Jean-Claude Jouanin

The isometric force that induces maximal surface muscle deoxygenation.

Abstract To determine the external force that induces maximal deoxygenation of brachioradialis muscle 32 trained male subjects maintained isometric contractions using the elbow flexor muscles up to the limit time (isotonic part of the isometric contraction, IIC) and beyond that time for 120 s (anisotonic part of the isometric contraction). During IIC each subject maintained relative forces of either 25% and 70% maximal voluntary contraction (MVC), 50% and 100% MVC, or 40% and 60% MVC. Muscle oxygenation was assessed using a near infrared spectroscope, and expressed as a percentage of the reference value (ΔO2rest) which was the difference between the minimal oxygenation obtained after 6 min of ischaemia at rest and the maximal reoxygenation following the release of the tourniquet. During IIC at 25% MVC, muscle oxygenation decreased to 17 (SEM 3)% ΔO2rest, then it levelled off [25 (SEM 1)% ΔO2rest]. After the point at which target force could not be maintained, reoxygenation was very weak. During IIC at 40%, 50%, 60%, and 70% MVC, the lowest muscle oxygenation values were obtained after 15–20 s of contraction and corresponded to −18 (SEM 6), −59 (SEM 12) −31 (SEM 6), and −29 (SEM 6)% ΔO2rest, respectively. For the contraction at 100% MVC, the lowest oxygenation [−19 (SEM 9)% ΔO2rest] was obtained while force was decreasing (69% MVC). During the anisotonic part of the isometric contractions, the greatest reoxygenation rate was obtained after 50% MVC IIC (P < 0.001). Our results showed that during isometric elbow flexions between 25% and 100% MVC, there was no linear relationship between external force and muscle oxygenation, and that the maximal deoxygenation of the brachioradialis muscle was obtained at 50% MVC.

Analysis of Heart Rate Variability after a Ranger Training Course

We studied the effects of prolonged physical activities on resting heart rate variability (HRV) during a training session attended by 23 cadets of the French military academy. This course lasts 1 month and is concluded by a 5-day field exercise simulation with physical and psychological stress. Data collection took place before (B) and immediately at the end (E) of the course. It included HRV recordings during a stand test (5 minutes lying down and 5 minutes standing), with a Polar R-R monitor, followed by blood sampling to assay plasma testosterone. The results (B and E) showed that the testosterone level fell by approximately 28.6 +/- 7%, indicating a high level of fatigue. During the stand test, the total power (TP) of the HRV spectrum increased in a supine position. The TP of B was 5,515.7 ms2 (SE, 718.4) and of E was 13018.9 ms2 (SE, 2,539.2; p < 0.001). High-frequency (HF) normalized values increased and low-frequency (LF) normalized values fell, regardless of position (HF normalized values and LF normalized values: supine, p < 0.01, p < 0.05; standing, p < 0.05, p < 0.01, respectively). LF:HF ratio fell 66.2 (SE, 12.9%; p < 0.01) in a lying position. During the time-domain analysis of HRV, differences between adjacent normal R-R intervals more than 50 milliseconds, expressed as a percentage, and differences between the coupling intervals of adjacent normal RR intervals increased in the lying position (p < 0.001). These results as a whole suggest that parasympathetic nervous system activity increases with fatigue.

Effect of fatigue on maximal velocity and maximal torque during short exhausting cycling

A group of 24 subjects performed on a cycle ergometer a fatigue test consisting of four successive all-out sprints against the same braking torque. The subjects were not allowed time to recover between sprints and consequently the test duration was shorter than 30 s. The pedal velocity was recorded every 10 ms from a disc fixed to the flywheel with 360 slots passing in front of a photo-electric cell linked to a microcomputer which processed the data. Taking into account the variation of kinetic energy of the ergometer flywheel, it was possible to determine the linear torque-velocity relationship from data obtained during the all-out cycling exercise by computing torque and velocity from zero velocity to peak velocity according to a method proposed previously. The maximal theoretical velocity (ν1) and the maximal theoretical torque (T1) were estimated by extrapolation of each torque-velocity relationship. Maximal power (Pmax) was calculated from the values of T0 and ν0 (Pmax = 0.25ν0T0). The kinetics of ν0, T0 and Pmax was assumed to express the effects of fatigue on the muscle contractile properties (maximal shortening velocity, maximal muscle strength and maximal power). Fatigue induced a parallel shift to the left of the torque-velocity relationships. The ν0, T0 and Pmax decreases were equal to 16.3%, 17.3% and 31%, respectively. The magnitude of the decrease was similar for ν0 and T0 which suggested that Pmax decreased because of a slowing of maximal shortening velocity as well as a loss in maximal muscle strength. However, the interpretation of a decrease in cycling ν0 which has the dimension of a maximal cycling frequency is made difficult by the possible interactions between the agonistic and the antagonistic muscles and could also be explained by a slowing of the muscle relaxation rate.

A dynamic network involving M1‐S1, SII‐insular, medial insular, and cingulate cortices controls muscular activity during an isometric contraction reaction time task

Magnetoencephalographic, electromyographic (EMG), work, and reaction time (RT) were recorded from nine subjects during visually triggered intermittent isometric contractions of the middle finger under two conditions: unloaded and loaded (30% of maximal voluntary contraction). The effect of muscle fatigue was studied over three consecutive periods under both conditions. In the loaded condition, the motor evoked field triggered by the EMG onset decreased with fatigue, whereas movement‐evoked fields (MEFs) increased (P < 0.01). Fatigue was demonstrated in the loaded condition, since (i) RT increased due to an increase in the electromechanical delay (P < 0.002); (ii) work decreased from Periods 1 to 3 (P < 0.005), while (iii) the myoelectric RMS amplitude of both flexor digitorum superficialis and extensor muscles increased (P < 0.003) and (iv) during Period 3, the spectral deflection of the EMG median frequency of the FDS muscle decreased (P < 0.001). In the unloaded condition and at the beginning of the loaded condition, a parallel network including M1‐S1, posterior SII‐insular, and posterior cingulate cortices accounted for the MEF activities. However, under the effect of fatigue, medial insular and posterior cingulate cortices drove this network. Moreover, changes in the location of insular and M1‐S1 activations were significantly correlated with muscle fatigue (increase of RMS‐EMG; P < 0.03 and P < 0.01, respectively). These results demonstrate that a plastic network controls the strength of the motor command as fatigue occurs: sensory information, pain, and exhaustion act through activation of the medial insular and posterior cingulate cortices to decrease the motor command in order to preserve muscle efficiency and integrity. Hum Brain Mapp, 2009.

Cardiovascular responses to leisure alpine skiing in habitually sedentary middle‐aged men

In order to evaluate the cardiovascular responses to leisure alpine skiing of habitually sedentary and not particularly active adult men, a series of continuous recordings of heart rate (HR) was performed on 10 subjects aged 51.0 +/- 1.3 years (mean +/- S.E.) during a 6-day ski trip at an altitude of 1000-2485 m. From the very first day, the subjects spontaneously adopted a regimen of intense physical activity, since 17.9% of the HR values recorded on the ski runs were higher than 85% of the maximal theoretical HR [THRmax (beats min-1) = 220--age (years)], which represented 19.5% of the actual skiing time. On day 2, 10.7% of the HR values were higher than 85% THRmax, or 17.9% of actual skiing time. For the entire ski trip, the mean HR during skiing was approximately 126 beats min-1, which corresponds to 75% THRmax. At rest, HR in the morning did not change significantly from days 2 to 6 (from 80 +/- 4 to 75 +/- 4 beats min-1, P > 0.05), whereas HR in the evening correlated significantly with the duration of physical activity during the day (r = 0.487, P < 0.001). In all cases, HR at rest was significantly higher than before the trip, except in the evening of day 4, the day on which the subjects skied less because they were fatigued after skiing in the morning. Systolic blood pressure (SBP) at rest was always lower than the control established before the trip, whether taken in the morning or in the evening.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiological effects of downhill skiing at moderate altitude in untrained middle-aged men

To evaluate whether occasional strong physical activity at moderate altitude for several consecutive days is acceptable in untrained middle-aged people, 10 men (age range, 46-59 years) underwent physical examinations before (control day, D0), during (D1-D8), and after 1 wk of leisure alpine skiing. With respect to D0, the resting concentration of plasma noradrenaline (NOR) increased transiently (p < 0.01) on D2 and then increased to a maximal value from D6-D8 (p < 0.01). There was no significant change in the concentration of adrenaline. Although maximal voluntary contraction of knee extensors diminished on D3 (P < 0.05), that of the digit flexors did not change. Heart rate (HR) and blood pressure at rest in the evening were always higher than control values except on D4 (forced rest). After the stay, there was a reduction in sympathetic activity. This was reflected by a return of NOR to its control value, a decrease in resting HR (64.2 [11.4] beats per minute [bpm]: control: 71.1 [10.1] bpm, P < 0.02), a tendency for triglyceride and insulin resistance to decrease, and a significant increase in alipoprotein A1/alipoprotein A2 (P < 0.01). Our results show that despite signs of fatigue on D3, the effects of physical activity that is relatively intense (HR > 70% maximal HR) together with mild hypoxia are well tolerated by untrained middle-aged men and that the controlled practice of downhill skiing may be accepted into a program to lower cardiovascular risk factors.

Influence of posture and training on the endurance time of a low-level isometric contraction

Classically, the critical force of a muscle (the relative force below which an isometric contraction can be maintained for a very long time without fatigue) is comprised of between 15 and 20% of its maximum voluntary contraction (MVC). However, some authors believe that the value is below 10% MVC. If such is the case, signs that accompany the establishment of muscle fatigue (EMG changes, continuous increase in systolic blood pressure [SBP] and heart rate [HR]) would have to appear more rapidly and with a higher intensity if the muscle is already partially fatigued at the start of maintaining a contraction at 10% MVC. Twelve healthy untrained participants carried out two isometric contractions with the digit flexors: one (test A) began with a maximum contraction sustained for 4 min followed without interruption by a contraction at 10% MVC for 61 min; the other (test B) was a contraction maintained at 10% MVC for 65 min. For test B, after an initial increase of 4 bpm with respect to at rest, HR remained stable until the end of contraction, SBP progressively increased by 24 mm Hg in 28 min, then remained unchanged until the end, and there were no significant changes in EMG (absence of spectral deviation towards low frequencies). For test A, in spite of the initial maximum contraction, changes in the parameters being studied (total maintenance time, HR, SBP, EMG) during maintenance at 10% MVC were identical to those for test B. The results show that (1) when the number and intensity of the co-contractions are minimized by applying an appropriate posture, it is possible to sustain an isometric contraction at 10% MVC for at least 65 min without the appearance of signs of muscle fatigue; (2) the critical force of the digit flexors is higher than 10% MVC.

Short Half-Life Hypnotics Preserve Physical Fitness and Altitude Tolerance During Military Mountainous Training

OBJECTIVE We study the effect of short half-life hypnotics (zaleplon or zolpidem against placebo) on altitude tolerance in 12 nonacclimated male soldiers (age, 22.1 +/- 0.8 years; height, 177.8 +/- 1.7 cm; weight, 69.8 +/- 1.7 kg). METHODS Soldiers were trained to practice mountaineering at high altitude (2,533-4,810 meters) during 3 periods (one per medication tested) of 4 days (D1-D4). In each period the nights were spent in a hut (3,613 m). Administration of hypnotics or placebo was then implemented at 9:45 p.m. Nocturnal arterial oxygen saturation (SaO2) and heart rate variability (HRV) were monitored. At 5:00 a.m. and 9:00 p.m. physical fitness was assessed using acute mountain sickness (AMS) score. At 5:00 p.m., a posteffort stand test was carried out to evaluate the orthoparasympathetic imbalance with fatigue. RESULTS Nocturnal SaO2 correlated negatively with morning AMS scores (R = -0.820, p < 0.02) and HRV analysis favored the sympathetic modulation. Posteffort stand tests revealed that sympathetic modulation attenuated from D2 to D3 in treated groups. CONCLUSION The present study provides evidence that zolpidem or zaleplon improves sleep and subsequent physical fitness at altitude.

Effect of co-contractions on the cardiovascular response to submaximal static handgrip.

Abstract The aim of this study was to investigate the influence of concomitant involuntary contractions of different muscles on heart rate (HR) and blood pressure (BP) during a sustained, submaximal handgrip. Nine male subjects [23.6 (0.4) years, 177.0 (1.5) cm, and 73.0 (2.7) kg, means (SE)] participated in the experiment. The maximal integrated electromyographic activity (IEMGmax) of four ipsilateral muscles, flexor digitorum (FD), biceps brachii (BB), rectus abdominalis (RA) and vastus lateralis (VA), was recorded. Then, after 30 min of rest, the subjects maintained a submaximal isometric handgrip for 2 min. Heart rate (HR), systolic (SBP) and diastolic (DBP) blood pressure and surface electromyography (EMG) of the four muscles were recorded. The amplitude and power spectrum of the EMG were analysed. During the handgrip the force was kept constant at 43 (1)% of the maximum voluntary contraction (MVC) only for 90 (12) s. After that time, the subjects were unable to maintain the target force which decreased continuously up to the end of the contraction (P < 0.01) with a residual force of 27 (3)% MVC at t=120 s. HR increased from 75 (3) beats · min−1 at rest to 109 (6) beats · min−1 at t=120 s (P < 0.01). SBP and DBP also increased from 112 (5) and 81 (2) mmHg to 176 (5) and 133 (7) mmHg, respectively (P < 0.01). The EMG activity rose significantly for both FD and BB with a moderate increase for RA and VL. In fact, the individual contributions of FD and BB to the EMG activity of the four muscles were 52 (2)% and 37 (2)%, respectively, whereas the RA and VL contributed only 9 (1)% and 1.4 (0.1)%. The amplitude ratio of FDs EMG to the total EMG activity of the four muscles from which recordings were made decreased with time from 72% to 33% (P < 0.01). The central commands level of activation, as reflected by the increased EMG activity of the four recorded muscles, was probably high enough to stimulate the cardiovascular centres through cortical spread (sometimes known as cortical irradiation). On the other hand, maintaining an isometric handgrip at 43% MVC reduced local muscle blood flow and metabolites known to stimulate type III and IV afferents then accumulated, which in turn induced a reflex-mediated elevation of blood pressure. However, the relative forces developed by the co-contracting muscles were of low intensity (less than 20% MVC) and short duration compared to those of the muscle group under study. These results suggest that the mass of the muscle groups recruited during a fatiguing submaximal handgrip contributes little to the cardiovascular response.

The vastus lateralis neuromuscular activity during all-out cycling exercise.

OBJECTIVE The objective of this work was to study modifications in motor control through surface electromyographic (sEMG) activity during a very short all-out cycling exercise. METHODS Twelve male cyclists (age 23+/-4 years) participated in this study. After a warm-up period, each subject performed three all-out cycling exercises of 6s separated by 2 min of complete rest. This protocol was repeated three times with a minimum of 2 days between each session. The braking torque imposed on cycling motion was 19 Nm. The sEMG of the vastus lateralis was recorded during the first seven contractions of the sprint. Time-frequency analysis of sEMG was performed using continuous wavelet transform. The mean power frequency (MPF, qualitative modifications in the recruitment of motor units) and signal energy (a quantitative indicator of modifications in the motor units recruitment) were computed for the frequency range 10-500 Hz. RESULTS sEMG energy increased (P0.05) between contraction number 1 and 2, decreased (P < or =0.05) between contraction number 2 and 3 then stabilized between contraction number 3 and 7 during the all-out test. MPF increased (P < or =0.05) during the all-out test. This increase was more marked during the first two contractions. CONCLUSIONS The decrease in energy and the increase in the sEMG MPF suggest a large spatial recruitment of motor units (MUs) at the beginning of the sprint followed by a preferential recruitment of faster MUs at the end of the sprint, respectively.