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Interdisciplinary Issue: Specificity

One of the most significant principles discovered by early kinesiology research is the principle of specificity. Specificity applies to the various components of fitness, training response, and motor skills. Motor learning research suggests that there is specificity of motor skills, but there is potential transfer of ability between similar skills. In strength and conditioning the principle of specificity states that the exercises prescribed should be specific, as close as possible to the movement that is to be improved. Biomechanics research that measures the angular kinematics of various sports and activities can be used to assess the similarity and potential specificity of training exercises. Given the peak angular velocities in Table 5.2, how specific are most weight training or isokinetic exercise movements that are limited to 500° per second or slower? The peak speed of joint rotations is just one kinematic aspect of movement specificity. Could the peek speeds of joint rotation in different skills occur in different parts of the range of motion? What other control, learning, psychological, or other factors affect the specificity of an exercise for a particular movement?

angular acceleration as an unbalanced rotary effect. An angular acceleration of -200 rad/s/s means that there is an unbalanced clockwise effect tending to rotate the object being studied. The angular acceleration of an isokinetic dynamometer in the middle of the range of motion should be zero because the machine is designed to match or balance the torque created by the person, so the arm of the machine should be rotating at a constant angular velocity.

Angular kinematics graphs are particularly useful for providing precise descriptions of how joint movements occurred. Figure 5.12 illustrates the angular displacement and angular velocity of a simple elbow extension and flexion movement in the sagittal plane. Imagine that the data repre-

Interdisciplinary Issue: Isokinetic Dynamometers

Isokinetic (iso = constant or uniform, kinetic = motion) dynamometers were developed by J. Perrine in the 1960s. His Cybex machine could be set at different angular velocities and would accommodate the resistance to the torque applied by a subject to prevent angular acceleration beyond the set speed. Since that time, isokinetic testing of virtually every muscle group has become a widely accepted measure of muscular strength in clinical and research settings. Isokinetic dynamometers have been influential in documenting the balance of strength between opposing muscle groups (Grace, l985).There is a journal (Isokinetics and Exercise Science) and several books (e.g., Brown, 2000; Perrin, 1993) that focus on the many uses of isokinetic testing. Isokinetic machines, however, are not truly isokinetic throughout the range of motion, because there has to be an acceleration to the set speed at the beginning of a movement that often results in a torque overshoot as the machine negatively accelerates the limb (Winter,Wells, & Orr, 1981) as well as another negative acceleration at the end of the range of motion.The effects of inertia (lossifidou & Baltzopoulos, 2000), shifting of the limb in the seat/restraints (Arampatzis et al., 2004), and muscular co-contraction (Kellis & Baltzopoulos, 1998) are other recent issues being investigated that affect the validity of isokinetic testing. It is important to note that the muscle group is not truly shortening or lengthening in an isokinetic fashion. Muscle fascicle-shortening velocity is not constant (Ichinose, Kawakami, Ito, Kanehisa, & Fukunaga, 2000) in isokinetic dynamometry even when the arm of the machine is rotating at a constant angular velocity. This is because linear motion of points on rotating segments do not directly correspond to angular motion in isokinetic (Hinson, Smith, & Funk, 1979) or other joint motions.

Figure 5.12. The angular displacement and angular velocity of a simple elbow extension/flexion movement to grab a book. See the text for an explanation of the increasing complexity of the higher-order kinematic variables.

sented a student tired of studying exercise physiology, who reached forward to grab a refreshing, 48-ounce Fundamentals of Biomechanics text. Note as we look at the kinematic information in these graphs that the complexity of a very simple movement grows as we look at the higher-order derivatives (velocity).

The elbow angular displacement data show an elbow extended (positive angular displacement) from about a 37° to about an 146° elbow angle to grasp the book. The extension movement took about 0.6 seconds, but flexion with the book occurred more slowly. Since the elbow angle is defined on the anterior aspect of a subject's arm, larger numbers mean elbow extension. The corresponding angular velocity-time graph represents the speed of extension (positive w) or the speed of flexion (negative w). The elbow extension angular velocity peaks at about 300 deg/s (0.27 sec) and gradually slows. The velocity of elbow flexion increases and decreases more gradually than the elbow extension.

The elbow angular acceleration would be the slope of the angular velocity graph. Think of the elbow angular acceleration as an unbalanced push toward extension or flexion. Examine the angular velocity graph and note the general phases of acceleration. When are there general upward or downward trends or changes in the angular velocity graph? Movements like this often have three major phases. The extension movement was initiated by a phase of positive acceleration, indicated by an increasing angular velocity. The second phase is a negative acceleration (downward movement of the angular velocity graph) that first slows elbow extension and then initiates elbow flexion. The third phase is a small positive angular acceleration that slows elbow flexion as the book nears the person's head. These three phases of angular acceleration correspond to typical muscle activation in this movement. This move ment would usually be created by a tri-phasic pattern of bursts from the elbow extensors, flexors, and extensors. Accelerations (linear and angular) are the kinematic variables closest to the causes (kinetics) of the motion, and are more complex than lower-order kinematic variables like angular displacements.

Figure 5.13 plots the ankle angle, angular velocity, plantar flexor torque, and REMG for the gastrocnemius muscle in a concentric-only and an SSC hop. Notice how only the SSC has a negative angular velocity (describing essentially the speed of the eccentric stretch of the calf muscles) and the dramatic difference in the pattern and size of the plantar flexor torque created.

Angular and linear kinematics give scientists important tools to describe and understand exactly how movement occur. Remember to treat the linear and angular measurements separately: like the old saying goes, "don't mix apples and oranges." A good example is your CD player. As the CD spins, a point near the edge travels a larger distance compared to a point near the center. How can two points make the same revolutions per minute and travel at different speeds? Easy, if you notice the last sentence mixes or compares angular and linear kinematic variables. In linear kinetics we will look at the trigonometric functions that allow linear measurements to be mapped to angular.

Biomechanists usually calculate angular kinematic variables from linear coordinates of body segments with trigonometry. There is another simple formula that converts linear to angular kinematics in special conditions. It is useful to illustrate why the body tends to extend segments prior to release events. The linear velocity of a point on a rotating object, relative to its axis of rotation, can be calculated as the product of its angular velocity and the distance from the axis to the point (called the radius): V = w • r. The special condition for using this

Figure 5.13. Ankle angle, angular velocity, torque, and rectified EMG in a concentric-only (PFJ: plantar flexion jump) and SSC hop exercise (RJ: rebound jump). Figure reprinted permission of Sugisaki et al. (2005).

formula is to use angular velocity in radians/second. Using a dimensionless unit like rad/s, you can multiply a radius measured in meters and get a linear velocity in meters/second.

The most important point is to notice that the angular velocity and the radius are equally important in creating linear veloci ty. To hit a golf ball harder you can either use a longer club or rotate the club faster. We will see in chapter 7 that angular kinetic analysis can help us decide which of these two options is best for a particular situation. In most throwing events the arm is extended late in the throw to increase the linear velocity of a projectile. Angular ki netics is necessary to understand why this extension or increase in the radius of segments is delayed to just before release.

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