Co-contractions in antagonistic hindlimb muscles during simulated step cycle rates

To determine whether the step cycle rate during locomotion may be limited by mechanical interactions of antagonistic muscles about the ankle, muscle activation patterns were simulated through electrical stimulation and muscle forces monitored in the rat lateral gastrocnemius-soleus (LGS) and tibialis anterior (TA) muscles. Simulations were made in acute experiments on Nembutal-anesthetized rats. Based on EMG data recorded from free-moving rats at 3 treadmill speeds, SOL mean cycle durations, cycles per second (eps), burst durations, and TA onset times following SOL EMG onset were determined. With muscle nerve stimulation based on these temporal patterns, muscle force was monitored from the detached tendons or directly at the approximate insertion sites. To investigate mechanical interactions attributed to the slow-twitch SOL muscle, forces were measured with the LGS intact, and after the SOL was denervated or the tendon was cut. With tendons detached, TA contraction force temporally overlapped with LGS or LG contraction force over periods ranging between 20 and 50 ms at 3.4 cps, 0-10 ms at 4.1 cps, and 0-10 ms at 4.6 cps. LGS force-time traces were asymmetrical, i.e. return time toward baseline force was more prolonged than the time to reach peak force. Without contributions to force from SOL activation, LG rate of relaxation was consistently increased and rate of rise and fall in force-time patterns were more symmetrical. Co-contractions were not observed when LGS contractions followed TA activation. At all cycle rates, forces remained above baseline (range 0.5-4.0 N) between contractions in LGS.(ABSTRACT TRUNCATED AT 250 WORDS)