Pressor response to static and dynamic knee extensions at equivalent workload in humans

Area of Science:

• Exercise Physiology
• Cardiovascular Regulation
• Skeletal Muscle Metabolism

Background:

• Static exercise was traditionally thought to induce a greater pressor response than dynamic exercise.
• Recent studies suggest repetitive muscle contractions recruiting small muscles evoke a greater response than sustained contractions.
• It was unclear if sustained contractions recruiting large muscles would elicit a greater pressor response.

Purpose of the Study:

• To investigate whether sustained isometric contractions recruiting large muscle groups induce a greater pressor response compared to repetitive contractions.
• To compare cardiovascular responses, including blood flow and arterial pressure, under different isometric exercise protocols.
• To explore the role of metabolite accumulation in mediating pressor responses during isometric exercise.

Main Methods:

• Nine subjects performed isometric knee extensions using large muscle groups.
• Protocols included 2-minute sustained contractions at 20% and 40% maximal voluntary contraction (MVC) and 4-minute repetitive contractions at 40% MVC (1s on, 1s off).
• Measurements included femoral arterial blood flow (BF), oxygen consumption (VO2), and mean arterial pressure (MAP) during exercise and post-exercise occlusion.

Main Results:

• Compared to repetitive contractions of equivalent duration and total tension-time integral (TTI), sustained contractions showed significantly lower femoral arterial blood flow (Delta BF) and oxygen consumption (Delta VO2).
• However, sustained contractions resulted in a greater increase in mean arterial pressure (Delta MAP) compared to repetitive contractions.
• Post-exercise occlusion revealed a significantly greater Delta MAP after sustained contraction, suggesting greater pressor response and potential metabolite accumulation.

Conclusions:

• Sustained isometric contractions recruiting large muscle groups elicit a greater pressor response than repetitive contractions.
• This heightened pressor response during sustained contractions is likely attributed to greater accumulation of metabolites within the active skeletal muscles.
• Findings challenge previous assumptions and highlight the importance of contraction type and muscle group size in cardiovascular responses to isometric exercise.