Deciphering the metabolic and mechanical contributions to the exercise-induced circulatory response: insights from eccentric cycling.

Metabolic demand and muscle mechanical tension are closely coupled during exercise, making their respective drives to the circulatory response difficult to establish. This coupling being altered in eccentric cycling, we implemented an experimental design featuring eccentric vs. concentric constant-load cycling bouts to gain insights into the control of the exercise-induced circulatory response in humans. Heart rate (HR), stroke volume (SV), cardiac output (Q), oxygen uptake (V(.-)(O(2))), and electromyographic (EMG) activity of quadriceps muscles were measured in 11 subjects during heavy concentric (heavy CON: 270 +/- 13 W; V(.-)(O(2)) = 3.59 +/- 0.20 l/min), heavy eccentric (heavy ECC: 270 +/- 13 W, V(.-)(O(2)) = 1.17 +/- 0.15 l/min), and light concentric (light CON: 70 +/- 9 W, V(.-)(O(2)) = 1.14 +/- 0.12 l/min) cycle bouts. Using a reductionist approach, the circulatory responses observed between heavy CON vs. light CON (difference in V(.-)(O(2)) and power output) was ascribed either to metabolic demand, as estimated from heavy CON vs. heavy ECC (similar power output, different V(.-)(O(2))), or to muscle mechanical tension, as estimated from heavy ECC vs. light CON (similar V(.-)(O(2)), different power output). 74% of the Q response was determined by the metabolic demand, also accounting for 65% and 84% of HR and SV responses, respectively. Consequently, muscle mechanical tension determined 26%, 35%, and 16% of the Q, HR, and SV responses, respectively. Q was significantly related to V(.-)(O(2)) (r(2) = 0.83) and EMG activity (r(2) = 0.82; both P < 0.001). These results suggest that the exercise-induced circulatory response is mainly under metabolic control and support the idea that the level of muscle activation plays a role in the cardiovascular regulation during cycle exercise in humans.