Proceedings of The Physiological Society

Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PCA300

Poster Communications

The effect of muscle-specificity and muscle length on the modulation of corticospinal excitability during passive ankle movement in humans

J. Škarabot1, P. Ansdell1, C. Brownstein1, G. Howatson1, S. Goodall1, R. Durbaba1

1. Northumbria University, Newcastle upon Tyne, United Kingdom.

Modulation in corticospinal excitability has been shown in passive movement of the upper limb, but data on lower limb muscles is scarce. Whilst differences in neuronal behaviour between upper and lower limb muscles have been established, differences also exist between the muscles of the lower limb. For example, neuronal behaviour of the antagonist about the ankle such as soleus (SOL) and tibialis anterior (TA) might differ due to differences in afferent input (1,2) and distribution of corticomotoneuronal projections (3,4). Additionally, the neuronal behaviour between static position and passive shortening and lengthening may also depend on the muscle length at the point of stimulation (5). Thus, the purpose of this study was to assess corticospinal excitability during static positioning and passive plantar and dorsiflexion in SOL and TA, respectively, with stimulation delivered at short, intermediate and long muscle lengths. Healthy, young volunteers (n = 12; 6 females) were recruited. All testing was performed on the right limb with the subject seated in an isokinetic dynamometer. During passive shortening and lengthening, the ankle was moved through a range-of-motion (ROM) of 20° (10° of plantarflexion and dorsiflexion; 0° = ankle joint at 90°) at 5°×s−1. Transcranial magnetic stimuli (TMS) were delivered with the ankle in a static position and during passive movement at 0°, 7.5° plantarflexion (long and short muscle length for TA and SOL, respectively) and 7.5° dorsiflexion (short and long muscle length for TA and SOL, respectively) to elicit motor evoked potentials (MEPs). In addition, maximal muscle responses (Mmax) were elicited via supramaximal stimulation of the peripheral nerve in each of the three static positions and were used to normalise MEPs. TMS stimulus intensity was set to 1.2 × resting motor threshold (separately for TA and SOL) and responses were recorded separately for each muscle. MEP/Mmax was not modulated between static positioning, passive lengthening or shortening in SOL (0.02 ± 0.003 for all; p = 0.121). In TA, MEP/Mmax was facilitated during passive shortening (0.17 ± 0.02) compared to static positioning (0.10 ± 0.02; p = 0.023) and passive lengthening (0.09 ± 0.02; p < 0.001). Muscle length at the point of stimulation had no effect on MEP/Mmax in TA (p = 0.787). These results suggest a differential intrinsic modulation of corticospinal excitability between TA and SOL during passive movement, which is possibly mediated by differences in feedback from spindle receptors causing changes at the supraspinal level, and/or dissimilar, non-uniform distribution of direct corticomotoneuronal projections whereby TA is more susceptible to facilitation (3,4). Alternatively, the fascicle length changes across the ROM might differ for SOL and TA, resulting in different quantity of afferent input.

Where applicable, experiments conform with Society ethical requirements