A key symptom of muscular dystrophy has been reversed by scientists in the lab.
They identified a compound that eliminated the prolonged muscle tensing known as myotonia in mice bred with a common form of the condition.
The researchers say the discovery raises hope myotonia could be reversed in muscular dystrophy patients even after it has become well established.
The University of Rochester study is published online by the Journal of Clinical Investigation.
Myotonic dystrophy - the most common form of muscular dystrophy in adults - is characterised by progressive muscle wasting and weakness, particularly in the lower legs, hands, neck, and face.
It undermines a key mechanism which controls electrical activity in the muscles, destroying their ability to relax and resulting in myotonia - often one of the first signs of disease.
This can be particularly severe in the hand muscles, causing a patient''s grip to lock. Relaxation restored
The Rochester team designed a molecule that, when injected into mice with myotonic dystrophy, restored a critical control mechanism, and with it the ability of muscles to relax.
Researcher Dr Charles Thornton, director of the university''s Neuromuscular Disease Center, said: "This work should provide hope and encouragement to people with myotonic dystrophy and their families.
"It encourages us to believe that other parts of the disease could be reversible as well.
"As we move forward, we should not be content to keep this condition from getting worse. We should set our set our sights on making it better."
The Rochester team pinpointed a genetic flaw responsible for myotonia five years ago.
This disrupts the intricate and finely balanced flow of tiny charged particles responsible for electrical activity in cells.
One key "channel" is effectively disabled, causing electrical signals in the muscles to stay jammed in the "on" mode.
The compound - morpholino - developed by the researchers in collaboration the biotechnology company Gene Tools, restores the proper genetic instructions for building the disabled channel. Prolonged effect
When it was injected into mice muscle cells the channel was restored, and the myotonia all but disappeared.
Dr Thurman Wheeler, who led the research, said: "This material is incredibly stable in the cells once we get it inside the muscle.
"And the effects are surprisingly prolonged, which makes it potentially more attractive as a treatment."
However, the Rochester team said more work was needed before this new approach could be tested in people.
The first priority will be to develop a better method of delivering the compound throughout the body.
Dr Marita Pohlschmidt, director of research at the Muscular Dystrophy Campaign, described the research as "exciting and reassuring".
But she added: "We must be reminded that these experiments have been carried out in an animal model.
"We do however look forward to further research to see whether this kind of approach can be found to have an effect in people with the condition."