What is the most common reason why people can't perform a single-joint exercise correctly, assuming the load is light?

The most common reason why individuals frequently struggle to perform a single-joint exercise correctly, particularly when the load is light, is poor motor control. Motor control refers to the ability of the nervous system to manage and coordinate movements. Even with light resistance, if a person lacks adequate motor control, they may be unable to execute the exercise with proper form and technique. This can lead to inefficiency in movement patterns and an increased risk of injury.

Poor motor control often manifests in various ways, including improper activation of muscles, poor stabilization of joints, and difficulty maintaining proper posture throughout the exercise. These issues are more pronounced with single-joint movements because they require a higher degree of precision compared to multi-joint exercises. Therefore, even if a person has good flexibility, is motivated, and has warmed up adequately, the underlying motor control deficiencies will likely prevent them from successfully completing the exercise with the correct technique.