shortened stride

["Field, Shannon" <Shannon.Field@alliedsignal.com>]

What is the most likely cause of a shortened stride on one side?  Last
night, we noticed my gelding was a little off when reaching forward with
his left front at the trot (head bobs a little and stride is shorter).
He had been blazing around the night before, bucking and galloping while
playing in the arena.  Would this be enough to cause a strained muscle
and have this effect?  Or could this be the result of a chiropractic or
shoeing problem?  I'm worried I just haven't noticed this before and it
may have been occuring.  An experienced horse friend came by to see him
for the first time last night and immediately caught it.  He was
recently treated by a vet with acupunture and injections in his right
hip, hock and stifle for pain that seemed to stem from an old injury we
can't identify.  He's only 5 and loves the trail.  Any input would be
appreciated.

Shannon