Final answer:
Tightness in the tibialis posterior, peroneus longus, peroneus brevis, adductors, hamstrings, and lateral hip muscles such as the piriformis can cause pronation or external rotation during a single leg squat. Muscles work in pairs, and tightness may indicate weakness in opposing muscle groups like the quadriceps.
Step-by-step explanation:
When performing a single leg squat, pronation or external rotation can indicate tightness in several muscles. The muscles most commonly associated with this movement pattern include the tibialis posterior, which can become overworked and tight when compensating for excessive pronation. Additionally, tightness in the peroneus longus and peroneus brevis may contribute to external rotation of the leg. Pronation can also implicate an imbalance where the adductors (adductor longus, adductor magnus, and gracilis) and hamstrings (biceps femoris, semitendinosus, and semimembranosus), along with the gluteus maximus and the muscles in the lateral hip such as the piriformis, need to be assessed for tension.
Muscles come in pairs, with the quads and hamstrings serving as antagonistic pairs for knee movement. It's worth noting that tightness can result from overactivity of certain muscles due to compensatory movements, or from weakness in their antagonists. For example, excessive pronation during a squat can indicate weakness in the quadriceps, particularly the vastus medialis, which works to stabilize the patella and control lower limb alignment.