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.

initial speed when it is dropped is 150 ft/s

now at the same moment the acceleration due to gravity is perpendicular to its velocity

so as we know that

now we have centripetal acceleration a = 32 ft/s^2 as its perpendicular to velocity

while tangential acceleration at the same moment will be zero

so

now radius of curvature is given as

so the radius of curvature just after it is dropped is 703.125 ft