Sternomastoid muscle twitch maximum relaxation rate: prolonged slowing with fatigue and post-tetanic acceleration

1. The maximum rate of relaxation of stimulated twitches (twitch maximum relaxation rate) of the sternomastoid muscle was compared with its frequency-force curve (expressed as the ratio of force produced at a stimulation frequency of 20 Hz to that produced at 50 Hz, the 20:50 ratio) before and after fatiguing exercise of the sternomastoid muscle in 10 normal subjects. The mean (±) fresh state twitch maximum relaxation rate was 9.51 (± 0.64)% force loss/10 ms and the mean (± sem) 20:50 ratio was 73.4 (± 2.6)%. 2. The mean twitch maximum relaxation rate fell to 71.5 (± 2.2)% of the pre-fatigue value at 2 min after exercise, recovering to 73.1 (± 1.6)% by 5 min, 78.4 (± 2.34)% by 10 min and 80.6 (± 2.70)% by 60 min. The 20:50 ratio fell to 72.3 (± 3.56)% of pre-fatigue levels at 10 min after exercise and recovered to 78.8 (± 3.16)% by 60 min. There was no significant difference in the percentage falls between the two techniques. 3. Further studies were performed to determine the response of the twitch maximum relaxation rate to a prior brief tetanic stimulus or a brief maximal voluntary contraction of the muscle. Both tetanic stimulation and maximal voluntary contraction accelerated the twitch maximum relaxation rate to over 140% of the fresh state value. Both manoeuvres temporarily returned the slowed twitch maximum relaxation rate after fatiguing exercise back to the fresh state value. 4. The effect of fatiguing exercise on the time course of recovery of quadriceps twitch maximum relaxation rate was also studied in two subjects. The twitch maximum relaxation rate had not recovered to fresh state levels by 5 min after exercise in both subjects. At 10 min, one subject had recovered to the fresh state value, but the other had recovered to only 85% of the fresh state value. Both recovered to fresh state values by 1 h. 5. We conclude that the twitch maximum relaxation rate of the sternomastoid as measured from a twitch contraction has a prolonged time course of recovery after fatiguing exercise, and may follow the same time course of recovery as low-frequency fatigue. Tetanic stimulation and voluntary contractions may temporarily accelerate the relaxation rate, and therefore a distinction should be made between relaxation rates measured after a twitch contraction (twitch maximum relaxation rate) and those after tetanic stimulation or a maximum voluntary contraction (maximum relaxation rate).