University of Houston • University of Houston-Clear Lake • ISSO Annual Report Y2002—pp. 87-88

Muscle Reflexes Are Modulated Preparatory Neuromuscular Activation Levels

Charles S. Layne (UH)

THIS PROJECT IS DESIGNED TO INVESTIGATE WHETHER VARYING patterns of mechanical stimulation to the soles of the feet have an impact on subsequent neuromuscular activation. The basic concept underlying the project is that sensory input can be used to modulate neuromuscular activation levels and that different stimulation patterns can result in different amounts of activation. If supported, a new rehabilitation technology can be developed that relies on controlled patterns of foot sensory input to generate muscle contractions.

Muscle contractions are a prerequisite for the prevention of disuse muscle atrophy. The loss of muscle mass (atrophy) is a common occurrence for astronauts and bedridden individuals. Lower limb atrophy and associated loss of motor control present substantial risks to crew health and mission objectives. Currently, the leading countermeasure for atrophy is resistance exercise; however, this is a time-consuming strategy. Previous evidence indicates that mechanical stimuli to the soles enhances neuromuscular activity and thereby can possibly attenuate muscle atrophy. These results suggest foot pressure could be used as a complementary countermeasure to the current exercise regime, producing a more time-efficient strategy.

In this study, data were collected from twenty seated subjects whose foot rested on a dynamic foot pressure device (DFPD). The DFPD uses small solenoids embedded within an aluminum frame to provide mechanical stimuli to the sole. The solenoids are computer-operated and can be independently controlled to provide stimulation of varying spatial and temporal patterns. In the current study two different experimental protocols were used. In both protocols the dependent measures were peak surface electromyographic (EMG) amplitude and positive integrated area in volts/seconds (PIA).

In one protocol, a stimulus was applied to one area of the sole (heel). Immediately after this stimulus, a second stimulus was applied to a different area of the sole (lateral ridge). Each stimulus consisted of a solenoid applying a mechanical stimulus to the sole for 250 milliseconds. Stimulation "doublets" were randomly applied twenty times within a 60-second time period. This protocol was designed to determine if the prior application of a stimulus to one area of the sole influenced the response to the second stimulus applied to the different area. EMG data obtained in response to second stimulus were compared to EMG obtained when that same site was stimulated without prior stimulation of another site. Preliminary analysis indicates that the response to a second stimulus that has been immediately preceded by a stimulus to another area of the sole is attenuated relative to a baseline response. These data suggest that there is a temporal and spatial relationship between stimuli and subsequent neuromuscular responses.

A second protocol was designed to investigate the role of muscle spindle input on neuromuscular responses to the mechanical stimulation of the sole. In this protocol, spindle input was modified by changing the ankle angle prior to stimulation of a single site on the sole. This stimulation site was under the fifth metatarsal, and the stimulus was again applied for 250 ms. Researchers applied stimulus twenty times within a 60-second trial. Since muscle spindles respond to muscle length, varying the ankle angle such that the ankle extensor muscles were held in either a shortened or elongated length changes muscle spindle input. Spindle input was modulated because it is known to have a direct impact upon muscle reflexes, particularly the mono-synaptic stretch reflex. As hypothesized, when the muscle was shortened, the resulting neuromuscular response was attenuated relative to when the muscle was held in a stretched position. Analyses are ongoing.

Current data clearly indicate that the temporal and spatial pattern of mechanical stimulation can interact with an existing neural "environment" that, itself, is subject to modification, to influence muscle activation levels in response to stimulation. There are optimal patterns of stimulation that result in the greatest response to mechanical stimulation of the sole. Once these patterns are more fully identified, they can be incorporated into a rehabilitative device that utilizes sole stimulation.

Two UH graduate students, Andrew Abercromby and Katey Forth, participated in this study. Their project is described in an article in The Daily Cougar (May 29, 2002), official student newspaper of the University of Houston. <http://www.stp.uh.edu/vol67/142/news/news2.html>

Presentations
Layne, C. S., K. E. Forth, and A. F. Abercromby. "Spatial Factors Influence the Generation of Neuromuscular Responses to Foot Stimulation," 14th Humans in Space Conference, Banff, Alberta, Canada, May, 2003. (In review.)
Layne, C. S., K. E. Forth, and A. F. Abercromby. "Does Varying Muscle Spindle Input Modify Neuromuscular Responses to Foot Stimulation?" 14th Humans in Space Conference, Banff, Alberta, Canada, May, 2003. (In review.)
Layne, C. S., A. P. Mulavara, J. J. Bloomberg, K. E. Forth, M. F. Baxter, J. J. Houser, and I. B. Kozlovskaya. "Using Dynamic Foot Stimulation During Space Flight as a Countermeasure to Muscle Degradation," 24th Annual International Conference of the IEEE/EMBS and Annual Fall Meeting of the Biomedical Engineering Society, Houston, TX, Oct., 2002. (Invited.)
Layne, C. S., K. E. Forth, M. F. Baxter, and J. J. Houser. "Enhanced Neuromuscular Activity from Mechanical Foot Stimulation. World Space Congress, Houston, TX, Oct., 2002.

Funding and proposals
Barbieri, E., H. A. Malki, C. S. Layne, W. Shireen, and F. Attarzadeh. "Prototype EVRA Machine for Rehabilitation." Grants to Enhance and Advance Research (GEAR), University of Houston, May, 2003, $22,840.
"Continued Development of a Foot Pressure Device." NASA mini-grant, $53,000 (pending).
"Countermeasures During Bedrest." Joint proposal with European scientists, European Space Agency (ESA). Funding to UH dependent on matching by US interests (pending). <http://spaceflight.esa.int/users/downloads/facilities-doc/bed_RA-03-LTBRall.htm>

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Institute for Space Systems Operations - Y2002 Annual Report
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