Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC180
The rapid neural response to slow platform tilts in standing man.
T. Osborne1, M. Lakie1
1. University of Birmingham, Birmingham, United Kingdom.
Gurfinkel et al (1995) have previously described the reactions of standing human subjects to slow tilts of the support surface. Averaged records showed that a constant velocity platform tilt of 0.05°/s initially resulted in a relatively fast body inclination in the direction of the tilt. After a period of around 3-5 seconds, stronger resistance to body inclination began. Gurfinkel et al suggested the cause of this initial body deviation was due to mechanical yielding of the calf muscle/tendon/joint unit under conditions of support surface tilt. Neural intervention occurred only as the body slowed after the initial inclination. The recent discovery (Casadio et al., 2005;Loram & Lakie, 2002) that ankle stiffness is inadequate on its own to provide stable standing has lead us to reinvestigate this behaviour. Subjects (n = 5, 3 male) stood quietly with eyes closed on custom built footplates controlled by a servo motor which generated slow rotations in the sagital plane around the axis of the ankle joint. Trials consisted of a baseline period lasting between 10 and 15 seconds, followed by a constant velocity (0.05°/s) toes-up, or toes-down tilt lasting 10 seconds. 20 toes-down, and 20 toes-up tilts were completed for each subject. We combined high quality ultrasound imaging of the soleus and gastrocnemius muscles with a novel automatic tracking technology to provide a continuous measure of muscle length change for the duration of the initial response. Ankle and platform angles were measured by laser retroreflective rangefinders and surface EMG recorded from the soleus and gastrocnemius muscles. Our averaged results of a toes-down tilt of the support surface (Fig 1. top) confirm that the initial rapid body deviation, as measured by ankle angle (Fig 1. middle), lasted around 3 seconds. This was accompanied by a shortening of the soleus and gastrocnemius muscles (Fig 1. bottom). Conversely, toes-up tilts resulted in a corresponding backward body inclination accompanied by a lengthening of the calf muscles. Changes in calf muscle EMG were observable within 500 ms of the start of the ramp. Mechanically, the calf muscles would be expected to lengthen as the body leant forwards, and shorten as the body leant backwards. Out results show the opposite demonstrating that neural rather than purely mechanical processes are involved in the initial response to slow support surface tilts.
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