Heart Rate and Condition

by Frederick Harper, PhD

Some horses start the work day with a bang, but fade toward the end. They're trained to the harness, but are not physically fit, or conditioned. Out-of-condition work horses fall into two categories:

• horses that are not worked during the week, but are worked hard on weekends;
  
  
• horses that are worked consistently during their work season, but are not in condition when the season starts.

The conditioning of a work horse involves its cardiovascular, respiratory, muscular, and skeletal systems. The cardiovascular system, the first body system to get into shape, takes six to eight weeks. The respiratory and muscular systems shape up next. The bone system takes longest, and may not be fully conditioned until the end of the season. So just when the horse is in peak condition, it becomes idle again. A horse that does field work in summer and logging in winter is in better shape than a horse that's allowed to loaf during the off season.

Conditioning prepares the horse to work at its potential without becoming fatigued or exhausted. Until a work horse is properly conditioned, muscle fatigue causes it to stumble and slip more, and sustain more injuries. An unfit horse lathers easily, while a conditioned horse has clear, watery sweat. Unfit horses may breathe heavily—their nostrils flare and their flanks heave as they breathe rapidly and deeply.

When a horse moves, its muscles contract by converting chemical energy into mechanical energy. For this process to occur, the blood must transport oxygen from the lungs to the muscles and other body tissues. The more intensely the horse works, the more oxygen its body needs. The more oxygen the horse's body needs, the faster its heart must beat.

Any exercise accelerates the heart rate to as much as 60 beats per minute. A heart rate over 150 beats per minute indicates energy production without oxygen, which can't last long without the horse becoming fatigued. A horse's maximum heart rate is 250 beats per minute. Overall, the heart rate of a heavy horse is lower than that of a lighter horse.

Since high temperatures and humidity increase the heart rate, a horse must be properly conditioned if it is expected to work during the summer months. A properly conditioned horse can work more intensely and for a longer time at a higher heart rate without becoming fatigued. After work, a conditioned horse should have a heart rate of no more than 70 beats per minute within 10 minutes after coming to rest.

Heart rate is a good measure of a horse's condition in two ways:

1. As the horse becomes better conditioned, its heart rate is reduced when the horse does specific work.
   
   
2. The better conditioned the horse, the quicker its heart rate returns to its normal resting level after work stops.

Frederick Harper, PhD, is an Extension horse specialist at the University of Tennessee in Knoxville. This article appeared in the Summer 1995 issue of Rural Heritage.