What is the primary difference between anticipatory postural adjustments (APAs) and compensatory postural adjustments (CPAs), and when does each occur during a voluntary movement?

• A. APAs involve only distal muscles; CPAs involve proximal muscles.
• B. APAs occur before movement to prepare the body and maintain stability; CPAs occur after onset to restore balance.
• C. APAs are conscious reflexes; CPAs are automatic.
• D. APAs occur after movement to restore balance; CPAs occur before movement to prepare balance.

Answer: (B)

Anticipatory postural adjustments are preplanned, feedforward responses that prepare the body for a voluntary movement by activating postural muscles before the prime mover to keep the center of mass over the base of support. They occur before the movement starts, helping to maintain stability as the action begins. Compensatory postural adjustments are feedback responses that occur after the movement has begun (or after a perturbation) to restore balance and stability when the disturbance has already occurred. An example is the coordinated activation of trunk and leg muscles before reaching to prevent a loss of balance; if balance is upset during or after the movement, compensatory adjustments kick in to bring you back to a stable posture. The other options don’t fit because APAs are not limited to distal muscles, they’re not about conscious versus automatic control, and the timing described in the last choice is reversed.