Physiological responses to sustained isometric contractions during one- and two-armed work. Role of muscle mass on cardiovascular responses to isometric work.

Cardiovascular and ventilatory responses were investigated during one- and two-armed isometric contractions at the same level of maximum voluntary contraction force (MVC). Significant differences were observed in heart rate, blood pressure, pulse arrival time, and oxygen intake in the contractions as compared with each other. These findings contradict the generally accepted view that cardiovascular responses at the same relative force in isometric muscle contractions, irrespective of the size of muscle mass engaged, are equal.

Cholinergic and adrenergic influences upon cat skeletal muscle performance induced by stimulation of the anterior hypothalamus.

The influence exerted by stimulation of the anterior hypothalamus (zone of cholinergic vasodilatation) on the performance of triceps surae during tetanic contraction has been investigated. Hypothalamic stimulation, if combined with indirect muscle stimulation, improves muscle performance, i.e. slows down the rate of decrease of contraction. The effect is abolished by atropine (0.5 mg/kg) and insensitive to propanolol (0.1 mg/kg). The cholinergic influence is only effective if hypothalamic stimulation coincides with the beginning of motor nerve stimulation. In some of our experiments stimulation of the same central structures elicited another influence on muscle performance, associated with the release of adrenaline into the blood. In this case, contrary to the cholinergic influence, the force of contraction increases only when hypothalamic stimulation does not start earlier than 100 s after the initiation of contraction (phase of slow decline of tension). The effect is insensitive to atropine (0.1 mg/kg) and abolished by propanolol (0.1 mg/kg).

On the mechanism of an increase of muscle performance and of vasodilation during emotional stress in man.

The mechanism of the increase of muscle performance and of vasodilation during emotional stress was studied. The "emotional" increment of voluntary performance does not depend on the level of blood supply to the working muscles, and the effect is maintained under conditions of cessation of arterial inflow. Augmentation of muscle performance is also observed during emotional stress when isometric contraction is evoked by electrical tetanic stimulation of the nerve, when the number of muscle fibres participating in the evoked response does not increase during emotional stress. The "emotional" vasodilation is greatly reduced in patients suffering from McArdle's syndrome, in whom the normal course of glycolysis in muscles is disrupted. It is suggested that acetylcholine liberated from sympathetic fibres causes the activation of glycolysis in muscles, which in its turn induces vasodilation in resting skeletal muscles and increase of muscle performance during emotional stress.