Neural Adjustments during Repeated Braking and Throttle Actions on a Motorcycle Setup.

The aim of the study was to assess neuromuscular changes during an intermittent fatiguing task designed to replicate fundamental actions and ergonomics of road race motorcycling. Twenty-eight participants repeated a sequence of submaximal brake-pulling and gas throttle actions, interspaced by one maximal brake-pulling, until failure. During the submaximal brake-pulling actions performed at 30% MVC, force fluctuations, surface EMG, maximal M-wave (Mmax) and H-reflex were measured in the flexor digitorum superficialis. At the end of the task, the MVC force and associated EMG activity decreased (P<0.001) by 46% and 26%, respectively. During the task, force fluctuation and EMG activity increased gradually (106% and 61%, respectively) with respect to the pre-fatigue state (P≤0.029). The Mmax first phase did not change (P≥0.524), whereas the H-reflex amplitude, normalized to Mmax, increased (149%; P≤0.039). Noteworthy, the relative increase in H-reflex amplitude was correlated with the increase in EMG activity during the task (r=0.63; P<0.001). During the 10-min recovery, MVC force and EMG activity remained depressed (P≤0.05) whereas H-reflex amplitude and force fluctuation returned to pre-fatigue values. In conclusion, contrarily to other studies, our results bring forward that when mimicking motorcycling brake-pulling and gas throttle actions, supraspinal neural mechanisms primarily limit the duration of the performance.

Plyometric Exercises: Optimizing the Transfer of Training Gains to Sport Performance.

Rapid force production and its transmission to the skeleton are important factors in movements that involve the stretch-shortening cycle. Plyometric exercises are known to augment this cycle and thereby improve the neuromechanical function of the muscle. However, the training exercises that maximize translation of these gains to sports performance are not well defined. We discuss ways to improve this transfer.

Age-related changes in sensory and motor components of the Hoffmann-reflex pathway of the flexor carpi radialis.

Ageing is accompanied by numerous changes within the sensory and motor components of the muscle spindle pathway. To further document these age-related changes, this study compared the characteristics of the Hoffmann (H) reflex and M wave, evoked with several pulse durations, between young and old adults. The H-reflex and M-wave recruitment curves were recorded at rest in the flexor carpi radialis of 12 young (21-36 years) and 12 older adults (62-80 years). For each pulse duration (0.05, 0.2 and 1 ms), the maximal M-wave (MMAX ) and H-reflex (HMAX ) amplitude, the M-wave amplitude associated with HMAX (MHmax ) and the H-reflex amplitude for a stimulus intensity evoking an M-wave of 5% MMAX (HM5% ) were measured. The strength-duration time constant and response threshold were estimated from the charge/stimulus-duration relation for the H reflex and M wave. Results indicate that varying pulse duration mainly induces a similar effect on H-reflex and M-wave recruitment curves between young and older adults. Regardless of pulse duration, old adults had lesser HMAX (p = 0.029) and HM5% (p < 0.001) but greater MHmax (p < 0.001). The H-reflex and M-wave response thresholds were greater in old than young adults (p = 0.003), but the strength-duration time constant was lesser in old than young adults for the H reflex (p = 0.048) but not the M wave (p = 0.21). These results suggest greater age-related changes in the sensory than the motor component of the H-reflex pathway, which may be indicative of a greater loss of sensory than motor axons or alterations of synapses between Ia afferents and motor neurones.

Revisiting the use of Hoffmann reflex in motor control research on humans.

Research in movement science aims at unravelling mechanisms and designing methods for restoring and maximizing human functional capacity, and many techniques provide access to neural adjustments (acute changes) or long-term adaptations (chronic changes) underlying changes in movement capabilities. First described by Paul Hoffmann over a century ago, when an electrical stimulus is applied to a peripheral nerve, this causes action potentials in afferent axons, primarily the Ia afferents of the muscle spindles, which recruit homonymous motor neurons, thereby causing an electromyographic response known as the Hoffmann (H) reflex. This technique is a valuable tool in the study of the neuromuscular function in humans and has provided relevant information in the neural control of movement. The large use of the H reflex in motor control research on humans relies in part to its relative simplicity. However, such simplicity masks subtleties that require rigorous experimental protocols and careful data interpretation. After highlighting basic properties and methodological aspects that should be considered for the correct use of the H-reflex technique, this brief narrative review discusses the purpose of the H reflex and emphasizes its use as a tool to assess the effectiveness of Ia afferents in discharging motor neurones. The review also aims to reconsider the link between H-reflex modulation and Ia presynaptic inhibition, the use of the H-reflex technique in motor control studies, and the effects of ageing. These aspects are summarized as recommendations for the use of the H reflex in motor control research on humans.

Changes of agonist and synergist muscles activity during a sustained submaximal brake-pulling gesture.

We analyzed the time course of changes in muscle activity of the prime mover and synergist muscles during a sustained brake-pulling action and investigated the relationship between muscle activity and braking force fluctuation (FF). Thirty-two participants performed a continuous fatiguing protocol (CFP) at 30% of maximal voluntary contraction (MVC) until failure. Surface electromyography was used to analyze root mean square (RMS) values in the flexor digitorum superficialis (FD), flexor carpi radialis (FC), extensor digitorum communis (ED), extensor carpi radialis (EC), brachioradialis (BR), biceps brachii (BB), and triceps brachii (TB). The FF and RMS in all muscles increased progressively (P<0.01) during the CFP, with sharp increments at time limit particularly in FD and FC (P<0.001). The RMS of the FD and FC were comparable to the baseline MVC values at time limit, in comparison to the other muscles that did not reach such levels of activity (P<0.003). The three flexor/extensor ratios used to measure coactivation levels decreased significantly (P<0.001). In contrast to RMS, MVC was still depressed at the minute 10 of recovery. The results suggest that the time limit was mainly constrained by fatigue-related mechanisms of the FD and FC but not by those of other synergist and antagonist muscles.

Effects of postactivation potentiation on mechanical output and muscle architecture during electrically induced contractions in plantar flexors.

This study investigated the effects of postactivation potentiation (PAP) on the force output and muscle architecture in plantar flexor muscles. The mechanical response to a single electrical stimulus (twitch) and to two (doublet) and three (triplet) stimuli (10-ms interpulse interval) was measured before and after a 6-s maximal voluntary contraction (MVC). Ultrasound imaging was used to measure fascicle length and pennation angle of the gastrocnemius medialis at rest and during the electrically induced contractions. Immediately after the conditioning MVC, twitch peak force (+40%) and its maximal rate of force development (+57%) and relaxation (+62%) were greater than before the MVC (P < 0.001). The PAP extent was less for the doublet than for the twitch and for the triplet than for the doublet (P < 0.05). Whereas none of the architectural parameters changed at rest, fascicle shortening and increase in pennation angle during contractions were greater after than before the conditioning MVC, with a greater extent (P < 0.001) during the twitch (+28% and +58%, respectively) compared with the doublet (+16% and +36%) and the triplet (+12% and +14%). Overall, our results indicate that the effect of the conditioning MVC on mechanical output and muscle architecture decreased from the twitch to the triplet in plantar flexor muscles. The decreased PAP observed during doublet and triplet compared with twitch indicates that the benefit of this mechanism to the enhancement of muscle performance becomes progressively less effective during successive muscle activation.NEW & NOTEWORTHY Postactivation potentiation (PAP), a form of activity-dependent potentiation, is defined as the acute enhancement of muscle contractile properties induced by a conditioning contraction, usually a maximal voluntary contraction. The purpose of our study was to investigate the effects of PAP on force production and muscle architecture in plantar flexor (PF) muscles. The results indicate that the extent of PAP decreased from the twitch to the triplet in PF muscles. Furthermore, this study shows that fascicle shortening and the increase in pennation angle were greater after the maximal voluntary contraction for the twitch, the doublet, and the triplet, with more pronounced effects for the twitch.

Effects of tendon vibration and age on force reproduction task performed with wrist flexors.

The sense of force is suggested to rely in part on proprioceptive inputs when assessed with a force reproduction task. The age-related alterations in proprioceptive system could, therefore, alter the sense of force. This study investigated the effects of tendon vibration on a force reproduction task performed with the wrist flexors in 18 young (20-40 year) and 18 older adults (60-90 year). Participants matched a target force (5% or 20% of their maximal force) with visual feedback of the force produced (target phase), and reproduced the target force without visual feedback (reproduction phase) after a 5-s rest period with or without vibration. The force reproduction error was expressed as the ratio between the force produced during the reproduction and the target phases. For the trials with vibration, the error was expressed as the ratio between the force produced during the reproduction phase performed with and without vibration. Tactile acuity was assessed with a two-point discrimination test. The error was greater at 5% than at 20% contraction intensity (p < 0.001), and in older [56.5 (32.2)%; mean (SD)] than in young adults [33.5 (13.6)%] at 5% (p = 0.002) but not 20% target (p = 0.46). Tendon vibration had a greater effect at 5% than 20% contraction intensity, and in older [41.7 (32.4)%, p < 0.001] than young adults [20.0 (16.1)%]. Tactile acuity was lesser in older than young adults (p < 0.001). The results support the contribution of proprioception in the sense of force, and highlight a decrease in performance with ageing restricted to low-force contractions.

Distribution of motor unit properties across human muscles.

The purpose of our review was to compare the distribution of motor unit properties across human muscles of different sizes and recruitment ranges. Although motor units can be distinguished based on several different attributes, we focused on four key parameters that have a significant influence on the force produced by muscle during voluntary contractions: the number of motor units, average innervation number, the distributions of contractile characteristics, and discharge rates within motor unit pools. Despite relatively few publications on this topic, current data indicate that the most influential factor in the distribution of these motor unit properties between muscles is innervation number. Nonetheless, despite a fivefold difference in innervation number between a hand muscle (first dorsal interosseus) and a lower leg muscle (tibialis anterior), the general organization of their motor unit pools, and the range of discharge rates appear to be relatively similar. These observations provide foundational knowledge for studies on the control of movement and the changes that occur with aging and neurological disorders.

Distinguishing between Fatigue and Fatigability in Multiple Sclerosis.

Fatigue is one of the most common debilitating symptoms reported by persons with multiple sclerosis (MS). It reflects feelings of tiredness, lack of energy, low motivation, and difficulty in concentrating. It can be measured at a specific instant in time as a perception that arises from interoceptive networks involved in the regulation of homeostasis. Such ratings indicate the state level of fatigue and likely reflect an inability to correct deviations from a balanced homeostatic state. In contrast, the trait level of fatigue is quantified in terms of work capacity (fatigability), which can be either estimated (perceived fatigability) or measured (objective fatigability). Clinically, fatigue is most often quantified with questionnaires that require respondents to estimate their past capacity to perform several cognitive, physical, and psychosocial tasks. These retrospective estimates provide a measure of perceived fatigability. In contrast, the change in an outcome variable during the actual performance of a task provides an objective measure of fatigability. Perceived and objective fatigability do not assess the same underlying construct. Persons with MS who report elevated trait levels of fatigue exhibit deficits in interoceptive networks (insula and dorsal anterior cingulate cortex), including increased functional connectivity during challenging tasks. The state and trait levels of fatigue reported by an individual can be modulated by reward and pain pathways. Understanding the distinction between fatigue and fatigability is critical for the development of effective strategies to reduce the burden of the symptom for individuals with MS.

Forearm muscles fatigue induced by repetitive braking on a motorcycle is best discriminated by specific kinetic parameters.

Maneuvering a motorcycle in racing conditions or for prolonged time is sufficiently demanding that on many occasions forearm muscles reach a state of functional failure when riders cannot properly brake or operate the throttle. This study intends to discriminate which ones of the several dynamometric parameters used in the literature to characterize the Force-time (F-t) curve during voluntary contractions are more sensitive to neuromuscular fatigue in simulated motorcycle-riding conditions. Thirty-three adults performed an intermittent fatiguing protocol (IFP) that simulated the brake-pulling and throttle-twisting actions, by using a hydraulic system equipped with a pressure sensor. Sixty pressure-time (P-t) curve parameters, including the rate of pressure development (RPD) and area under the curve were measured to characterize the time course of the braking maximal voluntary contraction (MVC). Two types of variables were used to analyze the P-t curve: 1) Times interval (from 0 to 30-50-100-500-1000 and 2000 ms); 2) Percentages of MVC (10-30-60-90%MVC). Overall significant (p ≤ 0.05) fatigue-related declines were observed only at time intervals longer than 100 ms and contraction intensities higher than 30%MVC. Strong and significant linear declines (p < 0.001) were observed at 500 ms and 1 s for normalized pressures, as well as for the ratio RPD60%MVC/MVC (p < 0.003) throughout the IFP. Our results suggest considering RPD at time windows of 0-500 ms and 0-1 s, and contraction intensities comprised between 30% and 60% of MVC, as more suitable criteria to study fatigue-related decrements in performance rather than the classical MVC force.

Changes in corticospinal excitability during the preparation phase of ballistic and ramp contractions.

KEY POINTS: Changes in corticospinal excitability prior to a contraction may depend on its characteristics, including the rate of torque development. This study compared the specific modulation of cortical and spinal excitability during the preparation phase (last 500 ms before contraction) of fast (ballistic) and ramp contractions of ankle dorsiflexors, using transcranial magnetic stimulation and peripheral nerve stimulation. The results indicate earlier changes at the cortical than at the spinal level during the preparation phase of both contraction types. However, these adjustments are delayed prior to ballistic relative to ramp contractions. This study suggests that the time course of change in cortical and spinal excitability during the preparation phase of a voluntary action is specific to the intended rate of torque development of the upcoming contraction. ABSTRACT: The present study investigated cortical and spinal excitability during the preparation phase of ballistic (BAL) and ramp (RAMP) isometric contractions. To this end, young adults performed BAL and RAMP (1500 ms torque rise time) contractions, reaching a similar torque level, with the ankle dorsiflexor muscles. Transcranial magnetic stimulation of the motor cortex was randomly applied to record motor evoked potentials (MEP) in the tibialis anterior during the last 500 ms preceding the contraction (n = 16). Short-interval intracortical inhibition (SICI; n = 10) and spinal motor neurone excitability (F-wave occurrence; n = 8) were also assessed during this period. Data were averaged over 100 ms time windows beginning 500 ms prior to the onset of contractions. An increase in MEP amplitude and a decrease in SICI were observed from the 200-100 ms and 300-200 ms time windows prior to BAL and RAMP contractions (P < 0.05), respectively, with greater changes prior to RAMP than to BAL within the 300-200 ms time window (P < 0.05). F-wave occurrence, used to assess spinal motor neurone excitability, increased prior to RAMP (200-100 ms, P < 0.05) but not BAL contractions. Data obtained in a few participants during the last 100 ms confirmed a delayed and steeper rise in corticospinal excitability prior to BAL contractions. These results indicate earlier changes at the cortical than at the spinal level, with delayed changes prior to BAL contractions. This study suggests that the time course of change in cortical and spinal excitability during the preparation phase of a voluntary action is specific to the intended rate of torque development of the upcoming contraction.

Strength Training: In Search of Optimal Strategies to Maximize Neuromuscular Performance.

Training with low-load exercise performed under blood flow restriction can augment muscle hypertrophy and maximal strength to a similar extent as the classical high-load strength training method. However, the blood flow restriction method elicits only minor neural adaptations. In an attempt to maximize training-related gains, we propose using other protocols that combine high voluntary activation, mechanical tension, and metabolic stress.

Comparison of Plyometric Training With Two Different Jumping Techniques on Achilles Tendon Properties and Jump Performances.

Laurent, C, Baudry, S, and Duchateau, J. Comparison of plyometric training with two different jumping techniques on Achilles tendon properties and jump performances. J Strength Cond Res 34(6): 1503-1510, 2020-This study compared the influence of 10 weeks of plyometric training with 2 different jumping techniques on Achilles tendon properties and the height achieved in drop jumps (from 20, 40, and 60 cm) and countermovement jumps (CMJ). Subjects were allocated to 2 training groups (n = 11 in each group) and 1 control group (CON, n = 10). One training group kept the knees extended (KE) during ground contact, whereas the other training group flexed the knees to ∼80-90° (KF). Achilles tendon stiffness was assessed with ultrasonography, and jump performance was derived from force platform recording. Training increased jump height (p < 0.01) in both groups. The increase for the 20-cm drop jump was greater (p < 0.05) for the KE group (11.3%) thanfor the KF group (6.3%), with no statistical difference between groups for the 40- and 60-cm drop jumps. Contact time during the 20-cm drop jump decreased (∼8%; p < 0.01) after training, with no difference between the training groups. The increase in CMJ height was greater (p = 0.05) for the KF group (17.5%) than for the KE group (11.8%). Achilles tendon stiffness increased (32%; p < 0.001) for the KE group but not for the KF group (11%; p = 0.28). There was a positive association (p < 0.001) between the changes in tendon stiffness and jump height for 20-cm drop jump in both KE group (r = 0.49) and KF group (r = 0.62). None of these parameters changed in CON group. In conclusion, the extent of increase in jump height (20-cm drop jump and CMJ) and in Achilles tendon stiffness after training differed between the 2 jumping techniques.

Aftereffects of prolonged Achilles tendon vibration on postural control are reduced in older adults.

AIM: To assess the change in the contribution of proprioceptive signal from leg muscles in postural control with ageing. METHODS: Fifteen young (~23 yr) and fifteen older adults (~68 yr) participated in Experiment 1, which consisted of recording the mean position of the centre of pressure (CoP), CoP path length, CoP velocity, and the amplitude of the Hoffmann (H) reflex and maximal M wave (MMAX) in the soleus muscle during upright standing, before and after 1 h of bilateral Achilles tendon vibration applied in seated posture. Eight young (~24 yr) and eight older adults (~67 yr) participated in Experiment 2 consisting of recording H-reflex and MMAX in seated posture before and after the 1-h vibration procedure used in Experiment 1. RESULTS: Immediately after the 1-h vibration, the mean CoP position shifted forward in both groups (p < 0.05), with a greater magnitude of change (% pre-vibration) in young [mean(SD); 74(41)%] than older adults [44(40)%; p < 0.05]. The CoP path length and velocity only increased in young adults after vibration (p < 0.05). The H-reflex amplitude decreased only in young adults after vibration [before: 35(12); after: 16(13)% Mmax, p < 0.05] during upright standing (Experiment 1), whereas it decreased similarly (p > 0.05) in young [before: 47(12)% Mmax; after: 28(17)% Mmax] and older adults [before: 34(13)% Mmax; after: 21(14)% Mmax] in seated posture (Experiment 2). CONCLUSION: Prolonged Achilles tendon vibrations lead to lesser postural perturbation in older than in young adults, supporting the assumption of a decreased reliance on leg muscle proprioception in postural control with ageing.

Electrical Stimulation of Muscle: Electrophysiology and Rehabilitation.

The generation of action potentials in intramuscular motor and sensory axons in response to an imposed external current source can evoke muscle contractions and elicit widespread responses throughout the nervous system that impact sensorimotor function. The benefits experienced by individuals exposed to several weeks of treatment with electrical stimulation of muscle suggest that the underlying adaptations involve several physiological systems, but little is known about the specific changes elicited by such interventions.

Efficacy of a new strength training design: the 3/7 method.

AIM: This study investigated the efficacy of a new strength training method on strength gain, hypertrophy, and neuromuscular fatigability. METHODS: The training exercise consisted of elbow flexion against a load of ~ 70% of one repetition maximal (1RM). A new method (3/7 method) consisting of five sets of an increasing number of repetitions (3 to 7) during successive sets and brief inter-set intervals (15 s) was repeated two times after 150 s of recovery and compared to a method consisting of eight sets of six repetitions with an inter-set interval of 150 s (8 × 6 method). Subjects trained two times per week during 12 weeks. Strength gain [1RM load and maximal isometric voluntary contraction (MVC)], EMG activity of biceps brachii and brachioradialis, as well as biceps' brachii thickness were measured. Change in neuromuscular fatigability was assessed as the maximal number of repetitions performed at 70% of 1RM before and after training. RESULTS: Both 3/7 and 8 × 6 methods increased 1RM load (22.2 ± 7.4 and 12.1 ± 6.6%, respectively; p < 0.05) and MVC force (15.7 ± 8.2 and 9.5 ± 9.5%; p < 0.05) with a greater 1RM gain (p < 0.05) for the 3/7 method. Normalized (%Mmax) EMG activity of elbow flexors increased (p < 0.05) similarly (14.5 ± 23.2 vs. 8.1 ± 20.5%; p > 0.05) after both methods but biceps' brachii thickness increased to a greater extent (9.6 ± 3.6 vs. 5.5 ± 3.7%; p < 0.05) for the 3/7 method. Despite subjects performing more repetitions with the same absolute load after training, neuromuscular fatigability increased (p < 0.05) after the two training methods. CONCLUSION: The 3/7 method provides a better stimulus for strength gain and muscle hypertrophy than the 8 × 6 method.

Modulation of the Hoffmann reflex in soleus and medial gastrocnemius during stair ascent and descent in young and older adults.

BACKGROUND: The Hoffmann (H) reflex can provide relevant information on spinal control of leg muscles during locomotor tasks in young and older adults. RESEARCH QUESTION: Is the H reflex in the leg muscles differently modulated during stair gait in young and older adults? METHOD: The H reflex in soleus (SOL) and medial gastrocnemius (MG) (normalized to the maximal M-wave amplitude obtained during upright standing; Mmax) was recorded in 19 young and 18 older adults during upright standing, and stair ascent and descent of a 3-step staircase. RESULTS: H-reflex amplitude during upright standing was greater in young than older adults for SOL (48% vs. 26% Mmax; p = 0.001) and MG (23% vs. 14% Mmax; p = 0.02). When data were averaged across groups during stair ascent, H-reflex amplitude in SOL increased from 15% Mmax at the beginning of the stance phase to 29% Mmax at mid-stance, then decreased to be 4% Mmax in the swing phase. During stair descent, H-reflex amplitude was maximal (20% Mmax) at the beginning of the stance phase, decreased to 5% Mmax at the end of stance, and increased to 11% Mmax in the swing phase. Similar adjustments were observed for the H reflex in MG for both ascent and descent. H-reflex modulation during gait cycle (relative to upright standing) is less pronounced in older adults (p < 0.05). However, no difference was observed between subgroups of young and older adults matched for H-reflex amplitude in upright standing. In both groups, H-reflex modulation was not associated with changes in background electromyographic activity. SIGNIFICANCE: This study indicates that the H reflex is modulated within the stair gait cycle during ascent and descent. Although its magnitude was slightly reduced, the overall modulation of the H reflex is not affected in healthy older adults.

Anodal transcranial direct current stimulation does not influence the neural adjustments associated with fatiguing contractions in a hand muscle.

PURPOSE: The objective of the current study was to investigate the mechanisms responsible for the briefer time to failure of a submaximal contraction (C2) when performed 60 min after a similar contraction (C1), and the influence of anodal transcranial direct current stimulation (a-tDCS) applied over the motor cortex on these mechanisms. METHODS: In two sessions, ten adults sustained two isometric contractions (35% of maximum) to failure with the abductor pollicis brevis (APB). Before C2, either a-tDCS or sham stimulation was applied over the motor cortex. Fatigue-related changes in Hoffmann (H) and long-latency (LLR) reflexes, motor-evoked potential (MEP) induced by transcranial magnetic stimulation and associated silent period (SP), maximal motor wave (Mmax), voluntary activation (VA), electromyographic (EMG) activity and peak force (PT3) evoked by a 3 pulse-train (100 Hz) were investigated. RESULTS: The results indicate that regardless of session, the time to failure was briefer (- 13%, p < 0.05) for C2 than C1, with no a-tDCS effect. During C1, MEP amplitude, SP duration and LLR amplitude increased, H-reflex amplitude did not change, and Mmax, VA and PT3 decreased (p < 0.05). Except for EMG activity that was greater during C2 than C1 (p < 0.001), all variables were similar in C1 and C2 (p > 0.05), and recovered their initial values after the 60-min rest, except PT3. CONCLUSIONS: The results of the current study indicate that a-tDCS did not influence corticospinal excitability and time to failure of C2 when performed with the APB. These observations may reflect a peripheral origin of the briefer C2 time to failure in the APB.

The slack test does not assess maximal shortening velocity of muscle fascicles in humans.

The application of a series of extremely high accelerative motor-driven quick releases while muscles contract isometrically (i.e. slack test) has been proposed to assess unloaded velocity in human muscle. This study aimed to measure gastrocnemius medialis fascicle shortening velocity (VF) and tendinous tissue shortening velocity during motor-driven quick releases performed at various activation levels to assess the applicability of the slack test in humans. Gastrocnemius medialis peak VF and joint velocity recorded from 25 participants using high frame rate ultrasound during quick releases (at activation levels from 0% to 60% of maximal voluntary isometric torque) and during fast contractions without external load (ballistic condition) were compared. Unloaded joint velocity calculated using the slack test method increased whereas VF decreased with muscle activation level (P≤0.03). Passive and low-level quick releases elicited higher VF values (≥41.8±10.7 cm s-1) compared with the ballistic condition (36.3±8.7 cm s-1), while quick releases applied at 60% of maximal voluntary isometric torque produced the lowest VF These findings suggest that initial fascicle length, complex fascicle-tendon interactions, unloading reflex and motor-driven movement pattern strongly influence and limit the shortening velocity achieved during the slack test. Furthermore, VF elicited by quick releases is likely to reflect substantial contributions of passive processes. Therefore, the slack test is not appropriate to assess maximal muscle shortening velocity in vivo.

Acute Effect of Noradrenergic Modulation on Motor Output Adjustment in Men.

PURPOSE: To determine the role of noradrenergic modulation in the control of motor output, we compared the acute effect of reboxetine (REB), a noradrenaline reuptake inhibitor, to a placebo (PLA) on knee extensors motor performance and cortical and spinal excitability. METHODS: Eleven young men took part in two randomized experiments during which they received either 8 mg of REB or a PLA. The torque produced during a maximal voluntary contraction (MVC) and its variability (i.e., coefficient of variation) during submaximal contractions ranging from 5% to 50% MVC were measured. Paired electrical (PES) and transcranial magnetic stimulation (TMS) were used to assess changes in voluntary activation during MVC, and corticospinal (motor-evoked potential (MEP)) and spinal excitability (Hoffmann (H) reflex) during contraction at 20% MVC. RESULTS: MVC torque and torque steadiness increased respectively by 9.5% and 24% on average in REB compared with PLA condition (P < 0.001). Voluntary activation tested by TMS and PES was greater (~3%; P < 0.05) in REB than PLA condition. The increase in voluntary activation in REB condition was significantly correlated with subjects' initial voluntary activation level when tested by TMS (r = -0.62; P = 0.048) and PES (r = -0.86; P < 0.01). The maximal amplitudes of H reflex and MEP and the slope of their recruitment curves were enhanced by REB (P < 0.05). The ratio between the TMS-induced EMG silent period and the corresponding MEP (silent period/MEP) was reduced in REB condition (P < 0.01). CONCLUSIONS: The present findings indicate that voluntary activation and accuracy in force control can be increased by an enhanced level of noradrenaline concentration. This improvement in motor performance is accompanied by changes located at both cortical and spinal levels.