This lower limb muscle, which attaches to the calcaneus via the calcaneal tendon and plantar flexes the foot when the knee is extended, is the

Gastrocnemius

Explanation:

The gastrocnemius muscle is a two-headed muscle found in the back region of the lower leg of humans. It runs from the knee to the heel.

Gastrocnemius is the lower limb muscle, which connects to the calcaneus via the calcaneal tendon and plantar flexes the foot when the knee is extended.