EFFECTS OF PRIOR ARM EXERCISE ON PULMONARY GAS-EXCHANGE KINETICS DURING HIGH-INTENSITY LEG EXERCISE IN HUMANS

For moderate work rates (i.e, below the lactate threshold, theta(L)), oxygen uptake ((V) over dot(O2)) approaches the steady state mono-expo nentially. At higher work rates, the (V) over dot(O2) kinetics are mor e complex, reflecting the delayed superimposition of an additional, sl ow component. The mechanisms of this 'slow' component are poorly under stood. It has been demonstrated, however, that while a prior bout of s upra-theta(L) cycling (with a 6 min recovery) does not significantly a ffect the (V) over dot(O2) time course for a subsequent sub-theta(L) b out, it significantly speeds the (V) over dot(O2), response to a subse quent supra-theta(L) bout (Gausche, Harmon, Lamarra & Whipp, 1989; Ger bino, Ward & Whipp, 1996). These investigators proposed that this spee ding was a result of improved muscle perfusion during the exercise tra nsient, possibly related to the residual metabolic acidaemia still pre sent at the start of the subsequent exercise bout. To determine whethe r speeding of the (V) over dot(O2) kinetics could also be induced by a bout of prior high-intensity exercise performed at a remote site (e.g . the arms), subjects each performed two 6 min bouts of high-intensity cycling (leg exercise: LE) at a work rate equivalent to 50 % of 'Delt a LE' (the difference between maximum (V) over dot(O2).LE and theta(L, LE)). On one occasion this was preceded by a 6 min period of cycling a t 50 % Delta LE and, on another, by a similar period of arm-crank exer cise (arm exercise: AE) at 50 % Delta AE; in each case, the work bouts were separated by 6 min of unloaded pedalling. Pulmonary gas exchange variables were derived breath-by-breath. During unloaded pedalling an d at minute 6 of each work bout, arterialized venous blood samples wer e drawn from the dorsum of the heated hand or at the wrist for analysi s of pH, lactate, pyruvate, noradrenaline (NAdr), adrenaline (Adr), an d potassium (K+). The difference in li,(V) over dot(O2) between minute 6 and 3 of each work bout (Delta(V) over dot(O2[6-3])) and the 'parti al' O-2 deficit (O-2 Def) provided indices of the slow phase of (V) ov er dot(O2) kinetics. The initial AE and LE bouts resulted in similar d egrees of metabolic (lactic) acidaemia; the residual acidaemia at the end of the subsequent 6 min recovery phase was also similar for the tw o protocols, as were [K+], [Adr] and [NAdr]. The subsequent LE bouts w ere associated with reductions in both Delta(V) over dot(O2),([6 3]) a nd O-2 Def, relative to control, with the effect being more marked whe n the work was preceded by a prior LE bout than a prior AE bout: Delta (V) over dot(O2),([6-3]) averaging 32 and 56 % of control, respectivel y, and O-2 Def 71 and 81 %. Consequently, the increase in [lactate] an d decrease in pH induced in this second LE bout were smaller when prec eded by prior leg exercise than prior arm exercise. It is therefore co ncluded that while metabolic acidaemia induced at a site remote from t he legs is associated with a less prominent slow phase of the (V) over dot(O2) kinetics for high-intensity leg exercise, a component specifi c to the involved contractile units appears to exert the dominant effe ct. The mechanisms underlying this response are, however, presently un certain.