In Graves' disease, increased muscle tension and reduced elasticity of affected muscles is primarily caused by active muscle contraction

In three patients with Graves' disease of recent onset, length-tension diagrams were made during surgery for squint under eyedrop anesthesia, while the other eye looked ahead, into the field of action, or out of the field of action of the muscle that was measured. The affected muscles were found to be very stiff when the other eye looked straight ahead. Unexpectedly, the affected muscle did not shorten any further (one case) or only 6-7 mm (two cases) when the other eye looked into the field of action of the muscle, whereas it lengthened 9-10 mm when the other eye looked out of the field of action of the muscle. This means that the raised muscle tension and reduced elasticity of affected muscles in these cases of Graves' disease of recent onset are caused by active muscle contraction. It has been found previously that hyperthyroidism, induced in rats, casues transitions from slower to faster myosin-heavy chain (MHC) types. The findings seem compatible with a transition of slow into fast muscle fiber types in Graves' disease.