Biomedical subjects

Jacques Duchateau

Publications and source records attributed to Jacques Duchateau.

16 recordsLinked to original sources

PubMed2007

Postactivation potentiation in a human muscle: effect on the rate of torque development of tetanic and voluntary isometric contractions.

Postactivation potentiation (PAP), a mechanism by which the torque of a muscle twitch is increased following a conditioning contraction, is well documented in muscular physiology, but little is known about its effect on the maximal rate of torque development and functional significance during voluntary movements. The objective of this study was to investigate the PAP effect on the rate of isometric torque development of electrically induced and voluntary contractions. To that purpose, the electromechanical responses of the thumb adductor muscles to a single electrical stimulus (twitch), a train of 15 pulses at 250 Hz (HFT(250)), and during ballistic (i.e., rapid torque development) voluntary contractions at torque levels ranging from 10 to 75% of maximal voluntary contraction (MVC) were recorded before and after a conditioning 6-s MVC. The results showed that the rate of torque development was significantly (P < 0.001) increased after the conditioning MVC, but the effect was greater for the twitch ( approximately 200%) compared with the HFT(250) ( approximately 17%) or ballistic contractions (range: 9-24%). Although twitch potentiation was maximal immediately after the conditioning MVC, maximal potentiation for HFT(250) and ballistic contractions was delayed to 1 min after the 6-s MVC. Furthermore, the similar degree of potentiation for the rate of isometric torque development between tetanic and voluntary ballistic contractions indicates that PAP is not related to the modality of muscle activation. These observations suggest that PAP may be considered as a mechanism that can influence our contractions during daily tasks and can be utilized to improve muscle performance in explosive sports.

Adaptation, Physiological↗

PubMed2006

Specific modulation of motor unit discharge for a similar change in fascicle length during shortening and lengthening contractions in humans.

This study examines the effect of a change in fascicle length on motor unit recruitment and discharge rate in the human tibialis anterior during shortening and lengthening contractions that involved a similar change in torque. The dorsiflexor torque and the surface and intramuscular electromyograms (EMGs) from the tibialis anterior were recorded in eight subjects. The behaviour of the same motor unit (n=63) was compared during submaximal shortening and lengthening contractions performed at a constant velocity (10 deg s-1) with the dorsiflexor muscles over a 20 deg range of motion around the ankle neutral position. Muscle fascicle length was measured non-invasively using ultrasonography. Motor units that were active during a shortening contraction were always active during the subsequent lengthening contraction. Furthermore, additional motor units (n=18) of higher force threshold that were recruited during the shortening contraction to maintain the required torque were derecruited first during the following lengthening contraction. Although the change in fascicle length was linear (r2>0.99), and similar for both shortening and lengthening contractions, modulation of discharge rate differed during the two contractions. Compared with an initial isometric contraction at short (11.9+/-2.4 Hz) or long (11.7+/-2.2 Hz) muscle length, discharge rate increased only slightly and stayed nearly constant throughout the lengthening contraction (12.6+/-2.0 Hz; P<0.05) whereas it augmented progressively and more substantially during the shortening contraction, reaching 14.5+/-2.5 Hz (P<0.001) at the end of the movement. In conclusion, these observations indicate a clear difference in motor unit discharge rate modulation with no change in their recruitment order between shortening and lengthening contractions when performed with a similar change in muscle fascicle length and torque.

Adult↗

PubMed2006

Training adaptations in the behavior of human motor units.

The purpose of this brief review is to examine the neural adaptations associated with training, by focusing on the behavior of single motor units. The review synthesizes current understanding on motor unit recruitment and rate coding during voluntary contractions, briefly describes the techniques used to record motor unit activity, and then evaluates the adaptations that have been observed in motor unit activity during maximal and submaximal contractions. Relatively few studies have directly compared motor unit behavior before and after training. Although some studies suggest that the voluntary activation of muscle can increase slightly with strength training, it is not known how the discharge of motor units changes to produce this increase in activation. The evidence indicates that the increase is not attributable to changes in motor unit synchronization. It has been demonstrated, however, that training can increase both the rate of torque development and the discharge rate of motor units. Furthermore, both strength training and practice of a force-matching task can evoke adaptations in the discharge characteristics of motor units. Because the variability in discharge rate has a significant influence on the fluctuations in force during submaximal contractions, the changes produced with training can influence motor performance during activities of daily living. Little is known, however, about the relative contributions of the descending drive, afferent feedback, spinal circuitry, and motor neuron properties to the observed adaptations in motor unit activity.

Action Potentials↗

PubMed2006

Voluntary activation during maximal contraction with advancing age: a brief review.

It is well established that the loss of muscle mass (i.e. sarcopenia) is the primary factor contributing to the reduction in muscle force with ageing. Based on the observation that force declines at a faster rate than muscle mass, neural alterations are also thought to contribute to muscle weakness by reducing central drive to the agonist muscles and by increasing coactivation of the antagonist muscles. Researchers have attempted to quantify the contribution of impaired voluntary drive to the decline in muscle force using superimposed electrical stimulation during maximal voluntary contractions (MVCs) and by recording surface electromyographic (EMG) activity. Although reduced voluntary activation of agonist muscles and increased coactivation of antagonist muscles during a MVC have been reported with advancing age, such changes are not supported by all studies. These discrepancies may be explained by differences in sensitivity between the methods used to assess voluntary activation, as well as differences between the characteristics of the study population, the muscle group that is tested, and the type of contraction that is performed. The objective of this review is to summarize current knowledge regarding the activation of agonist and antagonist muscles during MVC in elderly and to try to clarify the disparities in literature concerning the influence of a possible deficit in voluntary activation on the maximal force capacity of muscles in elderly adults.

Adult↗

PubMed2006

Age-related fatigability of the ankle dorsiflexor muscles during concentric and eccentric contractions.

This study compares the fatigability of the ankle dorsiflexors during five sets of 30 maximal concentric and eccentric contractions in young and elderly adults. The torque produced by the ankle dorsiflexors and the average surface electromyogram (aEMG) of the tibialis anterior were continuously recorded. The contribution of central and peripheral mechanisms to muscle fatigue was tested before, after each set of contractions, and during a 30 min recovery period by the superimposed electrical stimulation method. The compound muscle action potential (M-wave), the mechanical response to single (twitch) and paired (doublet) stimulation, and the postactivation potentiation were also recorded. Compared with young subjects, elderly adults exhibited a greater loss of torque for concentric (50.2 vs. 40.9%; P<0.05) and eccentric (42.1 vs. 27.1%; P < 0.01) contractions. Although young subjects showed a lesser decrease in torque during the eccentric compared with concentric contractions, elderly adults experienced similar fatigability for the two types of contractions despite a comparable depression in the EMG activity of both groups and contraction types (10-20%). As tested by the interpolated-twitch method and aEMG/M-wave ratio, voluntary activation was not altered during either type of contraction or for either age group. During the two fatigue tasks, only elderly adults experienced a decrease in M-wave area (26.4-35.4%; P < 0.05). All together, our results suggest that the fatigue exhibited by both young and elderly adults during maximal concentric and eccentric contractions mainly involved peripheral alterations and that elderly adults may also have experienced a decline in neuromuscular propagation.

Action Potentials↗

PubMed2006

Neural aspects of muscle stretching.

Neural mechanisms contribute significantly to the gains that occur in the range of motion about a joint with stretching exercises. In the acute condition, lengthening of a muscle-tendon unit decreases spinal reflex excitability, which reduces passive tension and increases joint range of motion. Similarly, participation in a stretch-training program decreases tonic reflex activity and increases flexibility.

Humans↗

PubMed2005

Change in muscle fascicle length influences the recruitment and discharge rate of motor units during isometric contractions.

This study examines the effect of fascicle length change on motor-unit recruitment and discharge rate in the human tibialis anterior (TA) during isometric contractions of various intensities. The torque produced during dorsiflexion and the surface and intramuscular electromyograms (EMGs) from the TA were recorded in eight subjects. The behavior of the same motor unit (n = 59) was compared at two ankle joint angles (+10 and -10 degrees around the ankle neutral position). Muscle fascicle length of the TA was measured noninvasively using ultrasonography recordings. When the ankle angle was moved from 10 degrees plantarflexion to 10 degrees dorsiflexion, the torque produced during maximal voluntary contraction (MVC) was significantly reduced [35.2 +/- 3.3 vs. 44.3 +/- 4.2 (SD) Nm; P < 0.001] and the average surface EMG increased (0.47 +/- 0.08 vs. 0.43 +/- 0.06 mV; P < 0.05). At reduced ankle joint angle, muscle fascicle length declined by 12.7% (P < 0.01) at rest and by 18.9% (P < 0.001) during MVC. Motor units were activated at a lower recruitment threshold for short compared with long muscle fascicle length, either when expressed in absolute values (2.1 +/- 2.5 vs. 3.6 +/- 3.7 Nm; P < 0.001) or relative to their respective MVC (5.2 +/- 6.1 vs. 8.8 +/- 9.0%). Higher discharge rate and additional motor-unit recruitment were observed at a given absolute or relative torque when muscle fascicles were shortened. However, the data indicate that increased rate coding was mainly present at low torque level (<10% MVC), when the muscle-tendon complex was compliant, whereas recruitment of additional motor units played a dominant role at higher torque level and decreased compliance (10-35% MVC). Taken together, the results suggest that the central command is modulated by the afferent proprioceptive information during submaximal contractions performed at different muscle fascicle lengths.

Action Potentials↗

PubMed2005

Spinal reflexes and coactivation of ankle muscles during a submaximal fatiguing contraction.

This study examined the involvement of spinal mechanisms in the control of coactivation during a sustained contraction of the ankle dorsiflexors at 50% of maximal voluntary contraction. Changes in the surface electromyogram (EMG) of the tibialis anterior and of two antagonist muscles, the soleus and lateral gastrocnemius, were investigated during and after the fatigue task. Concurrently, the compound action potential (M-wave) and the Hoffmann reflex of the soleus and lateral gastrocnemius were recorded. The results showed that the torque of the ankle dorsiflexors and the average EMG of the tibialis anterior during maximal voluntary contraction declined by 40.9 +/- 17.7% (mean +/- SD; P < 0.01) and 37.0 +/- 19.9% (P < 0.01), respectively, at task failure. During the submaximal fatiguing contraction, the average EMG of both the agonist and antagonist muscles increased, leading to a nearly constant ratio at the end of the contraction when normalized to postfatigue values. In contrast to the monotonic increase in average EMG of the antagonist muscles, the excitability of their spinal reflex pathways exhibited a biphasic modulation. The amplitude of the Hoffman reflexes in the soleus and lateral gastrocnemius increased to 147.5 +/- 52.9% (P < 0.05) and 166.7 +/- 74.9% (P < 0.01), respectively, during the first 20% of the contraction and then subsequently declined to 66.3 +/- 44.8 and 74.4 +/- 44.2% of their initial values. In conclusion, the results show that antagonist coactivation did not contribute to task failure. The different changes in voluntary EMG activity and spinal reflex excitability in the antagonist muscles during the fatiguing contraction support the concept that the level of coactivation is controlled by supraspinal rather than spinal mechanisms. The findings indicate, however, that antagonist coactivation cannot simply be mediated by a central descending "common drive" to the motor neuron pools of the agonist-antagonist muscle pairs. Rather, they suggest a more subtle regulation of the drive, possibly through presynaptic mechanisms, to the motoneurons that innervate the antagonist muscles.

Adolescent↗

PubMed2005

Aging does not affect voluntary activation of the ankle dorsiflexors during isometric, concentric, and eccentric contractions.

This study examines the age-related deficit in force of the ankle dorsiflexors during isometric (Iso), concentric (Con), and eccentric (Ecc) contractions. More specifically, the contribution of neural and muscular mechanisms to the loss of voluntary force was investigated in men and women. The torque produced by the dorsiflexors and the surface electromyogram (EMG) from the tibialis anterior and the soleus were recorded during maximal Iso contractions and during Con and Ecc contractions performed at constant angular velocities (5-100 degrees/s). Central activation was tested by the superimposed electrical stimulation method during maximal voluntary contraction and by computing the ratio between voluntary average EMG and compound muscle action potential (M wave) induced by electrical stimulation (average EMG/M wave). Contractile properties of the dorsiflexor muscles were investigated by recording the mechanical responses to single and paired maximal stimuli. The results showed that the age-related deficit in force (collapsed across genders and velocities) was greater for Iso (20.5%; P < 0.05) and Con (38.6%; P < 0.001) contractions compared with Ecc contractions (6.5%; P > 0.05). When the torque produced during Con and Ecc contractions was expressed relative to the maximal Iso torque, it was significantly reduced in Con contractions and increased in Ecc contractions with aging, with the latter effect being more pronounced for women. In both genders, voluntary activation was not significantly impaired in elderly adults and did not differ from young subjects. Similarly, coactivation was not changed with aging. In contrast, the mechanical responses to single and paired stimuli showed a general slowing of the muscle contractile kinetics with a slightly greater effect in women. It is concluded that the force deficit during Con and Iso contractions of the ankle dorsiflexors in advanced age cannot be explained by impaired voluntary activation or changes in coactivation. Instead, this age-related adaptation and the mechanisms that preserve force in Ecc contractions appeared to be located at the muscular level.

Adaptation, Physiological↗

PubMed2004

Postactivation potentiation influences differently the nonlinear summation of contractions in young and elderly adults.

The force enhancement of a twitch after a maximal conditioning muscle contraction [i.e., postactivation potentiation (PAP)] is reduced with aging, but its influence on the summation of force in response to repetitive stimulation at different frequencies is not known. The purpose of this work was to compare the electrically evoked mechanical responses of the tibialis anterior muscle between young and elderly adults after a 6-s maximal voluntary contraction (MVC). The results showed that, immediately after the conditioning MVC, twitch torque and its maximal rate of development and relaxation were significantly enhanced in both groups, but the magnitude of potentiation was greater in young (148.0 +/- 14.2, 123.7 +/- 16.5, and 185.4 +/- 36.5%, respectively) compared with elderly adults (87.4 +/- 15.2, 63.8 +/- 9.9, and 62.9 +/- 11.0%, respectively). This age-related difference in potentiation of the twitch disappeared completely 1 min after the conditioning MVC. The potentiation of torque and speed-related parameters in response to two- and three-pulse trains, delivered at a constant interval of 10 ms (100 Hz), was less than for a single pulse for both groups. In young adults, the magnitude of PAP on the successive individual mechanical contributions within a train of stimuli declined progressively such that the third contribution did not differ significantly from the same contribution before the conditioning MVC. In contrast, the second and third contributions did not potentiate (P > 0.05) in elderly adults. Although these contributions did potentiate significantly at a lower frequency of stimulation (20 Hz) in the two groups, the difference in PAP between young and elderly adults still persisted. This overall attenuation of potentiation with aging, however, appears to have a moderate influence on the decrement of the muscular performance.

Adaptation, Physiological↗

PubMed2004

Preceding muscle activity influences motor unit discharge and rate of torque development during ballistic contractions in humans.

To investigate the effect of initial conditions on the modulation of motor unit discharge during fast voluntary contractions, we compared ballistic isometric contractions of the ankle dorsiflexor muscles that were produced from either a resting state or superimposed on a sustained contraction. The torque of the dorsiflexors and the surface and intramuscular EMGs from the tibialis anterior were recorded. The results showed that the performance of a ballistic contraction from a sustained contraction ( approximately 25% maximal voluntary contraction (MVC)) had a negative effect on the maximal rate of torque development. Although the electromechanical delay was shortened, the EMG activity during the ballistic contraction was less synchronized. These observations were associated with a significant decline in the average discharge rate of single motor units (89.8 +/- 3.8 versus 115 +/- 5.8 Hz) and in the percentage of units (6.2 versus 15.5% of the whole sample) that exhibited double discharges at brief intervals (= 5 ms). High-threshold units that were not recruited during the sustained contraction displayed the same activation pattern, which indicates that the mechanisms responsible for the decline in discharge rate were not restricted to previously activated units, but appear to influence the entire motor unit pool. When a premotor silent period (SP) was observed at the transition from the sustained muscular activity to the ballistic contraction (19% of the trials), these adjustments in motor unit activity were not present, and the ballistic contractions were similar to those performed from a resting state. Together, these results indicate that initial conditions can influence the capacity for motor unit discharge rate and hence the performance of a fast voluntary contraction.

Adult↗

PubMed2004

Effect of static stretch training on neural and mechanical properties of the human plantar-flexor muscles.

To determine the contributions of neural and mechanical mechanisms to the limits in the range of motion (ROM) about a joint, we studied the effects of 30 sessions of static stretch training on the characteristics of the plantar-flexor muscles in 12 subjects. Changes in the maximal ankle dorsiflexion and the torque produced during passive stretching at various ankle angles, as well as maximal voluntary contraction (MVC) and electrically induced contractions, were recorded after 10, 20, and 30 sessions, and 1 month after the end of the training program. Reflex activities were tested by recording the Hoffmann reflex (H reflex) and tendon reflex (T reflex) in the soleus muscle. Training caused a 30.8% (P < 0.01) increase in the maximal ankle dorsiflexion. This improved flexibility was associated (r(2) = 0.88; P < 0.001) with a decrease in muscle passive stiffness and, after the first 10 sessions only, with a small increase in passive torque at maximal dorsiflexion. Furthermore, both the H- and T-reflex amplitudes were reduced after training, especially the latter (-36% vs. -14%; P < 0.05). The MVC torque and the maximal rate of torque development were not affected by training. Although the changes in flexibility and passive stiffness were partially maintained 1 month after the end of the training program, reflex activities had already returned to control levels. It is concluded that the increased flexibility results mainly from reduced passive stiffness of the muscle-tendon unit and tonic reflex activity. The underlying neural and mechanical adaptation mechanisms, however, showed different time courses.

Adult↗

PubMed2004

Postactivation potentiation in human muscle is not related to the type of maximal conditioning contraction.

The mechanical performance of a muscle can be enhanced by preceding contractile activity, such as occurs with postactivation potentiation. To investigate whether the type of contraction influences the extent of potentiation, the effects of 6-s maximal isometric (ISO), concentric (CON), and eccentric (ECC) maximal voluntary contractions (MVC) on the muscle twitch were compared in the tibialis anterior of nine subjects. The study also examined the effect of postactivation potentiation on the force evoked by the second (C2) and third (C3) responses of two-pulse (PT2) and three-pulse (PT3) trains that were delivered at a 10-ms interpulse interval. The results showed that immediately after the conditioning MVC, twitch torque (Pt) and its maximal rate of torque development (+dPt/dt) and relaxation (-dPt/dt) were significantly enhanced, without any change in contraction time (CT), half-relaxation time ((1/2)RT), and compound muscle action potential (M wave). The extent of Pt potentiation was similar for all MVC modalities, and the mean maximal values ranged from 150% to 180%. Furthermore, postactivation potentiation was greater for the single pulse compared with PT2 and PT3 responses. All parameters returned to initial values within 7-10 min. Although Pt (or C1) was potentiated more than was C2 and C3, its decline over time was proportionally more rapid than those for C2 and C3. We conclude that postactivation potentiation was not related to the type of conditioning MVC under these experimental conditions. The observation that postactivation potentiation increased C1 more than C2 and C3 indicates that a saturation process limits the extent of potentiation during the summation of successive responses to a train of stimuli. These results have practical application in the design of functional electrical stimulation protocols.

Action Potentials↗

PubMed2004

Muscle fatigue and the mechanisms of task failure.

An alternative approach in the study of muscle fatigue is to address the question, "What causes task failure during a fatiguing contraction?" This approach is described by considering how variation in the type of load supported and contraction intensity influence both the time to task failure and the centrally mediated adjustments in reflex activity and motor unit behavior.

Electromyography↗

PubMed2002

Reflex regulation during sustained and intermittent submaximal contractions in humans.

To investigate whether the intensity and duration of a sustained contraction influences reflex regulation, we compared sustained fatiguing contractions at 25 % and 50 % of maximal voluntary contraction (MVC) force in the human abductor pollicis brevis (APB) muscle. Because the activation of motoneurones during fatigue may be reflexively controlled by the metabolic status of the muscle, we also compared reflex activities during sustained and intermittent (6 s contraction, 4 s rest) contractions at 25 % MVC for an identical duration. The short-latency Hoffmann(H) reflex and the long-latency reflex (LLR) were recorded during voluntary contractions, before, during and after the fatigue tests, with each response normalised to the compound muscle action potential (M-wave). The results showed that fatigue during sustained contractions was inversely related to the intensity, and hence the duration, of the effort. The MVC force and associated surface electromyogram (EMG) declined by 26.2 % and 35.2 %, respectively, after the sustained contraction at 50 % MVC, and by 34.2 % and 44.2 % after the sustained contraction at 25 % MVC. Although the average EMG increased progressively with time during the two sustained fatiguing contractions, the amplitudes of the H and LLR reflexes decreased significantly. Combined with previous data (Duchateau & Hainaut, 1993), the results show that the effect on the H reflex is independent of the intensity of the sustained contraction, whereas the decline in the LLR is closely related to the duration of the contraction. Because there were no changes in the intermittent test at 25 % MVC, the results indicate that the net excitatory spinal and supraspinal reflex-mediated input to the motoneurone pool is reduced. This decline in excitation to the motoneurones, however, can be temporarily compensated by an enhancement of the central drive.

Action Potentials↗

PubMed2002

Neural adaptations with chronic activity patterns in able-bodied humans.

The purpose of this article is to review the neural adaptations that occur in able-bodied humans with alterations in chronic patterns of physical activity. The adaptations are categorized as those related to cortical maps, motor command, descending drive, muscle activation, motor units, and sensory feedback. We focused on the adaptations that occur with such activities as strength training, limb immobilization, and limb unloading. For these types of interventions, the adaptations are widely distributed throughout the nervous system, but those changes that are observed with strength training are often not the converse of those found with reduced-use protocols.

Adaptation, Physiological↗