2 legs versus 4 legs. What is the difference?

Bipeds and Quadrupeds - What’s the difference?

We both have four limbs, we have a lot of the same muscles, so structurally what’s really the difference between us and our furry friends?

As bipeds we walk on two feet. In theory this means 50% of our body weight is transmitted through our legs to the ground when standing, and our legs move alternately to each other during gait. There is a small amount of rotation though the lower part of our spine, and some rotation and torqueing through our pelvis. The way we achieve forward momentum is by swinging our leg forward, and allowing gravity to do the majority of the work with minimal muscle activation. This allows for a very energy efficient, albeit somewhat lazy gait pattern. However, there are consequences to this. We are naturally built to be a bit unstable, with tall bodies over a very narrow base of support, and a very high center of gravity. To allow us to keep our center of gravity we require a lordosis, or curvature in the lower part of our backs. Poor postural habits, muscle weakness, and various lifestyle factors can cause this lordosis to increase, which can cause compression of the joints in our lower back, pain, and arthritis. Because most of the movement and most of the compression happens near the bottom of the spine, this tends to be where humans have the most wear and tear.

As quadrupeds, your dog uses four legs to walk and run. The exact pattern of foot placement depends on the speed of their gait, and they may have between one and three feet on the ground at any given time. This greatly increases their stability, however every step requires some degree of spinal movement. Faster gait patterns require greater spinal flexion and extension. By extending their spine they are able to stretch out and increase their stride length, and by flexing their spine they can push off harder with their legs to increase power output. The discs and ligaments in the spine must absorb all of this load. If these soft tissues structures start to fail, it can lead to disc herniations and IVDD.

Dogs tend to stand with 60% of their body weight in their front legs, and 40% in their back legs. As such they often develop stiffness and muscle tension in the upper part of the back. Humans also tend to develop issues here, however this is more often related to poor postural habits related to sitting.

In addition to having more of their body weight in their front legs, dogs have to rely on the ligaments in their neck and their neck muscles to keep their head upright, as opposed to humans who rely on the passive effects of gravity to keep our head oriented over our bodies. This means dogs are more prone to soft tissue injuries and muscle spasm in their neck, whereas humans would be more prone to compression types of injuries in their spine.