Differential effects of contracting muscle mass and relative exercise intensity on arterial plasma potassium concentration during and following incremental arm and leg cycling exercise.

Whilst arm cycling is commonly used during clinical testing and rehabilitation, the associated changes in arterial plasma K+ concentration ([K+]a) and thus possible risks of associated hyperkalaemia and hypokalaemia are unknown. During exercise, K+ is released from contracting skeletal muscles and simultaneously taken up by inactive muscles, with [K+]a increasing markedly during intense exercise. Hence during exercise [K+]a should be influenced by both the contracting muscle mass and the relative exercise intensity. We therefore investigated the effects of incremental one-arm (1ARM) and two-leg (2LEGS) cycling on [K+]a, and on lactate ([Lac-]), hydrogen ([H+]), adrenaline and noradrenaline. Eight healthy participants performed 1ARM, rested (3.0 ±0.2h, mean±SE) then performed 2LEGS, with radial arterial blood sampled at rest, during common submaximal workrates (25, 50W), peak workrate and 1–30min post-exercise. During exercise at 50W, VO2, [K+]a and [Lac-]a were higher during 1ARM than 2LEGS (P<0.05). In contrast, at peak exercise, 2LEGS (272±15W) elicited higher VO2, [K+]a, [Lac-]a, [adrenaline]a and [noradrenaline]a than 1ARM (57±4W) (P<0.05), with [H+]a elevated only in 2LEGS (P<0.05). A curvilinear response for [K+]a versus %VO2peak was similar between modes, until diverging close to VO2peak. The slope of the log[K+]a versus %VO2peak regression was higher in 2LEGS (P<0.01), indicating greater [K+]a throughout exercise utilising a larger muscle mass, and similarly for [Lac-]a. In conclusion, during exercise utilising a smaller contracting muscle mass, [K+]a and [Lac-]a were greater at the same absolute submaximal work rate compared to utilising a large muscle mass. Whilst responses were similar between modes when expressed against relative exercise intensity, [K+]a and [Lac-]a were higher during exercise with a large muscle mass. Hence, for submaximal exercise, the relative intensity is more important in determining [K+]a and [Lac-]a, but the size of the contracting muscle mass still exerts a positive effect, whilst during peak exercise, the size of the active muscle mass was more important in determining [K+]a and [Lac-]a. Furthermore, arm cycling did not induce marked disturbances in [K+]a during or after exercise, suggesting minimal associated myocardial risks and thus supporting its safe use in clinical and aged populations.