What is the principle of balance and stability?

We have seen than angular kinetics provides mathematical tools for understanding rotation, center of gravity, and rotational equilibrium. The movement concept of balance is closely related to these angular kinetic variables. Balance is a person's ability to control their body position relative to some base of support (Figure 7.13). This ability is needed in both static equilibrium conditions (e.g., handstand on a balance beam) and during dynamic movement (e.g., shifting the center of gravity from the

rear foot to the forward foot). Balance can be enhanced by improving body segment positioning or posture. These adjustments should be based on mechanical principles. There are also many sensory organs and cognitive processes involved in the control of movement (balance), but this section focuses on the mechanical or technique factors affecting balance and outlines application of the Principle of Balance.

Before we apply this principle to several human movements, it is important to examine the mechanical paradox of stability and mobility. It turns out that optimal posture depends on the right mix of stability and mobility for the movement of interest. This is not always an easy task, because stability and mobility are inversely related. Highly stable postures allow a person to resist changes in position, while the initiation of movement (mobility) is facilitated by the adoption of a less stable posture. The skilled mover learns to control the position of their body for the right mix of stability and mobility for a task.

The biomechanical factors that can be changed to modify stability/mobility are the base of support, and the position and motion of the center of gravity relative to the base of support. The base of support is the two-dimensional area formed by the supporting segments or areas of the body (Figure 7.14). A large base of support provides greater stability because there is greater area over which to keep the body-weight. Much of the difficulty in many gymnastic balancing skills (e.g., handstand or scale) comes from the small base of support on which to center bodyweight.

The posture of the body in stance or during motion determines the position of the center of gravity relative to the base of support. Since gravity is the major external force our body moves against, the horizon

tal and vertical positions of the center of gravity relative to the base of support are crucial in determining the stability/mobility of that posture. The horizontal distance from the edge of the base of support to the center of gravity (line of action of gravity) determines how far the weight must be shifted to destabilize a person (Figure 7.15a). If the line of gravity falls outside the base of support, the gravitational torque tends to tip the body over the edge of the base of support. The vertical distance or height of the center of gravity affects the geometric stability of the body. When the position of the center of gravity is higher, it is easier to move beyond the base of support than in postures with a lower center of gravity. Positioning the line of gravity outside the base of support can facilitate the rotation of the body by the force of gravity (Figure 7.15b).

Biomechanical studies of balance often document the motion of the two important forces of interest, body weight and the reac tion force under the base of support. Video measurements using the segmental method measure the motion of the center of gravity over the base of support. Imagine where the center of gravity would be and how it would move in the base of supports illustrated in Figure 7.14. Force platforms allow the measurement of the misnomer center of pressure, the location of the resultant reaction force relative to the base of support. In quiet standing, the center of gravity sways around near the center of the base of support, while the center of pressure moves even faster to push the weight force back to the center of the base of support. The total movement and velocities of these two variables are potent measures of a person's balance.

Recall that the inertia (mass and moment of inertia), and other external forces like friction between the base and supporting surface all affect the equilibrium of an object. There are also biomechanical factors (muscle mechanics, muscle moment arms,

Figure 7.15. The position of the line of gravity relative to the limits of the base of support determines how far the weight must be shifted for gravity to tend to topple the body (a) or the size of the gravitational torque helps create desired rotation (b).

Figure 7.15. The position of the line of gravity relative to the limits of the base of support determines how far the weight must be shifted for gravity to tend to topple the body (a) or the size of the gravitational torque helps create desired rotation (b).

angles of pull, and so on) that affect the forces and torques a person can create to resist forces that would tend to disrupt their balance. The general base of support and body posture technique guidelines in many sports and exercises must be based on integration of the biological and mechanical bases of movement. For example, many sports use the "shoulder width apart" cue for the width of stances because this base of support is a good compromise between stability and mobility. Wider bases of support would increase potential stability but put the limbs in a poor position to create torques and expend energy, creating opposing friction forces to maintain the base of support.

The Principle of Balance is based on the mechanical tradeoff between stability and mobility. The Principle of Balance is similar to the Coordination Continuum because the support technique can be envisioned as a continuum between high stability and high mobility. The most appropriate technique for controlling your body depends on where the goal of the movement falls on the stability-mobility continuum. Coaches, therapists, and teachers can easily improve the ease of maintaining stability or initiating movement (mobility) in many movements by modifying the base of support and the positions of the segments of the body. It is important to note that good mechanical posture is not always required for good balance. High levels of skill and muscular properties allow some people to have excellent balance in adverse situations. A skater gliding on one skate and a basketball player caroming off defenders and still making a lay-up are examples of good balance in less than ideal conditions.

Imagine that a physical therapist is helping a patient recover from hip joint replacement surgery. The patient has regained enough strength to stand for short lengths of time, but must overcome some discomfort and instability when transition-ing to walking. The patient can walk safely between parallel bars in the clinic, so the therapist has the patient use a cane. This ef fectively increases the base of support, because the therapist thinks increasing stability (and safety) is more important. If we combine angular kinetics with the Principle of Balance, it is possible to determine on what side of the body the cane should be held. If the cane were held on the same (affected) side, the base of support would be larger, but there would be little reduction in the pain of the hip implant because the gravitational torque of the upper body about the stance hip would not be reduced. If the patient held the cane in the hand on the opposite (unaffected) side, the base of support would also be larger, and the arm could now support the weight of the upper body, which would reduce the need for hip abductor activity by the recovering hip. Diagram the increase in area of the base of support from a single-leg stance in walking to a single-leg stance with a cane in each hand. Estimate the percentage increase in base of support area using the cane in each hand.

Classic examples of postures that would maximize mobility are the starting positions during a (track or swimming) race where the direction of motion is known. The track athlete in Figure 7.16 has elongated his stance in the direction of his start, and in the "set" position moves his center of gravity near the edge of his base of support. The blocks are not extended too far backwards because this interacts with the athlete's ability to shift weight forward and generate forces against the ground. For a summary of the research on the effect of various start postures on sprint time, see Hay (1993). Hay also provides a good summary of early research on basic footwork and movement technique factors in many sports.

In many sports, athletes must take on defensive roles that require quick movement in many directions. The Principle of Balance suggests that postures that foster mobility over stability have smaller bases of support, with the center of gravity of the

body not too close to the base of support. When athletes have to be ready to move in all directions, most coaches recommend a slightly staggered (one foot slightly forward) stance with feet about shoulder width apart. Compare the stance and posture of the volleyball and basketball players in Figure 7.17. Compare the size of the base of support and estimate the location of the center of gravity in both body positions. What posture differences are apparent, and are these related to the predominant motion required in that sport? Bases of support need only be enlarged in directions where stability is needed or the direction of motion is known.

There are movement exceptions to strict application of the Principle of Balance because of high skill levels or the interaction of other biomechanical factors. In well-learned skills like walking, balance is easily maintained without conscious attention over a very narrow base of support. Gymnasts can maintain balance on very small bases of support as the result of considerable skill and training. A platform div-

Interdisciplinary Issue: Gender Differences

It is generally considered that the lower center of gravity in women gives them better balance than men.What is the biomechanical significance of the structural and physiological differences between men and women? While there is substantial research on the physiological differences between the genders, there is less comparative research on the biomechanical differences. Motor control and er-gonomic studies have observed significant differences in joint angles during reaching (Thomas, Corcos, & Hasan, 1998) and lifting (Lindbeck & Kjellberg, 2000). Greater interest in gender differences seems to focus on issues related to risk of injury, for example, to like the anterior collateral ligament (ACL) (Charlton, St. John, Ciccotti, Harrison, & Schweitzer, 2002; Malinzak, Colby, Kirkendall,Yu, & Garrett, 2001).

Figure 7.17. Comparison of the ready positions of a basketball player and a volleyball player. How are the mechanical features of their stance adapted to the movement they are preparing for?

Application: Inverse Dynamics of Walking

The ground reaction forces measured by force platforms in walking are used in clinical biomechanics labs to calculate net forces and torques in joints (inverse dynamics). For the sagittal and frontal planes illustrated (Figure 7.18), can you see how the typical ground reaction force creates a knee flexor and adductor torques in stance? Can you draw the moment arms relative to the knee joint axis for these forces? The stance limb activates muscles to create a net knee extensor torque to support body weight in the sagittal plane (A), and a knee abductor torque to stabilize the knee in the frontal plane (B).

er doing a handstand prior to a dive keeps their base of support smaller than one shoulder width because extra side-to-side stability is not needed and the greater shoulder muscle activity that would be required if the arms were not directly underneath the body. Another example might be the jump shot in basketball. Many coaches encourage shooters to "square up" or face the basket with the body when shooting. Ironically, the stance most basketball players spontaneously adopt is staggered, with the shooting side foot slightly forward. This added base of support in the forward-backward direction allows the player to transition from pre-shot motion to the primarily vertical motion of the jump. It has also been hypothesized that this stagger in the stance and trunk (not squaring up) helps the player keep the shooting arm aligned with the eyes and basket, facilitating side-to-side accuracy (Knudson, 1993).

Balance is a key component of most motor skills. While there are many factors that affect the ability to control body mobility and stability, biomechanics focuses on

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