Time Needed to Change the Isometric Force Production of a Response

Abstract

This study investigated the time needed to change a motor program that specified the elbow flexor muscles to gradually increase the isometric force production from 15% to 75% of one's maximum voluntary contraction (MVC). A double-stimulation paradigm was used with the restriction that subjects (N = 12) be at 15% of their MVC before the presentation of the first stimulus. Subjects reacted to the first stimulus (randomly presented) by gradually increasing their isometric force from 15% to 75% of their MVC and then reacted to the second stimulus by altering the force production in one of four ways: (a) increasing the force to the 75% level rapidly instead of gradually, (b) discontinuing the increase and maintaining the level of force attained, (c) discontinuing all force production, or (d) reversing the direction of force so that it is produced by the elbow extensors. The data revealed that more time was needed to increase the force rapidly than to perform any of the other three conditions.     

Rate of Force Development in the Handgripping Muscles by Females as a Function of Fatigue Level

Abstract

Twenty-eight young, adult women were tested for maximum voluntary contraction (MVC) of the grip flexors and were then given five fatigue trials in which the task was to maintain a maximum isometric contraction until the strength level had been degraded to either 80%, 60%, or 40% MVC. At that time, the subject relaxed and within one second was commanded to generate a single contraction as rapidly and forcefully as possible. Testing was also conducted in the nonfatigued state (1.00 MVC). Forces were determined at 4, 8, 16, 32, 64, 128, 256, 512, and 1,024 msec following the initial deflection of the pen from the baseline. Normalized force values were calculated by dividing the force expressed at the various time intervals by the maximum force generated under that condition at 1,024 msec. For both absolute and normalized rate of force development, the highest rate was attained between 32–64 msec. During that time period, the rate of absolute force development was reduced 10% at the 80% MVC level, 30% at 60% MVC, and 53% at 40% MVC. There was a definite pattern of depressed rate of absolute force development up to 256 msec at the higher fatigue levels. For normalized force, fatiguing a muscle to 80% MVC had no effect except at 64 msec where a greater percentage of maximal force was attained than at 1.00 MVC. Beyond that level of fatigue, however, normalized force tended to be adversely affected as fatigue progressed. The time required to develop either 50% or 70% of maximal force was significantly longer for the 40% MVC level than for the remaining conditions. In addition, the time necessary to develop 70% of maximal force was longer for the 60% MVC fatigue level than for the 80% MVC level.     

Isometric training with maximal co-contraction instruction does not increase co-activation during exercises against external resistances

Abstract

The present investigation verified that strength is improved by a training programme consisting of repetitions of maximal isometric voluntary co-contractions without increasing co-activations during contractions against external resistances. Ten participants performed 12 training sessions (four sets of 6 × 4 second maximal isometric co-contraction of the elbow flexor and extensors, 3 days a week for 4 weeks). Surface electromyograms of triceps and biceps brachii were collected during maximal voluntary isometric elbow flexion and extension against a force transducer. Maximal voluntary isometric force increased significantly after training, by 13.8 ± 6.0% (extension) and 9.6 ± 9.5% (flexion), but the observed increases in EMG of agonist muscles during maximal voluntary contraction were not significant. No significant changes in the levels of co-activation of the elbow flexors and extensors were observed. No significant change was observed for all the parameters in a control group of ten participants. These results indicated that the strength improvements after co-contraction training occur without increases in co-activation level.     

Characteristics of the Achilles Tendon Reflex Following Isometric and Isotonic Exercise

Abstract

An investigation was conducted to determine the effects of fatiguing exercise upon the Achilles tendon reflex. Eleven subjects performed one of four exercise tasks on each of four separate days. The exercise conditions involved low-intensity isometric, high-intensity isometric, low-intensity isotonic or high-intensity isotonic exercise. The low-intensity tasks required a 25% MVC load, while the high-intensity conditions required a 50% MVC load. Results showed that low-intensity isometric exercise reduced reflex force, as well as the time needed to reach peak force, while high-intensity exercise produced an enhancement of reflex force. Half-relaxation time was faster after subjects executed low-intensity isometric exercise, but generally somewhat prolonged following high-intensity isometric exercise. Thus, the Achilles tendon reflex may be either enhanced or depressed depending upon the type and intensity of exercise performed.     

优秀乒乓球运动员肘关节等动屈伸运动时拮抗肌共激活现象研究

目的:研究优秀乒乓球运动员和普通在校大学生肘关节拮抗肌活动在等动屈伸过程中的差异。方法:以8名优秀乒乓球运动员和8名普通高校大学生为研究对象,利用Biodex等动测试仪和Noraxon表面肌电仪记录上肢肘关节等动屈伸过程中作为拮抗肌的肱二头肌和肱三头肌的力量特征和表面肌电信号特征。肘关节伸肌和屈肌分别在最大等长收缩、15°/s、30°/s、60°/s、120°/s、180°/s、240°/s条件下进行3次最大等动离心屈伸运动。以标准化的均方根振幅(RMS)和标准化的峰值力矩作为评价指标。结果:对于大学生和优秀乒乓球运动员来说,随着肘关节速度的增加,两组受试者的屈伸肌力矩都呈下降趋势,大学生表现为速度大于60°/s时伸肌力矩大于屈肌力矩(P<0.05),优秀乒乓球运动员表现为伸肌力矩低于屈肌力矩,但没有统计学差别(P>0.05)。大学生和优秀乒乓球运动员都表现为在向心收缩时不同速度下随着主动肌力矩下降,拮抗肌激活水平表现为逐渐增高,且所有线性拟合系数r2>0.7。优秀乒乓球运动员拮抗肌肱三头肌的激活水平(在MVC时:10.1%±5.2%,240°/s时:15.1%±6.6%)要显著低于普通高校大学生(MVC时:29.3%±8.8%,240°/s时:38.0%±15.1%)。而作为拮抗肌的肱二头肌激活水平在普通大学生和优秀乒乓球运动员之间没有统计学差异(P>0.05)。优秀乒乓球运动员拮抗肌/主动肌肌电活动比要显著低于普通大学生(P<0.05)。结论:与普通大学生相比,优秀乒乓球运动员肘关节拮抗肌肱三头肌的激活水平要更低,这可能是优秀乒乓球运动员对肘关节周围肌肉进行长期训练的结果。而两者拮抗肌肱二头肌激活水平没有统计学差异,其原因可能是由于两者在日常活动中肱二头肌作为拮抗肌经常为了克服地心引力受到同样的刺激造成的。     

Women show similar central and peripheral fatigue to men after half-marathon*

Women are known to be less fatigable than men in single-joint exercises, but fatigue induced by running has not been well understood. Here we investigated sex differences in central and peripheral fatigue and in rate of force development (RFD) in the knee extensors after a half-marathon run. Ten male and eight female amateur runners (aged 25–50 years) were evaluated before and immediately after a half-marathon race. Knee extensors forces were obtained under voluntary and electrically evoked isometric contractions. Maximal voluntary isometric contraction (MVC) force and peak RFD were recorded. Electrically doublet stimuli were delivered during the MVC and at rest to calculate the level of voluntary activation and the resting doublet twitch. After the race, decreases in MVC force (males: ?11%, effect size [ES] 0.52; females: ?11% ES 0.33), voluntary activation (males: ?6%, ES 0.87; females: ?4%, ES 0.72), and resting doublet twitch (males: ?6%, ES 0.34; females: ?8%, ES 0.30) were found to be similar between males and females. The decrease in peak RFD was found to be similar between males and females (males: ?14%, ES 0.43; females: ?15%, ES 0.14). Half-marathon run induced both central and peripheral fatigue, without any difference between men and women. The maximal and explosive strength loss was found similar between sexes. Together, these findings do not support the need of sex-specific training interventions to increase the tolerance to neuromuscular fatigue in half-marathoners.     

主动肌和拮抗肌功能转变瞬间的肌电平均功率频率变化

目的:通过对主动肌和拮抗肌功能转变瞬间肌电平均功率频率的观察,探讨中枢控制因素在决定MPF变化中的独立性作用.方法:10名健康男性志愿者在完成60%MVC强度的疲劳性等长屈肘运动负荷试验后,瞬间转变肌肉工作性质,完成60%MVC强度的疲劳性等长伸肘运动,分别记录肱二头肌和肱三头肌的sEMG信号,观察两次疲劳试验以及瞬间转变过程中肌电MPF等sENG指标变化.结果:等长屈肘和等长伸肘快速转变的瞬间,主动肌和拮抗肌MPF等各项sEMG信号分析指标均发生了显著性改变,表现出明显的跃变现象;无论是转变之前,还是转变之后的等长疲劳试验过程中,主动肌与拮抗肌的MPF、C(n)和%DEF均表现出类似的变化规律,而拮抗肌AEMG则维持相对恒定.结论:中枢运动控制是导致疲劳过程中MPF单调递减变化的独立性作用因素,其可能的作用机制是CNS将主动肌与拮抗肌作为一组控制肌群实施同步控制.     

运动性肌肉疲劳过程中主动肌与拮抗肌sEMG相干性分析

目的:通过静态疲劳负荷实验过程中主动肌与拮抗肌sEMG的相干性分析探索运动性肌肉疲劳过程中中枢神经系统对主动肌与拮抗肌共神经输入(common neural inputs)同步支配的变化特征。方法:以10名健康男性青年志愿者为研究对象,记录受试者以50%MVC负荷强度静态屈肘运动诱发肌肉疲劳过程中主动肌肱二头肌与拮抗肌肱三头肌的sEMG,为考查疲劳因素对相干性分析结果的影响作用,对记录的sEMG按运动持续时间平均分为两段,分别对两段sEMG进行相干性分析处理。结果:在疲劳负荷实验过程中,肱二头肌与肱三头肌MF指标随运动持续时间表现出显著性的单调递减变化趋势。从相干性分析结果看,在beta频段和gamma频段内,运动后半段肱二头肌与肱三头肌sEMG相干函数值要明显小于运动前半段,而tremor频段内无显著性差异。结论:在50%MVC静态负荷屈肘运动致肌肉疲劳过程中,随着负荷运动持续时间的增加,中枢神经系统对主动肌肱二头肌与拮抗肌肱三头肌共神经输入同步支配下降,这可能是由于主动肌与拮抗肌脊髓运动神经元兴奋性及运动皮层对脊髓运动神经元激活能力随疲劳发展的下降不同步性及为维持既定的收缩负荷,中枢神经系统对主动肌与拮抗肌运动单位的募集策略采取不同的调节方式造成的。     

Knee extension fatigue attenuates repeated force production of the elbow flexors

Abstract

Non-local muscle fatigue has been demonstrated with unilateral activities, where fatiguing one limb alters opposite limb forces. Fewer studies have examined if non-local fatigue occurs with unrelated muscles. The purpose of this study was to investigate if knee extensors fatigue alters elbow flexors force and electromyography (EMG) activity. Eighteen males completed a control and fatiguing session (randomised). Blood lactate was initially sampled followed by three maximal voluntary contractions (MVC) with the elbow flexors and two with the knee extensors. Thereafter, subjects either sat (control) or performed five sets of bilateral dynamic knee extensions to exhaustion using a load equal to the dominant limb MVC (1-min rest between sets). Immediately afterwards, subjects were assessed for blood lactate and unilateral knee extensors MVC, and after 1 min performed a single unilateral elbow flexor MVC. Two minutes later, subjects performed 12 unilateral elbow flexor MVCs (5 s contraction/10 s rest) followed by a third blood lactate test. Compared to control, knee extensor force dropped by 35% (p < 0.001; ES = 1.6) and blood lactate increased by 18% (p < 0.001; ES = 2.8). Elbow flexor forces were lower after the fatiguing protocol only during the last five MVCs (p < 0.05; ES = ～0.58; ～5%). No changes occurred between conditions in EMG. Elbow flexor forces significantly decreased after knee extensors fatigue. The effect was revealed during the later stages of the repeated MVCs protocol, demonstrating that non-local fatigue may have a stronger effect on repeated rather than on single attempts of maximal force production.     

Baumgartner's Modified Pull-Up Test For Male and Female Elementary School Aged Children

Abstract

Nineteen college men and women (aged 18–23 yrs, × = 21.1) were studied to ascertain the force-time components of a rapid maximal voluntary isometric contraction (MVC) for ankle dorsiflexors, knee extensors, elbow flexors, wrist flexors and hand grip. Standardized isometric strength testing protocol was used. After a practice period subjects were instructed to make an MVC without jerking and as quickly as possible, for each of the muscle actions noted above. Force readings were taken from a load cell assembly through an analogue-to-digital converter and analyzed to yield time values for MVC, 3/4 MVC, 1/2 MVC, and 1/4 MVC. The results indicated significant differences (p < .01) between the responses of the men and women, with the women reaching full MVC more rapidly than the men. Also, there were significant differences among the five muscle groups tested, with the wrist flexor muscle group developing MVC most quickly, while the knee extensors took the longest time to full MVC. Based upon these data it may be concluded that the time to full MVC differs between men and women, and also that the time to full MVC differs among the five muscle groups tested in this experiment.     

Do aerobic characteristics explain isometric exercise-induced neuromuscular fatigue and recovery in upper and lower limbs?

This study investigated the relationships between aerobic characteristics and (i) neuromuscular fatigue induced by 2-min sustained isometric maximal voluntary contractions (MVC) and (ii) subsequent recovery, in the upper and lower limbs. In a pseudo-randomized order, eleven healthy males completed four sessions on different days: maximal incremental cycling test (100 W + 40 W every 2 min); maximal arm-cranking test (50 W + 20 W every 2 min); and 2-min sustained isometric MVCs of the knee extensors (KE) and elbow flexors (EF). Neuromuscular assessment was performed with transcranial magnetic and peripheral nerve stimulation to evaluate central and peripheral neuromuscular factors of fatigue and the subsequent recovery. Peak oxygen uptake, gas exchange threshold and the corresponding power outputs were correlated with recovery of voluntary force after the 2-min KE MVC. Regression analysis showed that power output at the gas exchange threshold alone explained 72% of the variability in ?recovery of KE voluntary force. No relationships with fatigue or recovery in EF were observed. These results suggest that participants with greater aerobic capacities experience the same amount of fatigue and faster recovery of voluntary force in KE but not EF. The potential reasons behind the relationship in KE but not EF are discussed.     

Changes in muscle activity with increasing running speed

Electromyographic (EMG) activity of the leg muscles and the ground reaction forces were recorded in 17 elite male middle-distance runners, who performed isometric maximal voluntary contractions (MVC) as well as running at different speeds. Electromyograms were recorded from the gluteus maximus, vastus lateralis, biceps femoris, gastrocnemius and tibialis anterior. The results indicated that the averaged EMG (aEMG) activities of all the muscles studied increased (P < 0.05) with increasing running speed, especially in the pre-contact and braking phases. At higher speeds, the aEMG activities of the gastrocnemius, vastus lateralis, biceps femoris and gluteus maximus exceeded 100% MVC in these same phases. These results suggest that maximal voluntary contractions cannot be used as an indicator of the full activation potential of human skeletal muscle. Furthermore, the present results suggest that increased pre-contact EMG potentiates the functional role of stretch reflexes, which subsequently increases tendomuscular stiffness and enhances force production in the braking and/or propulsive phases in running. Furthermore, a more powerful force production in the optimal direction for increasing running speed effectively requires increased EMG activity of the two-joint muscles (biceps femoris, rectus femoris and gastrocnemius) during the entire running cycle.     

Changes in muscle activity with increasing running speed

Electromyographic (EMG) activity of the leg muscles and the ground reaction forces were recorded in 17 elite male middle-distance runners, who performed isometric maximal voluntary contractions (MVC) as well as running at different speeds. Electromyograms were recorded from the gluteus maximus, vastus lateralis, biceps femoris, gastrocnemius and tibialis anterior. The results indicated that the averaged EMG (aEMG) activities of all the muscles studied increased (P?<?0.05) with increasing running speed, especially in the pre-contact and braking phases. At higher speeds, the aEMG activities of the gastrocnemius, vastus lateralis, biceps femoris and gluteus maximus exceeded 100% MVC in these same phases. These results suggest that maximal voluntary contractions cannot be used as an indicator of the full activation potential of human skeletal muscle. Furthermore, the present results suggest that increased pre-contact EMG potentiates the functional role of stretch reflexes, which subsequently increases tendomuscular stiffness and enhances force production in the braking and/or propulsive phases in running. Furthermore, a more powerful force production in the optimal direction for increasing running speed effectively requires increased EMG activity of the two-joint muscles (biceps femoris, rectus femoris and gastrocnemius) during the entire running cycle.     

Differences in activation patterns between eccentric and concentric quadriceps contractions.

Previous studies analysing electromyograms (EMGs) from indwelling electrodes have indicated that fast-twitch motor units are selectively recruited for low-intensity eccentric contractions. The aim of this study was to compare the frequency content of surface EMGs from quadriceps muscles during eccentric and concentric contractions at various contraction intensities. Electromyograms were recorded from the rectus femoris, vastus lateralis and vastus medialis muscles of 10 men during isokinetic (1.05 rad x s(-1)) eccentric and concentric knee extension contractions at 25%, 50%, 75% and 100% of maximal voluntary contraction (MVC) for each contraction mode. Additionally, isometric contractions (70 degrees) were performed at each intensity. The mean frequency and root mean square (RMS) of the surface EMG were computed. Mean frequency was higher for eccentric than concentric contractions at 25% (P < 0.01), 50% (P < 0.01) and 75% (P < 0.05) but not at 100% MVC. It increased with increasing contraction intensity for isometric (P < 0.001) and concentric (P < 0.01) contractions but not for eccentric contractions (P = 0.27). The EMG amplitude (RMS) increased with increasing contraction intensity similarly in each contraction mode (P < 0.0001). Higher mean frequencies for eccentric than concentric contractions at submaximal contraction intensities is consistent with more fast-twitch motor units being active during eccentric contractions.     

Angle-specific impairment of elbow flexors strength after isometric exercise at long muscle length

In this study, we examined the long-term reductions in maximal isometric force (MIF) caused by a protocol of repeated maximal isometric contractions at long muscle length. Furthermore, we wished to ascertain whether the reductions in MIF are dependent on muscle length--that is, are the reductions in MIF more pronounced when the muscle contracts at a short length. The MIF of the elbow flexors of seven young male volunteers was measured at five different elbow angles between 50 degrees and 160 degrees. On a separate day, the participants performed 50 maximal voluntary isometric muscle contractions with the elbow flexors at a lengthened position; that is, with the shoulder hyperextended at 45 degrees and the elbow joint fixed at 140 degrees. Following this exercise, the MIF at the five elbow angles, range of motion, muscle soreness and plasma creatine kinase activity were measured at 24 h intervals for 4 days. On day 1, the decline in MIF was higher at the more acute elbow angles of 50 degrees (42 +/- 8%) and 70 degrees (39 +/- 8%; both P<0.01) than at 90 degrees (26 +/- 4%) and 140 degrees (16 +/- 3%; both P<0.01). No significant reduction in MIF was evident at an elbow angle of 160 degrees. Maximal isometric force at an elbow angle of 140 degrees was fully restored on day 3, whereas at an angle of 50 degrees it remained depressed for the 4 day observation period. Restoration of MIF was a function of the elbow angle, with force recovery being less at the smaller angles. The range of motion was decreased by 14 +/- 2 degrees on day 1 (P<0.01) and did not return to baseline values by day 4. Muscle soreness ratings remained significantly elevated for the 4 day period. Serum creatine kinase peaked on day 1 (522 +/- 129 IU, P<0.01) and decreased thereafter. We conclude that the disproportionate decrease in MIF at the small elbow angles and the length-specific recovery in MIF after repeated maximal isometric contractions at long muscle length may be explained by the presence of overstretched sarcomeres that increased in series compliance of the muscle, therefore causing a rightward shift of the force-length relationship.     

Eccentric exercise-induced muscle weakness abolishes sex differences in fatigability during sustained submaximal isometric contractions

BackgroundFemales are typically less fatigable than males during sustained isometric contractions at lower isometric contraction intensities. This sex difference in fatigability becomes more variable during higher intensity isometric and dynamic contractions. While less fatiguing than isometric or concentric contractions, eccentric contractions induce greater and longer lasting impairments in force production. However, it is not clear how muscle weakness influences fatigability in males and females during sustained isometric contractions.MethodsWe investigated the effects of eccentric exercise-induced muscle weakness on time to task failure (TTF) during a sustained submaximal isometric contraction in young (18–30 years) healthy males (n = 9) and females (n = 10). Participants performed a sustained isometric contraction of the dorsiflexors at 35° plantar flexion by matching a 30% maximal voluntary contraction (MVC) torque target until task failure (i.e., falling below 5% of their target torque for ≥2 s). The same sustained isometric contraction was repeated 30 min after 150 maximal eccentric contractions. Agonist and antagonist activation were assessed using surface electromyography over the tibialis anterior and soleus muscles, respectively.ResultsMales were ∼41% stronger than females. Following eccentric exercise both males and females experienced an ∼20% decline in maximal voluntary contraction torque. TTF was ∼34% longer in females than males prior to eccentric exercise-induced muscle weakness. However, following eccentric exercise-induced muscle weakness, this sex-related difference was abolished, with both groups having an ∼45% shorter TTF. Notably, there was ∼100% greater antagonist activation in the female group during the sustained isometric contraction following exercise-induced weakness as compared to the males.ConclusionThis increase in antagonist activation disadvantaged females by decreasing their TTF, resulting in a blunting of their typical fatigability advantage over males.     

Angle-specific impairment of elbow flexors strength after isometric exercise at long muscle length

Abstract

In this study, we examined the long-term reductions in maximal isometric force (MIF) caused by a protocol of repeated maximal isometric contractions at long muscle length. Furthermore, we wished to ascertain whether the reductions in MIF are dependent on muscle length — that is, are the reductions in MIF more pronounced when the muscle contracts at a short length. The MIF of the elbow flexors of seven young male volunteers was measured at five different elbow angles between 50° and 160°. On a separate day, the participants performed 50 maximal voluntary isometric muscle contractions with the elbow flexors at a lengthened positions that is, with the shoulder hyperextended at 45° and the elbow joint fixed at 140°. Following this exercise, the MIF at the five elbow angles, range of motion, muscle soreness and plasma creatine kinase activity were measured at 24 h intervals for 4 days. On day 1, the decline in MIF was higher at the more acute elbow angles of 50° (42±8%) and 70° (39±8%; both P<0.01) than at 90° (26±4%) and 140° (16±3%; both P<0.01). No significant reduction in MIF was evident at an elbow angle of 160°. Maximal isometric force at an elbow angle of 140° was fully restored on day 3, whereas at an angle of 50° it remained depressed for the 4 day observation period. Restoration of MIF was a function of the elbow angle, with force recovery being less at the smaller angles. The range of motion was decreased by 14±2° on day 1 (P<0.01) and did not return to baseline values by day 4. Muscle soreness ratings remained significantly elevated for the 4 day period. Serum creatine kinase peaked on day 1 (522±129 IU, P<0.01) and decreased thereafter. We conclude that the disproportionate decrease in MIF at the small elbow angles and the length-specific recovery in MIF after repeated maximal isometric contractions at long muscle length may be explained by the presence of overstretched sarcomeres that increased in series compliance of the muscle, therefore causing a rightward shift of the force-length relationship.     

System approach to games and competitive playing

Abstract

The force enhancement of muscle twitch contraction after a maximal voluntary contraction (MVC) has been defined as post-activation potentiation. However, the effects of post-activation potentiation on ballistic movements have not been studied extensively, or the underlying neurophysiologycal mechanism. In the current study, we examined post-activation potentiation and spinal H-reflex excitability in the soleus muscle. Mechanical power during explosive ballistic plantar flexions was measured in 14 males before and after 5 s, 4 min, and 10 min of isometric conditioning (EPF_pre, EPF_5s, EPF_4min, EPF_10min, respectively). Four sessions corresponding to four different protocols of isometric conditioning were conducted. The protocols were different in the intensity (10% vs. 100% of MVC) and duration (7 vs. 10 s) of the isometric conditioning. The results showed a significant enhancement in mechanical power in EPF_4min compared with EPF_pre, only when the isometric conditioning was performed at 100% of MVC for 10 s. No significant changes were observed in the H-related parameters (e.g amplitude, threshold, H/M ratio) after the isometric conditioning. Our results show that to obtain a post-activation potentiation during explosive ballistic movements, the intensity and duration of the isometric conditioning must be controlled. Moreover, the improvement in mechanical power is not related to spinal H-reflex excitability.     

Neuromuscular variables affecting the magnitude of force loss after eccentric exercise

The aim of this study was to examine neuromuscular variables contributing to differences in force loss after participants were exposed to the same relative bout of eccentric exercise. Thirty-six males performed 50 maximal eccentric contractions of the elbow flexors and were stratified into high responders (n = 10) and low responders (n = 10) based on force loss 36 h after exercise. Maximal voluntary isometric contractions (MVCs) and electromyography (EMG) were measured at baseline and 36 h after exercise. During eccentric exercise, mean peak torque, mean end-range torque from the final 25% of each trial and total angular impulse were computed over 25 contractions in each of two bouts. The slope of the change in these values for each 25 eccentric contractions was calculated for each participant using linear regression. At baseline, MVC was not different between groups (low responders: 97.0 +/- 9.6 N x m; high responders: 82.7 +/- 6.4 N x m; P = 0.08). High responders demonstrated a 68% (range 62-78%) reduction in MVC and low responders a 39% (29-48%) reduction after exercise. Peak torque, end-range torque and total angular impulse were 13%, 40% and 33% higher, respectively, in the low than in the high responders (peak torque: P = 0.0002; end-range torque: P < 0.0001; total angular impulse: P < 0.001). The rate of decline in peak torque slope was greater in high than in low responders (P = 0.044). In conclusion, lower peak torque, end-range torque and total angular impulse during eccentric contractions and a greater peak torque slope may identify high responders to eccentric exercise.     

Neural mechanisms of strength increase after one-week motor imagery training

The neural mechanisms explaining strength increase following mental training by motor imagery (MI) are not clearly understood. While gains are mostly attributed to cortical reorganization, the sub-cortical adaptations have never been investigated. The present study investigated the effects of MI training on muscle force capacity and the related spinal and supraspinal mechanisms. Eighteen young healthy participants (mean age: 22.5?±?2.6) took part in the experiment. They were distributed into two groups: a control group (n?=?9) and an MI training group (n?=?9). The MI group performed seven consecutive sessions (one per day) of imagined maximal isometric plantar flexion (4 blocks of 25 trials per session). The control group did not engage in any physical or mental training. Both groups were tested for the isometric maximal plantar flexion torque (MVC) and the rate of torque development (RTD) before and after the training session. In addition, soleus and medial gastrocnemius spinal and supraspinal adaptations were assessed through the recording of H-reflexes and V-waves, with electrical stimulations of the posterior tibial nerve evoked at rest and during MVC, respectively. After one week, only the MI training group increased both plantar flexion MVC and RTD. The enhancement of muscle torque capacity was accompanied by significant increase of electromyographic activity and V-wave during MVC and of H-reflex at rest. The increased cortical descending neural drive and the excitability of spinal networks at rest could explain the greater RTD and MVC after one week of MI training.