UK Microgravity Research

The influence of gravity on the control of voluntary movement in man - the effect of dance training

Dr N. J. Davey

This is a neurophysiological study aimed at understanding the corticospinal (voluntary) control of movement in man. We shall examine three subjects during parabolic flight in 0G, 1G 2G environments. Subjects will have had different experiences of both dance-training and parabolic flight. The project addresses the influence of gravitational fields on the corticospinal (voluntary) control of limb muscles and how these are coordinated with stabilising postural adjustments. In other words, if we extend our left arm horizontally our centre of gravity is disturbed. If we do nothing to correct for this we will topple over. In fact, we contract out back and trunk muscles on the other side of our body to maintain our stability and stop us falling over.

We are able to test the corticospinal pathways from the brain, via the spinal cord to the muscles using a technique called transcranial magnetic stimulation (TMS). TMS induces electrical currents in the brain and these are sufficient to excite some of the large corticospinal neurones that project down the spinal cord and connect (via synapses) to the motoneurones controlling each of the body's muscles. We can record the responses from the muscles to the TMS using surface electromyography (EMG). The size of the EMG output will vary according to how excitable each of these synapses is. The synaptic excitability can alter according to what inputs from other sources (other parts of the brain, sensory receptors, vision, ears etc, etc) the synapse is receiving. So, for example, if the subject is making a voluntary effort in a muscle, the response to TMS will be very much larger because the pathway to that muscle is much more excitable than when the muscle is at rest. In this study, we shall record EMG from the back muscles and the arm muscles and examine how the size of the responses vary according to gravitational conditions and when the arm is extended or relaxed. In this way we shall be able to understand how important gravity is to the voluntary maintenance of posture, particularly when the limbs are moved.

Our hypothesis is that shared corticospinal (from the motor cortex in the brain) input to back and limb muscles helps maintain trunk stability when a limb is moved and that this control pathway may be enhanced by particular forms of training such as that undertaken by dancers. Our ground-based study (Davey et al., 2002) has shown that these pathways operate in a similar way in both lying and standing postures, suggesting that postural reflexes alone may not be responsible for this compensation. Gravity-related reflexes (eg: vestibular (ears) or proprioceptive (joint and muscle)) will be modified by the prevailing gravitational field and the dependence on corticospinal control may become greater. Individuals with a greater ability to control posture (such as dancers) may have an enhanced corticospinal control of postural muscles and perform better when gravitational cues are lost (zero G) or altered (2 G).

We have completed two parabolic flights as a test-bed for our EMG recording apparatus in these extreme conditions. We have recently been granted permission by the French Health Authority (MEDES) to use TMS in parabolic flight and plan another three flights in March 2003. Clearly, if the work leads to greater understanding of interaction of limb and trunk muscles, it will provide insights into the mechanisms behind physiotherapy techniques that are used to rehabilitate individuals following neuro-trauma such as stroke or spinal cord injury.

REFERENCE
Davey NJ, Lisle R, Loxton-Edwards B, McGregor AH, Nowicky AV (2002) Activation of back muscles during abduction of the contralateral arm in man. Spine 27:1355-1360