Heteronymous reflex responses in a hand muscle when maintaining constant finger force or position at different contraction intensities
Introduction
When an individual performs a submaximal isometric contraction with the first dorsal interosseus (FDI) either to produce a constant force against a rigid restraint (force task) or to maintain the position of the index finger against a constant load (position task), the amplitude of the stretch reflex (Doemges and Rack, 1992a, Maluf et al., 2007) and the tendon-tap reflex (T-reflex) (Maluf et al., 2007, Jordan et al., 2007) are similar during both tasks. In contrast, when a reflex was elicited in the FDI by electrical stimulation of the median nerve (heteronymous H-reflex), the amplitude of the reflex response was larger during the position task than during the force task (Maluf et al., 2007, Jordan et al., 2007). Similar results have been observed with single motor units recorded in FDI in response to the T-reflex and median nerve H-reflex (Jordan et al., 2007).
These divergent results for electrically and mechanically induced reflexes might be attributable either to the reflex pathway involved (i.e., homonymous for the stretch and T-reflexes and heteronymous for the median nerve stimulation) or to the type of stimulus applied (mechanical and electrical stimulus). Previous work, however, suggests similar modulation of the afferent input onto the motor neuron pool from the homonymous and heteronymous pathways (Meunier and Pierrot-Deseilligny, 1989). Moreover, the electrically induced reflex response is more sensitive to presynaptic inhibition compared with stretch and tendon-tap reflexes (Morita et al., 1998). If the different adjustments in the T- and H-reflexes during the force and position tasks are attributable to presynaptic inhibition, the greater amplitude of the heteronymous H-reflex during the position task should be present during contractions performed at different contraction intensities as presynaptic inhibition does not change with contraction force (Meunier and Pierrot-Deseilligny, 1989). Similarly, the absence of a difference in the T-reflex across tasks should not change with contraction force. If confirmed, these results suggest that changes in the efficacy of presynaptic inhibition, as observed in elderly adults (Butchart et al., 1993, Earles et al., 2001, Tsuruike et al., 2003), stroke patients (Aymard et al., 2000), individuals with spasticity (Morita et al., 2001), and in healthy subjects after a few weeks of limb immobilization (Lundbye-Jensen and Nielsen, 2008) and the consumption of ethanol (von Dincklage et al., 2007), could compromise motor performance.
The aim of the study was to compare heteronymous reflex responses evoked in the FDI by electrical and mechanical stimuli when the force and position tasks were performed at three target forces. These results indicate a similar modulation of homonymous and heteronymous afferent input onto the motor neuron pool of the agonist muscle, but a different adjustment of presynaptic inhibition during the force and position tasks. In addition, our results underscore that the use of the H-reflex method appears as a better tool to investigate fine differences in the afferent synaptic input onto the motor neuron pool between tasks compared with the mechanically evoked reflex.
Section snippets
Materials and methods
After informed consent was obtained, experiments were conducted on 22 subjects (9 women) aged between 18 and 37 yr (25.0 ± 5.7 yr; mean ± SD). None of the participants reported any signs of neurological disorder or cardiovascular disease. Subjects were all right-handed and were asked to refrain from exercising the arm muscles for 24 h before testing. The Human Subjects Committee at the University of Colorado in Boulder approved the experimental procedures.
Results
Nineteen of the 22 subjects who participated in the study were included in the statistical analysis of the reflex responses induced by the median nerve stimulation (11 men; 25.0 ± 5.7 years; 165.7 ± 12.7 cm; 65.8 ± 10.6 kg) and 18 were included in the analysis of the tendon-tap of APB (8 women; 25.2 ± 5.8 years; 167.9 ± 13.8 cm; 65.6 ± 10.9 kg). Data were excluded for the electrically induced reflex from three subjects. In two subjects, the difference in EMG activity between the force and position tasks
Discussion
The main finding of this study was the greater response for the heteronymous H-reflex during the position task compared with the force task performed at the three contraction forces. In contrast, there was no difference between tasks in the size of the heteronymous T-reflex at any contraction force. In combination with previous work, these results indicate that the task difference in reflex responsiveness depends more on the type of stimulus applied than the reflex pathway used, and are
Acknowledgments
The work was supported by an award (NS043275) from the National Institute of Neurological Disorders and Stroke to R.M. Enoka.
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