Estimating peak strains associated with fast exercise in thoroughbred racehorses

Area of Science:

• Equine orthopedics
• Bone physiology
• Biomechanics

Background:

• Third metacarpal bones (MC3) in Thoroughbred racehorses are prone to damage.
• Exercise-induced thickening of the dorsal cortex (DC) of MC3 may not be sufficient to prevent high bone strains.
• The adaptive capacity of MC3 to racing speeds remains unclear.

Purpose of the Study:

• To correlate dorsal cortex (DC) proportionate size in racehorses with surface strains during racing.
• To determine if DC thickness plateaus with sustained high-speed exercise.
• To assess if this plateau limits bone strain to safe levels.

Main Methods:

• Longitudinal radiographic analysis of MC3 in 40 racehorses over 2-5 years.
• Measurement of dorsal cortex (DC), medulla (M), and palmar cortex (PC) thickness.
• Calculation of a relative thickness index (RI) to estimate bone strains at racing speeds.

Main Results:

• DC thickness reached a plateau after a mean of 501 days, with significant periosteal bone growth.
• No further changes in DC thickness or RI were observed after the plateau.
• Estimated peak strains at 12 m/sec were -2492 microstrains, suggesting strains exceed 3000 microstrains at racing speeds.

Conclusions:

• Experienced racehorses experience bone strains exceeding normal mammalian limits.
• The DC may not adapt sufficiently to prevent damage during high-speed exercise.
• Further research into protective mechanisms is needed to reduce bone injury in racehorses.