Refinement of a protocol to induce reliable muscle cramps in the abductor hallucis.

Objective: To test the reliability of immediate replication of muscle cramp characteristics induced with different electrical stimulation protocols. Approach: Five (age 33.8 +/- 5.7 y, 60% male) and ten (age 47.4 +/- 11.7 y, 60% male) participants completed independent discovery and validation cohorts, respectively. This was to identify a protocol that resulted in consistent muscle cramp characteristics (discovery), and to examine the test-retest reliability of the identified protocol (validation). Electrical stimulation (150 burst) at abductor hallucis motor-point was used to induce muscle cramps with 4 Hz increments in stimulation frequency (8-32 Hz) or until muscle cramp was first evident, followed by refinement (2 and 1 Hz) until at least two muscle cramps occurred. This defined the cramp threshold frequency, and concurrent electromyogram activity and duration of the cramp were quantified. The discovery cohort involved three separate randomised sessions where intervals between stimulation was 60, 90, and 120 s respectively. In each session, four randomised electrical stimulation protocols were completed. Stimulation current was fixed at 10, 20, and 30% higher than m-wave stimulation current (protocols 1-3 respectively), or randomised within 4 Hz steps (protocol 4) to minimise any order effect. Main results: We were able to immediately replicate tolerable muscle cramp at least twice. Discovery cohort demonstrated (i) incremental changes in stimulation frequency (protocols 1-3 vs. protocol 4, i.e. order effect), and (ii) changes in stimulation current with differing protocols did not significantly alter the prevailing muscle cramp characteristics, and (iii) defining the muscle cramp characteristics elicits good-to-excellent inter-observer reliability. The validation cohort's test-retest reliability and the minimum detectable change were improved for all muscle cramp characteristics when immediately replicated more than twice at the lowest stimulation frequency. Significance: This study provides evidence for a reliable method for inducing repeatable muscle cramps in abductor hallucis.