Application of the F-Response for Estimating Motor Unit Number and Amplitude Distribution in Hand Muscles of Stroke Survivors

Abstract

The F-response was used in this study to assess changes in the first dorsal interosseous (FDI) muscle after a hemispheric stroke. The number of motor units and their sizes were estimated bilaterally in 12 stroke survivors by recording both the compound muscle action potential (CMAP) and F wave responses. These F waves were induced by applying a large number of electrical stimuli to the ulnar nerve. The amplitude distribution of individual motor unit action potentials (MUAPs) was also compared between paretic and contralateral muscles. When averaged across all the subjects, a significantly lower motor unit number estimate was obtained for the paretic FDI muscle ( 88 ±13) compared with the contralateral side ( 139 ±11) ( ). Pooled surface MUAP amplitude analysis demonstrated a right-skewed distribution for both paretic (kurtosis 3.0) and contralateral (kurtosis 8.52) muscles. When normalized to each individual muscle's CMAP, the surface MUAP amplitude ranged from 0.22% to 4.94% (median 1.17%) of CMAP amplitude for the paretic muscle, and from 0.13% to 3.2% (median 0.62%) of CMAP amplitude for the contralateral muscle. A significant difference in MUAP outliers was also observed between the paretic and contralateral muscles. The findings of this study suggest significant motor unit loss and muscle structural reorganization after stroke.

Figure 1

An example of four surface MUAPs extracted from F wave responses in the paretic FDI muscle of Subject 9. All detected surface MUAPs have identical size, shape and latency. The F responses were evoked in submaximal electrical stimulation with M wave amplitude 30% of CMAP. Note that upper part shows the raw F responses superimposed on M wave afterpotentials. The displaced baseline was removed in detected surface MUAPs as shown in the bottom.

Figure 2

Comparisons of the CMAP amplitude and motor unit number estimation (MUNE) between the paretic and contralateral muscles. CMAP amplitude (mean ± standard error): 11.55 ± 0.94 mV (paretic), 13.96 ± 1.00 mV (contralateral), p < 0.01. MUNE: 88 ± 13 (paretic), 139 ± 11 (contralateral), p < 0.01.

Figure 3

Left: plot of ratio of MUNE vs ratio of CMAP amplitude (R² = 0.45, p < 0.02). Right: plot of ratio of MUNE vs ratio of pinch force (R² = 0.26, p = 0.09).

Figure 4

Surface MUAP amplitude distribution for paretic and contralateral muscles. Surface MUAPs were pooled across all the subjects: 197 MUAPs from paretic side and 181 MUAPs from contralateral side. The MUAP amplitude was normalized to each muscle’s CMAP amplitude.