Transcutaneous Electrical Neuromuscular Stimulation (TENS) works in dogs by sending low level currents to an area of muscle that has been injured or weakened due to lack of use, stimulating it and forcing it to contract.
This approach is also used in the rehabilitation of dogs with spinal cord injuries. You should consult your vet or physiotherapist for instructions on their proper placement and use.
The application of heat is used to reduce stiffness and muscle spasms, increase blood flow and relieve pain. Unlike cold therapy, heat therapy is applied after the initial swelling and inflammation (a.k.a. irritation) stage of an injury.
Heat therapy is most commonly applied to joints: shoulders, elbows and wrists in the front legs; and hips, knees and ankles in the back legs. It can be used on any part of your dog's body where muscle damage, strain or spasm occurs. For example, a dog with a disc problem in the spine (neck or back) may have muscle spasms, which heat therapy can relieve.