There are two causes of body movement. First, an external force can act on the person or animal; a strong gust of wind, for example, may cause the movement of living organisms. Second, through biological machinery such as muscles, force is produced internally, leading to movement. The truly interesting aspect of our movement within a physical environment is that without exception, our movement is a consequence of both muscles generating force internally and the environment producing forces on our body externally.
How then does the nervous system deal with the generation of instructions to the muscles to control movement? In other words, what is the “language” for motor commands? Let’s begin with a straw person model, which we know cannot be correct but is nonetheless useful to enhance our understanding. Perhaps the motor commands specify an exact sequence of muscular forces along with the actual temporal pattern of these forces. This type of idea is seen in the impulse-variability theory for the speed–accuracy trade-off. This model is based upon the assumption that motor commands specify the intensity and duration of muscular contractions.
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Nikolai Bernstein argued that the central nervous could not directly specify muscular forces. His argument is quite simple. Imagine flexing and extending your elbow joint in rhythm to a metronome. In one case you are moving in the horizontal (transverse) plane so that gravity is neutral. In the second case, you move in the vertical (sagittal) plane. In this latter case, the effects of gravity are dependent upon whether you are extending the elbow (moving down) or flexing the elbow (moving up). In these two conditions of planar motion, the movement trajectories are the same, but the actual internally generated muscular forces are different. In other words, motor commands cannot be specific because we cannot know (predict) the exact nature of external forces acting upon us. Motor commands must be more general.
Delos Wickens, in a classic experiment published in 1938, showed that motor commands are not linked directly to muscle commands. Human research subjects placed their index finger on a button, with the palm of the hand facing down. A stimulus was presented, and a short time later, the index finger received a mild electrical shock. In a short time, the subjects were conditioned to avoid the shock. What did these people learn? Did they learn to contract a muscle, or did they learn to move away? Wickens then had these subjects put their index finger on the button but this time with the palm up. If in fact the individuals had developed motor commands to contract their flexor muscle of the index finger, they would not avoid the shock. If the individual’s motor commands were written to “move away”—if, in other words, the motor commands were a spatial code—then the participant would easily avoid the shock. For9 out of 10 subjects, the movement was immediate and away. In other words, at the highest levels of the central nervous system (CNS), the motor commands were set up in reference to what is called allocentric space (with reference to the external spatial world) compared to muscular space.
Are specific muscular commands unimportant? The answer is negative. One can design a very easy experiment to capture this point. Take your favorite pen, and sign your name on a piece of paper. Now, use your nondominant hand and sign your name. Research clearly has shown that these two signatures look very similar but at the same time are different. One’s nondominant hand signature is not as clear as that of one’s dominant hand. In other words, motor commands have generality (the spatial commands can be transferred to different effectors) and also have specificity. Specificity means that one’s nervous system has specialized commands for well-practiced muscular patterns. These commands might be much more motor specific. We do not know much about the nature of this type of specificity.
Another way to examine the nature of motor commands is to make the assumption that human beings strive to minimize a particular characteristic of movement. For example, to minimize movement time (MT), muscular contractions would produce force as quickly as possible and then decelerate (“jam on the brakes”) at the last possible instant. This is the driving strategy of a typical 16-yearold. However, unless there is a sense of urgency, it seems as though human beings attempt to move to minimize some aspect of the energy expended. The way to do this is to move smoothly. Smooth is defined as lacking changes in acceleration. Pointto-point movement trajectories look as though the minimization principle is related to reducing changes in acceleration. Movement trajectories tend to show a bell-shaped velocity profile so that there is a period of positive acceleration and then a negative acceleration. Scientists are not in full agreement concerning the actual muscular and/or neural quantity that is minimized, but it is clear that biological motion is geared to reduce energy expenditure.
Finally, motor commands must possess a high degree of generality. Handwriting is the most popular example. People can sign their name on a whiteboard, a check, a letter, and using a mouse with a computer screen. Signatures are large or small, written fast or slow, but a signature maintains the spatial–temporal pattern that makes it “yours.” What type of motor commands allows humans to do this? Perhaps it makes sense to think of a set of motor commands like a set of rules, or equations, that define the trajectory in space and time. These rules, or equations, are scalable so that the essence of our signature or our tennis forehand exists, but the particular muscle fibers, or muscle units, are organized independently of these rules or commands. Thus, our motor commands are not in muscle language, which allows for flexibility as well as specificity.
- Flash, T., & Hogan, N. (1985). The coordination of arm movements: An experimentally confirmed mathematical model. The Journal of Neuroscience, 5,1688–1703.
- Wickens, D. D. (1938). The transferance of conditioned excitation and conditioned inhibition from one muscle group to the antagonistic muscle group. Journal ofExperimental Psychology, 22, 101–123.