Do doctors usually perform ultrasounds to diagnose muscle strains as part of the diagnosis in sports medicine?

No: Usually if a doctor is concerned that the injury is a sprain or a tear if will order an MRI

No they do not: When a patient presents with muscle strain/sports injury, the history & exam by physician is usually adequate to make diagnosis. If not, xrays and /or MRI's may be ordered for further investigation. Ultrasounds are used to evaluate masses and differentiate between solid & fluid filled masses, such as gallstones or kidney stones. Also used for evaluation arterial flow/obstruction/aneurysm screening

They may : Usually completely depends on the doctor...

U/S IN SPORTS: U/S modalities are becoming more popular through medicine & sports medicine is no different, particularly with the increased optics (resolution) of U/S probes. A skilled ultrasound operator & U/S interpreter may be able to identify edema associated with muscle injuries but it would require a level of training that many providers don't have. Image shows U/S of rotator tendon injury w/ edema.