What is Center of Mass?

What is Center of Mass?

The center of mass, often used interchangeably with center of gravity, is the theoretical point where all of an object's mass is evenly balanced in every direction. For a symmetrical object like a barbell plate, it sits at the geometric center. For a human body it is not a fixed anatomical spot: standing upright with arms at your sides, it lies roughly in the pelvis, around the level of the second sacral vertebra, but it shifts the instant you move. Raise your arms overhead and it rises; bend forward and it moves forward, even outside your body entirely in a deep bow or a gymnastics arch. When you pick up a load, the relevant point becomes the combined center of mass of you plus the load, weighted toward the heavier object. This matters for two reasons that run through almost everything in lifting. First, balance: you stay upright only while the vertical line through your center of mass falls within your base of support, the area under and between your feet. Let it drift past your toes or heels and you must step, tip, or fall. Second, leverage: the horizontal distance between the load's center of mass and a given joint is the moment arm, and it determines how much torque your muscles must produce to control that joint. A bar held close to your body has short moment arms and feels manageable; the same bar drifting away from you suddenly loads your joints far more heavily. Understanding where the combined center of mass sits, and keeping it organised over the midfoot, is the mechanical logic behind cues like keep the bar close and stay balanced over your feet.

How it works

Gravity pulls on every particle of your body and your load, but the net effect can be represented as a single downward force acting through the combined center of mass. Whether you stay balanced depends on where the vertical projection of that point, sometimes called the line of gravity, meets the ground relative to your base of support. In quiet standing your base is the outline of and area between your feet, and postural muscles make constant tiny adjustments to keep the line of gravity inside it. Widen your stance and you enlarge the base, making balance easier; rise onto your toes and you shrink it, making balance harder. This is the foundation of standing balance and of every loaded lift. In the squat and deadlift the practical target is to keep the combined center of mass of the lifter and barbell balanced over the midfoot throughout the movement. Because the bar is heavy relative to body segments, the system's center of mass sits close to the bar, so in effect this means keeping the bar tracking over the midfoot. If the bar drifts forward of the midfoot, it moves away from the hips and knees, lengthening the moment arms and sharply increasing the torque, and therefore the muscular effort, needed to control those joints; the lift feels heavier and the balance shifts onto the toes. Drift too far back and you tip toward the heels. This is why a vertical bar path over the midfoot is efficient: it keeps the load's leverage on each joint as short as the lift allows, so more of your strength goes into moving the weight rather than fighting a long lever. It also explains technique differences. A low-bar squat places the bar lower on the back and pairs with more forward torso lean, keeping the bar over the midfoot; a high-bar squat sits more upright with the bar higher, but both organise the system mass over the midfoot. In the deadlift the bar starts over the midfoot and the hips find the height that keeps it balanced there off the floor. Balance and leverage, both governed by the center of mass, are two sides of the same principle: control where the combined mass sits, and you control both stability and how hard the lift loads your joints.

How to apply it

• Keep the bar over the midfoot: On squats and deadlifts, track the bar in a near-vertical line over the middle of your foot. This keeps the combined center of mass balanced and the moment arms on your hips and knees as short as the movement allows, so the lift is efficient.
• Keep the load close: The nearer a load stays to your body, the shorter its horizontal distance from your joints and the less torque you must resist. Let a bar or dumbbell drift away and the same weight loads your joints far more heavily and pulls your balance forward.
• Manage your base of support: Balance holds only while the line through your center of mass stays over your feet. A wider stance gives a larger, more stable base for heavy or unstable loads; a narrow base demands more precise balance and control.
• Set hip height to balance the system: In the deadlift, the right starting hip height is the one that puts the bar over your midfoot and your shoulders slightly ahead of it. Proportions differ, so use bar balance over the midfoot, not a fixed hip position, as the guide.
• Account for the load, not just yourself: Once you hold weight, balance is about the combined center of mass of you plus the load, shifted toward the heavier object. Carries, front-loaded and back-loaded lifts each move that point, so brace and position around where the system mass actually sits.

Types

Worked example

How bar position over the midfoot changes the leverage on your joints. These are simplified numbers to show the principle, not exact readings: as the bar drifts forward of the midfoot, the horizontal distance to the hip grows, and the torque your hips must resist grows with it, even though the weight on the bar never changes.

The load on the bar is identical in every row; only the leverage changes with where the mass sits. Keeping the bar over the midfoot minimises the moment arm and keeps the line of gravity in the middle of your base of support, which is why it is both the most balanced and the most efficient position to lift from.

Center of mass vs base of support

Balance is the relationship between these two. You stay upright while the line through your center of mass falls inside your base of support. You can improve balance either by controlling where the mass sits or by adjusting the base beneath it, and heavy lifting uses both.

By goal

• Powerlifters and strength athletes: Treat bar-over-midfoot as a non-negotiable checkpoint on the squat and deadlift. Keeping the system center of mass balanced there minimises the leverage on your hips and knees, so more of your strength moves the bar. Film from the side and watch the bar track a near-vertical line.
• General lifters and beginners: Learn to feel balanced pressure across the whole foot rather than pitching onto the toes or heels. Keeping loads close to your body shortens the leverage on your joints and makes lifts feel lighter and safer, which is one of the highest-value habits to build early.
• Balance and older adults: Standing balance is about keeping your center of mass over your base of support. Training a stable, controllable base, and practising controlled weight shifts and carries, builds the balance and joint control that protect against stumbles and support confident movement under load.

Common misconceptions

• "The center of mass is a fixed spot in the body." It moves constantly. It sits near the pelvis in relaxed standing but shifts with every change in posture and can even fall outside the body, such as in a deep bend. Adding a load moves the combined center of mass toward the heavier object.
• "Keeping the bar close is just a comfort cue." It is mechanics, not preference. The horizontal distance from the load to a joint is the moment arm that sets the torque your muscles must resist. A bar drifting away lengthens that lever, so the same weight suddenly loads your joints far more heavily.
• "Balance depends only on strong legs." Balance is governed by keeping the line through your center of mass within your base of support, using continuous sensory feedback and small muscular corrections. Leg strength helps, but control of where the mass sits, and the size of your base, are what actually keep you upright.
• "Center of mass and center of gravity are different in lifting." In everyday gravity they coincide, so the terms are used interchangeably in training. They only diverge in a non-uniform gravitational field, which is irrelevant in the gym. For a lifter, center of mass and center of gravity mean the same practical balance point.

Related terms

Center of Mass FAQ

What is the center of mass?

The center of mass is the single point where all of a body's mass is balanced, and through which gravity effectively acts. For a standing person it lies near the pelvis, but it moves as you change position. With a load, the relevant point is the combined center of mass of you plus the weight.

Where is the center of mass in the human body?

Standing upright with arms at your sides, it sits roughly in the pelvis, around the level of the second sacral vertebra. It is not fixed, though: raising your arms, bending, or holding a weight all move it, and in some positions it falls outside the body entirely.

Why does the bar stay over the midfoot in the squat and deadlift?

Because the bar is heavy, the combined center of mass sits close to it, and keeping that point over the midfoot keeps you balanced and the leverage on your joints short. Drift forward and the moment arms lengthen, sharply increasing the torque your muscles must resist.

What is the difference between center of mass and center of gravity?

In everyday conditions on Earth they are the same point and the terms are used interchangeably. They only differ in a non-uniform gravitational field, which never applies in the gym. For lifting, treat center of mass and center of gravity as the same balance point.

How does the center of mass affect balance?

You stay balanced only while the vertical line through your center of mass falls within your base of support, the area under and between your feet. If that line drifts past your toes or heels, you must step or fall. A wider stance enlarges the base and helps.

Why does keeping a weight close make it feel lighter?

The horizontal distance from the load to your joints is the moment arm, which sets the torque your muscles must produce. Holding a weight close keeps that distance short, so the same load demands less muscular effort. Let it drift away and the effective load on your joints rises.

Does the center of mass move during a lift?

Yes, continuously. As your limbs and trunk change position through a rep, your body's center of mass shifts, and the combined center of mass of you and the bar shifts with it. Good technique keeps that combined point tracking over the midfoot throughout the movement.

How can I use center of mass to lift better?

Keep the bar over your midfoot and close to your body so leverage stays short, set your stance to give a stable base for the load, and film lifts from the side to check the bar travels a near-vertical path. These habits improve both balance and efficiency.

References

1. Swinton PA, et al. A biomechanical analysis of straight and hexagonal barbell deadlifts using submaximal loads. J Strength Cond Res, 2011. PubMed 21659894
2. Hales ME, Johnson BF, Johnson JT. Kinematic analysis of the powerlifting style squat and the conventional deadlift during competition: is there a cross-over effect between lifts? J Strength Cond Res, 2009. PubMed 19910816
3. Schoenfeld BJ. Squatting kinematics and kinetics and their application to exercise performance. J Strength Cond Res, 2010. PubMed 20182386
4. Winter DA, Patla AE, Frank JS. Assessment of balance control in humans. Med Prog Technol, 1990. PubMed 2138696