While seated, place your feet flat on the floor while bending the knees so that the toes go behind the vertical plane of the knees. (Z shaped legs basically) P

While seated, place your feet flat on the floor while bending the knees so that the toes go behind the vertical plane of the knees. (Z shaped legs basically) Place your hand on the back of the upper portion of the calf, right under the knee. Keeping the foot on the floor and the leg in the same Z position, try to "slide" it backwards with as much force as possible but so it doesn't actually slip. You will feel muscle tension with your hand. That's the gastrocnemius muscle. You can probably contract it at will as well, so much that it cramps, that's the one. You don't want to flex it on either side of the calf. Now lean your body onto your leg (same position) with your elbow on top of the knee. Try to lift your heel up with your weight on top of the knee, while not tensing the gastroc (feeling it using the opposite hand), i.e. avoid trying to move the foot backwards, only lift the heel up. I am pretty sure this loads up the soleus muscle instead, that sits underneath the gastroc. You can feel it tense up if you place the opposite hand around the lower part of the calf, above the ankle, to the sides of where the curve of the gastroc transitions into the achilles tendon. Now this is pretty difficult to do without putting your weight on top of your leg, but I think after you identify the correct muscle it becomes much easier to do without tools. I had some success by trying to push the floor away with the balls of the feet while raising the heel and monitoring the gastroc with the opposite hand. After you do some of those loaded seated calf raises, the soleus muscle tends to become tense and stays tense for a while, you can feel it especially in the lower calf. This is probably how it eats up so much energy. Another way to load these muscles is to sit, bend your knees and spread them out while raising the heels, lean forward and place your elbows on your spread knees, shifting some of your weight on top of them. Then try rocking forwards and backwards while moving your heels up and down. Your lower calves will quickly start to burn, but they take a while to truly tire out. Which I guess is the point. :)