Didier Gamet

Motor unit recruitment and EMG power spectra during ramp contractions of a bifunctional muscle

Surface electromyograms (EMGs) were analysed on the short and long head of the biceps brachii (BBSH and BBLH) during single (F and S) or dual (F+S) flexion and supination tasks. It was confirmed, by the analysis of EMG root-mean-square (RMS) values, that the highest activations of BBSH and BBLH were obtained during a maximal dual task. This study was essentially concerned with the analysis of power spectra data obtained during progressive or ramp contractions (RCs). The shape of the power spectra established during the first second of the RCs differs between F, S and F+S tasks. Differences in mean power frequency (MPF) calculated during RCs would be representative of a recruitment of motor units (MUs) that is, at least partly, task-dependent. In order to compare MPF values calculated from RCs performed under different mechanical conditions (F, S and F+S), MPF-RMS(PSD) relationships have been established (RMS(PSD) being defined as the power spectrum density RMS). Both BBSH and BBLH exhibited initial MPF values higher in supination RC than in flexion RC. Because of plateau values reached at the same level of muscle activation whatever the task performed, the slope of the MPF-RMS(PSD) relationship was lower in S than in F. These results are in favour of MU recruitment that is, at least partly, different in F and in S conditions. Dual submaximal tasks seem to mix the activation of the F and S subpopulations of MUs as revealed by the spectral parameters obtained during F+S ramp contractions. This study could find some implication in the field of muscle rehabilitation or reinforcement.

Reproducibility of kinetics of electromyogram spectrum parameters during dynamic exercise

During incremental exercise on a cycle ergometer a study was made of the reproducibility of changes in electromyographic activity (EMG) of human quadriceps muscles. Seven subjects performed three periods of incremental exercise either every 2 weeks (G1: four subjects) or 6 weeks (G2: three subjects). Each test was normalized with respect to the maximal aerobic power (MAP) of the subject. It consistd of a quasilinear increase in mechanical power (from 20% to 100% MAP) during 8 min. For rectus femoris muscle activity, changes in total power spectrum (PEMG) and in mean power frequency (MPF) were fitted by a 3rd order polynomial function (named profile) and normalized with respect to the maximal value for PEMG and to the mean value for MPF. A curvilinear increase was found for PEMG. The MPF kinetics varied from one subject to another. These changes were either a continuous increase, or a continuous decrease or an increase followed by a decrease. Only for G2, was MAP determined before each test in order to update the limits of the test. A good reproducibility of PEMG was shown by its mean magnitude [95.88%, (SD 3.92)] and by computing a mean correlation coefficient between profiles two by two [r2=0.948 (SD 0.028)n=21]. Intraclass coefficient correlation (ICC) calculated for each subject indicated a high level of reproducibility for five of the seven subjects (ICC>0.80). No clear effect of MAP updating on PEMG and MPF profile reproducibility was observed. Thus it is suggested that MPF kinetics may characterize a subject at a given moment while PEMG kinetics may illustrate a normal profile, and they may both characterize EMG changes for a population during incremental exercise.

Electromyogram power spectrum during dynamic contractions at different intensities of exercise

SummaryDuring dynamic contractions performed on a cycle ergometer, we studied the influence of motor unit (MU) recruitment on the electromyographic (EMG) spectral content by exerting mechanical power of different intensities, which was chosen to remain below the maximal aerobic power (VO2max). The spectral parameters: EMG total power (PEMG), mean (MPF) and median (MED) power frequencies, which are the most representative of the EMG spectral content, were calculated according to the EMG activity of the vastus medialis muscle (VM) and soleus muscle (SOL) of the right leg. For VM and SOL, PEMG increased linearly with exerted power demonstrating an enhancement of MU recruitment. Moreover these relationships were less scattered when exerted power was expressed as a percentage of VO2max. Changes in MPF and MED with varying exercise intensities were different from one subject to another. For a set of subjects, MPF and MED were found to be independent of exerted power. Although VM and SOL muscles are different in fibre type composition, similar results were obtained for both EMG activities. We have concluded that for dynamic contractions performed at different intensities below VO2max, the recruitment of the MU has a poor effect on the EMG spectral content whatever the predominant type of fibre.

Biomechanical study of impact engagement in rugby union scrummaging of front row player

Rugby union scrum is considered as an important phase of the game and it epitomizes the game physical nature. Due to very high mechanical strains, the scrum is also a severe cervical and shoulder injuries source for the front row player. Most scrummaging researches have mainly focused on forces generated by forward pack (Milburn 1993). In a previous experimental study (Piscione 2006), the impact phase has shown that the force kinetics shapes are close to a rebound phenomenon shape. The present study consists on evaluating a mechanical model, based on the Kelvin–Voigt model, in order to model the impact engagement.

‘Dynamic’ transverse mechanical properties of rat skeletal muscle under in vivo compression

In a general purpose, studies about effects of external mechanical strains on muscle aimed to investigate sores formation, rise of muscle pathologies by long term pressure application, impact damage, muscle fatigue (1)... Few studies concerning mammalian muscles have investigated twitch contraction under in vivo compression either on rat muscle fibre bundles using high hydrostatic pressure (2) or on human biceps using a compressive cuff (3). Our purpose was to study the effect of in vivo compression on twitch contraction with a particular interest to the transverse force developed by compressed muscle.

Tendon jerk, passive stiffness and muscle spindle efficacy in immature rats

In prepubertal children tendon jerk and stretch reflexes were found to be lower than those of adults which were explained, at least partly, by the lower stiffness of the triceps surae quantified in children (Grosset et al. 2007). It was also suggested that differences in spindle sensitivity contributed to the stretch reflex differences. In rats, we have the capacity to investigate reflex activities, muscle stiffness and spindle solicitation and thus to propose respective contributions of muscle stiffness and spindle efficacity in the reflex activation of muscles.

Passive stiffness of rat soleus muscle from weaning to senescence

During growth or during ageing, skeletal muscle undergoes numerous adaptations. Meanwhile, there are few data on the skeletal muscle adaptations throughout life, and particularly with regard to passive mechanical properties. From stress–strain relationship, Kovanen (1989) reported a very slight increase in ultimate tensile strength and tangent modulus in rat soleus muscle between 1 and 24 months whereas Bensamoun et al. (2006) reported a decrease in dynamic tension and static tension measured with relaxation test between 4 and 12 months. The aim of this study is to characterise passive mechanical properties of rat soleus muscle over a lifetime thanks to an incremental stepwise test and a stretch release test.

Transverse mechanical properties of rat skeletal muscle under in vivo ramp compression

Studies about effects of external mechanical strains on muscle aimed to investigate skin or muscle pathologies by long term pressure application, impact damage, muscle fatigue (1)... Few studies concerning mammalian muscles have investigated twitch contraction under in vivo compression either on rat muscle fibre bundles using high hydrostatic pressure (2) or on human biceps using a compressive cuff (3) but no data were available on twitch transverse. In our previous work (4), axial twitch and its transverse force in rat muscle were studied during in vivo compression relaxation tests with short duration (30s). Our present purpose was to study the effects of ramp compression and its cumulative effects on axial and transverse force.

Adaptability of passive stiffness and spindle solicitation along the life of sedentary rats

We have undertaken a systematic quantification of neuromechanical parameters for the soleus muscle of sedentary rats ranging in age between 21 days and 24 months with the intent to compare the kinetics of adaptation for various parameters due to maturation, and then to aging. Among the studied parameters are the spindle afferent discharges recorded from peripheral electroneurograms (ENG) and the passive stiffness, knowing that the spindle solicitation depends on the stiffness of the passive structures linked to the muscle receptors. Recently, we have proposed an index of spindle efficacy (Rosant and Pérot 2006) which relates the spindle discharges to the incremental stiffness calculated at the stretch conditions used to record the ENGs. A deterioration of this spindle efficacy has been found in aging rats (Rosant et al. 2007). In this presentation this spindle efficacy will be calculated all along the life.

Strength and fatigability of rat soleus muscle to weaning until senescence

Development and aging are associated to progressive changes in skeletal muscle morphology and performance (Close 1964, Hort-Legrand et al. 2001, Doherty 2003). Also, we have undertaken a systematic quantification of mechanical and electrophysiological parameters for the soleus muscle (SOL) of sedentary rats ranging in age between 21 days and 24 months with the intent to compare the kinetics of adaptation for the different parameters due to maturation, and then to aging. Among the different parameters studied, evolution of strength and fatigability in soleus muscle is studied.