Abstract
The silent period induced by cutaneous electrical stimulation of the digits has been shown to be task-dependent, at least in the grasping muscles of the hand. However, it is unknown if the cutaneous silent period is adaptable throughout muscles of the entire upper limb, in particular when the task requirements are substantially altered. The purpose of the present study was to examine the characteristics of the cutaneous silent period in several upper limb muscles when introducing increased whole-body instability. The cutaneous silent period was evoked in 10 healthy individuals with electrical stimulation of digit II of the right hand when the subjects were seated, standing, or standing on a wobble board while maintaining a background elbow extension contraction with the triceps brachii of ~5% of maximal voluntary contraction (MVC) strength. The first excitatory response (E1), first inhibitory response (CSP), and second excitatory response (E2) were quantified as the percent change from baseline and by their individual durations. The results showed that the level of CSP suppression was lessened (47.7 ± 7.7% to 33.8 ± 13.2% of baseline, p = 0.019) and the duration of the CSP inhibition decreased (p = 0.021) in the triceps brachii when comparing the seated and wobble board tasks. For the wobble board task the amount of cutaneous afferent inhibition of EMG activity in the triceps brachii decreased; which is proposed to be due to differential weighting of cutaneous feedback relative to the corticospinal drive, most likely due to presynaptic inhibition, to meet the demands of the unstable task.
Highlights
A single pulse of high intensity electrical current delivered to the digits of the hand produces a period of decreased electromyographic (EMG) activity, or a cutaneous silent period (CSP), in muscles of the upper limb during voluntary contractions [1,2,3]
The results showed that the level of CSP suppression was lessened (47.7 ± 7.7% to 33.8 ± 13.2% of baseline, p = 0.019) and the duration of the CSP inhibition decreased (p = 0.021) in the triceps brachii when comparing the seated and wobble board tasks
Results demonstrated no habituation within the abductor digiti minimi (ADM) (t(9) = 2.202, p = .055), abductor pollicis brevis (APB) (t(9) = -1.231, p = .250), extensor carpi radialis (ECR) (t(9) = .447, p = .666), flexor carpai radialis (FCR) (t(9) = -.244, p = .813), and triceps brachii lateral head (TRI) (t(9) = 1.948, p = .083), which falls in line with previous investigations[19]
Summary
A single pulse of high intensity electrical current delivered to the digits of the hand produces a period of decreased electromyographic (EMG) activity, or a cutaneous silent period (CSP), in muscles of the upper limb (e.g. thenar, triceps brachii) during voluntary contractions [1,2,3]. The cutaneous receptive fields activated by the stimulus, and the afferent fibers carrying the activation signal, aid in the coordination of activity in one or more muscles best suited to protect the area of tissue from the noxious stimuli [12, 13]. This is referred to as the modular organization of nociceptive reflexes, and the basic theory can be applicable whether the end result is excitation or inhibition of a given muscle (e.g. excite flexor, inhibit extensor). The evidence presented further supports the function of the CSP as an adaptable spinal inhibitory reflex that can help protect the hand or limb from harm
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