Development of Ballistic Skills
Movements can be classified by their EMG and force curve signatures. Three movement categories are (1) ramped movements in which force is gradually and steadily applied through continuously graded muscle firing; (2) discontinuous, slow movements that accelerate gradually, then decelerate, and then accelerate again, such as when tracking an object with a hand; and (3) ballistic movements that are rapid, all-out, forceful movements that reach peak acceleration within milliseconds of their initiation (Brooks, 1983; Desmedt, 1983).
In simple ballistic movements, such as a rapid reach for an object, the motion is started by an initial strong impulse in the agonist muscle. The agonist firing then stops, but the movement continues due to the momentum of the limb. A burst by the antagonist muscle subsequently causes the limb to decelerate. In targeting tasks, there may be a second burst in the agonist to correct the limb placement. This creates a signature triphasic pattern of muscle activation: agonist, antagonist, agonist (Palmer, Cafarelli, & Ashby, 1994).
In contrast to single-limb movements, ballistic skills involving the total body are quite complex. Typical sport skills are examples of ballistic skills. Of these, movements in which the performer projects an object have been of considerable interest to both biomechanists and developmentalists. Striking (as in batting and swinging a racket), kicking (as in football and soccer), and throwing (as in handball and baseball or softball) have all received study. These complex movements tend to begin proximally with action of the trunk muscles; energy is then passed from segment to segment of the open kinetic chain until it reaches the distal effector, usually the hand or foot. During striking, kicking, and throwing, this energy exchange is superimposed on a running or stepping base of support.
Another interesting commonality across these complex projectile skills is that they employ a backswing to place the body segments in position to move forward. In the advanced form of striking, throwing, and kicking, the backswing and forward swing partially overlap, and proximal segments begin to move forward while more distal segments are still moving backward. The advanced form of these movements also involves strong acceleration of the distal segment just milliseconds before contact or release, which yields high distal angular velocity. This acceleration seems to result partially from the inertia of each distal segment against the motion of adjacent proximal segments. The lagging segment stretches the agonist muscles of the distal segment, which may in turn excite reflexes that augment the muscular contractions in the distal segment (Roberts & Metcalfe, 1968). As we will see, lag is one aspect of object projection that develops only gradually over time. It is so important for effective ballistic movement that Southard (2002a, 2002b) used lag as a collective variable in his studies of throwing.
In this chapter we focus on the development of striking, throwing, and kicking. Most developmental studies on ballistic projectile skills have been based in developmental sequence theory, although a few studies are beginning to address issues from a dynamic systems perspective. At the close of this chapter we discuss future directions in this area.
Intertask Developmental Sequence
The only ballistic skills that have been studied in relation to their intertask development are overarm throwing and striking. The approach used in this research is quite different from the intertask approach used to study locomotion (see chapter 6). For locomotion, the developmental sequence is formed by ordering each locomotor skill according to its initial appearance in the movement repertoire. For throwing and striking, the two tasks are compared to see where participants are located within a component developmental sequence at the same point in time in each skill. Do the participants show the same developmental level in the two skills, or do they develop faster in one skill than they do in the other?
Langendorfer (1987a) compared the development of trunk and humerus action in overarm throwing with development of the same action in overarm striking. He asked 58 boys aged 1.5 to 10.3 years to throw a ball forcefully and to strike a tennis ball suspended 6 inches (15 cm) above their head forcefully with a lightweight racket. He then looked at side and rear film views to categorize the boys’ movements in each skill. Figure 7.1 shows his results. From the cross-sectional data we can infer how the longitudinal data might look. The data on trunk action (see figure 7.1a) suggested that children advance in throwing before they do in striking: The boys started out at a primitive level in both skills and then advanced to level 2 throwing before achieving level 2 striking and advanced to level 3 throwing before reaching level 3 striking. The cross-sectional data on humerus action (see figure 7.1b) suggested that children begin at level 1 in both skills and then move ahead in either skill until they reach level 2 in both skills. Then, however, they progress to level 3 in throwing before they progress to level 3 in striking.
Langendorfer (1987a) learned from his data that he could not compare forearm action between striking and throwing. Rather, the action of the racket, which is the distal segment in striking, was more comparable to the forearm action of throwing. Figure 7.1c shows the developmental comparison for forearm action. While some children exhibited different relationships early on, more progressed in striking before they progressed in throwing. Then throwing took the developmental lead for more children.
The finding on forearm action demonstrates how equipment can affect a person’s developmental level. Using Newell’s notions of person constraints (see chapter 3), we can think of equipment as an extension of the body. Comparing hammering with a hammer with pounding with a fist demonstrates how an implement changes the inertial characteristics of the arm and hand. This analysis also applies to prosthetic limbs. In both cases the performer needs to learn how to incorporate the implement into the actual movement pattern. Langendorfer’s (1987a) data suggest that at first the implement assists the child in achieving a higher developmental level in the distal component, probably because its greater inertia causes lag. Later, though, the handle length may hinder change to a higher developmental level by making it more difficult to delay that lag until the body has reached front facing (see tables 7.1 and 7.4).
The last component that Langendorfer (1987a) compared across the two skills was stepping. As we can see in figure 7.1d, the developmental relationship for stepping was less clear. Langendorfer attributed this result to a degree of unreliability in the component sequence: Stepping tended to be variable across trials in both throwing and striking, making intertask comparisons unreliable.
The arrows in figure 7.1 reflect Langendorfer’s (1987a) speculation on the longitudinal course that these two tasks might display while developing. The only actual longitudinal information that we have regarding this question was reported by Roberton (1982). She compared the development of trunk action within sidearm striking (as opposed to overarm striking) and overarm throwing. Interestingly, the trunk action of four children did follow the developmental path that Langendorfer’s cross-sectional overarm data suggested: The children progressed one level in their throwing trunk action and then caught up to that same level in their striking trunk action. Then they progressed again in throwing before catching up in striking.