Final answer:
Weakness in the hip flexor (L1-2) can cause a lean in the swing phase of movement.
Step-by-step explanation:
Weakness in the hip flexor muscles at the L1-2 level can lead to a lean in the swing phase of walking or running. The hip flexors are responsible for lifting the thigh towards the chest during walking and running. When these muscles are weak, the leg may not lift properly, causing a lean or imbalance during the swinging motion of the leg.