 This looks pretty normal, people walking. But if we stop to examine what we're witnessing, this is amazing. And a little bit weird. Most terrestrial mammals walk on four legs, which makes sense. It's more stable, it's faster, and more efficient. And yet, we humans have evolved to walk on two legs. And through a set of anatomical adaptations, we've made it work. A balance between stability and mobility, between efficiency and performance. These competing forces have shaped the anatomy of our entire musculoskeletal system. All of these adaptations, together with our brains and the use of our hands, have set us on the path to not only survive on this planet, but thrive in all climates and environments. So why did this transition to bipedal gait happen? It turns out we needed our hands to feed ourselves, rather than use them for locomotion. But that's another story. Today, let's have a look at the anatomy of walking and standing. Let's see how the conflict between mobility and stability, between performance and efficiency, has been resolved. When we stand, remarkably little energy is required to maintain an upright position, even on two relatively spindly legs. And when we walk, our bodies move smoothly through space, with little up and down or side to side motion. So, how do we efficiently distribute our body's weight to our lower limbs? Well, first we need to stabilize our center of gravity, regardless of where, how, or if we're even moving. In humans, the body's center of gravity is in the pelvis, immediately anterior to the second sacral vertebra, right here. It's centered over our feet, and much of the anatomy of the lower limb helps to maintain it there. Curvatures in the vertebral column center the weight of our upper body and transmit it through the pelvis to our thigh. The femur, however, is angled to bring the knees closer to the midline, under the center of gravity, and over the feet. If we bend our hips and knees, our muscles are working to carry the weight of the body. We can't do this for long. Now, when we stand upright like this, we can do this for hours. Well, maybe not today. To make standing easy, our joints lock into place, which transfers the work away from muscles and onto our ligaments and bones. Standing is energy efficient and mainly passive. Let's see what happens when the joints unlock and we begin to walk. When we walk, the body's center of gravity barely changes. This is indicated by the line drawn by the marker. In fact, our center of gravity only moves about 5 centimeters up and down. Here stability wins. So how do joints and muscles collaborate to minimize fluctuations and maximize efficiency? It starts with the pelvis. As the swing leg moves forward, the center of gravity moves upward over the standing leg. To counter this upward movement, the pelvis simply tilts downward towards the swing leg. Abductor muscles in the standing leg limit this downward movement and stabilize the drop. Let's have a look at these abductors in dissection. This is a dissection of the gluteal region behind the hip joint. To see the major abductors, we need to reflect gluteus maximus. Here is gluteus medius and when I reflect that, you can see gluteus minimus. Both of these abductors of the hip joint run from the lateral aspect of the pelvis to the greater trochanter of the femur and control downward movement of the pelvis. We stabilize the pelvis with these muscles. It's a way to maintain mobility when we walk. As we step, the swing leg moves forward and that side of the pelvis rotates forward with it while the pelvis associated with the stance leg stays behind. This rotation on the swing leg side effectively lengthens our stride and helps to prevent excessive drop in the center of gravity. We achieve this rotation through the forward momentum of the swing leg along with back and abdominal muscles. A somewhat surprising feature of walking is that the knee of the stance limb is not fully extended and does not lock as the body moves over it. It stays slightly flexed. This keeps our movement fluid and also keeps the center of gravity low and the body more stable. Now let's have a look at the large flexors and extensors of the knee. Here in the anterior compartment is the quadriceps. The tendon of this muscle reaches over the joint and extends the knee. Here in the posterior compartment are the hamstrings. Together, they flex the knee. During each step, the knee of the swing leg is brought close to the midline so that the foot is planted in front of the other foot. This limits lateral deflection in the center of gravity during walking. Large adductor muscles in the thigh are actually responsible for moving the knee closer to the midline. Let's take a closer look at these muscles. This is the medial compartment of the thigh. It contains the adductors which bring the knee closer to the midline. Everything we've talked about till now has served to stabilize our center of gravity and keep it near the midline. But if we left it at that, our feet would also point inwards. And we're not walking to the midline. We want to move forward. So how do we keep our gait pointed forwards? We need to introduce some lateral rotation to balance things out. Lateral rotation in the hip joint keeps the swing foot pointed directly forward counteracting the inward rotation of the pelvis. Let's see how this works. This is the gluteal region on one side of the body. I'm going to reflect the large gluteus maximus so that we can see the small rotators deep to the muscle. Here they are. These keep our stance balanced and the entire limb oriented so that the foot faces forward. Now that the center of gravity is stable and the foot is oriented forward, we're ready to put our foot down. First contact is with the heel. Gravity is on our side. And honestly, we just fall onto our foot. A controlled fall that we'll have to stabilize. As the foot hits the ground, both the knee and the ankle joints need to be stabilized. Cruciate and other ligaments, together with the mighty quadriceps muscle, prevent the knee from lunging forward too much. The ankle is more of a true hinge joint and is restricted in its mobility by a set of strong ligaments. It's still the smallest and least stable of the 3 meter joints in the lower limb. It doesn't lock when we stand like the knee and hip joint do. And its decreased mobility when walking makes it vulnerable to injury. After the heel hits the ground, weight is transferred onto the sole of the foot. This lower limb is now our stance leg. Next, the heel lifts off the ground, followed by the toes. This movement requires powerful muscles in the posterior compartment of the leg. Together, this is the forward propulsion of what once again becomes the swing leg. Here is the posterior compartment of the leg. The biggest muscles are the gastrocnemius here and soleus here. Their function is to lift the heel off the ground and begin to propel the body forward. Underneath these more superficial muscles are others. Their tendons pass into the foot where they attach. These muscles help with the controlled roll off of the foot and transition the stance leg to being the swing leg, lifting it off the ground. This incredible cycle allows us to walk with ease and fluidity wherever the road may take us.