F-wave responses and recruitment at low intensities: Muscle-specific excitability profiles of intrinsic hand muscles

F-waves are late motor responses traditionally elicited via supramaximal stimulation. However, submaximal stimulation may offer a more sensitive probe for assessing spinal motoneuron excitability. This study characterizes F-wave recruitment curves using graded submaximal stimulation across various intrinsic hand muscles.

F-waves were recorded from the abductor pollicis brevis (APB), abductor digiti minimi (ADM), and first dorsal interosseous (FDI) in 15 healthy subjects (ages 35-78). Four stimulus intensities were employed-10%, 20%, 30%, and 100% of the maximal compound muscle action potential (CMAP) amplitude-with 20 stimuli delivered per intensity. Analysis focused on F-wave frequency, amplitude, and matched F-wave/CMAP ratios (F-wave/CMAPm), where the F-wave and CMAP were recorded at identical stimulus intensities.

Most parameters correlated positively with stimulus intensity; however, F-wave/CMAPm consistently decreased as intensity increased. The ADM exhibited a distinct recruitment profile, with F-wave persistence rising steeply at low intensities and reaching near-saturation at 30%, while showing no significant amplitude differences between submaximal levels. In contrast, the APB and FDI displayed more linear recruitment patterns for both persistence and amplitude.

Submaximal stimulation reliably elicits F-waves and reveals muscle-specific excitability profiles. The ADM's high excitability suggests a more readily recruitable motoneuron pool. Furthermore, the inverse relationship between F-wave/CMAPm and stimulus intensity suggests the early recruitment of larger, highly excitable Type II motoneurons. This refined recruitment curve approach may provide valuable insights into the "split-hand" phenomenon observed in amyotrophic lateral sclerosis.