Why did horse legs become longer and more specialized for running?

The evolution of the horse toward longer, more specialized legs was primarily an adaptation for swift running in open environments, such as the grasslands that became widespread in North America during the Miocene Epoch.12

These anatomical changes provided significant advantages for survival and efficiency:

• Increased Speed: Lengthening the lower leg bones and concentrating muscle mass higher up on the limb reduced the weight at the end of the leg. Because a leg acts like a pendulum, this reduction in distal weight decreases inertia, allowing for faster oscillation and a more efficient stride.34
• Specialized Movement: The limbs evolved to become more rigid, restricting movement primarily to a fore-and-aft plane. This specialization, supported by the fusion of lower leg bones like the radius and ulna, improved stability and energy efficiency during high-speed running.25
• Weight Distribution: Over time, horses transitioned from padded, multi-toed feet to walking on the tips of a single digit. This shift, combined with the development of a spring-like mechanism in the foot, allowed for fuller limb extension and provided the necessary support for the animal's weight during rapid movement.1256