The muscle spindle has a threshold in which it produces muscle tension. Application of an ice pack lowers the temperature of the skin which in turns raises the threshold at which the muscle spindle produces tension. By raising this threshold, the muscle spindle becomes less excitable and decreases muscle tension. This means that an ice pack or similar can be very effective in reducing a muscle spasm or a cramp due to its effects in decreasing muscle tension.

Luis

Source:Orthopedic Physical Therapy Secrets, Second Edition