Are motor units with different activation thresholds spatially distributed in human gastrocnemius medialis?

It has recently been shown that, in human medial gastrocnemius [MG], the fibres of motor units activated during standing occupy localised territories in the longitudinal plane (1). Coupled with the muscle's pennate fascicle architecture there is potential for units with similar intrinsic properties to be concentrated at different points along its length. Aponeurosis material properties and fascicle geometries tend to vary along the length of MG (2-3). Fibres may therefore be arranged within the muscle to occupy a local environment which optimizes their intrinsic properties. Recent evidence indicates that fibres of units activated during standing are more highly represented in the distal portion of MG (4), suggesting low threshold units are preferentially represented in this muscle region. We further investigate this observation by testing whether fibres of motor units with different activation thresholds have different spatial distributions along the proximal-distal axis of MG. Twelve participants completed the study which was approved by the Local Ethics Committee. Surface EMGs were recorded using 128 electrodes placed over MG. Electrically evoked twitches were elicited by stimulating the tibial nerve branch. Pulses were delivered for 50s with current pulse amplitude increased every 10 stimuli to elicit an increase in muscle activation. Representative ankle torques were calculated from recorded force plate data. The degree of MG stimulation was determined from M-wave root mean square [RMS] values. The location of the highest M-waves along the length of the electrode grid was calculated using barycentre coordinates from the spatial distributions of the incremental M-wave values computed for each stimulation level. There was a positive linear relationship between mean ankle plantar flexion torque and M-wave amplitude [p<0.01, R2=0.98]. Increasing stimulation amplitude therefore increased the number of activated motor units. In eight participants the highest M-wave amplitudes occurred in proximal electrode channels for low amplitude stimulation and in more distal channels at higher amplitudes. As M-wave amplitude is spatially associated to the distribution of activated muscle fibres, the shifts in RMS distribution suggest that, in these participants, there was regional organization along the proximal-distal muscle axis of muscle units according to their activation threshold. In three participants larger M-waves moved to proximal electrode channels with increasing stimulation amplitude. In one participant no spatial shifts in M-wave distribution occurred. Such variability between participants warrants further investigation to establish whether it is the result of the experimental methodology or represents variation across the general populatio

Variations in the Spatial Distribution of the Amplitude of Surface Electromyograms Are Unlikely Explained by Changes in the Length of Medial Gastrocnemius Fibres with Knee Joint Angle

This study investigates whether knee position affects the amplitude distribution of surface electromyogram (EMG) in the medial gastrocnemius (MG) muscle. Of further concern is understanding whether knee-induced changes in EMG amplitude distribution are associated with regional changes in MG fibre length. Fifteen surface EMGs were acquired proximo-distally from the MG muscle while 22 (13 male) healthy participants (age range: 23-47 years) exerted isometric plantar flexion at 60% of their maximal effort, with knee fully extended and at 90 degrees flexion. The number of channels providing EMGs with greatest amplitude, their relative proximo-distal position and the EMG amplitude averaged over channels were considered to characterise changes in myoelectric activity with knee position. From ultrasound images, collected at rest, fibre length, pennation angle and fat thickness were computed for MG proximo-distal regions. Surface EMGs detected with knee flexed were on average five times smaller than those collected during knee extended. However, during knee flexed, relatively larger EMGs were detected by a dramatically greater number of channels, centred at the MG more proximal regions. Variation in knee position at rest did not affect the proximo-distal values obtained for MG fibre length, pennation angle and fat thickness. Our main findings revealed that, with knee flexion: i) there is a redistribution of activity within the whole MG muscle; ii) EMGs detected locally unlikely suffice to characterise the changes in the neural drive to MG during isometric contractions at knee fully extended and 90 degrees flexed positions; iii) sources other than fibre length may substantially contribute to determining the net, MG activatio

Familial STAG2 germline mutation defines a new human cohesinopathy

We characterize a novel human cohesinopathy originated from a familial germline mutation of the gene encoding the cohesin subunit STAG2, which we propose to call STAG2-related X-linked Intellectual Deficiency. Five individuals carry a STAG2 p.Ser327Asn (c.980 G > A) variant that perfectly cosegregates with a phenotype of syndromic mental retardation in a characteristic X-linked recessive pattern. Although patient-derived cells did not show overt sister-chromatid cohesion defects, they exhibited altered cell cycle profiles and gene expression patterns that were consistent with cohesin deficiency. The protein level of STAG2 in patient cells was normal. Interestingly, STAG2 S327 is located at a conserved site crucial for binding to SCC1 and cohesin regulators. When expressed in human cells, the STAG2 p.Ser327Asn mutant is defective in binding to SCC1 and other cohesin subunits and regulators. Thus, decreased amount of intact cohesin likely underlies the phenotypes of STAG2-SXLID. Intriguingly, recombinant STAG2 p.Ser327Asn binds normally to SCC1, WAPL, and SGO1 in vitro, suggesting the existence of unknown in vivo mechanisms that regulate the interaction between STAG2 and SCC1

Equilíbrio estático e dinâmico em bailarinos: revisão da literatura

A dança envolve integração de movimento, equilíbrio postural e aspectos relacionados ao controle postural. Informações sobre o equilíbrio em bailarinos são de grande importância, pois eles são considerados modelos de controle postural. O objetivo foi revisar estudos sobre equilíbrio postural estático e dinâmico em bailarinos, caracterizando o controle e a dependência visual desses atletas para a manutenção do equilíbrio. Para isso, foi realizada uma revisão nas bases de dados PubMed, SciELO, Lilacs e Science Direct, considerando o período entre 1997 a 2013, utilizando os descritores equilíbrio, controle postural, plataforma de forças, ballet, bailarinos clássicos e aferência visual. Foram selecionados 18 artigos capazes de fornecer dados quantitativos para avaliação do equilíbrio nesses atletas classificados pelo nível de evidência científica Oxford. A literatura revisada mostra completa concordância quanto ao efeito da retirada da informação visual sobre a estabilidade postural de bailarinos considerados como executantes altamente treinados. Estudos mostrando a comparação do equilíbrio de bailarinos com outras técnicas desportivas confirmaram um padrão específico de equilíbrio nesses indivíduos. Entretanto, associando-se à restrição visual, bailarinos apresentaram maior deslocamento do centro de pressão comparado a outras modalidades desportivas, sugerindo maior dependência visual para a manutenção do equilíbrio. Bailarinos apresentam menor oscilação postural em relação a indivíduos não treinados e indivíduos treinados em outras práticas desportivas, com maior dependência visual para manutenção do equilíbrio