Sensory enhancement amplifies interlimb cutaneous reflexes in wrist extensor muscles

Abstract

Interlimb neural connections support motor tasks such as locomotion and cross-education strength training. Somatosensory pathways that can be assessed with cutaneous reflex paradigms assist in subserving these connections. Many studies show that stimulation of cutaneous nerves elicits reflexes in muscles widespread across the body and induces neural plasticity after training. Sensory enhancement, such as long-duration trains of transcutaneous stimulation, facilitates performance during rehabilitation training or fatiguing motor tasks. Performance improvements due to sensory stimulation may be caused by altered spinal and corticospinal excitability. However, how enhanced sensory input regulates the excitability of interlimb cutaneous reflex pathways has not been studied. Our purpose was to investigate the effects of sensory enhancement on interlimb cutaneous reflexes in wrist extensor muscles. Stimulation to provide sensory enhancement (2-s trains at 150 Hz to median or superficial radial nerves) or evoke cutaneous reflexes (15-ms trains at 300 Hz to superficial radial nerve) was applied in different arms while participants (n = 13) performed graded isometric wrist extension. Wrist extensor electromyography and cutaneous reflexes were measured bilaterally. We found amplified inhibitory reflexes in the arm receiving superficial radial and median nerve sensory enhancement with net reflex amplitudes decreased by 709.5% and 695.3% repetitively. This suggests sensory input alters neuronal excitabilities in the interlimb cutaneous pathways. These findings have potential application in facilitating motor function recovery through alterations in spinal cord excitability enhancing sensory input during targeted rehabilitation and sports training.NEW & NOTEWORTHY We show that sensory enhancement increases excitability in interlimb cutaneous pathways and that these effects are not influenced by descending motor drive on the contralateral side. These findings confirm the role of sensory input and cutaneous pathways in regulating interlimb movements. In targeted motor function training or rehabilitation, sensory enhancement may be applied to facilitate outcomes.

Keywords: cutaneous reflexes; interlimb reflexes; sensory enhancement.

Fig. 1.

Overview of experimental protocol. Muscle contraction level and stimulation applied during each condition are presented in separate rows. Muscle contraction level (10, 25, or 35% of maximal muscle activation, EMG_MVC) and stimulated nerve (superficial radial nerve, SR; median nerve, MED) are presented in separate columns for the dominant and nondominant arm, respectively. The single lightning bolt represents short-duration (15 ms) and high-frequency (300 Hz) reflex stimulation, whereas the series of lightning bolts represent long-duration (2 s) and low-frequency (150 Hz) stimulation for sensory enhancement.

Fig. 2.

Muscle activity during a typical trial with the windows chosen for data analysis. A: activity of wrist extensor carpi radialis muscle (ECR EMG). Data were rectified and low-pass filtered at 100 Hz. Artifact from the sensory enhancement was removed. The window used for cutaneous reflex analysis is indicated within the gray-shaded area. B: data used for cutaneous reflex analysis in the same trial. Horizontal dashed line represents the average value of prestimulation EMG. Early-latency reflex (peak response latency ~50–75 ms) is averaged over a 10-ms window (dark gray) centered around the lowest value. Net reflex is the cumulated average value in the 150-ms window (light gray) poststimulation period.

Fig. 3.

EMG traces of an individual participant at each condition during Task 1 [sensory-enhanced arm performed 10% maximal voluntary contraction (MVC), reflex-stimulated arm performed graded contraction contraction]. Solid, dashed, and dotted lines are averages across 15 sweeps of cutaneous stimulation when reflex-stimulated arm contracted at 10%, 25%, and 35% of EMG_MVC, respectively. Shaded areas represent SD of 15 sweeps at each contraction level. Stimulation artifacts were removed.

Fig. 4.

EMG traces of an individual participant at each condition during Task 2 [sensory-enhanced arm performed graded contraction, reflex-stimulated arm performed 10% maximum voluntary contraction (MVC)]. Solid, dashed, and dotted lines are averages across 15 sweeps of cutaneous stimulation when sensory-enhanced arm contracted at 10%, 25%, and 35% of EMG_MVC, respectively. Shaded areas represent SD of 15 sweeps at each contraction level. Stimulation artifacts were removed.

Fig. 5.

Effects of sensory enhancement and background muscle activity on early-latency and net reflex amplitudes with the reflex-stimulated arm performing graded contraction (Task 1). A and C: solid bar graphs showing results from the sensory-enhanced arm. B and D: patterned bar graphs showing results from the reflex-stimulated arm. For A–D, the y-axis represents normalized reflex amplitudes and the x-axis represents different conditions. Different shades in each bar graph represent the level of muscle contraction performed in the reflex-stimulated arm. *P < 0.05, significant effect of condition. #P < 0.05, significant effect of background muscle activity.

Fig. 6.

Effects of sensory enhancement and background muscle activity on early-latency and net reflex amplitudes with the sensory-enhanced arm performing graded contraction (Task 2). A and C: solid bar graphs showing results from the sensory-enhanced arm. B and D: patterned bar graphs showing results from the reflex-stimulated arm. For A–D, the y-axis represents normalized reflex amplitudes and the x-axis represents different conditions. Different shades in each bar graph represent the level of muscle contraction performed in the sensory-enhanced arm. *P < 0.05, significant effect of condition. #P < 0.05, significant effect of background muscle activity.