|Gaping maw and excessive noise optional.|
Agility shouldn't be the most intense workout of your dog's week. Heck, they should barely break a (metaphorical) sweat during practice. Practice is a time where you hone handling and obstacle skills. It is not a time where you tire out your dog. A tired dog is more prone to make mistakes, and mistakes can lead to injury.
Compliment your dog's competitive hobbies with strengthening and conditioning exercises outside of the ring. Body awareness exercises and tricks are perfect tools to teach your dogs how to use their bodies safely, and conditioning builds muscle where you need it so they can withstand the impact of jumping, turning and hitting contacts.
I'm a huge proponent of keeping our dogs at a healthy weight and keeping them fit year-round. Ribs should be easily felt and muscles should be well developed. Keep practice sessions short and stop before your dog tires. Our pets don't live forever, but this is one way to lengthen the quality time we have with them.