Soleus H-reflex recruitment is not altered in persons with chronic spinal cord injury

Abstract

Objective: To determine whether spasticity in persons with spinal cord injury (SCI) is associated with elevated monosynaptic reflex excitability.

Design: One-way experimental.

Setting: Research laboratory.

Participants: Convenience sample of 9 subjects (8 men, 1 woman) with chronic and complete SCI and 20 persons (14 men, 6 women) with no neurologic impairment. Subjects with SCI exhibited lower-extremity spasticity as indicated by velocity-dependent increased resistance to passive muscle stretch, abnormally brisk deep tendon reflexes, involuntary lower-extremity flexion and/or extension spasms, and clonus.

Intervention: Soleus H-reflex recruitment curves were elicited in all subjects.

Main outcome measures: Soleus H-reflex threshold (HTH), gain (HGN), and amplitude (HPP).

Results: There was no difference between subjects with and without SCI in HTH, HGN, or HPP.

Conclusions: Spasticity in people with chronic and complete SCI was not associated with increased excitability of the connections between Ia afferent projections and motoneurons. Factors extrinsic to these connections may have a role in spasticity caused by SCI.

Fig 1

Representative examples of H-reflexes, M waves, and recruitment curves from an ABLE subject (A, B) and an SCI subject (C, D). (A, C) Single sweeps of raw data. Numbers to the right of each sweep indicate the stimulation intensity at which each response was obtained. H-reflexes and M waves were observed at appropriate latencies after the stimulation artifact. (B, D) Circles (●) represent H-reflex peak-to-peak amplitude; squares (■) are M-wave peak-to-peak amplitude. Values are presented as a proportion of the maximum M-wave and are plotted against stimulation intensity. Each point represents the mean response observed at a given stimulation intensity. Solid lines are the best-fit lines calculated from the rising portion of H-reflex and M-wave recruitment. The equations of these lines were used to calculate HTH and HGN. See Methods. Note the similarity in the appearance of the recruitment curves for the 2 subjects.

Fig 2

Group mean ± standard error data for (A) HTH, (B) HGN, and (C) HPP. There were no between-group differences (SCI vs ABLE) in these measures (P≥.05). Abbreviations: MGN, m-wave gain; MTH, M-wave threshold.

Fig 3

Correlations among HTH, HGN, and HPP. Correlation coefficients (r) are shown for the pooled data (SCI, ABLE). *Correlation differed significantly from zero (P≤.05). Relationships among the dependent measures were similar in both groups. Legend: ○, ABLE subjects; +, SCI subjects.