Final answer:
The proper collateral ligament of the metacarpophalangeal (MCP) joint is tightest at full flexion, which provides stability and resists valgus stress.
Step-by-step explanation:
The proper collateral ligament of the metacarpophalangeal (MCP) joint is known to be tightest at full flexion. This is because the ligament is biomechanically aligned to resist valgus stress and provide stability to the joint in this position. Particularly, in the context of the MCP joint of the fingers, this tightness helps to grip objects more securely, allowing the flexor tendons to work effectively. The ligament ensures that there is no excessive sideways movement of the joint during flexion which could otherwise lead to joint instability or damage.