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Muscle use during dynamic knee extension: implication for perfusion and metabolism
1Autonomic and Cardiovascular Control Laboratory, Muscle Biology Laboratory, Department of Exercise Science, The University of Georgia, Athens, Georgia 30602, USA. cray@med.hmc.psghs.edu
Journal of Applied Physiology (Bethesda, Md. : 1985)
|September 8, 1998
Summary
Dynamic one-legged knee extension (DKE) exercise recruits more quadriceps femoris muscle mass as work rate increases. Blood flow is directed to newly recruited muscle, not increased perfusion of engaged muscle, supporting aerobic metabolism.
Area of Science:
- Exercise Physiology
- Muscle Physiology
- Biomechanics
Background:
- Dynamic one-legged knee extension (DKE) is a common model for studying physiological responses to aerobic exercise.
- Muscle blood flow in DKE is often normalized to total quadriceps femoris mass, irrespective of exercise intensity.
- This normalization contradicts the principle that greater force production involves recruitment of larger muscle volumes.
Purpose of the Study:
- To investigate muscle utilization within the quadriceps femoris during dynamic one-legged knee extension (DKE) at varying work rates.
- To determine if increased exercise intensity leads to greater muscle recruitment or enhanced perfusion of already active muscle fibers.
Main Methods:
- Six healthy subjects underwent magnetic resonance imaging (MRI) of the quadriceps femoris before and after 4 minutes of DKE at 20 W and 40 W.
- Muscle activation and recruitment were assessed by measuring changes in T2-weighted MRI signals, indicating water content shifts due to metabolic activity.
- Quadriceps femoris muscle volume was calculated from MRI data.
Main Results:
- The percentage of quadriceps femoris with elevated T2 signals increased significantly with work rate, from 16% at rest to 54% at 20 W and 94% at 40 W.
- Quadriceps femoris muscle volume increased by 11.4% after 40-W DKE, suggesting muscle swelling due to increased blood volume.
- Extrapolation indicated that approximately 1,301 cm³ of quadriceps femoris were engaged at 20 W and 2,292 cm³ at 40 W.
Conclusions:
- Increased power output during aerobic DKE is primarily achieved through the recruitment of additional motor units and muscle fibers within the quadriceps femoris.
- Muscle blood flow during submaximal DKE appears to be maximal and directed towards newly recruited muscle tissue, rather than increasing perfusion in already active muscle.
- These findings challenge the conventional normalization of muscle blood flow relative to total muscle mass and highlight the importance of considering muscle recruitment patterns in exercise physiology research.
Keywords:
Non-programmatic