A comparison of the moment arms of pelvic limb muscles in horses bred for acceleration (Quarter Horse) and endurance (Arab).

This study investigates how selective breeding for specific performance characteristics has affected the muscular structures in the hind limbs of two horse breeds: the Quarter Horse, bred for acceleration, and the Arab, bred for endurance. The research suggests that selective breeding has resulted in significant physical differences that offer each breed advantages in their respective specialties.

Methodology and Hypothesis

• The researchers hypothesized that the Quarter Horse, bred for quick accelerations, would have longer moment arms, the perpendicular distance from the line of action of the force to the pivot point, in their hind limb extensor muscles compared to the Arab. The longer the moment arm, the more torque a force can generate, so a longer moment arm in the hind limb extensor muscles could mean greater potential for fast, powerful muscular contractions.
• This study involved dissection of six hind limbs from both Quarter Horses and Arabs to determine muscle moment arm lengths for several key muscles: gluteus medius, biceps femoris, semitendinosus, vastus lateralis, gastrocnemius (medialis and lateralis) and tibialis cranialis.

Findings

• Results revealed that the moment arms of the biceps femoris (acting at the hip) and the gastrocnemius lateralis (acting at the stifle or knee) were significantly longer in the Quarter Horse, aligning with their hypothesis.
• However, the remaining muscle moment arm lengths were similar in both breeds, suggesting that other factors might also influence performance abilities.
• The Quarter Horse muscles were all found to be capable of generating greater muscle moments due to their larger physiological cross-sectional areas (PCSAs), meaning they have greater isometric force potential. This suggests that PCSA may be a better determinant of muscle torque than moment arm length in horses.
• The Arab horses had a greater muscle fascicle length to moment arm ratio (MFL : MA ratio) for all muscles except the gastrocnemius and tibialis cranialis. This could mean that Arab horses’ muscles are optimized to function at slower contraction velocities and generate greater force outputs, facilitating endurance as opposed to power output.