My friend tore her achilles tendon (back of leg) a couple of years ago, and it was hanging by a thread. She had it set in a cast in a toe pointing straight down position to allow it to heal. She was warned that it could snap easily in its weakened state if the muscle was stressed.
If it had snapped, the leg muscle would have detached from the back of the foot and sprung up into her leg, meaning messy surgery rooting around to find the end, then pulling it back down and sewing the two ends of the tendon together, followed by a much longer healing process.
Fortunately, that surgery wasn't necessary because she put no stress on it until it had healed. But she was immobilized for nearly 6 months, and most irritated by the awkward position the cast held her foot in.