Balance Overview

Balance refers to the ability to maintain the line of gravity (the vertical line from the center of mass) of a body within the base of support. The ability to main maintain balance has both injury and performance implications. Failure to maintain balance can result in a fall and subsequent injury. According to the Center for Disease Control and Prevention each year, millions of individuals age 65 and older fall. In fact, more than one out of four older people falls each year and these falls are serious and costly:

Each year, 3 million older people are treated in emergency departments for fall injuries.¹
Over 800,000 patients a year are hospitalized because of a fall injury, most often because of a head injury or hip fracture.¹
Each year at least 300,000 older people are hospitalized for hip fractures.²
More than 95% of hip fractures are caused by falling⁸, usually by falling sideways.³
Falls are the most common cause of traumatic brain injuries (TBI).⁴
In 2015, the total medical costs for falls totaled more than $50 billion. Medicare and Medicaid shouldered 75% of these costs.⁵

Given the high number of falls in the elderly a significant amount of biomechanics research is focused on fall prevention. In addition to fall prevention, balance is an important component of sport performance. For example, balance is essential for generating proper GRFs for propulsion and lifting, resisting checking or blocking, and maximizing mobility for quick movements in sprinting and cutting.

Before we dive into our discussion of the biomechanics of balance, it is important to clarify some terms:

Stability—measure of the difficulty of displacing an object from equilibrium
Balance—a state in which opposing forces are equal so that they effectively cancel each other out and stability is maintained
Posture—a composite of the positions of all the joints of the body at any given moment.
Postural control—ability to maintain stability of the body segments in response to forces that threaten to disturb the body’s structural equilibrium
Perturbation—any sudden change in conditions that displaces the body posture away from equilibrium

Factors that Influence Stability - Sensory & Motor Factors

Maintaining balance requires the coordination of input from multiple sensory systems and motor output. The first step in maintaining balance is to sense changes of spatial orientation with respect to the base of support. Subsequently corrective motor responses must be initiated to maintain the line of gravity within the base of support. In terms of sensory input, our ability to maintain balance relies heavily on the vestibular, somatosensory, and visual systems. The vestibular system relays information about motion, head position, and spatial orientation; hence, it is the primary sensory system that helps us to maintain our balance. Further aiding in the regulation of balance is the somatosensory system, which provides information about joint position and the position of different body parts relative to each other. The somatosensory system also relays pressure and vibratory information from skin and joints, which helps provide information regarding position and movement relative to the support surface. Lastly, the visual system aids in the regulation of balance by relay information about body and head motion, as well as spatial location relative to objects.

Age related balance impairments stem from a decline in the ability to rapidly detect changes in spatial orientation and initiate corrective motor responses⁶. As a result, the elderly have a considerably increased risk of falls⁷.

Stability Strategies

The central nervous system (CNS) implements “strategies” that are intended to be selectively applied to various levels of instability. The primary criteria that determine what strategy is used depend on 1.) ability to maintain stability and 2.) energetic cost. Essentially the body wants to maintain stability at all costs (to prevent injury), but it will do so in an energetically efficient manner.

When discussing these stability strategies, they are in reference to quiet stance, in which the body has normal postural sway that is maintained by postural alignment, associated joint stiffness and muscle tone.

Postural sway with the eyes open and the eyes closed

An illustration of postural sway with the eyes open and eyes closed.

There are a variety of different ways that stability strategies can be classified. Here are some of the most common:

Proactive vs. Reactive Stability Strategies

Proactive (anticipatory) responses occur in anticipation of internal or external destabilizing forces
Reactive (compensatory) responses occur as reactions to external displacement of the body’s center of gravity

Proactive strategies allow for more time to evaluate a situation and make corresponding postural changes to try and prevent a fall. Whereas reactive strategies are implemented in the moment that a destabilizing force is applied, so there is minimal time to make postural changes with the goal of maintaining stability.

The sport of football provides great examples of proactive and reactive strategies. Let’s use another football example to illustrate these two strategies. For example, a running back is anticipating that they may be tackled so they use proactive strategies, such as covering the ball with both hands and increasing their base of support in the direction of a potential tackle. Alternatively, a quarterback who is being sacked, does not see the tackle coming, so they only have the option of relying on reactive strategies in which they react and try to make postural changes as they are being tackled.

Fixed Support vs. Change-in-Support Strategies

Another way of classifying stability strategies is to classify them as various fixed support or change in support strategies. Fixed support strategies utilize patterns of muscle activity in which the base of support remains fixed during the perturbation and recovery of equilibrium. With fixed support strategies stability is regained through movements of parts of the body but the feet remain fixed on the base of support. Specifically, muscles respond in an attempt to restore the line of gravity to a position within the base of support. Alternatively, change-in-support strategies include stepping or grasping, meaning that the base of support looses contact with a surface, with the goal of enlarging the body’s base of support so that it remains under the body’s center of gravity.

Fixed Support Strategies can be further divided into the ankle and hip strategies. The “Ankle Strategy” is used to respond to minimal instabilities and consists of small dorsi- and plantar-flexion movements at the ankle, which shift mass anteriorly or posteriorly. This is the simplest and most rapid response because it only requires rotation at the ankle. The ankle strategy has a relatively low energetic cost, but the trade-off is that it only results in small shifts in mass and hence can only be used to respond to small perturbations. The other fixed support strategy is the “Hip Strategy”, which consists of flexion and extension at the hip. The hip strategy is more complex and slower than the ankle strategy because it requires activation of more and larger muscle groups. Correspondingly, the hip strategy has a higher metabolic cost, but also shifts more mass, so it can be used to respond to larger perturbations.

When fixed support strategies are not adequate for maintaining stability then Change-In Support strategies are utilized. Change in support strategies require the individual to leave the support surface in some manner, such as stepping or grasping. These strategies are considerably more complex because multiple segments are involved, and they also take longer to initiate than fixed-support strategies. However, they are the best option for responding to large perturbations.

Ankle, hip and stepping stability strategies

The fixed-support strategies (the ankle and hip strategies) and the change-of-support or the stepping strategy. Obtained from Inverse optimal control for redundant systems of biological motion.

The Biomechanics of Maintaining Stability

An important aspect to consider when examining balance is rotational stability. When examining rotational stability, the body is modeled as a solid object that rotates (tips) about a pivot or tipping point (axis of rotation). We can further examine the ability to maintain stability by comparing the stabilizing and destabilizing torques that act in opposing directions about this pivot point. The main stabilizing force that we will focus on in this course is the weight force; thus, the stabilizing torque is the product of the weight force and the moment arm (perpendicular distance) from the line of action from the weight force to the pivot point. In our discussion of torque, we saw that torque was associated with rotation: however, when evaluating stability, the stabilizing torque (torque created by weight) does not actually create rotation because the ground creates a counterforce that prevents this rotation. Alternatively, destabilizing forces are applied in such a manner that a destabilizing torque is created that will attempt to tip the body about the pivot point (create a fall). The product of this destabilizing force and its moment arm, which is the perpendicular distance from the destabilizing force to the pivot point, is the destabilizing torque. In our previous discussion of torque, we saw that movement was always in the direction of net torque and the same will apply here.

τ_stabilizing > τ_{destabilizing}

F_stabilizingr_stabilizing> F_{destabilizing}r_{destabilizing}

Let’s take for example a football tackle. In this case Player B is applying a destabilizing force to Player A in the attempt to tackle or cause Player A to fall. We will assume that Player A is a rigid object, and if they were to rotate during the tackle, they would pivot about the side of their foot (axis of rotation). We can determine Player A’s ability to maintain balance by comparing the magnitude of the stabilizing torque (the torque created by the weight of Player A) and the destabilizing torque (the torque created by force applied by Player B). Remember that the stabilizing torque (torque created by weight), does not actually create rotation, because the ground prevents this rotation, whereas the destabilizing torque causes Player A to rotate posteriorly. For Player A to maintain stability:

τ_weight > τ_{Player B}

Biomechanical Factors that Influence Stability

Now that we have some background on the concepts of balance and stability, we will move on to discussing biomechanical factors that help maintain stability. The topics that we will cover are:

Mass
Base of Support
Height of COM

Mass:

One of the most simple concepts regarding balance is that size matters, specifically objects with more mass are more stable than lighter objects. Increased mass is a common strategy to maintain the stability of an object and we see that larger objects are more stable and difficult to displace. For example, the large size of sumo wrestlers and football defensive linemen makes them more stable. However, it is not always possible to add mass and greater mass may limit mobility.

Size of the Base of Support:

Another biomechanical factor that has important implications for stability is the base of support (BOS). The BOS is the area bounded by a perimeter formed by the object’s points of contact with the support surface. The base of support (BOS) is the area bounded by a perimeter formed by the object’s points of contact with the support surface

The BOS is an important concept to understand an individual's ability to maintain balance, as. If an individual’s line of gravity passes outside of the BOS a fall occurs. We can make a variety of postural changes to alter the size of the BOS in order to maintain stability and we can also shift mass to ensure that our line of gravity stays within the BOS.

balance will be maintained as long as the line of gravity stays within the base of support

When a destabilizing torque is applied, we can increase the size of the BOS in the direction of the destabilizing force in an effort to maintain stability. This is commonly referred to as “widening your stance”. The widened BOS increases the size of the stabilizing moment arm, which increases the stabilizing torque and helps make an individual more stable.

Widening one's stance increases the stabilizing moment arm.

Height of COM:

The final biomechanical factor that influences stability that we will cover is the height of the COM. It is common to hear “lower your center of mass” from coaches in sports where stability is important (e.g. football, hockey, soccer). Lowering the COM helps to reduce the effect of a destabilizing torque, by reducing the destabilizing moment arm. The smaller destabilizing torque makes it more likely that the stabilizing torque will be adequate for maintaining stability.

Lower the COM decreases the moment arm of the destabilizing torque

To summarize there are 3 main postural changes that help maintain stability:

Add mass—increases F_weight
Widen stance—increases r_weight
Lower COM—decreases r_{destabilizing}

Supplementary

Dynamic Stability

Our discussion of balance and stability has thus far focused on situations where minimal motion occurs, which is termed static equilibrium. However, there are situations where forces due to movement help to maintain stability, which is termed dynamic stability. In dynamic situations it is possible to maintain balance when line of gravity is outside of BOS because alternative forces (inertial, centripetal, kinetic friction) balance torques. For example, take the skier in the figure below. The line of gravity is well outside the contact points of the skis with the snow, but they maintain stability because the centripetal force is pulling them in the opposite direction.

In dynamic situations the line of gravity may move outside of the base of support (BOS) without a loss of stability.

Posture

In this section of the course we will cover the topic of posture. Remember that posture is the position of the body at a given moment. An ideal posture is stable, minimizes localized tissue loading and reduces energetic cost by having minimal muscle activation. While we often think about posture in maintaining a static position, we are constantly change posture to move, alter loading on tissues and help us to maintain stability. We have already talked about postural changes that are made to maintain the line of gravity within the BOS, or to help maximize the stabilizing torque when responding to a destabilizing torque.

When evaluating posture, the line of gravity (weight force vector) serves as a reference for the analysis of the effects of this force on the body segments. In this manner we identify where the line of gravity falls relative to a joint and this indicates in what direction that the weight force vector wants to create rotation. For example, in the figure below we can see that the weight force vector falls anterior to the ankle, indicating that if this was the only force it would create dorsiflexion. Thus, to maintain a static (not moving) posture, the plantar flexors would need to be activated to neutralize this motion.

τ_weight = τ_{plantar flexion}

muscle activation neutralize torque created by the line of gravity

The line of gravity falls anterior to the ankle with the ideal standing posture. This wants to create dorsiflexion at the ankle (body pivots forward over stationary foot). The plantar flexors are activated to neutralize this motion.

Ideal Posture

Lower Body:

We often evaluate postural changes in reference to what is described as normal or ideal posture, which exhibits minimal torque at the major joints in the body. In the sagittal plane, the gravity line is located anterior to the ankle joint, which produces an ankle dorsiflexion moment and must be canceled out by activating the ankle plantar flexors. At the knee, the line of gravity falls anterior to the joint producing a knee extensor moment. However, because we lock our knees when standing we can rely on passive tension of the posterior knee ligaments to stop this extension, so there no muscle activity is required. The line of gravity falls slightly posterior to the hip joint, producing a hip extensor moment that is neutralized by passive tension of the iliofemoral ligament and hence no muscle activity is required.

Upper body:

Considerable interest is often placed on examining spinal posture. In the ideal posture the line of gravity passes through the midline of the trunk. In the sagittal plane the line of gravity passes through the bodies of the cervical and lumbar vertebrae and anterior to the thoracic vertebrae. This creates the highest amount of stress on the thoracic area and wants to further curve the thoracic curvature. Ligamentous structures and passive muscle tension do not produce enough force to counter the gravitational moment so the back extensors must be activated to counteract this flexor moment.

Location of the line of gravity in an ideal posture

Postural Abnormalities

Postural abnormalities are deviations from the ideal posture and result in increased loading on various joints, muscles and ligaments. In the ideal posture the line of gravity falls directly through or close to most of the major joints in the body. This minimizes the length of the moment arm to weight force vector, thereby reducing the amount of torque created by the weight force, and hence reducing the amount of torque that must be neutralized by muscle activation or passive muscle or ligament tension. In the case of postural abnormalities, the line of gravity is typically moved further from the joint centers, which increases the moment arm and correspondingly increases the amount of torque created by the weight force. With the increased torque created by weight there is an increase in the degree of muscle activation or passive muscle or ligament tension that is required to neutralize this moment.

Postural Abnormalities

Muscle Imbalance

In addition to shifts of the weight force vector it is important to consider the role of muscle imbalances when evaluating posture. At most joints in the body there are several nearby muscle attachments that are responsible for the variety of movements. Tightness or weakness in any of these muscles can result in imbalance at the joint. Take for example the pelvic girdle. Activation of either the back extensors or hip flexors will cause the pelvis to tilt anteriorly, whereas activation of the abdominal muscles or hamstrings will cause the pelvis to tilt posteriorly. Weakness or tightness in any of these muscles will result in an imbalance that leads to pelvic tilt.

Muscle imbalance can result in the rotation of segment. In this case, muscle imbalance at the pelvis results in anterior tilt.

References:

Centers for Disease Control and Prevention, National Center for Injury Prevention and Control. Web–based Injury Statistics Query and Reporting System (WISQARS) [online]. Accessed August 5, 2016.
Healthcare Cost and Utilization Project (HCUP). 2012. Agency for Healthcare Research and Quality, Rockville, MD. http://hcupnet.ahrq.govExternal. Accessed 5 August 2016.
Parkkari J, Kannus P, Palvanen M, Natri A, Vainio J, Aho H, Vuori I, Järvinen M. Majority of hip fractures occur as a result of a fall and impact on the greater trochanter of the femur: a prospective controlled hip fracture study with 206 consecutive patients. Calcif Tissue Int, 1999;65:183–7.
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When no attribution is noted, images are from the Public Domain or the work of the author.