Final answer:
The tibialis anterior is the muscle that demonstrates two periods of peak intensity during the gait cycle. It contributes to lifting the foot at the beginning of the swing phase and during heel strike. The gastrocnemius, while the largest muscle in the lower leg and active during the gait cycle, exhibits a different pattern of activity.
Step-by-step explanation:
The muscle that exhibits two periods of peak intensity during the gait cycle is the tibialis anterior. This muscle is located in the anterior compartment of the leg and is responsible for dorsiflexing and inverting the foot. During walking, the tibialis anterior contracts to lift the foot off the ground at the start of the swing phase (clearing the toes from the ground) and then again during the heel strike to help control the foot's placement on the ground. In contrast, the gastrocnemius muscle, which is one of the two muscles that pull upon the Achilles tendon, is more involved in plantarflexion of the foot, such as when pushing off the ground during walking or running. The other muscles, quadriceps and hamstrings, are involved in different phases of the gait cycle but do not exhibit two peaks of activity like the tibialis anterior does.
It's also interesting to note that the largest muscle in the lower leg is the gastrocnemius, which works with the soleus to form the calf muscle group. Together, the medial and lateral heads of the gastrocnemius muscle exert a force on the Achilles tendon that is primarily responsible for plantarflexion at the ankle. This powerful movement is necessary for propelling the body forward during walking, running, and jumping.