2 legs versus 4 legs. What is the difference?
Updated: Apr 19, 2020
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.
Any one who has dealt with an injury has likely experienced the effects of compensation from that injury. It’s a familiar story. You broke your leg, now your other leg has to bear 100% of your body weight while you are healing. Because of this, you require a gait aid, such as crutches, a walker, or in some case even a wheel chair. Now your knee on the other side is sore from taking the extra load. In humans, if you are compensating for a leg injury on one side, you typically develop issues in the contralateral knee and hip, pelvis, and lower part of your spine. If
you are compensating for an arm injury, you will just use your other arm, and it tends not to have much affect on your spine at all.
Dogs also will compensate for an injured leg, but how this looks and how it manifests itself is different. If they are unable to weightbear on one leg, they have 3 other legs to support their body weight. As such, injuries often do not slow them down too much, and often can make us underestimate the severity of their injury.
In dogs, the effect of compensation on the spine tends to be much greater. Because they will continue to ambulate on 3 legs, they will still walk and even run, but with a much less efficient gait pattern. There will be much greater excursion of spinal movement and torquing force through the spine, all the way from head to tail. If they injure a leg you will see two common compensation patterns: 1) Increased weight transfer to the contralateral side (ie/ if an injury on the right side, increased weightbearing on the left side); and 2) increased weightbearing in either the front end or back end (in the case of a hind limb injury increased front-loading, in the case of a front limb jury increased weightbearing in hind end). Let’s think about an example to consider what this means. If a dog injures his right back leg, you will see increased weightbearing in the left hind, and both front limbs. All of this extra weight in the front legs put more load through their shoulders, elbows, and carpus (wrists). What this means, is even though they might have a back leg injury, they may end up with pain in their upper back or even a front leg from compensation. If the original injury is left unmanaged and these abnormal movement patterns are allowed to remain, over time this may manifest itself as arthritis in the elbow, shoulders, or carpi due to increased stress on these joints.
All of this means we really must consider the whole dog in treatment of any injury!