Which of the following can impair a person's squat depth?

A stiff hip joint capsule can significantly impair a person's squat depth because it restricts the range of motion at the hip joint. When the hip joint lacks the necessary mobility due to stiffness, it becomes challenging for an individual to achieve a deep squat position. Proper squatting mechanics require adequate flexion and extension at the hips, and any limitation in this area can lead to compensatory movements or prevent the individual from squatting to the optimal depth.

The other options, although potentially important in their own right for overall movement quality and performance, do not directly limit squat depth in the same way. Strong quadriceps can actually support squatting by providing the necessary force to lift the body. Lack of coordination may affect the squat in terms of technique and balance but does not inherently limit the range of motion achievable at the hips. High flexibility can allow for deeper squats rather than impairing them, as increased flexibility in the hips and ankles facilitates better squat mechanics.