Effect of head-out water immersion on neuromuscular function of the plantarflexor muscles

INTRODUCTION: Electromyograms (EMGs) recorded during head-out immersion have demonstrated reduced signal amplitudes and decreased EMG/force ratios for both maximal and submaximal isometric contractions compared with measurements on dry land. Similar EMG changes have been found in spaceflight. Hypothesis: This study was designed to examine neuromuscular function in the legs during immersion with special reference to reflex sensitivity. METHODS: Recordings were made during plantarflexion in air and water. EMGs were recorded from surface and/or internal electrodes on the soleus (SOL) and medial gastrocnemius (MG) muscles during maximal voluntary contraction (MVC). Hoffman and Achilles tendon reflexes were measured during submaximal plantarflexion (50% MVC) in air and water. RESULTS: During immersion, MVC decreased 13% while EMG amplitude of the plantarflexor muscles decreased by 29% and 35% for SOL and MG, respectively. A similar trend was observed in measurements of Hofman and Achilles tendon reflexes. CONCLUSIONS: Head-out immersion induced a deterioration of neuromuscular function, perhaps by triggering inhibitory mechanisms. The origin of these mechanisms seems to be related mainly to effects of partial weightlessness, but hydrostatic pressure should also be considered.     

Steadiness training with light loads in the knee extensors of elderly adults

PURPOSE: This study was conducted to determine the effect of steadiness training with light loads in the knee extensors of elderly adults. METHODS: Twenty-one elderly adults (72 +/- 4.6 yr) performed 16 wk of closely supervised knee extensor training that consisted of lifting and lowering 30% of the one-repetition maximum (1-RM) load as steadily as possible (10 reps per set, three sets per session, three sessions per week). Nine subjects served in a control group. Unilateral and bilateral maximal voluntary contraction (MVC) force; 1-RM load; force fluctuations during submaximal isometric, concentric, and eccentric contractions; timed functional performance (gait, chair rise, stair ascent and descent); muscle volume via magnetic resonance imaging (MRI); and the electromyogram (EMG) were measured. RESULTS: The training group exhibited modest average gains in MVC force and 1-RM load; muscle volume and EMG were unaltered. Although isometric steadiness was unchanged on average, training elicited the greatest improvements in the least steady subjects. Force fluctuations during concentric and eccentric contractions were significantly reduced. Of 21 subjects, 14 responded to training with gains in 1-RM load greater than the typical change (6%) in the control group. Before training, these responders exhibited greater force during bilateral compared with unilateral contractions. The small changes in physical functional performance were similar for the training and control groups. The training group could lift the pretraining 1-RM load 4.6 times after training (5.6 times for responders). CONCLUSIONS: Steadiness training with the knee extensors thus produced neural adaptations that increased strength in elderly adults who exhibited bilateral facilitation, improved isometric steadiness in unsteady subjects, improved steadiness during concentric and eccentric contractions, and enhanced the ability to lift heavy loads repeatedly.     

Effects of eccentric versus concentric training on thigh muscle strength and EMG

The purpose of this study was to compare pure eccentric and concentric strength training regarding possible specific effects of muscle action type on neuromuscular parameters, such as a decreased inhibition during maximal voluntary eccentric actions. Two groups of young healthy adult men performed 10 weeks of either eccentric or concentric unilateral isokinetic knee extensor training at 90 degrees.s(-1), 4 sets of 10 maximal efforts, 3 days a week. Knee extensor torque and surface EMG from the quadriceps and hamstring muscle groups were collected and quantified in a window between 30 and 70 degrees knee angle (range of motion 90-5 degrees ) during maximal voluntary eccentric and concentric knee extensor actions at 30, 90, and 270 degrees.s(-1). Changes in strength of the trained legs revealed more signs of specificity related to velocity and contraction type after eccentric than concentric training. No major training effects were present in eccentric to concentric ratios of agonist EMG or in relative antagonist (hamstring) activation. Thus, for the trained leg, the muscle action type and speed specific changes in maximal voluntary eccentric strength could not be related to any effects on neural mechanisms, such as a selective increase in muscle activation during eccentric actions. Interestingly, with both types of training there were specific cross-education effects, that is, action type and velocity specific increases in strength occurred in the contralateral, untrained, leg, accompanied by a specific increase in eccentric to concentric EMG ratio after eccentric training.     

Quadriceps concentric and eccentric exercise 1: Changes in contractile and electrical activity following eccentric and concentric exercise

The purpose of this study was to determine whether or not losses of strength or endurance following eccentric and concentric exercise are associated with reduced excitation. The effects of eccentric and concentric work on maximal voluntary isometric contraction (MVC) and surface electromyogram (EMG) of the quadriceps were studied in 10 healthy male subjects following bench-stepping for 20 min with a constant leading leg. Prior to stepping and at 0, 0.25, 0.50, 0.75, 1, 3, 24 and 48 h afterwards the subjects performed a 30 s leg extension MVC with each leg during which the isometric force and the root mean square voltage of the EMG were recorded. In the eccentrically exercised muscles (ECC), MVC0-3 (force during the first 3 s of contraction) fell immediately after the bench-stepping exercise to 88+/-2% (mean+/-SE) of the pre-exercise value and remained significantly lower than the concentrically exercised muscles (p<0.05). The muscle weakness in the ECC could not be attributed to central fatigue as surface EMG amplitude at MVC0-3 increased during the recovery period. Muscle weakness after eccentric exercise appears to be due to contractile failure, which is not associated with a reduction in excitation as assessed by surface EMG. Muscular fatigue over 30 s did not change in the two muscle groups after exercise (p = 0.79), indicating that the ECC were weaker but not more fatiguable after exercise.     

Comparison between muscle power outputs exerted by concentric and eccentric contractions

Abstract Although muscle power output has been evaluated from various dynamic muscle contractions, muscle power outputs exerted by concentric and eccentric contractions due to different physiological mechanisms have not been examined thoroughly. This study aimed to reveal characteristics of eccentric power output exerted by elbow flexion, and to examine the relationship between eccentric and concentric outputs. Ten young males performed single power output tasks via concentric contraction with 40%, 50%, and 60% maximal voluntary contraction (MVC), and by eccentric contraction with 130% and 140% MVC. Power output due to concentric and eccentric contractions was measured using a power measurement instrument with a rotary encoder attached to the pulley. The eccentric contraction velocities in both loads were maximal, about 0.5 s, at about a 90° bend of the elbow joint after the onset of contraction. Although eccentric contraction with 140% MVC completed in about 0.5 s (range, 0.22–0.73 s), contractions with 130% MVC completed in about 8 s (range, 1.41–12.08 s) due to moderate contraction velocities. The reliability of the eccentric power parameters was found to be good, as well as that of the concentric power. The average velocity of eccentric contraction with 130% MVC correlated significantly with the peak power of concentric contraction with 60% MVC. In conclusion, eccentric power may be influenced more by maximal strength rather than muscle contraction velocity.     

Influence of electrical stimulation on dynamic forces of the arm flexors in strength-trained and untrained men

We examined the influence of activation and strength-training history on the force-velocity relationship. Over 4 days, maximal isometric (MVC) and concentric and eccentric constant angle isokinetic forces at 6 velocities were measured in the arm flexors of 6 strength-trained men and 6 untrained men under voluntary condition and with electromyostimulation (EMS) (2.5 kHz sine wave, 50 burst · s⁻¹, 50% duty cycle) superimposed on the voluntary muscle actions. Trained men were 31% stronger than untrained controls. Voluntary concentric forces decreased with increasing velocities and were less than MVC in both groups. Voluntary eccentric forces exceeded MVC only in the trained men. Untrained men tolerated 21.9 mA and trained men 31.3 mA of EMS current, yet tetanic forces were similar: 92.5 N versus 96.0 N. These data suggest that, independent of training status, percutaneous EMS inhibits voluntary forces of the arm flexors. Modifications of the EMS parameters may be necessary to allow for the more complete study of the influence of training history on muscle mechanics and the neural regulation of force production.     

Alteration in neuromuscular function of the plantar flexors following caffeine ingestion

The aim of this study was to compare the neuromuscular function of the plantar flexors following caffeine or placebo administration. Thirteen subjects (25 ± 3 years) ingested caffeine or placebo in a randomized, controlled, counterbalanced, double‐blind crossover design. Neuromuscular tests were performed before and 1 h after caffeine or placebo intake. During neuromuscular testing, rate of torque development, isometric maximum voluntary torque, and neural drive to the muscles were measured. Triceps surae muscle activation was assessed by normalized root mean square of the EMG signal during the initial phase of contraction (0–100 ms, 100–200 ms) and maximal voluntary contraction (MVC). Furthermore, evoked spinal reflex responses of the soleus muscle (H‐reflex evoked at rest and during MVC, V‐wave) and peak twitch torques were evaluated. The isometric maximum voluntary torque and evoked potentials were not different. However, we found a significant difference between groups for rate of torque development in the time intervals 0–100 ms [41.1 N·m/s (95% CI: 8.3–73.9 N·m/s, P = 0.016)] and 100–200 ms [32.8 N·m/s (95% CI: 2.8–62.8 N·m/s, P = 0.034)]. These changes were accompanied by enhanced neural drive to the plantar flexors. Data suggest that caffeine solely increased explosive voluntary strength of the triceps surae because of enhanced neural activation at the onset of contraction whereas MVC strength was not affected.     

Neuromuscular fatigue profile in endurance-trained and power-trained athletes

PURPOSE: This study examined the effects of training background on the relationship between the neuromuscular fatigue profile and maximal voluntary torque production in isometric, concentric, and eccentric contraction modes. METHODS: Before and after three sets of 31 isokinetic concentric knee extensions at 60 degrees .s(-1), voluntary and electrically induced contractions were recorded in 14 endurance-trained (ENDU) men (seven cyclists: age 25 +/- 2 yr, mass 70 +/- 8 kg, height 175 +/- 5 cm; and seven triathletes: age 27 +/- 4 yr, mass 71 +/- 5 kg, height 179 +/- 6 cm) and seven explosive power-trained men (EXPLO: age 24 +/- 1 yr, mass 73 +/- 5 kg, height 179 +/- 4 cm). Maximal knee-extension torque, activation level (twitch interpolation technique), electromyographic activity of agonist and antagonist muscles, and twitch contractile properties were assessed. RESULTS: At preexercise, the maximal voluntary isometric and concentric torques of EXPLO were greater than those of ENDU (P < 0.05). After the fatiguing exercise, significant isometric (18%; P < 0.01) and concentric (25%; P < 0.05) torque decreases in EXPLO were associated with, respectively, twitch torque (Pt) and maximal rate of twitch development (+dPt/dt) reductions (P < 0.01) and with an increase in the antagonist coactivation level (P < 0.01). No modification was observed for ENDU. Interestingly, the coactivation level was also increased (P < 0.01) in eccentric contraction for EXPLO, although the maximal eccentric torque decrease (P < 0.01) could not be specifically attributed to any group. CONCLUSION: The fatiguing exercise induced central and peripheral adaptations, but the mechanisms differed regarding the contraction mode. At pre- and postfatiguing exercise, it seems that the neuromuscular profile depends on the subject's training background and the contraction modes used to assess fatigue.     

Changes in motor unit characteristics after eccentric elbow flexor exercise

Morphological evidence suggests that fast-twitch fibers are prone to disruption of their membrane structures by eccentric exercise. However, it is unclear how this is reflected in the discharge rate and action potential propagation of individual motor units, especially at high contraction levels. High-density surface electromyograms were recorded from biceps brachii muscle and decomposed to individual motor unit action potentials at isometric contraction levels between 10% and 75% of maximal voluntary contraction (MVC) before intermittent maximal elbow flexor eccentric exercise, and two hours (2H), two days (2D) and four days (4D) post-exercise. Maximal voluntary force decreased by 21.3±5.6% 2H and by 12.6±11.1% 2D post-exercise. Motor unit discharge rate increased and mean muscle fiber conduction velocity decreased, at the highest isometric contraction levels only (50% and 75% of MVC) at 2H post-exercise. These results indicate that eccentric exercise can disturb the function of motor units active at high contraction levels in the early stages after exercise, which seems to be compensated by the central nervous system with an increase in neural drive during submaximal isometric contractions.     

Changes in the reflex excitability during and after a sustained, low-intensity muscle contraction

The purpose of the study was to evaluate the soleus H-reflex amplitude during and after a low-intensity isometric contraction. Twelve healthy, untrained subjects performed a 10-minute isometric plantar flexion at 20% of their maximum voluntary contraction torque output. The electromyogram, H-reflex, and maximum M-wave (Mmax) of the soleus muscle was recorded during and 10 minutes after the end of the contraction. The results indicated that the H-reflex increased significantly (mean +/- SEM: 44.7 +/- 16.6%, p < 0.05), but when the fatigue protocol was over, the H-reflex was depressed for the first 3 minutes, relative to the H-reflex that was recorded before fatigue, when the muscle was relaxed. The Mmax did not change significantly during the whole experiment. Furthermore, the stimulation frequency (0.1 vs. 0.3 Hz) did not have any significant effect on the H-reflex modulation. The results of the current study suggest that the reflex excitability is increased as fatigue develops, whereas this increase turns to depression for the first minutes of the recovery phase. The functional significance of these changes and the neural mechanisms which might be responsible are discussed.     

Whey protein hydrolysate augments tendon and muscle hypertrophy independent of resistance exercise contraction mode

In a comparative study, we investigated the effects of maximal eccentric or concentric resistance training combined with whey protein or placebo on muscle and tendon hypertrophy. 22 subjects were allocated into either a high‐leucine whey protein hydrolysate + carbohydrate group (WHD) or a carbohydrate group (PLA). Subjects completed 12 weeks maximal knee extensor training with one leg using eccentric contractions and the other using concentric contractions. Before and after training cross‐sectional area (CSA) of m. quadriceps and patellar tendon CSA was quantified with magnetic resonance imaging and a isometric strength test was used to assess maximal voluntary contraction (MVC) and rate of force development (RFD). Quadriceps CSA increased by 7.3 ± 1.0% (P < 0.001) in WHD and 3.4 ± 0.8% (P < 0.01) in PLA, with a greater increase in WHD compared to PLA (P < 0.01). Proximal patellar tendon CSA increased by 14.9 ± 3.1% (P < 0.001) and 8.1 ± 3.2% (P = 0.054) for WHD and PLA, respectively, with a greater increase in WHD compared to PLA (P < 0.05), with no effect of contraction mode. MVC and RFD increased by 15.6 ± 3.5% (P < 0.001) and 12–63% (P < 0.05), respectively, with no group or contraction mode effects. In conclusion, high‐leucine whey protein hydrolysate augments muscle and tendon hypertrophy following 12 weeks of resistance training – irrespective of contraction mode.     

Assessment of the H‐reflex at two contraction levels before and after fatigue

The aim of the present study was to compare the H‐reflex evoked at rest and at 20% maximal voluntary contraction (MVC) prior to and after fatiguing the lateral gastrocnemius (LG). The maximal H‐reflex and M‐wave were recorded in the LG, and soleus (SOL). Electrical evoked potentials were delivered to the posterior tibial nerve when muscles were inactivated and at 20% MVC. After fatigue, the H_max/M_max ratio of the fatigued LG was increased for both contraction levels (rest and 20% MVC) and remained unaltered for non‐fatigued SOL. Before fatigue, the H_max/M_max ratio of SOL was enhanced at rest compared with the H_max/M_max ratio at 20% MVC. No differences were observed for LG. Fatigue of a single muscle leads to increased spinal reflex activity of the homonymous muscle. Contrary to previous recommendations in the literature, there appears to be no benefit with regard to the H‐reflex amplitude in evoking electrical potentials during constant voluntary contractions at 20% MVC compared with inactivated muscles. The observed difference in SOL prior to fatigue was most likely due to hyperpolarization of the muscle fiber membrane.     

The relationship between isokinetic knee extension and flexion strength with soccer kick kinematics: an electromyographic evaluation

AIM: The purpose of this study was to investigate the relationship between isokinetic strength knee testing and soccer kick kinematics using electromyography (EMG). METHODS: Thirteen pubertal soccer players (age: 14.3+/-0.4 years) performed maximum instep soccer kicks, while knee angular position of the swinging leg was recorded using a twin-axis electrogoniometer. Bipolar surface EMG activity of the vastus medialis, vastus lateralis and biceps femoris (BF) muscles was recorded. The subjects also performed maximum knee extension and flexion efforts at concentric angular velocities of 1.04, 3.14 and 5.23 rad x s(-1) and eccentric angular velocities of 1.04 and 3.14 rad x s(-1). RESULTS: The correlation coefficients between isokinetic moments and knee angular velocity values during the kick ranged from 0.609 to 0.898 for concentric moments and from 0.431 to 0.612 for eccentric moments. Agonist EMG values during isokinetic tests ranged from 63.17+/-19.9% to 128.7+/-34.9% maximum voluntary contraction (MVC). Antagonist EMG levels ranged from 9.76+/-6.12% to 36.91+/-22.81% MVC. The corresponding EMG values during the soccer kick ranged from 12.78+/-6.8% to 122.34+/-61.5% MVC and increased as the foot approached the ball. CONCLUSION: Isokinetic tests at intermediate and fast angular velocities are adequate for monitoring strength training programs in soccer. However, muscle activation patterns differ between the two movements, especially those of the BF muscle.     

Central versus peripheral adaptations following eccentric resistance training

Aim of the present investigation was to study the effects of an eccentric training on the neuromuscular properties of the plantar-flexor muscles. The experiment was carried out on 14 males divided into two groups (eccentric and control). Eccentric training consisted of six sets of six eccentric contractions at 120 % of one maximal concentric repetition and it was performed four times a week during four weeks. Before and after the 4-wk period, the plantar-flexor torque and the associated electromyographic activity were recorded during voluntary contractions (isometric, concentric and eccentric) and electrically induced contractions (twitch and tetanus), in order to distinguish central from peripheral adaptations. For the eccentric group, voluntary torque significantly increased after training independent of the action mode (relative gains 14 - 30 %, p < 0.05). This was associated with an increase in agonist EMG activity during isometric action and a decrease in antagonist coactivation in concentric (-27 %) and eccentric actions (-22 %) (p < 0.05). Voluntary activation level significantly increased from 80 +/- 5 % to 91 +/- 2 % (p < 0.05). Some of the twitch contractile properties (peak torque and maximal rate of twitch tension relaxation) were significantly modified (p < 0.05), but no changes were observed for the tetanus characteristics. These results allowed to conclude that the torque gains observed after the present training were more likely associated to central adaptations, affecting both agonist and antagonist muscles.     

Relationship between strength qualities and short track speed skating performance in young athletes

This study analyzed the relationships between isometric as well as concentric maximum voluntary contraction (MVC) strength of the leg muscles and the times as well as speeds over different distances in 17 young short track speed skaters. Isometric as well as concentric single‐joint MVC strength and multi‐joint MVC strength in a stable (without skates) and unstable (with skates) condition were tested. Furthermore, time during maximum skating performances on ice was measured. Results indicate that maximum torques during eversion and dorsal flexion have a significant influence on skating speed. Concentric MVC strength of the knee extensors was higher correlated with times as well as speeds over the different distances than isometric MVC strength. Multi‐joint MVC testing revealed that the force loss between measurements without and with skates amounts to 25%, while biceps femoris and soleus showed decreased muscle activity and peroneus longus, tibialis anterior, as well as rectus femoris exhibited increased muscle activity. The results of this study depict evidence that the skating times and speeds are primarily influenced by concentric MVC strength of the leg extensors. To be able to transfer the strength onto ice in an optimal way, it is necessary to stabilize the knee and ankle joints.     

Why does knee extensor muscles torque decrease after eccentric-type exercise?

AIM: The purpose of this study was to re-examine central and peripheral origins of neuromuscular fatigue after a highly strenuous eccentric exercise of the knee extensor muscles (KE) using both voluntary/evoked contractions and electromyographic recordings (EMG). METHODS: Before, and 30 min after 15 min of intermittent one-logged downhill running, maximal percutaneous electrical stimulations (single twitch, 0.5 s tetanus at 20 Hz and 80 Hz) were applied to the femoral nerve of 10 male subjects. Electrically evoked superimposed twitches were delivered during isometric maximal voluntary contraction (MVC) to determine maximal voluntary activation (%VA). Vastus lateralis (VL), vastus medialis (VM) and biceps femoris (BF) EMG were recorded during MVC and quantified using the root mean square (RMS) value. M-wave characteristics were also determined. RESULTS: KE MVC and %VA decreased significantly with fatigue (-19.6+/-6.1%; P<0.001 and -7.8+/-6.6%; P<0.01, respectively). Peak tetanus tension at 20 and 80 Hz (P20 and P80, respectively) declined (P<0.001), concurrently with a decrement of the P20 x P80(-1) ratio (-37.3+/-16.6%; P<0.001). Antagonist muscle coactivation, RMS to M-wave peak-to-peak amplitude and MVC x P80(-1) ratios were unchanged after the fatiguing exercise. CONCLUSIONS: The results reveal that part of the large loss in MVC may have a central origin but most of the MVC decrement is due to the presence of low-frequency fatigue while possible contractile failure cannot be excluded.     

Fatigue and recovery at long and short muscle lengths after eccentric training

PURPOSE: To determine the effects of high-speed eccentric training of rat plantar flexor muscles on: 1) maximum (120 Hz) force at 90 degrees ankle position; 2) fatigue (40 concentric muscle actions, ROM 50 degrees) and recovery (6 concentric muscle actions) tested at short or long muscle lengths; and 3) low-frequency fatigue. METHODS: Training consisted of eccentric muscle actions from ankle positions of 140 degrees to 40 degrees (velocity approximately 400 degrees x s(-1)) followed by unresisted concentric muscle actions (5 x 10 repetitions, 5 d x wk(-1) for 6 wk). Fatigue was induced by concentric muscle actions with a rest of 12.5 s between muscle actions, and recovery consisted of equivalent concentric muscle actions performed every 5 min for 30 min. Low-frequency fatigue was measured 35 min after testing at 90 degrees ankle position by using the ratio of isometric force produced by 20- and 100-Hz stimulation frequencies. RESULTS: Eccentric training increased maximal isometric force per muscle weight by 22% whereas muscle weights were unchanged. In control muscles (C), isometric force immediately preceding each concentric muscle action decreased more at long lengths than at short lengths during the fatigue protocol; this length-dependent difference disappeared after 30 min of recovery. At short lengths, isometric force decreased less in trained muscles (T) (C: 78.4 +/- 3.6%; T: 59.6 +/- 4.4%) and recovered more during the following 30-min period (C: 84.7 +/- 2.5%; T: 95.4 +/- 2.8% of initial values). Changes in F20/F100 were smaller for trained muscles (C: 35.4 +/- 2.0%; T: 22.0 +/- 1.4%). CONCLUSIONS: High-speed eccentric training (5 d x wk(-1) for 6 wk) reduced fatigability and enhanced recovery mainly at long muscle lengths. It also reduced low-frequency fatigue, which may be attributed to alterations in intracellular calcium handling.     

Effect of resistance training on muscle fatigue and recovery in intact rats

PURPOSE: To examine the effect of resistance training on muscle fatigue from intermittent contractions and subsequent recovery in intact rats. METHODS: By using electrical stimulation, plantar flexor muscles were trained with eccentric and concentric contractions (5 x 10 repetitions, 5 d x wk(-1) for 6 wk) during ankle rotations. By using nerve stimulation, concentric contractions (40) imposed on isometric contractions (stimulation time, 1.9 s; rest period, 13.6 s; intermittent contractions) induced fatigue. During recovery, equivalent contractions were used every 5 min for 30 min. RESULTS: Training increased isometric forces (19% and 23% at ankle positions of 1.57 and 0.70 rad), but muscle weights were not changed. After training, smaller declines in isometric (control, 68.9+/-1.4%; trained, 58.8+/-2.9%) and average concentric force (control, 71.6+/-0.7%; trained, 65.5+/-2.8%) occurred from fatigue. Recovery for 5 min returned isometric and average concentric force to 61.7+/-2.2% and 65.1+/-2.5% of initial values for controls and 76.9+/-2.2% and 77.1+/-2.2% after training. After recovery for 30 min, these forces were 87.6+/-0.7% and 89.2+/-1.1% of initial values for controls and recovered almost completely (94.2+/-1.3% and 94.6+/-1.6%) in trained muscles. During fatigue, the decline in force during successive concentric contractions was larger after training (from 19.7+/-1.1% to 50.1+/-2.0%; controls, from 19.9+/-2.0% to 41.7+/-1.4%). Recovery of this decline in force was training-independent and complete within 5 min. CONCLUSIONS: Rat plantar flexor muscles adapt to 6 wk of 5 d x wk(-1) resistance training with: 1) increased isometric force, 2) smaller losses in isometric and average concentric force during fatigue, 3) larger force decline during concentric contractions during fatigue, and 4) improved recovery following fatigue. Different mechanisms might account for the recovery of the average concentric force and the decline in force during concentric contractions.     

膝关节屈肌和伸肌等速向心、等速离心及等长测试的研究

本文通过运用Ｃｙｂｅｘ－６０００型等速测力系统对２５名健康男青年膝屈肌和伸肌进行等速向心、等速离心及等长测试，来比较膝肌肉向心收缩、离心收缩和等长收缩的功能，并探讨其相互间的关系。结果表明：等速测试时，膝屈肌和伸肌离心收缩的肌力明显大于向心收缩。在膝屈曲６０°时，离心收缩和等长收缩的肌力同样明显大于向心收缩，但离心收缩与等长收缩相比无显著性差异。随运动速度增加，屈肌和伸肌向心收缩的肌力明显下降，而离心收缩的肌力无明显变化。     

Prior muscular exercise affects cycling pattern

The aim of this study was to investigate the effect of concentric or eccentric fatiguing exercise on cycling pattern. Eleven well trained cyclists completed three sessions of cycling (control cycling test [CTRL], cycling following concentric [CC] or eccentric [ECC] knee contractions) at a mean power of 276.8 +/- 26.6 Watts. Concentric and eccentric knee contractions were performed at a load corresponding to 80 % of one repetition maximum with both legs. Before and after CTRL, CC or ECC knee contractions and after cycling, a maximal voluntary contraction (MVC) test was performed. Cardiorespiratory, mechanical and electromyographic activity (EMG) of the rectus femoris, vastus lateralis and biceps femoris muscles were recorded during cycling. A significant decrease in MVC values was observed after CC and ECC exercises and after the cycling. ECC exercise induced a significant decrease in EMG root mean square during MVC and a decrease in pedal rate during cycling. EMG values of the three muscles were significantly higher during cycling exercise following CC exercise when compared to CTRL. The main finding of this study was that a prior ECC exercise induces a greater neuromuscular fatigue than a CC exercise, and changes in cycling pattern.