Thank you for the reply.
It is true that at the initial presentation, if a soft tissue injury was suspected, xrays would not have made much of a difference and the treatment of choice is usually strict rest and pain medication/anti-inflammatory for a week or two. If however, there was no improvement and if it appears the symptoms are worse, an xray is essential to make sure we are not missing anything.
A soft tissue injury would show no abnormalities, and depending on what is injured on the knee, that may or may not. That being said, and keeping his breed in mind, it would beneficial to look for signs of an intervertebral disc problem, especially since it occurred so acutely. Type one disc disease (due to a ruptured disc) can present with any combination or severity of signs including pain and loss of function. Type two disc disease (due to a bulging disc) is usually less severe and more chronic in nature. I worry that the fatigue, abnormal sitting posture and leg trembling may be his way of manifesting pain. And while it would seem not to present acutely, degenerative joint disease (aka arthritis) can often show many of these symptoms. Changes due to DJD would be evident on an xray, and may be suggestive of disc disease in some cases, so it would be worthwhile to at least take a look and to make sure we were not missing anything else that is not a common.
There are certain tests that can check his nerve reflexes (a toe pinch, a knee jerk reflex, a foot placement test, etc) and if any of these were not tested they should be as they can help to localize the source of the problem, especially if it involved the spine or nerve roots. At this point in time, I would consider getting input from a specialist, either in orthopedics or neurology if any of the nerve reflexes were involved. Even if it turned out that the problem was soft tissue in nature, orthopedic veterinarians have plenty of experience in rehab with these patients.
Here is a link with more information about disk disease in dogs. LINK HERE
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.
My posted replies are for general education only and not meant as a diagnosis. Only after a thorough veterinary examination can a diagnosis for your pet be made and specific treatments be advised or medications be prescribed.