The effects of repetitive contractions and ischemia on the ability to discriminate intramuscular sensation.

In order to investigate whether repetitive, low-level, muscular contractions or ischemia affect the ability of subjects to discriminate electrical stimuli delivered to the forearm musculature, a total of 25 experiments was performed on a group of five healthy subjects utilizing signal detection methodology. EMG needle electrodes were inserted into the forearm extensor musculature and the discriminability of two different pairs of constant current electrical stimuli were measured for sequential blocks of 100 trials, before and after the experimental interventions. Repetitive muscular activity consisting of a 15-min typing intervention at a rate of 60-80 wpm using a numeric keypad led to a significant decrease in the ability to discriminate the non-noxious stimulus pair in the block of trials immediately following the typing intervention, which then returned to pre-intervention discrimination levels following a 5-min break. The forearm ischemia significantly impaired the ability to discriminate both the noxious and non-noxious stimulus pair in the blocks of trials during and immediately after ischemia, which then returned to pre-intervention discrimination levels following a 5-min break. These experiments demonstrate that both repetitive muscular activity and ischemia acutely decrease the ability to discriminate intramuscular sensation. The mechanism may be due to decreased cortical processing, spinal cord sensitization or peripheral ischemia of large diameter afferents. These findings may be relevant to the physiological mechanisms underlying the development of overuse injuries.