Pain in the back of the foot is not necessarily a muscle strain. A muscle strain is an injury caused by a sharp contraction or excessive stretching of a muscle during exercise. It can easily occur during pull-ups and sit-up exercises. After a muscle strain, the strained area is in severe pain and can be felt with the hand as a hard mass of muscle tension, with significant pain to the touch, local swelling or subcutaneous bleeding, and significant restriction of movement.
However, there are many causes of instep pain, which may be soft tissue infection of the instep; there may also be lumbar disc herniation, which compresses the sciatic nerve causing instep pain.