How much does the human medial gastrocnemius muscle contribute to ankle torques outside the sagittal plane?

Ankle movements in the frontal plane are less prominent though not less relevant than movements in the plantar or dorsal flexion direction. Walking on uneven terrains and standing on narrow stances are examples of circumstances likely imposing marked demands on the ankle medio-lateral stabilization. Following our previous evidence associating lateral bodily sways in quiet standing to activation of the medial gastrocnemius (MG) muscle, in this study we ask: how large is the MG contribution to ankle torque in the frontal plane? By arranging stimulation electrodes in a selective configuration, current pulses were applied primarily to the MG nerve branch of ten subjects. The contribution of populations of MG motor units of progressively smaller recruitment threshold to ankle torque was evaluated by increasing the stimulation amplitude by fixed amounts. From smallest intensities (12-32 mA) leading to the firstly observable MG twitches in force-plate recordings, current pulses reached intensities (56-90 mA) below which twitches in other muscles could not be observed from the skin. Key results show substantial MG torque contribution tending to rotate upward the foot medial aspect (ankle inversion). Nerve stimulation further revealed a linear relationship between the peak torque of ankle plantar flexion and inversion, across participants (Pearson R>0.81; P<0.01). Specifically, regardless of the current intensity applied, the peak torque of ankle inversion amounted to about 13% of plantar flexion peak torque. Physiologically, these results provide experimental evidence that MG activation may contribute to stabilize the body in the frontal plane, especially under situations of challenged stabilit

A NOVEL SYSTEM OF ELECTRODES TRANSPARENT TO ULTRASOUND FOR SIMULTANEOUS DETECTION OF MYOELECTRIC ACTIVITY AND B-MODE ULTRASOUND IMAGES OF SKELETAL MUSCLES

Application of two dimensional surface electrode arrays can provide a means of mapping motor unit action potentials on the skin surface above a muscle. The resulting muscle tissue displacement can be quantified, in a single plane, using ultrasound imaging (US). Currently however, it is not possible to simultaneously map spatio-temporal propagation of activation and resulting tissue strain. In this manuscript we eveloped and tested a material that will enable concurrent measurement of 2D surface EMGs with US images. Specific protocols were designed to test the compatibility of this new electrode material both with EMG recording and with US analysis. Key results indicate that, for this new electrode material: i) the electrode-skin impedance is similar to that of arrays of electrodes reported in literature; ii) the reflection of ultrasound at the electrode-skin interface is negligible; iii) the likelihood of observing missing contacts, short-circuits and artefacts in EMGs is not affected by the US probe; iv) movement of tissues sampled by US can be tracked accurately. We therefore conclude this approach will facilitate multi-modal imaging of muscle to provide new spatio-temporal information regarding electromechanical function of muscle. This is relevant to basic physiology-biomechanics of active and passive force transmission through and between muscle, of motor unit spatio-temporal activity patterns, of their variation with architecture and task related function, and of their adaptation with ageing, training-exercise-disuse, neurological disease and injur

Distinct muscle fascicle length changes in feline medial gastrocnemius and soleus muscles during slope walking

On the basis of differences in physiology, e.g., histochemical properties and spindle density, and the structural design of the cat soleus (SO) and medial gastrocnemius (MG) muscles, we hypothesized that 1) fascicle length changes during overground walking would be both muscle and slope dependent, which would have implications for the muscles' force output as well as sensory function, and that 2) muscle-tendon unit (MTU) and fascicle length changes would be different, in which case MTU length could not be used as an indicator of muscle spindle strain. To test these hypotheses, we quantified muscle fascicle length changes and compared them with length changes of the whole MTU in the SO and MG during overground walking at various slopes (0, ± 25, ± 50, +75, and +100%). The SO and MG were surgically instrumented with sonomicrometry crystals and fine-wire electromyogram electrodes to measure changes in muscle fascicle length and muscle activity, respectively. MTU lengths were calculated using recorded ankle and knee joint angles and a geometric model of the hindlimb. The resultant joint moments were calculated using inverse dynamics analysis to infer muscle loading. It was found that although MTU length and velocity profiles of the SO and MG appeared similar, length changes and velocities of muscle fascicles were substantially different between the two muscles. Fascicle length changes of both SO and MG were significantly affected by slope intensity acting eccentrically in downslope walking (-25 to -50%) and concentrically in upslope walking (+25 to +100%). The differences in MTU and fascicle behaviors in both the SO and MG muscles during slope walking were explained by the three distinct features of these muscles: 1) the number of joints spanned, 2) the pennation angle, and 3) the in-series elastic component. It was further suggested that the potential role of length feedback from muscle spindles is both task and muscle dependent. Copyright © 2009 the American Physiological Society