Running

As far as I can see it is because my calf muscle is now absorbing the force of my foots impact with the ground, rather than the cushioning in my normal shoe.

As I have learnt to run with the cushioned shoe my calf has never adapted to take this repeated loading cycle, so it gets tired, and then once fatigued it starts to hurt.

Therefore, if I keep going barefoot running the muscles will increase in strength, and it will no longer hurt.

If there is a way I could adapt my technique to avoid this process then I am all ears.

Do you shorten your stride so that you land with your foot underneath you? In cushioned trainers, people land with the leg outstretched, relying on the bounce in the shoe's sole to carry them forward. Trying that barefoot/vff will hurt.

Are you trying too hard to stop your heel touching down, perhaps? It's important that the ball of the foot touches first, but toe and heel can make contact right after that. Some do try to avoid any heel contact, which can only be done by tensing calf muscles.

Do you land with a bent knee? Many barefoot runners use an exaggerated knee bend. It reduces the shock of impact and helps the thigh muscles to share the load and contribute to the spring back.