Final answer:
The tibialis anterior muscle in the anterior compartment of the leg decelerates the leg before heel strike to prevent the foot from slapping down. It achieves this by controlling plantar flexion of the ankle during the gait cycle.
Step-by-step explanation:
The muscle group responsible for decelerating the leg before the heel strike during normal human locomotion is the anterior compartment of the leg, primarily the tibialis anterior muscle. During the gait cycle, just before heel strike, the tibialis anterior contracts to control plantar flexion of the ankle, preventing the foot from slapping down onto the ground.
The anterior compartment of the leg also includes the extensor hallucis longus, the extensor digitorum longus, and sometimes the fibularis tertius. These muscles help in dorsiflexion of the foot and work to decelerate the leg and stabilize the ankle before the heel hits the ground. In contrast, the muscles of the posterior compartment like the gastrocnemius and soleus are responsible for plantar flexion and are not primarily involved in decelerating the leg before heel strike.