Knee extensor muscle oxygen consumption in relation to muscle activation.

Area of Science:

• Exercise Physiology
• Muscle Physiology

Background:

• Muscle oxygen consumption (mVO2) and fatigability are lower during sustained isometric contractions at shorter knee angles (30°) compared to longer ones (90°).
• At low torques, the rectus femoris (RF) muscle exhibits less mVO2 than vastus lateralis and medialis.

Purpose of the Study:

• To investigate if knee angle- and muscle-dependent activation explain the observed differences in mVO2 and fatigability.
• To quantify muscle activation using rectified surface EMG (rsEMG) and relate it to mVO2 during isometric contractions at various knee angles.

Main Methods:

• Nine healthy males performed isometric knee extensor contractions at 30°, 60°, and 90° knee angles.
• Maximal torque capacity (MTC) was determined via nerve stimulation.
• Muscle oxygen consumption (mVO2) was measured using near-infrared spectroscopy (NIRS) during contractions at 10-70% MTC.
• Rectified surface EMG (rsEMG) was recorded to assess muscle activation.

Main Results:

• Maximal mVO2 was significantly lower (57.9%) and reached later at 30° compared to 60° and 90° knee angles.
• Muscle activation (rsEMG) at 30° was only slightly reduced (18.0%) at the start of contraction compared to longer angles.
• At 10% MTC, the RF muscle showed a delayed increase in mVO2 and a smaller increase in rsEMG compared to the vasti muscles.

Conclusions:

• Reduced muscle activation does not fully explain the substantial decrease in oxygen consumption at shorter knee angles.
• The delayed increase in RF mVO2 at shorter knee angles appears to be related to less forceful activation of the RF muscle.