With racehorses, muscle fibers are classified as either slow twitch or fast twitch fibers. Slow twitch, also known as Type I fibers , are highly dependant on oxygen to generate energy-generating ATP. These fibers suit distance/endurance gallopers, as they are capable of reducing lactate acid buildup and it effects, which includes muscle pain and lowered form, function and speed. Fast twitch, or Type II fibers, are made up of two types of fibers called Type II A & II B fibers. The Type II A fibers are both high and low oxidative. These fibers are capable of utilizing both aerobic and anaerobic metabolism to produce energy for racing. Type II A fibers are used to maintain high speed but only lasts for short distances. The Type II B fibers are primarily anaerobic, which means they cant gather more energy from oxygen through a horses airways or energy system. These fibers are used to give the horse shorts bursts of speed. Neither class of Type II muscle fibers has the ability to reduce lactic acid as do Type I fibers, therefore, fatigue is reached in a shorter time.

All horses possess a combination of both types of fibers. But the actual percentage of each type of muscle fiber that a racehorse possesses primarily determines a racehorses preferred racing distance. A horse with a high percentage of Type 1 will be capable of racing over longer distances, but at a slower speed. The horse high percentages of Type 11 fibers will be better suited over shorter distances.

Looking at the muscles types affect on type and conformation, the sprinting Type 11 horse typically has bulkier and shorter muscles, which may also produce a shorter stride. The Type 1 horse has longer and smoother muscles, with lowered fat, which produces an overall leaner look. The shorter and bulkier muscles of the sprinting horse can reduce its flexibility, which can result in a a stronger, but shorter stride.

Lets look at traits of horses with a high percentage of each type in turn.

Type 1

• Endurance type horses suited over longer distances.
• Long and lean muscles.
• Calmer mentality.
• Lowered cruising speed
• Often races back in the field
• Keeps steadily coming, even when tiring towards the end of a race.
• Performs best off a fast pace.
• Slow early gate speed.
• May appear to be outpaced mid race.
• Excellent heart and lungs.
• Lowered risk of minor muscle tears.
• Usually pulls up well.
• Quick recovery

Type 11

• Sprint type of horse
• Can put on weight when spelling.
• Fiery mentality.
• Increased muscle mass through shorter, but stronger & bulkier muscles
• High cruising speed
• Races on pace
• Quick, but short sprint.
• Can tire like shot towards the end of a race.
• Performs best off a slow pace.
• Superior early gate speed
• May appear to be cruising mid race
• Lowered heart and lung power.
• Higher risk of muscle tears.
• Can pull up blowing as if unfit.
• Slower recovery, can be sore next day or when fitness is below 100%.
• Can improve with a quick let up

Equal combination of both types

• Mixed traits of all of the above