As the fitness work increases you will be doing more fast work. On level ground as the speed increases it is the front limb that takes more peak forces through the limb(1), this in turn stretches the tendons and ligaments more and can lead to tiny tears causing inflammation.
These peak forces through a forelimb have been measured as high as 1.3 tonnes when a horse is galloping at 38mph, that’s over 2 times their body weight in force going through one front leg!
Use Cryochaps after exercise to help those limbs recover for the next days training.
(1)The Journal of Experimental Biology 209, 4389-4397 Published by The Company of Biologists 200, Accepted August 2006:Effect of speed on stride parameters in racehorses at gallop in ﬁeld conditions; T. H. Witte