It decreases joint stiffness directly.
There was a 20% decrease in stiffness at 45
C as compared with 33C in rheumatoid finger joints, which correlated perfectly to both subjective and objective observation of stiffness.
Any stiffened joint and thickened connective tissues should respond in a similar fashion.

3. Spa Wellness also relieves muscle spasms.
Muscle spasms have long been observed to be reduced through the use of heat, be they secondary to underlying skeletal, joint, or neuro-pathological conditions.
This result is possibly produced by the combined effect of heat on both primary and secondary afferent from spindle cells and from its effects on Golgi tendon organs. The effects produced by each of these mechanisms demonstrated their peak effect within the therapeutic temperature range obtainable with radiant heat.

4. It produces pain relief.
Pain may be relieved via the reduction of attendant or secondary muscle spasms. Pain is also at times related to ischemia due to tension or spasm, which can be improved by the hyperemia that heat-induced vasodilatation produces. This then breaks the feedback loop, in which the ischemia leads to further spasm and then more pain.
Heat has been shown to reduce pain sensation by direct action on both free-nerve endings in tissues and on peripheral nerves. In one dental study, repeated heat applications eventually led to abolishment of the whole nerve response responsible for pain arising from dental pulp.
Heat may lead to both increased endorphin production and a shutting down of the so-called “spinal gate” of Melzack and Wall, each of which can reduce pain.