Does pre-conditioning improve bone quality in the juvenile racehorse?

Professional Animal Scientist, Sep 2002 by Hiney, K M, Potter, G D, Gibbs, P G, Bloomfiled, S M

Abstract

Sixteen Quarter Horse-type geldings were used to determine the effectiveness of a pre-conditioning program on altering bone density in young horses prior to race training. The pre-conditioning program began when the average age of the horses was 15 mo. Horses were exercised on a high speed treadmill for 14 wk and were subsequently placed into race training. Changes in density of the third metacarpal bone were determined via radiographic densitometry throughout pre-- conditioning and the race training regimen. Gain in total bone volume was greater in the group exercised during the pre-conditioning phase. Also, the pattern of bone density in different locations indicated that the exercised group might have been preferentially laying down more bone in the dorsal and medial cortices and less bone in the palmar cortex to achieve altered bone shape. Initiation of race training appeared to limit gains in bone density in comparison with the pre-conditioning phase.

(Key Words: Exercise, Bone Modeling, Training, Horse.)

Introduction

Because of financial pressures, many racehorses begin training as early as 18 mo of age. However, bone mineral content (BMC), breaking strength (13), metacarpal bone size, and ultrasound speed (3) may not peak until 5 to 6 yr of age. Because of skeletal immaturity, the 2-yr-old horse appears to be at great risk, with injury rates as high as 58.1% (9). Fatigue fractures of metacarpal III (MC III), more commonly known as bucked shins or dorsal metacarpal disease, are quite common in this age group with reported incidence rates of 70% (23). In the Australian Thoroughbred racing industry, shin soreness was listed as the most frequent injury or disease among 2-yr olds and was viewed as a major cause of time lost from training and racing (1). However, shin soreness is commonly accepted as a normal part of the training process (2). Such high rates of injury are of a humane concern and result in large economic losses to trainers and owners. Therefore, means must be explored to aid in minimizing the risk of injury in the young racehorse.

One of the causative factors in the development of bone injuries may be the inherent biological process of improving bone strength. Bone remodeling, which repairs damaged bone, is a sequential process in which bone resorption precedes bone formation. Nielsen et al. (21) observed a decrease in BMC as horses entered training followed by increases in BMC as training progressed. The lowest density in bone was observed 50 to 60 d following initiation of training, which Nielson et al. (21) indicated is the time when trainers in the racing industry typically introduce speed work. It was suggested that the high rate of injury observed in young horses may be due to the initiation of high speed training coinciding with the time point when bone density is lowest. Therefore, stimulation of the remodeling-- modeling process prior to race training using a treadmill training regimen may shift the drop in bone density due to occur at an earlier time point when the horse is under a reduced workload.

Materials and Methods

Animals and Management-- Phase I. Sixteen geldings of Quarter Horse-type breeding were paired according to age and BW and were randomly assigned to one of two treatment groups. Horses began the study when the average age was 467 d (range, 456 to 480). An initial acclimation period of approximately 2 mo preceded the start of the project, during which animals were gelded and accustomed to handling. Horses were fed a 60% concentrate:40% forage diet based on coastal bermudagrass hay and a commercially available pelleted concentrate (A&M 13% CP Horse Pellet; Producers Cooperative Association, Bryan, TX) at 2.5% of their BW/d. Diets were formulated to meet or exceed NRC (19) recommendations for yearling horses undergoing rapid growth. The horses were maintained on this diet throughout the study. Horses were kept in 14.3 x 14.3 m pens (four horses per pen); they were separated twice daily at 12-h intervals, individually fed the pelleted concentrate, and then returned to their pens where forage was group-fed. Animals were weighed weekly, and feed intake was adjusted as necessary according to BW gain. The horses were vaccinated and dewormed as routine for Central Texas and received routine hoof care.

Exercise Program. Prior to d 0 of the study, the exercise group was familiarized with the treadmill apparatus until accustomed to loading onto and walking (1.8 m/s) on the treadmill. The exercised group performed the prescribed exercise program 3 d/wk on alternating days for 14 wk; the control horses received no forced exercise. On non-working days, all horses had free exercise in their pens. Weeks 1 and 2 served as basic familiarization to performing all three gaits (walk, trot, and canter) on the treadmill with more intense work bouts starting at wk 3. Horses then performed two, 1-min sprints separated by 2 min of walking. Sprints increased in speed as the horses progressed through the protocol to a maximum of 12 m/s. To increase the vertical component of strain upon MC III, the treadmill was inclined to a 4% slope after a brief warm-up period.