Proceedings of The Physiological Society
University of Manchester (2010) Proc Physiol Soc 19, C112
Localised Changes in Muscle Shape in Response to Activation of Fast Motor Unit Populations
E. Hodson-Tole1, I. Loram1, T. Vieira2
1. Institute for Biomedical Research into Human Movement and Health, Manchester Metropolitan University, Manchester, United Kingdom. 2. Lab. for Eng. of the Nueromuscular System, Politecnico di Torino, Torino, Italy.
Figure 1. a) The position of the EMG surface electrode and proximal (dark grey) and distal (light grey) ultrasound probes over the MG, showing region of application of the stimulation pulses. Average and root mean square of averaged M-Waves recorded in each EMG channel are shown from one subject. b) Representation of the mean total movement recorded (N=7) in proximal (probe 1) and distal (probe 2) images of MG muscle. Values were calculated from the position of markers placed over representative images and tracked through all recorded frames of each trial. Each square represents the mean total movement of one marker on the image and values are normalised to the maximum mean movement recorded. The grids are displayed so that the muscle is represented proximal-distal along the vertical and deep-superficial along the horizontal. A scale bar is included, with darker colours indicating more movement.
As the physiological and mechanical properties of skeletal muscle motor units show great diversity, their distribution throughout a muscle could have consequences for force output and hence motor control. Here we investigate how faster motor units might be distributed within the human medial gastrocnemius (MG) muscle, by using an electrical stimulation protocol. Subjects stood on two footplates. Strapping was placed around their waist, securing them to a vertical board and enabling them to remain standing with minimal activity in the calf muscles. Two B-mode ultrasound probes were secured to the left leg along the length of the MG, ensuring as much of the muscle as possible was visualised. Adjacent to the probes a linear array of 16 silver bar (1×10 mm) surface electromyography (EMG) electrodes was secured. To activate populations of faster motor units, a series of bipolar current pulses (2 Hz, 200 μs duration) were delivered to the main branch of the posterior tibial nerve over a period of 400 sec. The stimulation amplitude was set at a low level (197.86±55.06mA, N=7) producing clear movement in the ultrasound images with subjects confident they could comfortably tolerate the stimulation for the whole recording period Ultrasound images were analysed using a recently published algorithm1 to track the position of an 8×10 grid of markers placed over a representative image from each muscle view. The total movement of each marker across the time course of each trial was calculated to quantify movement in different regions of the muscle. EMG data from each of the 15 electrode channels were averaged across stimuli to produce representative M-waves of localised muscle activation. The root mean square amplitude of M-waves detected from the different channels was calculated to identify the region(s) over which muscle activity was detected. M-waves were predominantly recorded in the proximal region of the muscle (Fig. 1a). The ultrasound images revealed that significantly more movement occurred in this region (ANOVA p<0.001, Fig. 1b). Our data therefore indicate that, in the human MG muscle, faster motor units are likely to occupy discrete, localized regions concentrated in the proximal portion of the muscle. Such spatial organization is likely to lead to a distribution of physiological and mechanical properties along the length of the muscle. In addition, the localized changes in muscle shape which will occur with activation of different combinations of motor unit populations will influence sensory feedback, from structures such as muscle spindles in different portions of the muscle, and hence there are implications for understanding aspects of motor control.
Where applicable, experiments conform with Society ethical requirements