(-1); between these phases, each lasting for 4s, the rate changed linearly within a 2-s interval. The following modes of muscle activation were applied: (i) stimulation of single filaments constituting approximately one-fifth to one-seventh of the total cross-section of the L(7) and S(1) ventral roots; (ii) the distributed stimulation of five similar filaments; and (iii) direct stimulation of muscle through bipolar wire electrodes. A relative drop in tension, the fatigue index, expressed as the ratio at the end of a fatigue session over its value at the beginning of the test, was used to quantify fatigue effects. The fatigue indices during low-rate stimulation were 0.56+/-0.03 (mean+/-S.D.) at the first phase and 0. 64+/-0.02 at the third phase, while during high-rate stimulation this parameter was only 0.32+/-0.02. The high-rate stimulation noticeably increased the mean tension during low-rate stimulation; the ratio between the reactions at the third and the first phases could be as much as two to three times greater than that at the beginning of the fatigue session. It was demonstrated that the potentiation was connected with after-effects of the rate-tension hysteresis. The hysteresis decreased with fatigue, the fatigue index for the rate-tension loop areas ranging from 0.39 to 0.52 (0.45+/-0. 05, mean+/-S.D.). The fatigue processes developed more quickly and intensively in the previously fatigued muscles: the obtained fatigue indices were 0.73+/-0.05 and 0.70+/-0.10 at the first and third phases, and 0.62+/-0.06 (mean+/-S.D.) at the second phase of stimulation, respectively. In the cases of distributed and direct stimulation applied to muscles in a fresh state, fatigue dynamics did not differ significantly from those observed during single-filament stimulation. In experiments with distributed stimulation applied to previously fatigued muscles, a powerful depression of the high-rate components was registered in several cases, which seemed to be connected with depressive effects at the level of nerve-muscle synaptic transmission.The effects of low- and high-frequency fatigue were studied in isometric conditions of muscle contraction. In addition to the well-known differentiation between low- and high-frequency fatigue effects, the complex pattern of efferent stimulation used allowed us to identify additional fatigue-related changes in the rate-tension hysteresis. This hysteresis seems to be one of the possible mechanisms directed to compensate for low-frequency fatigue in the muscle contraction.
Changes in the compound muscle action potentials of cat gastrocnemius muscle were studied during low- and high-frequency fatigue. Fatiguing session consisted of 25-28 repetitions of the standard single fatigue tests (1.5min interval between the tests) that included the part of continuous frequency-modulated stimulation preceded and followed by single stimuli evoking twitch contractions in the muscle. The rate of the continuous part was changed in accordance with symmetrical double-trapezoidal signal, including three successive phases of constant rate at 10, 40 and 10s(-1); between these phases of 4s duration the rate changed linearly within a 2s interval. During fatigue relative changes in compound muscle action potential waves were usually smaller than changes in tension. Within the same fatigue procedure applied to a fresh muscle, the drop in tension was as much as 35% for high-rate stimulation and 59-71% for low-rate stimulation, whereas the decrease of the peak-to-peak compound muscle action potential waves amplitudes did not exceed 10-20%. Compound muscle action potential waves underwent the most pronounced depression during high-rate stimulation, the decrease proceeding during the following phase of low-rate stimulation. The tension changes during long-lasting activation were different for low- and high-frequency fatigue, with more pronounced depression during low-rate stimulation. As a rule, compound muscle action potential waves changes followed opposite patterns. Compound muscle action potential waves progressively split up, which was probably associated with a continuous slowing of the action potentials in the most fatigable motor units and the subsequent disappearance of the reactions at least in part of the motor units. Hysteresis effects in muscle contraction seem to be able, at least in part, to compensate for some of the depressive effects appearing during conduction of action potentials in muscle fibres.Changes in the compound muscle action potentials were studied during development of the muscle fatigue. These changes showed pronounced dependency on stimulation rate allowing differentiating effects of low- and high-frequency stimulation of the efferents supplying muscle under study. At the same time the fatigue-related changes in the action potentials were noticeably smaller than changes in tension, thus supporting existing concepts in the field arguing that fatigue effects are mainly connected with corresponding activity-dependent changes in muscle contraction machinery.
The purpose was to examine the fatigue pattern of elite female soccer players after competitive games. Soccer players (n = 23) from the Danish women Premier League performed a countermovement vertical jump test, a repeated 30-m sprint test, and the Yo-Yo intermittent endurance level 2 (Yo-Yo IE2) test at rested state and after a competitive game. Average heart rate during the game was 86 +/- 1% of maximal heart rate with no differences between halves. Blood lactate was 5.1 +/- 0.5 mmol.L after the first half, which was higher (p