Association between knee extensor strength and EMG activities during squat movement

Area of Science:

• Biomechanics
• Human Physiology
• Exercise Science

Background:

• The quadriceps femoris (QF) muscle group is crucial for lower limb function.
• Understanding the relationship between muscle force and electrical activity is vital for assessing exercise intensity and functional capacity.

Purpose of the Study:

• To investigate the association between the force-generating capability of the quadriceps femoris (QF) and its surface EMG activity during body mass-based squat movements.
• To identify potential thresholds in knee extensor strength related to exercise intensity during squats.

Main Methods:

• Measured isometric knee extension torque (KET) during maximal voluntary contraction in 101 participants (19-90 years).
• Recorded surface EMG of rectus femoris and vastus lateralis during body mass squats, normalized to maximal voluntary contraction (%EMG(max)).
• Calculated QF %EMG(max) as an index of QF activity during squats and analyzed its relationship with KET relative to body mass (KET/BM).

Main Results:

• A nonlinear relationship was observed between QF %EMG(max) and KET/BM.
• A breakpoint was identified at 1.9 N·m·kg(-1) in the KET/BM relationship.
• For individuals with KET/BM below 1.9 N·m·kg(-1), QF %EMG(max) increased sharply as KET/BM decreased.

Conclusions:

• QF muscle activity during squats is dependent on its force-generation capability.
• Body mass squats are considered high-intensity exercises for individuals with KET/BM below 1.9 N·m·kg(-1).
• A threshold of 1.9 N·m·kg(-1) for knee extensor strength is suggested for maintaining ease in activities of daily living.