At First Level, only lengthening of the stride is called for. This requires
only that the horse lengthen his outline or frame and show a longer stride.
Engagement of the hindquarters is not called for. In any case the rhythm
should stay the same as either the working or collected trot. If it gets
faster, the horse is generally considered to be running, or on the forehand
with the hindquarters getting lighter (this assumes that the horse was
performing a working trot [about 7 mph] to begin with).
Beverley Fields, dressage judge and FEI dressage competitor, event rider,
trainer and instructor.