Intramuscular sensation in conscious human subjects: a qualitative and quantitative study utilizing signal detection...

To investigate whether human subjects could consciously perceive different levels of intramuscular sensation, EMG needle electrodes were inserted into the forearm extensor musculature. Qualitative descriptions of the types of sensations evoked at various current intensities were recorded. A series of preliminary experiments was performed in order to determine the optimum waveform, current intensity, duration and separation between adjacent current pairs that could reliably be discriminated by conscious human subjects. The methodology of signal detection theory was utilized to obtain the discrimination measure, d' (d prime). Values of d' were obtained for sequential blocks of trials consisting of randomly delivered blocks of 50 "strong" and 50 "weak" stimuli. The ability to discriminate both noxious and non-noxious stimuli was determined using d'. It was demonstrated that human subjects are able to qualitatively describe non-nociceptive sensations elicited by low levels of intramuscular stimulation. Subjects were able to reliably discriminate pairs of electrical stimuli separated in intensity by 0.4 dB over four sequential blocks of trials on the same day and on different days. This work demonstrates that signal detection theory methods can reliably measure sensory discrimination in skeletal muscle, for both noxious and non-noxious levels of electrical stimulation. A protocol is proposed for appropriate future use of this methodology to investigate alterations in muscle sensation following experimental interventions.