Introduction to Walks

Introduction to Walks So, walks are an important, um, exercise and uh character animation. Basically a walk features all the basic elements of mechanics, but you can also have a lot of variety and personality and acting involved once you master the- the basic walk. Now, we’ll be, uh, breaking down the various poses in uh walks, and the two most important ones are shown here. The uh contact pose, when both feet are on the ground the legs are farthest apart. And the passing position, which is one foot on the ground, the other foot is in the air as the leg swings under the torso. Now, there’s intermediate uh poses uh between these and those have a variety of names for the uh pose that transitions from contact into passing position and the one that transitions out of passing position, but for now we’ll just focus on these two. Now, a few things to understand about the uh vocabulary of walks: first we have the step and stride. A stride is simply uh two steps. So, one full cycle of a walk would be a stride. And we talk about the step length uh and the stride length, so basically the stride length is the distance that, uh, the character travels when walking one full cycle. Now, uh, probably more interesting is the gait. So, the gait indicates the timing of the motion for each foot or each leg, so it indicates, uh, what that leg is doing in time. So, we might start in the contact pose with both feet on the ground then transition and enter into the passing position and then leave the passing position, then both feet are on the ground for a certain time and so forth. So, uh, the typical timing uh for a walk is about, uh, half a second per step or about twelve frames per step, which makes um 24 frames for a stride or full cycle. Uh, there’s variations of course, so uh we can uh have fewer frames or more frames during the cycle, but this is uh the typical uh walking uh timing. And uh you can think of that in terms of the uh parade march time, which most uh parade music is set to, so uh music like “Tah tah tah tah tah tah tah tah tah tah…”, you uh just hum that to yourself and walk and you’ll see that you follow the typical timing. Now, let’s look at the um gait pattern here in a little more detail. So, uh, for uh typical walking the first contact pose is about 10% of the cycle. Then, uh, we have one foot in the air for 40%, uh, that completes one step. Then we have a contact pose again for 10%, and then the other leg is in the air for the remaining 40%. So, this is a very typical gait pattern for a normal walk. And, uh, you see here that, um, the contact uh time when the walk is faster, this uh percentage for the contact time tends to go down a little bit, but also the total amount of time, uh, for a uh stride uh goes down. Here’s, uh, another example of a different gait pattern. This is for a character walking with a painful uh leg. So, uh, with the um painful leg you might think that the natural tendency would be to the- to have that leg on the ground the minimum amount of time, but actually, uh, what happens is the uh passing position with that leg, uh, being the one on the ground that tends to decrease and the contact pose with both legs on the ground of tends to increase, um, and then the total amount of time for a stride increases significantly. In other words, the person walks rather slowly. So, uh, one of the ways of understanding uh this pattern is that, uh, the pressure on the leg has a spike during the contact pose and so in order to minimize that, uh, increase in- in pressure uh you walk slowly and extend the time of the contact pose as long as is practical in order to uh minimize those uh pressure variations. Anyway, the point here is that, uh, with different conditions uh all of that’s indicated in the uh- this gait pattern and timing. Now, when you walk faster, you tend to increase both the length of your steps, or your length of your stride, and then also the cadence or the rate at which you’re taking steps. So, uh, for example if your- if your slow walk has a stride length of 3 feet and stride rate of 2/3 of a stride per second- in other words, 2 strides every 3 seconds- uh, this works out to be a speed of about 2 feet per second, so this would be a slow walk. A fast walk might be you increase your stride length, uh, to 4 feet and then you increase your stride rate to uh 1 stride per second and the combination of those two doubles your speed to 4 feet per second. And, and at around a faster speed of about 6 to 7 feet per second, you don’t tend to walk, you- your pattern transitions into something very different which would be a run. So, the point here is that you don’t, um, walk faster simply by taking bigger steps or simply by changing your um cadence, but you actually do both. Let’s take a quick look at a few reference videos here. First, a normal walk. Now, a slow walk. Now, a fast walk. And finally, let’s look at that slow walk but we’ll play it at uh double-speed, uh, so it’s as if it was fast. So, uh, the uh slow walk uh played fast uh doesn’t look natural, it just looks like it’s being played at high speed. Now, the question would be, well, uh, when we’re walking, uh, why do we take the step-length uh that’s- that we choose. Uh, certainly it’s possible to take, uh, longer steps or shorter steps so, um, when we walk at a certain speed why do we uh take the step-length we uh that- that we do? Um, well, it turns out that the body naturally adjusts the step length to minimize the uh energy required to maintain a certain walking speed; so, that’s why it’s unnatural to walk fast by taking many many quick, short steps and it’s unnatural to walk slow by taking long steps, um, but with a very slow cadence. Now, this um energy requirement you can understand that, uh, there’s two competing factors here. Uh, when you move the leg forward out of the contact pose, uh, it takes some energy to um give that leg um speed. So, just to get the leg moving, the body has to um exert a force and so you- some energy is- is required. Now, once the leg is moving, it just continues moving so, uh, it actually takes less energy uh to take fewer long steps uh than it takes to take, uh, many short steps. So, in this regard, long steps are favorable in terms of the energy required to move the leg. On the other hand, uh, when you take long steps, uh, the body uh drops a large distance, um, in the contact pose and then has to rise a large distance into the passing position. And that requires energy. So, uh, raising and lowering the body, uh, going uh out of the contact into passing position to raise it, and then dropping back into contact uh pose. So, that’s it takes energy and uh long steps require more energy into in terms of raising and lowering the body. So, uh, we have these two competing factors and uh your body has to find uh the best balance between those two depending on the walking speed that you want to, uh, be going. And, uh, you can take measurements on take- put people on a treadmill; fix the speed of the treadmill. If you asked them to take short steps and walk quickly and measure their, um uh, energy consumption it turns out that takes more energy. If you take- have them take long steps with a slow cadence that takes more energy. This minimum in energy consumption happens to occur at the step-length that the body naturally, uh, finds when uh walking at that speed. So, in summary walking has, uh, two basic poses: the contact pose with both feet on the ground, and the passing position with one leg moving under the torso. Uh, both feet are on the ground about 20% of the time, uh, for a cycle. So, that’s about 10% for the first step, then 40% uh of the time one leg is in the air in first step, and then the pattern repeats another 10% both feet on the ground and then 40% percent, uh, one leg in the air. That’s the uh gait pattern we were looking at. Uh, when you walk faster, you tend to increase both the step-length and the rate at which you are taking steps or the cadence. It’s not natural to uh only change the step-length or only change the cadence. And then, uh, for uh different walking speeds, the body finds the step-length that happens to minimize the required energy consumption for that step length. That’s why um walking at a certain speed, there’s a certain step-length that feels natural, um, to the walk, uh so. Now, uh, it’s amazing but uh just over years of walking, uh, your body has naturally discovered this uh- these step lengths. Well, that’s a basic introduction to walks. We’ll be getting into a lot more details uh of the different elements in the next tutorial, so I’ll see you then.