Final answer:
During a seated heel raise, the gastroc muscle acts as the agonist because it is the primary muscle responsible for lifting the heel through plantar flexion of the foot. The gastroc muscle contracts to pull up on the Achilles tendon, facilitating this movement.
Step-by-step explanation:
During the seated heel raise (calf raise), the gastroc muscle (gastrocnemius) would be considered an agonist. This is because when you perform a calf raise, the gastroc muscle is the primary muscle responsible for the plantar flexion of the foot at the ankle joint, which is the action of raising your heel. The gastroc muscle contracts and pulls upward on the Achilles tendon, causing your heel to lift off the ground.
The synergist muscle helps an agonist work, as in the calf raise, the soleus also acts as a synergist to the gastrocnemius. However, in this context, the gastrocnemius is not acting as a synergist but as the primary mover. Additionally, the magnitude and direction of the total force on the Achilles tendon during this movement would be significant, as the gastroc muscle generates a force that facilitates plantar flexion. This type of movement could cause the elevation of the body on the toes when standing or contribute to propulsion during activities like walking, running, or jumping.