Actions of β2-adrenoceptor agonist drug on neuromuscular function after fatigue.

PURPOSE: The effects of β2-agonists on human skeletal muscle contractile function are unclear, more particularly in the presence of fatigue. Moreover, it remains to be ascertained whether central motor drive during voluntary contractions could counter for eventual contractile alterations induced by β2-agonists. This study explored central and peripheral neuromuscular adjustments of the quadriceps induced by β2-agonist terbutaline after fatiguing exercise.

METHODS: On 2 d, nine men performed 120 intermittent (5-s contraction/5-s relaxation) submaximal (50% maximal voluntary contraction [MVC]) voluntary contractions, 2 h after having ingested a terbutaline (8 mg) or placebo capsule, in a double-blind randomized order. The isometric knee extension peak torque of single twitch and tetanic (10 and 100 Hz) contractions and of MVC was recorded before (PRE) and after (POST) the fatiguing exercise. Twitch maximal rate of torque relaxation (-dT/dt) was also calculated. Central motor drive was estimated via quadriceps electromyogram recorded during the submaximal voluntary contractions.

RESULTS: In PRE and POST conditions, MVC and twitch peak torque were not modified by terbutaline. PRE twitch -dT/dt was 22% higher in terbutaline compared with placebo (P < 0.005), but it did not differ significantly between treatments at POST. The evoked torque of 10- and 100-Hz tetanus were significantly lower in terbutaline compared with placebo, regardless of time (-16% and -17% respectively, P < 0.05). Central motor drive during voluntary exercise did not differ between treatments.

CONCLUSIONS: These results demonstrate that terbutaline affected the contractile function of the quadriceps muscle during electrically evoked contractions both before and after fatiguing exercise. However, this did not impair the maximal voluntary force-generating capacity of the knee extensors and did not result in any compensatory adjustments of the central nervous system.