Knee joint angle affects EMG-force relationship in the vastus intermedius muscle

Area of Science:

• Biomechanics
• Human Physiology
• Neuromuscular Control

Background:

• The relationship between electromyography (EMG) and muscle force is crucial for understanding muscle function.
• The influence of knee joint angle on the EMG-force relationship of individual quadriceps femoris (QF) muscles remains unclear.

Purpose of the Study:

• To investigate how knee joint angle affects the EMG-force relationship of the four individual QF muscles during isometric knee extensions.
• To specifically examine the role of the vastus intermedius (VI) at different knee flexion angles.

Main Methods:

• Eleven healthy males performed isometric knee extensions at 20-100% maximal voluntary contraction (MVC).
• Measurements were taken at three knee joint angles: 90°, 120°, and 150°.
• Surface EMG of the four QF muscles was recorded and normalized to MVC.

Main Results:

• Normalized EMG for all four QF muscles was significantly lower at 150° compared to 90° and 120° (P < 0.05).
• The vastus intermedius (VI) showed significantly lower normalized EMG at 150° compared to the other three QF muscles (P < 0.05).
• These findings indicate a downward shift in the EMG-force relationship at 150°.

Conclusions:

• The EMG-force relationship of the quadriceps femoris synergists is angle-dependent, shifting downward at more extended knee positions.
• The vastus intermedius (VI) exhibits the greatest sensitivity to changes in muscle length among the QF muscles studied.