H. Bjurstedt

Cardiorespiratory and metabolic responses to positive, negative and minimum-load dynamic leg exercise

Cardiorespiratory and metabolic responses to steady-state dynamic leg exercise were studied in seven male subjects who performed positive and negative work on a modified Krogh cycle ergometer at loads of 0, 16, 33, 49, 98, and 147 W with a pedalling rate of 60 rpm. In positive work, O2 uptake increased with the ergometric load in a parabolic fashion. Net O2 uptake averaged averaged 220 ml-min-1 at 0 W (loadless pedalling), and was 75 ml-min-1 lower at the point of physiological minimum load which occurred in negative work at approximately 9 W. The O2 cost of loadless pedalling is for one-third attributed to the work of overcoming elastic and viscous resistance, the remaining part being due mainly to the work of antagonistic muscle contraction in the moving legs. Although at a given VO2, work rate was much higher in negative than in positive work, corresponding values for VE were similar, suggesting that the mechanical tension in working muscles is of little or no importance in the control of ventilation in steady-state exercise. Heart rate increased linearly with VO2 in both positive and negative work, with a steeper slope in negative work. Evidence is presented that none of the current definitions of muscular efficiency yields the true efficiency of muscular contraction in cycle ergometry, net efficiency calculation resulting in too low estimates, and work and delta efficiency calculations in overestimated values in the low-intensity work range, and in underestimated values in the high-intensity range.

Interactions between heart rate, psychomotor performance and perceived effort during physical work as influenced by beta-adrenergic blockade

Effects of a single intravenous dose of propranolol (0,25 mg/kg body weight) were examined in 15 healthy male subjects who performed three reaction-time tasks of different complexity, while pedalling at five work loads on a cycle ergometer. Comparisons between measurements after propranolol and after injection of a placebo solution showed a pronounced reduction of heart rate and an increase in catecholamine excretion following propranolol. Comparisons of psychomotor performance showed no significan difference between the propranolol and placebo conditions. Nor did self-estimates of perceived physical and task-induced efforts reveal any significant effects of propranolol. The results support the notion that heart rate is not a prominent cue for perceived effort.

Cardiac responses to lower body negative pressure and dynamic leg exercise

SummaryCardiac responses to dynamic leg exercise at 0, 50, and 100 W in the supine position were investigated with and without the lower portion of the body exposed to a pressure of −6.6 kPa (Lower Body Negative Pressure, LBNP). Resting values for heart rate (HR) and stroke volume (SV) were considerably higher and lower, respectively, during LBNP than in the control condition. At the transition from rest to the mildest exercise during LBNP SV showed a prompt increase by about 40%, but no significant change in the control condition. HR, which increased by 17 beats · min−1 in the control condition, showed during LBNP no change initially and subsequently a small but significant drop below its resting value. Steady-state values for HR at the various levels of exercise were not significantly affected by LBNP, whereas corresponding values for SV were considerably lowered, so that exercise values for cardiac output were about 3 l · min−1 less during LBNP than in the control condition. The reductions in SV and cardiac output indicate residual pooling of blood in intra- and extramuscular capacitance vessels of the legs. With a change from rest to exercise at 100 W during LBNP mean systolic ejection rate (MSER) increased by 67%, the relations between SV and MSER suggesting that ventricular performance was maintained by a combination of the Frank-Starling mechanism and enhanced contractile strength.

Graded Restriction of Blood Flow in Exercising Leg Muscles

Much of our knowledge about the effects of acute muscle ischemia on the respiration and central circulation in humans has been derived from studies in which the circulation in muscles has been completely arrested by the use of inflatable cuffs. Notable examples are the classical experiments by Alam & Smirk in the 30s (1937). They blocked the circulation of one forearm and found that the trapping of metabolites within the limb after exercise prevented the postexercise fall in arterial pressure. Later, Asmussen & Nielsen (1964) also used the cuff method during dynamic leg exercise, and presented evidence that both the pressor response and the hyperpnea of exercise may in part originate from stimulation of muscle afferents sensitive to local metabolic changes in ischemic muscle. Yet, complete blocking of the circulation to an exercising limb can only be maintained for short periods and may give rise to pain and other adverse effects with physiological consequences of mixed origin. There has been a need for a method by which the experimenter can induce partial restriction of blood flow in large muscle groups, and control both the degree and the duration of the restriction within wide limits.