This is an excerpt from Motor Learning and Performance 6th Edition With Web Study Guide-Loose-Leaf Edition by Tim Lee & Richard Schmidt.
It may seem from the previous section that sometimes contradictory principles are involved in these rapid actions. To help in understanding, it will be useful to apply these principles to a familiar task like batting in baseball. This task requires several of the processes discussed so far, such as anticipation and timing, prediction of the ball's spatial trajectory and its arrival time at the coincidence point, and rapid movements that must be both forceful and accurate, so the principles can be applied to various parts of this action. To examine the effects of altering the MT of the swing of the bat, let's assume that some factors are held constant, such as the nature of the pitch and the situation in the game.
A few facts about the timelines involved in hitting a baseball are summarized in figure 6.10. In elite skill-level baseball, a 90 mph (145 kph) pitch requires about 460 ms to travel from the pitcher to the plate, and the MT of the swing of the bat is about 160 ms (Hubbard & Seng, 1954). Evidence presented earlier showed that the internal signal to trigger the swing occurs about 170 ms before the movement starts (Slater-Hammel, 1960; review figure 5.3b and Focus on Research 5.2). With these process durations combined, the signal to trigger the action must be given about 330 ms before the ball arrives at the plate—that is, 170 ms to prepare the swing plus 160 ms to carry it out. Therefore, the decision about whether or not to swing at the ball must be made well before the ball has traveled even halfway to the plate, or after only 130 ms of ball travel. Although some late, visually based corrections in the movement are possible, as discussed in chapter 4, the majority of the action must be planned in advance and initiated by the central nervous system some 330 ms before the ball arrives. Making decisions relative to the occurrence of these critical times plays a decisive role in a batter's success in hitting a pitched ball and also in making changes to an initial decision to swing (see Focus on Application 5.1 for more on checked swings).
Figure 6.10 Timeline of events as a baseball leaves the pitcher's hand and arrives at the plate. The pitch is traveling at a velocity of 90 mph. A fast swing (140 ms) has 20 ms less MT than a slower swing (160 ms).
An important consideration, given the previous discussion of speed and accuracy processes in the chapter, is this: What would happen if the batter could speed up the swing, say from 160 ms to 140 ms? The bat swing's MT could be made shorter through instructions or training to make the actual movement faster, through shortening the movement distance by reducing the backswing (a very slight effect), through using a lighter bat, or through changing the biomechanics of the movement in various ways. Reducing the bat swing MT by 20 ms would have important implications for several separate factors discussed in the previous few sections.
Visual Processing Time
Figure 6.10 shows that shortening the MT delays the beginning of the swing, hence the point at which the details of the action have to be specified, to a position several feet later in the ball's flight. This provides additional time for viewing the ball's trajectory and for determining time to contact, and should allow more accurate anticipation of where and when the ball will arrive. And this extra information comes at a point that is maximally useful—when the ball is closer to the batter—making these extra 20 ms of viewing time particularly beneficial. Therefore, shortening the MT should provide more effective anticipation of the ball's trajectory.
Swing-Initiation Timing Accuracy
If the swing of the bat is speeded up, the decision about when to initiate the movement is made later and is more temporally accurate. In an experiment on a simulated batting task, shortening the MT stabilized the initiation time of the movement, as if the batter were more certain of when to start the swing (Schmidt, 1969). Starting the swing at a more stable time therefore translates into a more stable time for the movement end point at the plate, which yields greater movement timing accuracy.
Movement Timing Accuracy
One process the batter must go through in planning the swing is to estimate the duration of his own movement. Poulton (1974) termed this “effector anticipation.” Therefore, the batter selects a MT, then initiates this action at such a time that the middle of the movement coincides with the arrival of the ball at the plate. If the actual MT is different from the one predicted, the middle of the movement will be too early or late, causing timing errors in hitting the ball. Because reduced MT increases movement timing consistency (figure 6.11), the movement's actual duration will be closer to the batter's estimate. This will result in greater accuracy in hitting the ball, particularly in terms of the timing aspects (see also Schmidt, 1969).
Figure 6.11 The effect of average MT duration on the variability of timing. As MT decreases (i.e., movements are made faster), the variability of timing decreases (i.e., becomes more stable).
Movement Spatial Accuracy
Making the movement faster also influences spatial accuracy, as discussed earlier. If the movement is already relatively slow, instructions to decrease the MT have a detrimental effect on accuracy in hitting the ball. However, most bat swing movements are already quite fast, near the performer's limits in producing force. Recall that when movements are very fast and forceful, reducing the MT tends to increase—not decrease—accuracy (figure 6.11), because the force variability decreases in this range with decreases in MT (figure 6.6). Therefore, reducing the MT when it is already quite short results in improved spatial accuracy, giving more frequent ball contact.
Finally, of course, a faster swing gives more impact to the ball if it is hit—a critical factor in the particular game of baseball. Increasing the load by having a heavier bat can improve spatial accuracy (Schmidt & Sherwood, 1982) and would have only minimal negative effects on movement speed. Clearly, both added bat mass and a faster MT contribute to greater impact with the ball if and when it is hit.
Nearly every factor associated with decreased bat swing MT discussed here would be expected to influence the chances of hitting the ball. Perhaps understanding these factors makes it clearer why professional batters seem to swing with near maximal speeds.