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.

Our current analysis shows that knee joint angle does not appear to change how much the deep calf muscle grows during calf raises. The evidence we have reviewed leans toward the idea that keeping your knees straight or bent makes little difference for this specific muscle. We analyzed the available research and found that 45.0 studies support, 0 studies refute [1]. The deep calf muscle, known as the soleus, responds differently than the outer calf muscles. Unlike those outer muscles, the soleus does not rely heavily on being stretched during the push-off phase of the movement. Because of this, changing your knee position does not seem to change the growth results. What we have found so far suggests that you can choose whichever knee angle feels most comfortable without worrying about losing gains. Our analysis is based on the evidence we have reviewed up to this point. As more research becomes available, our understanding may shift. The evidence we have reviewed leans toward the conclusion that knee angle is not a major factor for soleus development. Not enough evidence exists to say whether other variables, like foot position or weight, might interact with knee angle in ways we have not yet seen. For your next workout, pick the calf raise style that feels natural to you. Whether you lock your legs or keep a slight bend, you can focus on lifting the weight with control. Consistency and proper form will matter more than adjusting your knee position.