I am grateful to Kevin Kirby and Precision Intricast
for permission to reproduce this Newsletter (you can buy the 2 books of newsletters off Precision Intricast):
Maintenance of Balance in Relaxed Bipedal Standing
The mechanisms by which an individual maintains balance during relaxed bipedal stance is a very important subject for podiatrists and other health professionals who treat patients with mechanically based foot or lower extremity pathology. The basic mechanical interactions that exist between the foot and the remainder of the body need to be understood before one can appreciate the more complex biomechanics of how an individual walks, runs or performs any weightbearing activity. Since relaxed bipedal stance is one of the least complex weightbearing activities that humans perform, it is this activity that should be first explored in detail in order to form a basis for comprehending the biomechanics of more complex weightbearing activities.
Figure 1. During relaxed bipedal stance, the center of mass (CoM) is normally positioned anterior to the ankle joint axis so that ankle plantarflexion moment from one or a combination of the ankle joint plantarflexors is necessary to counteract the dorsiflexion moment caused by gravity acting on the CoM (left). The center of gravity (CoG) is the vertical projection of the CoM on the ground. As long as the CoG is positioned within the base of support of the feet, balance can be maintained (right).
During bipedal standing, the feet support the body weight of the individual so that upright standing can be maintained. The base of support in bipedal standing is made up of the area bounded by the outline of the plantar aspects of the feet and the area directly between the two feet (Fig. 1). In order for the individual to maintain balance, the center of mass (CoM) of the body must be positioned somewhere over the base of support of the body. As long as the CoM of the body remains within the area bounded by the plantar feet and the area between the feet, stable equilibrium will be maintained since the individual can alter the magnitude and location of the ground reaction force (GRF) between the plantar foot and the ground to force the CoM to stay over the base of support (Winter, David A.: A.B.C. (Anatomy, Biome-chanics and Control) of Balance During Standing and Walking. Waterloo Biome-chanics, Waterloo, Ontario, Canada, 1995).
In order to more fully understand how the body maintains stable equilibrium, or balance, in bipedal standing, it is important to review some terms and concepts commonly used in biomechanics research and in force plate and plantar pressure measurement systems. When an individual is standing on two feet, each foot has varying magnitudes and locations of GRF depending on the structure of the feet and lower extremity, the contractile activity of the muscles of the foot and lower extremity and the location of the CoM in relation to the plantar feet. One method that a researcher may objectively measure the relative location of the GRF acting on the plantar feet when an individual is in bipedal stance is with a device called a force plate.
Figure 2. Force plates can measure the center of pressure (CoP). CoP is defined as the point location of the resultant ground reaction force vector acting on the plantar foot or feet. When a person stands on just the right foot on a force plate, the CoP will be under the plantar aspect of the right foot (A). When a person stands on a single force plate with feet equally weighted, the CoP is located between the two feet (B). When a person stands on just the left foot, the CoP moves to the plantar aspect of the left foot (C).
A force plate can measure the location, direction and magnitude of the resultant force acting on the plantar feet. In bipedal standing, the force plate will indicate a resultant force vector acting in a vertical direction with a magnitude equal to body weight and with the line of action of the resultant vector passing between the two feet (Fig. 2). The point at which this resultant vector intersects the force plate is defined as the center of force (Nigg, Benno M. and Walter Herzog (eds.): Biomechanics of the Musculoskeletal System. John Wiley and Sons, New York, 1994, pp. 226-227). Center of force is also more commonly known as center of pressure (CoP), which is probably the more commonly used term in force plate and plantar pressure analysis systems (Winter, p. 4).
Center of pressure is defined as the point location of all the vertical ground reaction force vectors acting on the plantar foot. CoP takes into account the location and magnitudes of all of the GRF vectors and is measured relative to a location on either the plantar foot or on the ground. If only one foot is in contact with the ground, then the CoP will be plantar to that foot. If an individual is standing with both feet on one force plate, then only one CoP can be measured and it will be located between the two feet, assuming approximately equal weight on each foot (Fig. 2). If two feet are on the ground, with each foot on a separate force plate, then each foot will have its own individual CoP (Winter, p. 4).
Another term commonly used in human balance studies is called center of gravity (CoG). Contrary to common usage in which the term center of gravity and center of mass are thought to be synonymous, in the biomechanics community CoG is often defined as the vertical projection of the CoM on the ground (Fig. 1). Since both CoP and CoG are located on the ground, then their relative positions to each other allows one to make assumptions about how alterations in the CoP and CoG to each other allows an individual to maintain their CoM in a balanced position during relaxed bipedal stance. The method of control by which the individual maintains balance with the feet side-by-side to each other is known as the “ankle strategy” since the CoM of the body can be maintained essentially over the feet by altering the rotational forces, or moments, acting across the ankle joint axis (Winter, p. 5).
One other important concept that is central to this discussion is that the human body may be modeled as an “inverted pendulum” during both standing and during walking activities (Winter, p. 5). A simple pendulum consists of an object that is attached to a fixed axis of rotation so that when the object is set into motion, the acceleration of gravity on the CoM of the object causes it to swing back and forth repeatedly under the axis of rotation (Cutnell, J.D., Johnson, K.W.: Physics. (3rd ed). John Wiley & Sons, New York, 1995, pp. 298-299). In the inverted pendulum model of bipedal standing, however, the CoM of the body sways back and forth about the ankle joint axis which is both under the influence of the acceleration of gravity acting on the CoM and under the influence of the muscles which directly cause plantarflexion and dorsiflexion moments across the ankle joint axis.
When an individual stands with their CoM positioned anterior to the ankle joint axis, the acceleration of gravity acting on the CoM of the body causes an ankle joint dorsiflexion moment (Fig. 1). When the CoM is positioned posterior to the ankle joint, gravity causes an ankle joint plantarflexion moment. It is only when an individual is standing so that the CoM is directly over the ankle joint axis that gravity causes neither a plantarflexion moment or a dorsiflexion moment across the ankle joint axis (Hicks, J.H.: The Three Weight Bearing Mechanisms of the Foot. Pages 161-191 in F.G. Evans (ed): Biomechanical Studies of the Musculoskeletal System. C.C. Thomas Co., Springfield, Ill. 1961).
Figure 3. When the center of mass (CoM) of the body is anterior to the ankle joint and the center of pressure (CoP) from ground reaction force (GRF) is positioned anterior to the center of gravity (CoG), there will be a net posterior acceleration of the CoM.
If the CoM is anterior to the ankle joint, then in order to maintain balance, an ankle joint plantarflexion moment must be produced by contractile activity of one of the muscles capable of producing an ankle joint plantarflexion moment (i.e. gastrocnemius, soleus, deep posterior compartment or peroneal muscles). The increased contractile activity from one or a combination of these muscles increases the ankle joint plantarflexion moment, which, in turn, causes the CoP to move anteriorly on the plantar foot. Therefore, the increase in ankle joint plantarflexor contractile activity causes the CoP to move anteriorly which acts to counterbalance the tendency for the individual to lean further forward due to the CoM being anterior to the ankle joint.
If the CoM is posterior to the ankle joint, then in order to maintain balance, an ankle joint dorsiflexion moment must be produced by contractile activity of one of the muscles capable of producing ankle joint dorsiflexion moment (i.e. anterior tibial, extensor hallucis longus, extensor digitorum longus and peroneus tertius muscles). The increased contractile activity from one or a combination of these muscles increases the ankle joint dorsiflexion moment, which, in turn, causes the CoP to move posteriorly on the plantar foot.
Therefore, the increase in dorsiflexor contractile activity causes the CoP to move posteriorly which acts to counterbalance the tendency for the individual to lean further backward due to the CoM being posterior to the ankle joint. It is only in the special circumstance when the CoM is directly over the ankle joint axis that gravity is not causing any ankle joint moment and, as a result, no muscular contractile activity of the ankle joint plantarflexors and dorsiflexors is necessary to maintain balance (Hicks, pp. 167-171).
Even though the body seems, at first glance, to be relatively still during periods of bipedal standing, the mechanics of this “quiet” activity is much more dynamic and complex. Careful measurement of the sway of the CoM of the body in the sagittal plane relative to the changes in the location of the CoP and muscular contractile activity shows that the individual will constantly alter the contractile activity of the ankle joint dorsiflexors and plantarflexors in order to alter the locations of the CoP to maintain balance (Winter, pp. 5-14).
Experimental studies of relaxed bipedal stance show that if the CoM is positioned anterior to the ankle joint and the individual shifts their CoP anterior to the CoG by increasing the contractile activity of their ankle joint plantarflexors, that a posterior acceleration of the CoM of the body will occur (Fig. 3). Posterior acceleration of the CoM then causes a progressive increase in displacement of the CoM in a posterior direction. If the CoM is moving posteriorly too rapidly then the ankle joint dorsiflexors may be activated to shift the CoP posterior to the CoM which, in turn, will cause an anterior acceleration of the CoM of the body. Anterior acceleration of the CoM, in this example, then causes, at first, a progressive deceleration of the posterior movement of the CoM that is then followed by a progressive acceleration of the CoM in an anterior direction. In this fashion, the body maintains sagittal plane balance during relaxed bipedal standing by causing an oscillation of the CoP anteriorly and posteriorly in response to the movements of the CoM in relation to the feet (Winter, p 5-14).
Another interesting aspect of the inverted pendulum model is that the horizontal acceleration of the CoM is proportional to the difference in distance between the CoP and CoG. In other words, the larger the distance between the CoP and CoG, the larger the acceleration of the CoM and the smaller the distance between the CoP and CoG, the smaller the acceleration of the CoM. Therefore, by mechanical necessity, the range of CoP movement must be greater than the range of CoG movement in order to allow enough distance between CoP and CoG to generate sufficient backward or forward acceleration of the CoM toward the center of the foot in order to maintain balance (Winter, p. 6).
In summary, the body is able to maintain sagittal plane balance during relaxed bipedal standing by selectively increasing or decreasing the contractile activity of the ankle joint plantarflexors and dorsiflexors to adjust the location of the CoP in relation to the CoM so that the CoM never falls outside the base of support of the body. Understanding the basic interrelationships involved in the balance control of bipedal standing helps create a basis for developing a better knowledge of the complex neuro-mechanical interrelationships which exist in other, more dynamic, weightbearing activities.
[Reprinted with permission from: Kirby KA.: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters
, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 133-136.]