## Segmental Interaction Principle

Human movement can be performed in a wide variety of ways because of the many kinematic degrees of freedom our linked segments provide. In chapter 5 we saw that coordination of these kinematic chains ranges along a continuum from simultaneous to sequential. Kinetics provides several ways in which to examine the potential causes of these coordination patterns. The two expressions of Newton's second law and the work-energy relationship have been employed in the study of the coordination of movement. This section proposes a Principle of Segmental Interaction that can be used to understand the origins of movement so that professionals can modify movement to improve performance and reduce risk of injury.

The Segmental Interaction Principle says that forces acting between the segments of a body can transfer energy between segments. The biomechanics literature has referred to this phenomenon in several ways (Putnam, 1993). The contribution of body segments to movement has been called coordination of temporal impulses (Hochmuth & Marhold, 1978), the kinetic link principle (Kreighbaum & Bar-thels, 1996), summation of speed (Bunn, 1972), summation or continuity of joint torques (Norman, 1975), the sequential or proximal-to-distal sequencing of movement (Marshall & Elliott, 2000), and the transfer of energy or transfer of momentum (Lees & Barton, 1996; Miller, 1980). The many names for this phenomenon and the three ways to document kinetics are a good indication of the difficulty of the problem

Application: Strength vs. Power

The force-velocity relationship and domains of strength discussed in chapter 4, as well as this chapter's discussion of mechanical power should make it clear that muscular strength and power are not the same thing. Like the previous discussion on power lifting, the common use of the term power is often inappropriate. Muscular strength is the expression of maximal tension in isometric or slow velocities of shortening.We have seen that peak power is the right combination of force and velocity that maximizes mechanical work. In cycling, the gears are adjusted to find this peak power point. If cadence (pedal cycles and, consequently, muscle velocity of shortening) is too high, muscular forces are low and peak power is not achieved. Similarly, power output can be submaximal if cadence is too slow and muscle forces high.The right mix of force and velocity seems to be between 30 and 70% of maximal isometric force and depends on the movement. Kinesiology professionals need to keep up with the growing research on the biomechanics of conditioning and sport movements. Future research will help refine our understanding of the nature of specific movements and the most appropriate exercise resistances and training programs.

and the controversial nature of the causes of human motion.

Currently it is not possible to have definitive answers on the linear and angular kinetic causes for various coordination strategies. This text has chosen to emphasize the forces transferred between segments as the primary kinetic mechanism for coordination of movement. Most electromyographic (EMG) research has shown that in sequential movements muscles are activated in short bursts that are timed to take advantage of the forces and geometry between adjacent segments (Feldman et al., 1998; Roberts, 1991). This coordination of muscular kinetics to take advantage of "passive dynamics" or "motion-dependent" forces (gravitational, inertial forces) has been observed in the swing limb during walking (Mena, Mansour, & Simon, 1981), running (Phillips, Roberts, & Huang, 1983), kicking (Roberts, 1991), throwing (Feltner, 1989; Hirashima, Kadota, Sakurai, Kudo, & Ohtsuki, 2002), and limb motions toward targets (Galloway & Koshland, 2002) and limb adjustments to unexpected obstacles (Eng, Winter, & Patla, 1997).

Some biomechanists have theorized that the segmental interaction that drives the sequential strategy is a transfer of energy from the proximal segment to the distal segment. This theory originated from observations of the close association between the negative acceleration of the proximal segment (see the activity on Segmental Interaction below) with the positive acceleration of the distal segment (Plagenhoef, 1971; Roberts, 1991). This mechanism is logically appealing because the energy of large muscle groups can be transferred distally and is consistent with the large forces and accelerations of small segments late in baseball pitching (Feltner & Dapena, 1986; Fleisig, Andrews, Dillman, & Escamilla, 1995; Roberts, 1991). Figure 6.22 illustrates a schematic of throwing where the negative angular acceleration of the arm (aA) creates