Hemodynamic responses to static and dynamic muscle contractions at equivalent workloads

We tested the hypothesis that static contraction causes greater reflex cardiovascular responses than dynamic contraction at equivalent workloads [i.e., same tension-time index (TTI), holding either contraction time or peak tension constant] in chloralose-anesthetized cats. When time was held constant and tension was allowed to vary, dynamic contraction of the hindlimb muscles evoked greater increases (means ± SE) in mean arterial pressure (MAP; 50 ± 7 vs. 30 ± 5 mmHg), popliteal blood velocity (15 ± 3 vs. 5 ± 1 cm/s), popliteal venous Pco₂ (15 ± 3 vs. 3 ± 1 mmHg), and a greater decrease in popliteal venous pH (0.07 ± 0.01 vs. 0.03 ± 0.01), suggesting greater metabolic stimulation during dynamic contraction. Similarly, when peak tension was held constant and time was allowed to vary, dynamic contraction evoked a greater increase in blood velocity (13 ± 1 vs. -1 ± 1 cm/s) without causing any differences in other variables. To investigate the reflex contribution of mechanoreceptors, we stretched the hindlimb dynamically and statically at the same TTI. A larger reflex increase in MAP during dynamic stretch (32 ± 8 vs. 24 ± 6 mmHg) was observed when time was held constant, indicating greater mechanoreceptor stimulation. However, when peak tension was held constant, there were no differences in the reflex cardiovascular response to static and dynamic stretch. In conclusion, at comparable TTI, when peak tension is variable, dynamic muscle contraction causes larger cardiovascular responses than static contraction because of greater chemical and mechanical stimulation. However, when peak tensions are equivalent, static and dynamic contraction or stretch produce similar cardiovascular responses.