Move Dem Bones!
Human movement can be downright dynamic, if not mechanical.
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What does it take to get your body moving, aside from coffee?
It helps to have some energy, of course. And it helps to have a goal of some kind -- something to stimulate your juices. You have to be motivated, right?
Beyond being hyped up, though, there are certain physical actions to take, involving muscles. All the logic in the world won't help a bit, here, if you can't activate the muscle tissue. You have to start the engines and get the muscles in gear. They have to be active and coordinated. Only then can you move. You won't go if the muscles work against each other!
Getting the muscles in gear means orchestrating them. This is mind business. It requires understanding, or know-what. You also need know-how. The muscles have to work together to accomplish a specific task. At any time, with any kind of skill, all your muscles are working, or carefully not doing anything -- being appropriately silent, inactive. This is auto-management. Or self-management. Whether you walk, talk, eat, or sleep, each element of your body has its own job to do -- an assigned duty that contributes to the overall responsibility. It's your job to manage the operation. Else why are you here? It's a job you accomplish by using your brain.
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To behave properly you have to know what you're doing, where you're going. But how do you accomplish this? Well, of course, that's what perception -- pattern recognition or information acquisition -- is all about. We're loaded with sensors. The environment pretty much dictates your action, so you have to know what's going on. And for this you need sensors and a brain. With them you generate representations -- simulations. The sensors clue you in on the nitty-gritty details -- the signals -- and the brain tells the sensors what to look for, or how to interpret the signals. The interpretations are the representations. And all representations are simulations.
Take the eyes, for instance.
For the eyes to see, they first have to be in good working order. And of course they must be open, which is to say that the lids have to be up and out of the way. And to see specific things, the eyes have to be pointed in the right direction, and focused.
To see what's in back of you, for instance, you have to turn your head -- or look in a rearview mirror if one is available. To see behind a barn you have to move around it. To look inside a box you have to open it. That's the nature of the beasts. Seeing requires an unimpeded view. (We haven't yet learned to see through "opaque" objects. But who knows when that will be possible, too! After all, everything is mostly space!)
But it's not enough for the eyes to be open, pointing in the right direction, having an unimpeded view. They also have to be focused on stuff and there has to be a brain telling them what to do and what to look for and what to see.
Eyes may be pointing at a tree, but what they see will depend on whether they are the eyes of a botanist, construction worker, traveler, dog, .... Each pair brings its own personal background, interests, motives, and needs to the scene. A selective process is at work here, digging out this or that bit of information, and each pair of eyes sees what's in its best interests at the moment. And of course all the efforts have to be coordinated.
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Your body moves as a consequence of an orderly, organized space-time pattern of signals to the muscles -- innervations and contractions of muscles, which apply forces to bones. Seizures show what can happen when the innervations are not coordinated. The process is obviously very complex on the face of it, but even more so because the bones are linked together and the action of a muscle at one point has a domino effect on the rest of the body. When you contract a muscle, this exerts forces in a chain against other parts of the body and, ultimately, against an external, resistive element of the environment, such as the ground or water. Even for the simple act of moving your hand, neural signals must be transmitted to keep the rest of your body steady. Any purposive action must take the interactions into account.
Just Walking
Consider the coordination problem just to walk!
Simply to walk, you must apply torque to the femur (upper leg bone) of the stepping leg in order to bend it at the hip and thrust it forward. As the leg moves forward, the lower part of your spine twists away from the swinging leg and this causes your pelvis to rotate as well. Since you intend to move directly forward and not rotate in a circle with your pelvis, you must compensate for the rotation and swing your leg in an inside-outside motion, which is to say you rotate the hip laterally, or outwardly, a small amount as you thrust it forward.
And of course as you thrust the upper leg forward, you drag the lower leg with it. You both translate the whole leg and rotate it about the hip, and you encounter opposition to both motions. For the linear component, opposition occurs in the form of linear inertia, due to the leg's mass. For the rotational component, there are two opposing factors: One is the leg's rotational (or moment of) inertia, due to the mass distribution of both the upper and the lower leg. The other is the countering torque due to gravity.
When you swing your leg you also flex your knee, or at least let the leg go limp, momentarily. You do this to reduce the rotational inertia of the leg and the countering effect of gravity. Bending the knee brings the mass of the lower leg closer to the hip and so reduces the counter torque. But you quickly stimulate the extensor muscles of the lower leg to project it forward, else the leg would remain completely limp and you'd end up on your face with no support for the footfall. The knee is flexed the most at the beginning of the step and is gradually extended until it's almost fully extended at the landing.
Just as with the entire leg, so too the thrusting action of the lower leg encounters opposition. First, the lower leg has its own inertia in the form of mass that opposes the translation of the knee. Second, there is opposition to the rotation of the leg about the knee, which takes the form of angular inertia and a gravitational counter torque about the knee.
At the same time, the forward thrusting effort exerts a force diagonally and backwards against the ground, contracting the extensor muscles of the supporting leg. The force works against the friction between your shoe and the ground. The force pushes the earth backwards relative to you, or propels you forward relative to the earth.
In addition, the forward step instantly creates an imbalance in that all of your weight is put on the supporting leg, which is displaced from your center of gravity. Because of gravity, the immediate effect is for the unsupported pelvis to lower and for your body to tilt forward and toward the unsupported side. To keep from caving in to the downswing torque, you must counter with an opposing torque around the supporting hip that raises the same-side trunk and shoulder and pulls your trunk backward. Loss of function at any point of the action could be destabilizing.
System Considerations
Because of the interconnectedness of the framework of bones and the contact with the physical environment, movements of the individual body parts can't be determined in isolation. Rather, the whole system has to be considered. This is a very complex mechanical problem for the nervous system to solve, involving a great mass of specialized neural elements. It's the reason that astronauts have to be re-trained for operations in deep space, where there is no purchase (no foothold). If intended movements are to be successful, there has to be cooperation among all the muscles. There must therefore be an overall plan, or mental map, that patterns the muscle action. The event must be managed. It's real brain work! Else how could you possibly be in control?
To refer to tennis, suppose you're hitting a forehand drive. In this event, the muscles of your hitting arm produce a torque, or arm-turning motion, that drives the racket head against the ball. Concurrently you are organizing to snap your wrist to accelerate the racket through the ball. The drive depends for its force partly on the strength and speed of your arm, but also partly on the strength of your torso, which supports the hitting arm, and partly on the stability of your legs, which power your torso. The various events occur in a series-parallel combination. It's the combination of forces that produces the racket's speed and effective mass. If, at the point of impact with the ball, you should suddenly relax the muscles in your wrist or shoulder or legs or back, power would be lost. Or if your feet should slip on the court, power would again be lost and the ball-racket contact would be disrupted. It would be as if you were in deep space.
Just how the brain controls the muscle forces to produce the coordinated movement isn't known, but it's clear that the computation is by no means trivial. For one thing, the force produced by each muscle depends on several factors: the size of the muscle, the number of its fibers, and the extent to which it is stretched. In addition, the muscle doesn't attach at a single point or at a precise fulcrum but rather is distributed over an area; so neither the magnitude of the resultant force nor the point of application are obvious. For another thing, a muscle may span more than one joint. For example, the leg adductor (the tensor fasciae latae) attaches to the upper edge of the pelvic girdle, runs down the side of the femur and attaches at the other end to the outer surface of the knee. Also, the action at any joint may be subject to reactive forces. To make matters even worse, the joints aren't simple hinges.
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