Heart Rate Training Zones explained

The MAXIMAL zone (90-100%) of max HR

Training at this intensity is the definition of speed work. Main benefit is increased neuromuscular coordination at race-specific speeds. Also the scene of extreme fatigue, work in this zone must be carefully controlled. In the US this is done every 6-12 days, more often in Australia and other countries albeit over softer surfaces.

The THRESHOLD zone (80-90% of max HR)
This is the pace at which the horse is still able to use lactic acid for energy, which delays the onset of fatigue during a race. Targeting gallops towards this zone will improve cruising speed in a race. Only accompished by a 2:00 lick in ELITE horses, others will need to slow down.

The AEROBIC zone (70-80% of max HR)
This intensity best develops lung function and improves the horse's ability to use oxygen to fuel movement. Exercise at this pace actually allows for the creation of new blood capillaries which aid in performance. Happens a lot during the 'legging up' phase of getting a horse ready for the races, but often neglected when racing commences.

The RECOVERY zone (60-70% of max HR)
Here is the optimal pace to train in which any lactic acid is flushed away, and the recovery processes are enhanced. Best used after a breeze for 60-90 seconds before exiting the track. Many jogs, especially indoors, are just a tad to slow to accomplish optimal recovery.

The definition of fitness is for your horse to constantly be able to increase his speed or distance, or both, while remaining in these heart rate zones. This is best accomplished with progressive loading (variation of speed, distance, frequency) while allowing for recovery and recuperation.

On the flip side, a horse that normally gallops at 25mph in the threshold zone that suddenly shows maximal heart rates at such a workload, could indeed be compromised and giving you a very early sign that something is amiss.