Doing calf raises with your knees bent builds more of the deep calf muscle than doing them with straight legs. This happens because bending your knees changes how the calf muscles stretch and push, making the exercise more effective for that specific muscle.

Doing calf raises with your knees straight builds more of your main calf muscle than doing them with your knees bent. This happens because keeping your knees straight puts the muscle in a better position to handle more tension and stretch, leading to bigger growth.

Doing calf raises with your knees straight or bent builds the deep calf muscle about the same amount. This happens because that specific muscle doesn't care as much about how stretched it is when you push off, unlike the outer calf muscle. So, you don't need to change your knee angle to get the best results for this muscle.

Working your muscles while they are fully stretched builds more muscle than working them while they are shortened. This stretching advantage is especially noticeable in muscles that cross two joints, like the back of your lower leg, but doesn't seem to matter for muscles that only cross one joint.

The outer part of your calf and the deeper calf muscle don’t grow better with either straight-knee or bent-knee calf raises — they respond similarly to both.

Your calf muscle is strongest at pushing off or bending your knee when your leg is completely straight. As you bend your knee more, it gets much weaker because the muscle becomes too shortened to generate full force.