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Center of Gravity
The center of gravity is the theoretical point through which the resultant gravitational force acting on the body is considered to pass. Its position changes with posture and movement and is fundamental to understanding balance, stability, gait, lifting, and postural control.
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SystemMusculoskeletal System
The center of gravity is the theoretical point through which the resultant force of gravity acting on the body can be considered to pass. It is a fundamental concept in biomechanics and functional anatomy because its position influences posture, balance, stability, movement, and the muscular effort required to maintain body position.
In a perfectly symmetrical rigid object with uniform density, the center of gravity may remain at a fixed location. The human body, however, is composed of multiple movable segments with different masses. Consequently, the position of the whole-body center of gravity changes continuously as the limbs, trunk, and head change position.
Understanding the center of gravity requires considering its relationship to the center of mass, line of gravity, base of support, gravitational torque, and postural control system.
Center of Gravity and Center of Mass
The center of mass is the theoretical point representing the average location of the mass of an object. The center of gravity is the point through which the resultant gravitational force acts.
Under the nearly uniform gravitational field encountered at the Earth's surface, the center of mass and center of gravity of the human body are effectively treated as the same point for most biomechanical applications.
Center of Gravity Versus Center of Mass
| Concept | Definition |
|---|---|
| Center of mass | Point representing the average distribution of body mass |
| Center of gravity | Point through which the resultant gravitational force is considered to act |
| Human biomechanics | The two are generally treated as coincident near the Earth's surface |
Location in the Human Body
There is no single anatomical landmark that permanently represents the center of gravity of the human body. Its location depends on body proportions, posture, limb position, and the distribution of mass.
In a person standing in the anatomical position, the whole-body center of gravity is commonly described as lying in the pelvic region, approximately anterior to the second sacral vertebral level. This is an approximation rather than a fixed anatomical point.
Why the Center of Gravity Changes
The human body is an articulated system. Each body segment has its own mass and segmental center of mass. Moving one segment changes the distribution of mass of the entire body and therefore changes the location of the whole-body center of gravity.
Factors Affecting the Center of Gravity
| Factor | Effect |
|---|---|
| Body proportions | Influence baseline mass distribution |
| Arm position | Moves whole-body center of gravity toward displaced arm mass |
| Leg position | Changes distribution of lower limb mass |
| Trunk position | Strongly affects center of gravity because trunk represents substantial body mass |
| External load | Creates a combined body-load center of gravity |
Segmental Centers of Mass
Each body segment, such as the head, trunk, upper arm, forearm, thigh, or leg, can be considered to have its own center of mass.
The whole-body center of gravity represents the combined effect of all these segmental masses and their positions.
Movement of a Limb
When a limb moves away from the body, the whole-body center of gravity generally shifts toward the displaced mass. The rest of the body may compensate by moving in the opposite direction to preserve balance.
This principle explains many apparently automatic postural adjustments that accompany voluntary movement.
Line of Gravity
The line of gravity is an imaginary vertical line extending downward from the body's center of gravity toward the ground.
Its relationship to the joints and the base of support determines many of the external moments that must be controlled by muscles, ligaments, and other tissues.
Gravity as an External Force
Gravity continuously accelerates body mass toward the Earth. During standing, the support surface generates an opposing ground reaction force that prevents the body from accelerating downward.
Postural stability depends on controlling the mechanical relationship between these forces.
Base of Support
The base of support is the area bounded by the parts of the body that contact the supporting surface.
During normal standing, the feet and the area between them define the base of support. During sitting, the thighs, buttocks, feet, and other contacting structures may contribute to the effective support area.
Center of Gravity and Base of Support
For static equilibrium, the vertical projection of the center of gravity must remain within the base of support. As the projection approaches the edge of the base, the margin available for additional displacement becomes smaller.
If it passes beyond the existing base of support, the person must create a new base, such as by stepping or grasping an external support, or balance will be lost.
Mechanical Concepts of Stability
| Feature | Effect on Stability |
|---|---|
| Large base of support | Generally increases stability |
| Small base of support | Reduces available stability margin |
| Low center of gravity | Generally increases mechanical stability |
| High center of gravity | Generally increases destabilizing potential |
| Projection near center of base | Provides larger margin in multiple directions |
| Projection near edge of base | Provides smaller margin before corrective action is required |
Lowering the Center of Gravity
Flexing the hips and knees lowers the body's center of gravity. This is commonly used when preparing to resist an external force or when performing movements requiring increased mechanical stability.
A lower center of gravity reduces the height through which the body's mass acts relative to the supporting surface.
Widening the Base of Support
Moving the feet farther apart enlarges the base of support, particularly in the direction of the widened stance. This allows the center of gravity to move through a greater horizontal range before its projection reaches the boundary of support.
Direction-Specific Stability
Stability is influenced not only by the total size of the base of support but also by its shape. A stance that is broad from side to side may provide substantial mediolateral stability while offering less additional stability in the anterior-posterior direction.
Limits of Stability
The limits of stability describe how far a person can intentionally displace the center of gravity while maintaining the existing base of support and remaining in control.
These limits are influenced by foot position, joint mobility, strength, sensory information, coordination, confidence, and characteristics of the support surface.
Postural Sway
During quiet standing, the center of gravity is not perfectly stationary. The body exhibits small continuous movements called postural sway.
These movements are controlled by ongoing activity of the sensory and motor systems and demonstrate that upright balance is a dynamic process.
Center of Pressure
The center of pressure is the location of the resultant ground reaction force acting beneath the support surface. It is distinct from the center of gravity.
During quiet standing, the nervous system changes muscular activity to shift the center of pressure and generate forces that regulate movement of the center of mass.
Center of Gravity Versus Center of Pressure
| Feature | Center of Gravity | Center of Pressure |
|---|---|---|
| Represents | Resultant location of gravitational action on body mass | Resultant location of pressure or ground reaction force under support |
| Location | Associated with body mass distribution | Located at supporting surface |
| Changes with | Body configuration and movement | Neuromuscular forces and weight distribution |
| Role in standing | Variable controlled by postural system | Mechanism used to influence center-of-mass acceleration |
Center of Gravity in Quiet Standing
During quiet standing, small movements of the center of gravity are controlled mainly through coordinated activity around the ankle and other joints.
The body can be approximated mechanically as an inverted pendulum for some analyses of quiet stance, although actual human postural control involves multiple joints and complex neural regulation.
Ankle Control
Small anterior and posterior movements of the body can be regulated through plantarflexor and dorsiflexor activity around the ankle.
These muscles generate torques that influence the relationship between the center of pressure and the body's center of mass.
Hip Control
Larger or faster disturbances may require substantial movement around the hips. Hip musculature can rapidly reposition the trunk and pelvis and alter the distribution of body mass.
Stepping
When the center of gravity cannot be controlled within the existing base of support, taking a step creates a new base beneath the moving body.
This is one of the most important protective mechanisms for preventing a fall.
Center of Gravity During Walking
Walking requires controlled progression of the body's center of mass. Unlike quiet standing, the goal is not to keep the center of gravity in one location but to move it forward efficiently while repeatedly changing the base of support.
During gait, the center of mass moves vertically and laterally as well as forward.
Base of Support During Gait
The base of support changes continuously during walking. Periods of double support occur when both feet contact the ground, while single-support periods occur when only one foot supports the body.
The nervous system coordinates foot placement so that the changing support pattern remains compatible with progression of the center of mass.
Single-Limb Support
During single-limb support, the base of support becomes substantially smaller. Muscles around the hip, knee, ankle, trunk, and foot work together to control the center of mass over the supporting limb.
Pelvic Control During Gait
The hip abductors of the stance limb help control the pelvis during single-limb support. Their action limits excessive dropping of the opposite side of the pelvis and contributes to control of the trunk and center of mass.
Center of Gravity During Running
Running includes periods during which neither foot contacts the ground. The center of mass therefore follows a more dynamic trajectory and cannot be continuously controlled through ground reaction forces during the flight phase.
Foot placement and muscular preparation before landing are important for subsequent stability and propulsion.
Center of Gravity During Sitting
Sitting lowers the center of gravity and generally enlarges the effective support area compared with standing, especially when the feet and back are supported.
However, reaching or leaning can move the center of gravity toward the boundaries of the available support.
Center of Gravity During Rising From a Chair
Standing up from a chair requires the center of mass to move forward and upward. The trunk initially flexes to shift body mass toward the feet before the lower limbs extend to raise the body.
Insufficient forward displacement can make it difficult to transfer the body successfully from the seat to the feet.
Center of Gravity During Lifting
When an external object is lifted, the body and object form a combined mechanical system. The location of the combined center of gravity depends on the masses of the body and object and their relative positions.
Holding a Load Close to the Body
Holding a load close to the body generally reduces the horizontal distance between the load and the relevant joints. This decreases the external moment that muscles must oppose.
For this reason, the position of a load can be as important biomechanically as its mass.
Combined Center of Gravity
When carrying a load, the combined center of gravity shifts toward the load. The body commonly compensates by changing trunk or limb position to keep the combined system controllable relative to the base of support.
Examples of Load-Induced Compensation
| Load Position | Typical Mechanical Effect |
|---|---|
| Load held anteriorly | Combined center shifts anteriorly, often producing posterior trunk compensation |
| Load carried on one side | Combined center shifts laterally, often producing compensatory trunk displacement |
| Load carried posteriorly | Combined center shifts posteriorly, often producing anterior compensation |
| Load held far from body | Increases external moment and muscular demand |
Center of Gravity and Joint Moments
Gravity can create an external rotational effect around a joint whenever its line of action does not pass directly through the joint axis.
This rotational effect, or moment, must often be opposed by muscles, ligaments, joint structures, or external support.
Moment Arm
The moment arm of gravity is the perpendicular distance between the line of gravitational force and the relevant joint axis.
A larger moment arm generally produces a larger gravitational moment for the same force.
Gravity and Muscular Demand
Changing posture alters gravitational moment arms. A position that moves a heavy body segment farther from a joint can substantially increase the muscular force required to hold that position.
Trunk Flexion
When the trunk flexes forward, the center of mass of the upper body moves anterior to the lumbar and hip joint axes. This increases the external flexion moment that must be controlled by posterior muscles and passive tissues.
Leaning and Counterbalancing
People frequently reposition one body segment to counterbalance movement of another. For example, reaching an arm forward may be accompanied by posterior displacement of the trunk or pelvis.
These compensations help control the whole-body center of gravity relative to the base of support.
Center of Gravity and Postural Alignment
In relaxed standing, the line of gravity passes near several major joints. Its exact relationship to each joint determines whether gravity creates a flexion or extension moment and how much muscular or ligamentous support is required.
Approximate Line of Gravity in Standing
| Region | General Relationship |
|---|---|
| Head and neck | Requires muscular and ligamentous control to maintain head orientation |
| Trunk | Position of line influences spinal and hip moments |
| Knee | Small changes in alignment alter flexion or extension moments |
| Ankle | Line generally creates a tendency toward forward rotation controlled substantially by plantarflexor activity |
Center of Gravity and Postural Control
The nervous system does not directly sense a single anatomical point called the center of gravity. Instead, it estimates body orientation and movement using vestibular, visual, and somatosensory information.
Motor responses are then generated to regulate body configuration and keep the center of mass trajectory compatible with the intended task.
Visual Contribution
Vision provides external references that help estimate body orientation and movement relative to the environment. Visual information can become especially important when somatosensory information from the support surface is unreliable.
Vestibular Contribution
The vestibular system provides information about head acceleration and orientation relative to gravity. This contributes to postural responses that stabilize the head and body.
Somatosensory Contribution
Proprioceptors and cutaneous receptors provide information about joint position, muscle length, muscle tension, pressure, and contact with the support surface.
Sensory Integration
The central nervous system integrates sensory information and adjusts muscular activity to regulate the position and movement of the body. The relative contribution of each sensory system changes according to environmental conditions.
Anticipatory Postural Adjustments
Before a voluntary movement that is expected to disturb balance, the nervous system can activate postural muscles in advance. These anticipatory postural adjustments help control the expected shift in the center of mass.
Reactive Postural Adjustments
Unexpected disturbances require rapid corrective responses. Depending on the magnitude of the disturbance, the body may use ankle, hip, stepping, or grasping strategies to regain control.
Center of Gravity in Different Postures
| Posture | General Mechanical Characteristic |
|---|---|
| Standing upright | Relatively high center of gravity over foot-based support |
| Wide stance | Expanded base of support |
| Crouching | Lower center of gravity |
| Sitting | Lower center of gravity and larger support area |
| Single-leg stance | Small base of support with increased control demands |
| Lying | Very large support area and low center of gravity |
Single-Leg Stance
Standing on one leg dramatically reduces the base of support. The body must reposition its center of gravity over the supporting foot while coordinating hip, trunk, knee, ankle, and foot musculature.
Tandem Stance
Placing one foot directly in front of the other creates a narrow base of support, particularly in the mediolateral direction. This increases the precision required to control the center of gravity.
Reaching
Reaching moves one or more body segments away from their initial positions and shifts the whole-body center of gravity. The nervous system may compensate through movements of the trunk, pelvis, or lower limbs.
External Perturbations
A push, pull, collision, or sudden movement of the support surface can accelerate the center of mass toward the boundary of the base of support.
Corrective muscular responses attempt to alter the body's momentum or reposition the base before balance is lost.
Momentum and Balance
During movement, stability cannot be understood solely by examining the instantaneous position of the center of gravity. The velocity and momentum of the body also matter.
A rapidly moving center of mass may require a step even before its vertical projection has moved far beyond the previous support region.
Center of Gravity and Sports
Athletes manipulate their center of gravity to improve stability, acceleration, direction changes, and control. Lowering the body and widening the stance can increase stability when resisting external forces.
Other activities deliberately move the center of gravity rapidly beyond the existing base of support to initiate locomotion, jumping, or directional change.
Jumping
During jumping, muscular forces accelerate the center of mass upward. Once the body leaves the ground, the trajectory of the whole-body center of mass is governed primarily by gravity until external contact is re-established.
Movement of the limbs can change body configuration in flight but does not independently redirect the ballistic trajectory of the whole-body center of mass without an external force.
Landing
During landing, the lower limbs flex to increase the time and distance over which the body's downward momentum is reduced. Coordinated control of the center of mass helps maintain stability while impact forces are absorbed.
Changing Direction
Changing direction requires forces that redirect the velocity of the center of mass. Athletes often lower the center of gravity and position the foot strategically to generate the required ground reaction forces.
Center of Gravity and Falls
A fall becomes likely when the body cannot adequately control the center of mass relative to the available support. Loss of balance may result from excessive displacement, high momentum, inadequate muscular force, delayed reactions, or sensory impairment.
Fall-Recovery Strategies
| Strategy | Mechanical Purpose |
|---|---|
| Ankle response | Adjusts center-of-pressure forces without changing base |
| Hip response | Rapidly redistributes body mass |
| Stepping | Moves base of support beneath displaced center of mass |
| Grasping | Adds an external point of support |
Clinical Assessment
Clinical examination of balance often challenges the relationship between the center of gravity and base of support. Tests may narrow the stance, remove visual information, alter the support surface, or require controlled movement.
Romberg Test
The Romberg test compares standing stability with the eyes open and closed. Removing visual input increases reliance on vestibular and somatosensory information.
Functional Reach
Functional reaching tasks assess how far an individual can voluntarily displace the body while maintaining control over the existing base of support.
Single-Leg Balance
Single-leg stance reduces the support area and increases the demands placed on postural control. Performance depends on sensory integration, strength, joint function, and neuromuscular coordination.
Effects of Musculoskeletal Impairment
Weakness, joint pain, restricted range of motion, deformity, or altered foot mechanics can reduce the ability to reposition the body or generate the forces required to control the center of gravity.
Effects of Sensory Impairment
Reduced vision, vestibular dysfunction, or loss of proprioceptive and plantar sensory information can impair the nervous system's estimate of body orientation and movement.
Effects of Neurological Impairment
Neurological disorders can affect sensory integration, motor planning, coordination, muscle activation, or the timing of corrective responses. These changes can alter control of the center of mass even when musculoskeletal strength is relatively preserved.
Age and Center-of-Gravity Control
Aging itself does not change the fundamental mechanics of the center of gravity, but age-associated changes in strength, sensory function, reaction time, joint mobility, and coordination can reduce the ability to control it effectively.
Practical Principles of Stability
| Principle | Biomechanical Explanation |
|---|---|
| Lower the body | Lowers center of gravity |
| Widen the stance | Enlarges base of support |
| Move base toward disturbance | Places support beneath displaced mass |
| Keep external load close | Reduces moment arm and excessive center shift |
| Orient stance toward expected force | Increases available support in relevant direction |
Functional Importance of the Center of Gravity
The center of gravity provides a useful biomechanical representation of how the body's mass behaves as a whole. Its position helps explain why different postures require different amounts of muscular activity and why changing limb position can alter balance.
Control of the center of gravity is essential for standing, sitting, reaching, lifting, walking, running, jumping, landing, and responding to unexpected disturbances.
Key Anatomical and Biomechanical Features
| Feature | Key Point |
|---|---|
| Center of gravity | Point through which resultant gravitational force acts |
| Center of mass | Average location of body mass |
| Approximate standing location | Pelvic region, often described near the level anterior to S2 |
| Line of gravity | Vertical line through center of gravity |
| Base of support | Area defined by supporting contacts |
| Lower center of gravity | Generally increases mechanical stability |
| Wider base | Generally increases stability in the widened direction |
| Movement of body segments | Changes whole-body center of gravity |
| External loads | Shift the combined center of gravity |
| Postural control | Regulates center-of-mass movement relative to support |
Anatomical and Functional Importance
The center of gravity links anatomy with biomechanics by describing the combined mechanical effect of the body's distributed mass. Because the human body consists of movable segments, its center of gravity changes whenever posture or body configuration changes.
Postural stability depends on controlling the center of gravity relative to the base of support. A wide base and low center generally provide greater mechanical stability, while a narrow base, high center, or rapid displacement increases the demands placed on the postural control system.
During movement, the center of gravity is intentionally displaced. Walking advances it from one support region to another, rising from a chair moves it forward and upward, lifting an object creates a new combined center, and stepping restores support beneath it when balance is threatened.
The center of gravity therefore provides a central framework for understanding posture, equilibrium, balance, gait, lifting mechanics, joint moments, postural adjustments, movement efficiency, and the interaction between the musculoskeletal and nervous systems.
Published on October 15, 2026
Last updated on October 15, 2026
Last updated on October 15, 2026