Reaction time can be measured during voluntary contractions with electrode array.

Reaction time (RT) is classically divided into premotor time (PMT) and electromechanical delay (EMD). However, the determination of the onset of electromyographic activity (EMG) during voluntary contraction remains questionable. In addition, the reliability of RT, PMT and EMD needs to be determined. Twelve participants performed two sessions of RT trials, separated by 5 min. RT was evaluated during voluntary isometric contractions of the elbow flexors, i.e., time between a light signal (stimulus) and the onset of the mechanical response. To assess EMD, an electrode array (64 channels) was used to accurately detect the onset of EMG activity. PMT represented the major part of the RT (~88%). Coefficients of variation were reasonably satisfactory for all parameters (range: 11·9-13·4%). The use of electrode array appears to be a relevant method to measure EMD. Moreover, sessions based on two trials are reliable enough to detect changes in RT components.

Muscle force loss and soreness subsequent to maximal eccentric contractions depend on the amount of fascicle strain in vivo.

AIM: Defining the origins of muscle injury has important rehabilitation and exercise applications. However, current knowledge of muscle damage mechanics in human remains unclear in vivo. This study aimed to determine the relationships between muscle-tendon unit mechanics during maximal eccentric contractions and the extent of subsequent functional impairments induced by muscle damage. METHODS: The length of the muscle-tendon unit, fascicles and tendinous tissues was continuously measured on the gastrocnemius medialis using ultrasonography, in time with torque, during 10 sets of 30 maximal eccentric contractions of plantar flexors at 45°s(-1) , in seventeen participants. RESULTS: Muscle-tendon unit, fascicles and tendinous tissues were stretched up to 4.44 ± 0.33 cm, 2.31 ± 0.64 cm and 1.92 ± 0.61 cm respectively. Fascicle stretch length, lengthening amplitude and negative fascicle work beyond slack length were significantly correlated with the force decrease 48 h post-exercise (r = 0.51, 0.47 and 0.68, respectively; P < 0.05). CONCLUSIONS: This study demonstrates that the strain applied to human muscle fibres during eccentric contractions strongly influences the magnitude of muscle damage in vivo. Achilles tendon compliance decreases the amount of strain, while architectural gear ratio may moderately contribute to attenuating muscle fascicle lengthening and hence muscle damage. Further studies are necessary to explore the impact of various types of task to fully understand the contribution of muscle-tendon interactions during active lengthening to muscle damage.

In vivo and in vitro investigations of heterozygous nebulin knock-out mice disclose a mild skeletal muscle phenotype.

Nemaline myopathy is the most common congenital skeletal muscle disease, and mutations in the nebulin gene account for 50% of all cases. Recent studies suggest that the disease severity might be related to the nebulin expression levels. Considering that mutations in the nebulin gene are typically recessive, one would expect that a single functional nebulin allele would maintain nebulin protein expression which would result in preserved skeletal muscle function. We investigated skeletal muscle function of heterozygous nebulin knock-out (i.e., nebulin(+/-)) mice using a multidisciplinary approach including protein and gene expression analysis and combined in vivo and in vitro force measurements. Skeletal muscle anatomy and energy metabolism were studied strictly non-invasively using magnetic resonance imaging and 31P-magnetic resonance spectroscopy. Maximal force production was reduced by around 16% in isolated muscle of nebulin(+/-) mice while in vivo force generating capacity was preserved. Muscle weakness was associated with a shift toward a slower proteomic phenotype, but was not related to nebulin protein deficiency or to an impaired energy metabolism. Further studies would be warranted in order to determine the mechanisms leading to a mild skeletal muscle phenotype resulting from the expression of a single nebulin allele.

Time-dependent effect of acute hypoxia on corticospinal excitability in healthy humans.

Contradictory results regarding the effect of hypoxia on cortex excitability have been reported in healthy subjects, possibly depending on hypoxia exposure duration. We evaluated the effects of 1- and 3-h hypoxia on motor corticospinal excitability, intracortical inhibition, and cortical voluntary activation (VA) using transcranial magnetic stimulation (TMS). TMS to the quadriceps cortex area and femoral nerve electrical stimulations were performed in 14 healthy subjects. Motor-evoked potentials (MEPs at 50-100% maximal voluntary contraction; MVC), recruitment curves (MEPs at 30-100% maximal stimulator power output at 50% MVC), cortical silent periods (CSP), and VA were measured in normoxia and after 1 (n = 12) or 3 (n = 10) h of hypoxia (Fi(O(2)) = 0.12). One-hour hypoxia did not modify any parameters of corticospinal excitability but reduced slightly VA, probably due to the repetition of contractions 1 h apart (96 ± 4% vs. 94 ± 4%; P = 0.03). Conversely, 3-h hypoxia significantly increased 1) MEPs of the quadriceps muscles at all force levels (+26 ± 14%, +24 ± 12%, and +27 ± 17% at 50, 75, and 100% MVC, respectively; P = 0.01) and stimulator power outputs (e.g., +21 ± 14% at 70% maximal power), and 2) CSP at all force levels (+20 ± 18%, +18 ± 19%, and +14 ± 22% at 50, 75, and 100% MVC, respectively; P = 0.02) and stimulator power outputs (e.g., +9 ± 8% at 70% maximal power), but did not modify VA (98 ± 1% vs. 97 ± 3%; P = 0.42). These data demonstrate a time-dependent hypoxia-induced increase in motor corticospinal excitability and intracortical inhibition, without changes in VA. The impact of these cortical changes on physical or psychomotor performances needs to be elucidated to better understand the cerebral effects of hypoxemia.

Differences in twitch potentiation between voluntary and stimulated quadriceps contractions of equal intensity.

This study compared the extent of twitch and M-wave potentiation (POT) between voluntary and stimulated quadriceps contractions performed at the same intensity. Sixteen healthy men completed 10-s isometric knee extensions at 40% of the maximal voluntary contraction torque under electrical stimulation and voluntary conditions. Single stimuli were delivered to the femoral nerve to evoke twitches before (PRE) and from 3 to 600 s after the end of each conditioning contraction. Changes in twitch contractile properties and M-wave characteristics were compared between the conditions. The extent of twitch peak torque POT was smaller for the stimulated (122+/-20% of PRE) than for the voluntary condition (133+/-20% of PRE). The magnitude of POT for the maximal rate of twitch torque development was also smaller for the stimulated trial. Rectus femoris M-wave amplitude was potentiated by the voluntary but not by the stimulated contraction. It was concluded that stimulated contractions resulted in smaller twitch and M-wave POT than voluntary contractions, despite equivalent torque output and duration. The spatially and temporally fixed recruitment of motor units with electrical stimulation and therefore the lower number of activated motor units compared with voluntary actions of equal intensity could explain the present findings.

GH responses to two consecutive bouts of neuromuscular electrical stimulation in healthy adults.

BACKGROUND: It is well established that repeated GHRH administration or repeated voluntary exercise bouts are associated with a complete blunting of GH responsiveness when the administration of the second stimulus follows the first one after a 2-h interval. AIM: To evaluate GH responses to neuromuscular electrical stimulation (NMES) in healthy adults. METHODS: Six volunteers (mean age+/-s.d. 31.7+/-5.5 years) were studied before and after two consecutive bouts of NMES exercise (a series of 20 contractions at the maximum of individual tolerance, frequency: 75 Hz, pulse duration: 400 mus, on-off ratio: 6.25-20 s) administered at a 2-h interval. RESULTS: Baseline GH levels (mean: 0.3+/-0.2 ng/ml) significantly increased after the first NMES (peak: 4.2+/-3.7 ng/ml), with a complete normalization after 120 min (0.3+/-0.3 ng/ml). The administration of the second bout of NMES of comparable characteristics also resulted in a significant GH increase (peak: 5.2+/-3.2 ng/ml), which was comparable with that observed after the previous one. GH net incremental area under the curve after the first and second bouts of NMES were not significantly different (155.1+/-148.5 and 176.9+/-123.3 ng/ml per h, P=0.785). CONCLUSIONS: Unlike repeated pharmacological stimuli and voluntary exercise bouts, subsequent sessions of NMES administered at a 2-h interval appear to circumvent feedback mechanisms and to re-induce the GH responses, thus indicating a possible different underlying mechanism elicited by different GH-releasing stimuli.

Random motor unit activation by electrostimulation.

Whether the involvement of motor units is different between surface neuromuscular electrostimulation and voluntary activation remains an unresolved issue. The aim of this pilot study was to verify if motor unit activation during electrostimulation is nonselective/random (i.e., without obvious sequencing related to fibre type), as recently suggested by Gregory and Bickel [6]. Sixteen healthy men randomly performed submaximal isometric contractions (10-s duration) of the quadriceps femoris muscle at 20, 40 and 60 % of maximal voluntary torque under both stimulated and voluntary conditions. During the contractions, paired stimuli were delivered to the femoral nerve (twitch interpolation technique) and the characteristics of the superimposed doublet were compared between the two conditions. For each torque level, time-to-peak torque was significantly longer (p range = 0.05 - 0.0002) during electrostimulation compared to voluntary contractions. Moreover, time-to-peak torque during voluntary trials decreased significantly when increasing the torque level from 20 to 60 % of maximal voluntary torque (p range = 0.03 - 0.0001), whereas it was unchanged during electrostimulation. In conclusion, over-the-muscle electrostimulation would neither result in motor unit recruitment according to Henneman's size principle nor would it result in a reversal in voluntary recruitment order. During electrostimulation, muscle fibres are activated without obvious sequencing related to fibre type.

A follow-up of GH-dependent biomarkers during a 6-month period of the sporting season of male and female athletes.

In order to verify the effects of the sporting season (entailing periods of training, competition, recovery, resting) on GH-dependent parameters in male and female athletes from different sporting disciplines, 47 male and female athletes (3 rowers, 5 swimmers, 7 alpine skiers, 3 soccer players, 7 middle distance runners, 14 sprinters, 4 triathletes, 1 road walker, 3 cyclists) were followed-up for a period of 6 months. Blood samples were taken every two months for the evaluation of IGF-I, N-terminal propeptide of type III procollagen (PIIINP) and C-terminal cross-linked telopeptide of type I collagen (ICTP). Abnormal IGF-I, PIIINP and ICTP levels were observed during the follow-up period in 7/100 (7%), 9/100 (9.0%) and 8/100 (8%) samples of the male group, respectively, and in 9/88 (10.2%), 1/88 (1.1%) and 0/88 (0%) samples of the female group, respectively. Abnormal levels appeared to be randomly distributed over the different periods of the sporting season and within male and female subjects, with the large majority of abnormal values being found in the younger athletes. Taking into account all the tests done during the 6-month period (no. 564), individual markers falling outside the normal range (for age) were observed in a small number of instances (34/564 tests done, 24/300 for males and 10/264 for females). When our method for the detection of exogenous recombinant GH (rhGH) administration, based on the concomitant determination of these three peripheral GH-dependent markers and on the attribution of specific scores, was applied in the same athlete at a given time point of the 6-month period, the prevalence of a positive score was extremely low (ie, 3/188 samples or 1.6%). Total positive scores were actually recorded in only three male athletes (2 swimmers and 1 skier, aged <21 yr) at one occasion during the 6-month period considered. In contrast, no total positive scores were found in female athletes (ie, 0/88 samples). In conclusion, the concentrations of IGF-I, PIIINP and ICTP were stable and not significantly modified during 6 months of a sporting season (entailing periods of training, competition, recovery, resting) in athletes from different sporting disciplines. Therefore our method, based on the concomitant determination of three peripheral GH-dependent biomarkers appears safe, acceptable, relatively inexpensive and repeatable (in case of positive or suspected values) immediately or at different intervals of the sporting season. Further additional studies are requested to precise the cut-off values for narrower age-class subdivisions in both genders in order to improve the proposed method.

Central and peripheral fatigue of the knee extensor muscles induced by electromyostimulation.

The main purpose of this study was to characterise neuromuscular fatigue induced by 30 contractions of the knee extensor muscles evoked by electromyostimulation (EMS). Twelve healthy subjects were tested before and after a typical EMS session (frequency: 75 Hz, on-off ratio: 6.25 s on-20 s off) used for quadriceps femoris muscle strengthening. Surface electromyographic (EMG) activity and torque obtained during maximal voluntary and electrically evoked contractions were analysed to distinguish peripheral from central fatigue. Maximal voluntary torque of the knee extensor muscles decreased approximately 20 % (p < 0.001) following EMS. In the same way, peak torque associated to single (p < 0.05) and paired (p < 0.001) stimuli as well as M-wave amplitude (p < 0.05) significantly decreased as a result of EMS. The raw EMG activity of both vastus lateralis and rectus femoris muscle recorded during maximal voluntary isometric contraction significantly decreased after the session (-17.3 and -14.5 %, respectively) whereas no changes were observed when EMG signals were normalised to respective M-wave amplitudes. Similarly, voluntary activation estimated by using the twitch interpolation technique was unchanged following EMS. In conclusion, a typical session of EMS of the knee extensor muscles mainly induced neuromuscular propagation failure while excitation-contraction coupling and neural mechanisms were not significantly affected. It is recommended to interpret surface EMG data together with the corresponding M wave, at least for the knee extensor muscles, in order to distinguish peripheral from central causes of fatigue.