This is because the strenuous exercise results in a buildup of lactate in the
muscles and bloodstream, which has to be disposed of. Slow trotting keeps
the blood moving faster, and therefore it is able to eliminate lactic acid
faster. Slow trotting also stimulates replacement of the phosphagen (source
of energy) stores.
My "Equine Fitness" bible has a chart that plots recovery, in terms of blood
lactate concentration, over a period of 70 minutes under three regimes: 70
minutes of standing, 70 minutes of walking, 30 minutes of trotting followed
by 40 minutes of walking. Recovery was fastest and lactate lowest with the
trot/walk routine. Walking was slower, but after 70 minutes almost identical
recovery, and standing resulted in slowest recovery with the highest lactate
concentration at the end of the 70 minute period. The book goes on to
recommend slow trotting during recovery periods when doing interval training.
This is all measured on flat ground, so I would guess walking uphill would be
equivalent to slow trotting to allow the horse to recover.
Linda Romander
San Francisco, Calif.