Vincent Martin

Neuromuscular Consequences of an Extreme Mountain Ultra-Marathon

We investigated the physiological consequences of one of the most extreme exercises realized by humans in race conditions: a 166-km mountain ultra-marathon (MUM) with 9500 m of positive and negative elevation change. For this purpose, (i) the fatigue induced by the MUM and (ii) the recovery processes over two weeks were assessed. Evaluation of neuromuscular function (NMF) and blood markers of muscle damage and inflammation were performed before and immediately following (n = 22), and 2, 5, 9 and 16 days after the MUM (n = 11) in experienced ultra-marathon runners. Large maximal voluntary contraction decreases occurred after MUM (−35% [95% CI: −28 to −42%] and −39% [95% CI: −32 to −46%] for KE and PF, respectively), with alteration of maximal voluntary activation, mainly for KE (−19% [95% CI: −7 to −32%]). Significant modifications in markers of muscle damage and inflammation were observed after the MUM as suggested by the large changes in creatine kinase (from 144±94 to 13,633±12,626 UI L−1), myoglobin (from 32±22 to 1,432±1,209 µg L−1), and C-Reactive Protein (from <2.0 to 37.7±26.5 mg L−1). Moderate to large reductions in maximal compound muscle action potential amplitude, high-frequency doublet force, and low frequency fatigue (index of excitation-contraction coupling alteration) were also observed for both muscle groups. Sixteen days after MUM, NMF had returned to initial values, with most of the recovery process occurring within 9 days of the race. These findings suggest that the large alterations in NMF after an ultra-marathon race are multi-factorial, including failure of excitation-contraction coupling, which has never been described after prolonged running. It is also concluded that as early as two weeks after such an extreme running exercise, maximal force capacities have returned to baseline.

Central and peripheral contributions to neuromuscular fatigue induced by a 24-h treadmill run

This experiment investigated the fatigue induced by a 24-h running exercise (24TR) and particularly aimed at testing the hypothesis that the central component would be the main mechanism responsible for neuromuscular fatigue. Neuromuscular function evaluation was performed before, every 4 h during, and at the end of the 24TR on 12 experienced ultramarathon runners. It consisted of a determination of the maximal voluntary contractions (MVC) of the knee extensors (KE) and plantar flexors (PF), the maximal voluntary activation (%VA) of the KE and PF, and the maximal compound muscle action potential amplitude (Mmax) on the soleus and vastus lateralis. Tetanic stimulations also were delivered to evaluate the presence of low-frequency fatigue and the KE maximal muscle force production ability. Strength loss occurred throughout the exercise, with large changes observed after 24TR in MVC for both the KE and PF muscles (-40.9+/-17.0 and -30.3+/-12.5%, respectively; P<0.001) together with marked reductions of %VA (-33.0+/-21.8 and -14.8+/-18.9%, respectively; P<0.001). A reduction of Mmax amplitude was observed only on soleus, and no low-frequency fatigue was observed for any muscle group. Finally, KE maximal force production ability was reduced to a moderate extent at the end of the 24TR (-10.2%; P<0.001), but these alterations were highly variable (+/-15.7%). These results suggest that central factors are mainly responsible for the large maximal muscle torque reduction after ultraendurance running, especially on the KE muscles. Neural drive reduction may have contributed to the relative preservation of peripheral function and also affected the evolution of the running speed during the 24TR.

Electrical stimulation for testing neuromuscular function: from sport to pathology

The use of electrical stimulation (ES) can contribute to our knowledge of how our neuromuscular system can adapt to physical stress or unloading. Although it has been recently challenged, the standard technique used to explore central modifications is the twitch interpolated method which consists in superimposing single twitches or high-frequency doublets on a maximal voluntary contraction (MVC) and to compare the superimposed response to the potentiated response obtained from the relaxed muscle. Alternative methods consist in (1) superimposing a train of stimuli (central activation ratio), (2) comparing the MVC response to the force evoked by a high-frequency tetanus or (3) examining the change in maximal EMG response during voluntary contractions, if this variable is normalized to the maximal M wave, i.e. EMG response to a single stimulus. ES is less used to examine supraspinal factors but it is useful for investigating changes at the spinal level, either by using H reflexes, F waves or cervicomedullary motor-evoked potentials. Peripheral changes can be examined with ES, usually by stimulating the muscle in the relaxed state. Neuromuscular propagation of action potentials on the sarcolemma (M wave, high-frequency fatigue), excitation–contraction coupling (e.g. low-frequency fatigue) and intrinsic force (high-frequency stimulation at supramaximal intensity) can all be used to non-invasively explore muscular function with ES. As for all indirect methods, there are limitations and these are discussed in this review. Finally, (1) ES as a method to measure respiratory muscle function and (2) the comparison between electrical and magnetic stimulation will also be considered.

Effects of Recovery Modes after Knee Extensor Muscles Eccentric Contractions

PURPOSE This study aimed to evaluate the benefit of using low-intensity running or electromyostimulation (EMS) to hasten the recovery process from eccentric-contraction-induced injury. METHODS Before and 30 min, 24 h, 48 h, and 96 h after a one-legged downhill run, electrical stimulations were applied to the femoral nerve of healthy volunteers. Superimposed twitches were delivered during isometric maximal voluntary contraction (MVC) to determine the voluntary activation level (%VA). For 4 d after the exercise, each subject performed either (i) 30 min of running at 50% VO2max, (ii) 30 min of low-frequency EMS on the lower limb extensor muscles, or (iii) passive recovery. RESULTS Recovery time courses of the different variables did not differ significantly among the three experimental conditions. MVC decreased 30 min after the exercise and did not recover thereafter (P < 0.001). Percent VA was depressed after the exercise (P < 0.05) but did not contribute to MVC decrement thereafter. Mechanical responses to 80- and 20-Hz stimulation (P80 and P20, respectively) were significantly reduced over time (P < 0.01 and P < 0.001, respectively). Interestingly, MVC, P20, and P80 decrements were not statistically different (-9.6 +/- 14.5%, -13.2 +/- 14.2%, and -12.3 +/- 11.3%, respectively) at 48 h, and the P20.P80(-1) ratio showed complete recovery at this time. CONCLUSIONS The different recovery modes had no significant effect on the recovery time course of contractile properties. The prolonged torque loss is mainly due to peripheral alterations. Our results suggest that an alteration of the excitation-contraction coupling might be involved during the first 2 d after the eccentric exercise. From 2 to 4 d, damage to force-generating structures could account for the remaining torque deficit.

Central fatigue assessed by transcranial magnetic stimulation in ultratrail running.

PURPOSE The well-established central deficit in ultraendurance running races is not understood. The use of transcranial magnetic stimulation (TMS) in parallel with peripheral nerve stimulation provides insight into the source of these central changes. The aims of this study were to determine the presence and magnitude of voluntary activation deficits, especially supraspinal deficits, after a mountain trail-running race and to determine whether this can be explained by simultaneous changes in corticospinal excitability and intracortical inhibition. METHODS Neuromuscular function (TMS and femoral nerve electrical stimulation) of the knee extensors was evaluated before and after a 110-km ultratrail in 25 experienced ultraendurance trail runners during maximal and submaximal voluntary contractions and in relaxed muscle. RESULTS Voluntary activation assessed by both femoral nerve electrical stimulation (-26%) and TMS (-16%) decreased and were correlated (P < 0.01). Decreases in potentiated twitch and doublet amplitudes were correlated with decreased voluntary activation assessed by TMS (P < 0.05). There was increased motor-evoked potential (MEP) amplitude (P < 0.05) without change in cortical silent period (CSP) elicited by TMS at optimal stimulus intensity. Conversely, CSP at suboptimal TMS intensity increased (P < 0.05) without concurrent change in MEP amplitude. CONCLUSIONS The present results demonstrate the development of a large central activation deficit assessed by TMS, indicating that cortical motoneurons are severely impaired in their ability to fire at optimal frequency or be fully recruited after an ultraendurance running race. MEP and CSP responses suggest a shift in the sigmoidal MEP-stimulus intensity relationship toward larger MEP at higher TMS intensity without change in inflection point of the curve and a left shift in the CSP-stimulus intensity relationship.

Are Females More Resistant to Extreme Neuromuscular Fatigue

PURPOSE Despite interest in the possibility of females outperforming males in ultraendurance sporting events, little is known about the sex differences in fatigue during prolonged locomotor exercise. This study investigated possible sex differences in central and peripheral fatigue in the knee extensors and plantar flexors resulting from a 110-km ultra-trail-running race. METHODS Neuromuscular function of the knee extensors and plantar flexors was evaluated via transcranial magnetic stimulation (TMS) and electrical nerve stimulation before and after an ultra-trail-running race in 20 experienced ultraendurance trail runners (10 females and 10 males matched by percent of the winning time by sex) during maximal and submaximal voluntary contractions and in relaxed muscle. RESULTS Maximal voluntary knee extensor torque decreased more in males than in females (-38% vs -29%, P = 0.006) although the reduction in plantar flexor torque was similar between sexes (-26% vs -31%). Evoked mechanical plantar flexor responses decreased more in males than in females (-23% vs -8% for potentiated twitch amplitude, P = 0.010), indicating greater plantar flexor peripheral fatigue in males. Maximal voluntary activation assessed by TMS and electrical nerve stimulation decreased similarly in both sexes for both muscle groups. Indices of knee extensor peripheral fatigue and corticospinal excitability and inhibition changes were also similar for both sexes. CONCLUSIONS Females exhibited less peripheral fatigue in the plantar flexors than males did after a 110-km ultra-trail-running race and males demonstrated a greater decrease in maximal force loss in the knee extensors. There were no differences in the magnitude of central fatigue for either muscle group or TMS-induced outcomes. The lower level of fatigue in the knee extensors and peripheral fatigue in the plantar flexors could partly explain the reports of better performance in females in extreme duration running races as race distance increases.

Insights into the Mechanisms of Neuromuscular Fatigue in Boys and Men

PURPOSE The aim of the present study was to investigate the role of central and peripheral factors in neuromuscular fatigue induced by repeated maximal contractions in children and adults. METHODS Eleven boys (9.9 ± 1.2 yr) and 12 men (23.9 ± 3.5 yr) completed a fatigue protocol consisting in a repetition of 5-s maximal isometric voluntary contractions (MVC) of the knee extensors separated by 5-s passive recovery periods until the generated torque reached 60% of its initial value. Single magnetic stimulations were delivered to the femoral nerve every five MVC to follow the course of voluntary activation level and the amplitude of the potentiated twitch torque (Qtw(pot)) and vastus lateralis and rectus femoris concomitant M-waves (Mmax). RESULTS Torque reached 60% of initial value after 49.5 ± 16.8 and 34.0 ± 19.6 repetitions in boys and men, respectively (P < 0.05). Furthermore, men showed significantly higher knee extensor MVC decline than boys between 50% and 90% of total repetitions (P < 0.05). Voluntary activation remained unchanged in men, whereas it decreased significantly in boys (P < 0.05). In contrast, whereas Qtw(pot) remained unchanged in boys, Qtw(pot) decreased progressively up to 60% of total repetitions in men (P < 0.001). Finally, Mmax remained unchanged for vastus lateralis and rectus femoris muscles in both groups. CONCLUSIONS Children experienced no apparent peripheral fatigue and higher central fatigue than adults. The greater fatigue resistance in children could be related to a strategy of the CNS aimed at limiting the recruitment of motor units to prevent any extensive peripheral fatigue.

Can muscle size fully account for strength differences between children and adults

although it is self-evident that young children produce less strength than adults during maximal voluntary contractions (MVC), it still remains unclear whether these differences persist when strength is normalized to dimensional changes throughout growth, i.e., whether there are significant

Effect of Muscle Length on Voluntary Activation Level in Children and Adults

PURPOSE The aim of the present study was to compare the effect of muscle length on the level of voluntary activation (VA) at short and long muscle lengths between children and adults. METHODS Thirteen prepubertal boys (10.2 ± 1.1 yr) and 10 men (23.9 ± 2.9 yr) performed 5-s maximal isometric voluntary contractions of the knee extensor muscles at three muscular angles (20°, 90°, 100°; 0°, full extension) interspersed with at least 60-s passive recovery periods. Single magnetic stimulations were delivered to the femoral nerve during maximal isometric voluntary contractions to determine the level of VA using the twitch interpolation technique. The specific torque was calculated as the absolute torque divided by thigh muscle mass, as assessed using dual-energy x-ray absorptiometry. Finally, the theoretical specific torque that could be produced with a complete (i.e., 100%) activation level (specific torque at 100% VA) was estimated from the values of specific torque and VA. RESULTS Results showed a higher specific torque in adults at 90° and 100° but not at 20°. Accordingly, VA was significantly higher in adults at 90° (94% ± 4% vs 88% ± 8%, P < 0.05) and 100° (93% ± 6% vs 86% ± 8%, P < 0.05), whereas no significant difference was observed at 20°. Interestingly, the specific torque at 100% VA was not different between groups whatever the joint angle. CONCLUSIONS The lower ability of children to fully activate their motor units at long muscle length could account for their lower specific torque because no difference in theoretical specific torque was observed between groups at 90° and 100°.

The bigger, the stronger? Insights from muscle architecture and nervous characteristics in obese adolescent girls

Background:Young obese youth are generally stronger than lean youth. This has been linked to the loading effect of excess body mass, acting as a training stimulus comparable to strength training. Whether this triggers specific adaptations of the muscle architecture (MA) and voluntary activation (VA) that could account for the higher strength of obese subjects remains unknown.Methods:MA characteristics (that is, pennation angle (PA), fascicle length (FL) and muscle thickness (MT)) and muscle size (that is, anatomical cross-sectional area (ACSA)) of the knee extensor (KE) and plantar flexor (PF) muscles were evaluated in 12 obese and 12 non-obese adolescent girls (12–15 years). Maximal isometric torque and VA of the KE and PF muscles were also assessed.Results:Results revealed higher PA (P<0.05), greater MT (P<0.001), ACSA (P<0.01), segmental lean mass (P<0.001) and VA (P<0.001) for KE and PF muscles in obese girls. Moreover, obese individuals produced a higher absolute torque than their lean counterparts on the KE (224.6±39.5 vs 135.7±32.7 N m, respectively; P<0.001) and PF muscles (73.3±16.5 vs 44.5±6.2 N m; P<0.001). Maximal voluntary contraction (MVC) was correlated to PA for the KE (r=0.46–0.57, P<0.05–0.01) and PF muscles (r=0.45–0.55, P<0.05–0.01). MVC was also correlated with VA (KE: r=0.44, P<0.05; PF: r=0.65, P<0.001) and segmental lean mass (KE: r=0.48, P<0.05; PF: r=0.57, P<0.01).Conclusions:This study highlighted favorable muscular and nervous adaptations to obesity that account for the higher strength of obese youth. The excess of body mass supported during daily activities could act as a chronic training stimulus responsible for these adaptations.