Mechanisms of muscle cramps: peripheral or central generation?

We analyzed the cramp threshold (i.e., the minimum frequency of electrical stimulation capable of inducing a cramp) and the behaviour of individual motor units during cramps electrically-elicited in the absence (intact condition) and presence (blocked condition) of a peripheral nerve block in 8 healthy subjects. The cramp threshold was significantly greater in the blocked than in the intact condition (18 ± 3 Hz vs. 13 ± 3 Hz; P = 0.01). Cramp duration and peak EMG amplitude in the intact condition (55.6 ± 19.2 s and 47.5 ± 24.8 µV) were significantly greater compared to the blocked condition (2.6 ± 1.3 s and 13.9 ± 8.8 µV; P < 0.01). All motor units identified in the blocked condition (n = 38) had a shorter interval of activity and a greater discharge rate compared to the intact condition (n = 37) (respectively, 1.1 ± 1.0 s vs. 29.5 ± 21.8 s, P < 0.0001; 25.7 ± 11.6 pps vs. 20.0 ± 5.9 pps; P < 0.05). The motor unit activity detected during the blocked condition corresponded to spontaneous discharges of the motor nerves, while in the intact condition the motor unit discharge patterns presented the typical characteristics of motor neuron discharges. These results indicate a spinal involvement at the origin of cramps and during their development.