... Actually sometimes complete ruptures only cause pain in the immediate post injury period. The best example I can give is people with significant tendinosis of the long head of the bicep tendon. This tendon goes from intra-articular to extra-articular and then down the arm in front. When inflamed it can be quite painful causing chronic anterior shoulder pain. For some, this tendon eventually ruptures; they have a shot of severe pain, swelling and bruising and then their chronic shoulder pain is gone. Their bicep muscle looks different, but the pain is better.
...Do not use only pain as your guide. Weakness or other functional deficits can also be a sign of a complete rupture with or without severe pain.
An MRI will definitely identify the extent of the muscle injury and it is a non-invasive test that gives you a lot of information.
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