When a heel strike runner switches to forefoot running, calf tenderness may occur because the calf muslces are used in a more functional manner in forefoot running, but are offline in heel strike running whereby heel strike running over-engages the knees to accelerate the entire body forward. In forefoot running, the calves, not the knee, are used to stabilize the foot, absorb impact and contribute elastic energy that actually saves energy. Its the new engagement of the calves that causes a newbie forefoot runner to have some minor calf pain, but this specific pain will resolve as soon as the calves adapt to the new running condition, which they will. Be patient. Dont resign yourself because forefoot running is proven over and over to be much better on the entire body in the long run than heel striking. Read more here! 
Forefoot Running and Calf Pain
