Hi Linh,
These various terms can be used loosely sometimes.
You mention a few:
intramuscular tear: I don’t think I would ever use this term as it implies the muscle has simply torn through the “meaty” part of the central muscle and I personally don’t think this is a thing.
When I see a muscle tear in the central part of a muscle (hamstring its a good example) it is always associated with one of the intramuscular tendon extensions (some people call them aponeurotic expansions) . They are natural anatomical extensions of the tendon which travel deep into the muscle. When the muscle tears from these intramuscular tendons I refer to this as a musculotendinous junction failure. Then it is also possible for the intramuscular tendon to fail itself and this is a more significant injury in terms of return to activity time. In these cases I describe the rupture of the intramuscular tendon. In all of these tears the mechanism is overloading of the muscle.
Contusion injury is completely different as this is not a result of overloading of the muscle but rather compression of the muscle from an external source (like being hit). In the case of a contusion the compression of the muscle causes capillaries inside the muscle to explode, leaking blood into the interstitial space of the muscle architecture. Sometimes larger vessels are damaged and a haematoma may result. The key difference is a muscle “tear” is associated with overloading stress and a contusion is associated with external compression.
Muscle strain injury, I think of as a minor version of an intramuscular tear where there is no significant ultrasound findings of architectural failure, but the patient still feels like they have damaged the muscle.
Another mechanism is delayed onset muscle soreness (DOMS) where the muscle is overloaded with too much exercise and reaches a state of metabolic exhaustion. It is the feeling you have in a muscle after you work too hard at the gym after not going for a while and the muscle aches or burns a few days after and then resolves. If you have a significant DOMS you will see similar findings to a contusion where the muscle looks a little more echogenic than normal and feels tight under the transducer. It usually affects the whole muscle where as a contusion can be more localised. The difference is really in the history. The contusion injury has a history of being struck and the DOMS has a history of overuse.
Steve