The Rhythm of Movement

“If you can walk, you can dance.”

Within this traditional African proverb lies a hidden truth about our nature as animals: our bodies are built to have an inherent, subconscious sense of rhythm. From an early age, we unknowingly develop a rhythm in the way we move, from crawling across to the floor to walking on two feet. As we mature, this repetitious pattern of muscle movements become automatic as we travel from one place to another – and most of the time, unless we are explicitly dancing down the sidewalk like Bruno Mars in a music video, we don’t pay much attention to the rhythmic nature of walking at all.

Our leg muscles, like all the muscles in our body below the head, make connections in the spine (Figure A). Networks of neurons in the spine control the rhythm of our walking (Figure B).

For us upright, bipedal Homo sapiens, ordinary locomotion consists of walking or running. In both cases, our feet take turns making contact with the floor, bearing the entirety of our weight in an alternating fashion. The muscles controlling the free leg move it forward through the air, while the muscles controlling the stable grounded leg exert an upward and forward force. Then, when the forward foot makes landfall, the legs switch duties. This repetitive action of the legs is a semi-autonomous process that scientists believe is controlled by neural circuits in the spinal cord called central pattern generators.

A central pattern generator is a network of neurons that communicate with the muscles of the body. In some ways, central pattern generators behave like a metronome – they help the body perform rhythmic and repetitive tasks, such as walking. Amazingly, these tasks can be carried out unconsciously!

But what about our four-legged animal friends? Most terrestrial mammals other than humans locomote using all four of their limbs. Compared to us, these quadrupeds make twice as many points of contact with the ground. They have to keep track of more legs with each tick of their movement metronome. And with these extra limbs, they gain unique modes of locomotion compared to our simple one-two step.

A horse, of course!

Humans are particularly interested in studying the locomotion of horses, partially because we keep them as domesticated pets and as farm helpers, and partially because they form the center of a multi-billion-dollar racing and equestrian industry. If you have ever watched the surprisingly-entertaining Olympic sport of dressage, you’ll quickly learn that domesticated horses can be trained to perform all sorts of dance-like movements, such as a trot standing still (called a piaffe) or a diagonal walk (called a half-pass). Horses’ ability to learn some types of locomotion even appears to be in their genes!

In nature, however, wild horses use one of four basic stepping patterns for locomotion: walk, trot, canter, or gallop. Each one allows the horse to move faster than the one before, and horses naturally adjust their gait based on their desired speed of travel. For example, a horse might use the walk pattern to graze lazily in search of a bite of grass for dinner, but would quickly switch to a gallop to escape finding themselves turned into dinner.

Each of these methods of locomotion (also called gaits) has a characteristic rhythm and is characterized by specific qualities.

1. The walk is a slow, evenly-paced, four-beat pattern where the hooves come down one at a time in sequence. Since three hooves make contact with the ground at the same time, walking is the most stable (pun intended) method of locomotion.
2. The trot is slightly faster than the walk. When trotting, we can hear two distinct beats, as the diagonal legs alternate between sharing the burden of the weight. A horse can maintain a trot for hours, and is the most common method of movement for wild horses when they travel long distances. With the trot, there may be a moment where the horse is completely off the ground. This is called the suspension phase, which we can only observe when they reach higher speeds.
3. The canter is what the horse uses to pick up speed. With this gait, they may be moving up to 15 mph. The canter is a complex three-beat pattern. The left hindfoot makes the initial contact with the ground. Pushing off this foot, the right hindfoot and left front foot touch down at the same time. Finally, the right front foot comes down, pushing the horse off into the air for the suspension phase. This brief moment of air time explains the
   

   A galloping horse. Would you notice the video repeats itself if not for the numbers in the corner?

   pause in the three beat rhythm.

4. The gallop is the fastest form of movement. As the horse starts to accelerate out of the canter, the diagonal pair of legs land at slightly different times. This gait turns into the gallop, which produces a four-beat step with a characteristic swing rhythm. Although horses can gallop at over 40 mph (almost twice as fast as Olympic champion Usain Bolt’s record-setting speed of 23.3 mph), a horse might only be able to maintain this pace for a few minutes at most because it uses a lot of energy.

Horses use these gaits the same way an accelerating car switches gears on the highway. Each gait is tuned for a specific range of speeds, optimized to achieve the best balance between energy consumption, coordination, and velocity. If the horse tries to trot too quickly, it will use much more energy than if it switches to a canter. Thanks to the central pattern generators believed to be in the horse’s spinal cord, gait switching is entirely unconscious: Horses are all equipped with automatic transmission.

Be it human, horse, or any other creature, movement has a natural rhythm to it. Walking and running are possible because of special adaptations that primitive nervous systems developed long ago in our evolutionary past. In many ways, these adaptations are constant reminders that our bodies are meant for dancing!