Calves

The gastrocnemius originates from the femoral condyles and inserts on the calcaneus via the Achilles tendon. It's biarticular: it crosses both the knee and the ankle. This means it works more when the knee is extended (standing) and less when the knee is flexed (seated). The soleus originates from the tibia and the fibula and also inserts on the calcaneus. It's monoarticular and works independently of knee position.

This anatomical difference determines how to train them. The standing calf raise (machine, Smith machine, or single-leg with a dumbbell on a step) primarily hits the gastrocnemius because the knee is extended. The seated calf raise hits the soleus because the flexed knee puts the gastrocnemius in a shortened position, reducing its ability to generate force. For complete calves you need both exercises.

The soleus is composed mainly of slow-twitch fibers, so it responds well to high reps (15-20) and long times under tension. The gastrocnemius has more fast-twitch fibers and responds better to heavier loads and moderate reps (8-12). The calves are muscles used to working all day (walking is essentially a repeated calf raise), so they need above-average stimuli to grow. Full ROM, controlled eccentric, and a forced contraction at the top matter more than absolute load.