Final answer:
The tibialis anterior muscle is responsible for the lengthening contraction between heel strike and foot flat during walking, as it controls the foot's lowering through dorsiflexion.
Step-by-step explanation:
The muscle that will undergo a lengthening contraction between heel strike and foot flat during the walking cycle is the tibialis anterior. This muscle is located in the anterior compartment of the leg and is responsible for dorsiflexion of the foot. The tibialis anterior helps control the foot as it gently lowers to the ground, transitioning from heel strike to foot flat, and this movement involves a lengthening contraction, commonly known as an eccentric contraction, where the muscle lengthens while under tension.
During this phase, while the tibialis anterior is controlling the movement, other muscles from different compartments of the leg such as the fibularis longus, fibularis brevis (both in the lateral compartment), and various muscles from the posterior compartments like the gastrocnemius and soleus may also be active but in different roles.