Biomechanics is the application of mechanical principles to living organisms, and its core principles explain how forces, motion, and energy interact to produce human movement. Whether you’re a student, athlete, coach, or just curious, understanding these principles gives you a framework for analyzing everything from walking posture to a baseball pitch.
Newton’s Laws as the Foundation
Every principle in biomechanics traces back to Newton’s three laws of motion. The first law states that a body stays still or keeps moving at the same speed unless a force acts on it. Your body sitting in a chair remains motionless until your muscles generate enough force to stand. The second law ties force, mass, and acceleration together: applying more force to the same body mass produces greater acceleration. This is why a sprinter who generates more ground reaction force accelerates faster out of the blocks. The third law says every force has an equal and opposite reaction. When your foot pushes down and backward against the ground, the ground pushes up and forward against your foot, propelling you ahead.
These laws also govern what happens when you’re not moving. To stay balanced while standing, all the forces acting on your body must cancel out to zero, and all the rotational forces (moments) around any joint must also sum to zero. If they don’t, you either start moving or you fall. This concept of static equilibrium is what early biomechanists like Giovanni Borelli used to calculate the forces acting at human joints centuries ago.
Kinematics vs. Kinetics
Biomechanics splits movement analysis into two branches. Kinematics describes the geometry of motion without worrying about what causes it. The variables here are displacement (how far something moves), velocity (how fast), and acceleration (how quickly the speed changes). A motion-capture camera system tracking reflective markers on your body is a kinematic tool, measuring where each body segment is in space over time.
Kinetics, by contrast, focuses on the forces that cause or resist movement. Force platforms embedded in a lab floor measure the ground reaction forces your feet produce, reported in newtons, along with the rotational moments at each joint, measured in newton-meters. Together, kinematics and kinetics give the full picture: kinematics tells you what the movement looks like, and kinetics tells you what’s driving it.
Lever Systems in the Body
Your musculoskeletal system is built from levers. Every lever has three components: a pivot point (the joint), an effort (muscle force), and a load (the weight being moved). The arrangement of these three elements determines the class of lever and its mechanical properties.
- Class 1 levers place the pivot between the effort and the load, like a seesaw. The joint where your skull meets your spine works this way. Your neck muscles at the back of the skull provide the effort to lift the weight of your head. When those muscles relax, your head nods forward.
- Class 2 levers place the load between the pivot and the effort, like a wheelbarrow. Standing on your tiptoes is the classic example: the pivot is at your toe joints, the load is your body weight in the middle of your foot, and your calf muscles pulling on the Achilles tendon supply the effort. This arrangement gives you a mechanical advantage, meaning the muscle force needed is less than the weight being lifted.
- Class 3 levers place the effort between the pivot and the load. A biceps curl is the textbook case: the elbow is the pivot, the biceps attaches close to the elbow, and the weight is out at your hand. There’s no mechanical advantage here because the muscle must produce more force than the load. The tradeoff is speed and range of motion. Your hand travels much farther and faster than the point where the muscle attaches, which is why this lever type dominates in throwing and swinging movements.
Balance and Stability
Balance is the ability to keep your line of gravity, an imaginary vertical line dropping down from your center of gravity, within your base of support. Your base of support is the area between and beneath your points of contact with the ground. Stand with your feet together and that base is small, making balance harder. Step your feet apart and the base widens, making you more stable.
Three factors determine how stable you are. First, a lower center of gravity increases stability, which is why wrestlers and linemen crouch. Second, a wider base of support helps, which is why you instinctively widen your stance on a rocking boat. Third, positioning your center of gravity closer to the middle of your base gives you more room to sway before losing balance. Dynamic balance during walking is more demanding because your base of support shrinks to just your feet, and your center of mass is constantly shifting forward with each stride.
Force, Stress, and How Tissues Respond
When force is applied to biological tissue, the tissue deforms. How much it deforms, and whether it bounces back, depends on the tissue’s mechanical properties. The relationship between stress (force per unit area) and strain (the amount of deformation) follows a predictable curve in materials like bone.
In the elastic region of this curve, the tissue returns to its original shape when the load is removed, like a rubber band snapping back. No damage occurs. If loading continues past this elastic limit into the plastic region, permanent structural changes happen. The tissue doesn’t fully recover its shape even after the load is removed. The size of this plastic region tells you how much a tissue can deform before it breaks. Bone, for example, has a relatively small plastic region compared to tendon, meaning bone is more brittle and fails more suddenly. Understanding where on this curve a tissue sits during an activity helps explain injury mechanisms: a stress fracture is the result of repeated loading that pushes bone past its elastic limit over time.
Work, Power, and Athletic Performance
Work and power describe how effectively your muscles convert effort into movement. Mechanical work is force multiplied by the distance over which that force is applied, measured in joules. If you push harder against the ground during a squat or push over a greater distance by squatting deeper, you do more work.
Power adds a time component. It equals force multiplied by velocity, measured in watts. This is why power matters more than raw strength in explosive sports. Two athletes might produce the same total work during a vertical jump, but the one who produces it faster, generating higher velocity at takeoff, jumps higher. The takeoff velocity uniquely determines jump height because once your feet leave the ground, the only force acting on you is gravity.
Muscle power depends on two properties: the maximum force a muscle can produce and its maximum shortening speed. These follow an inverse relationship described by the force-velocity curve. At very high loads, muscles contract slowly. At very light loads, they contract quickly. Peak power output occurs somewhere in between. This relationship is why training for power requires a mix of heavy-load strength work and high-speed movement practice.
Projectile Motion and Throwing
Once an object leaves your hand or your feet leave the ground, it becomes a projectile governed by gravity and air resistance alone. Three factors at the moment of release determine the projectile’s path: release speed, release angle, and release height. In throwing sports, release speed is the dominant variable, and the entire kinetic chain from legs through trunk to arm is organized to maximize it.
In overhead throwing, stride length correlates with ball velocity. The separation between hip rotation and shoulder rotation at foot contact drives trunk rotation speed, which in turn drives arm speed. At peak preparation, the shoulder reaches close to 180 degrees of external rotation while maintaining about 90 degrees of abduction and more than 90 degrees of elbow flexion. Deviations from these positions don’t just reduce velocity; they shift stress to vulnerable structures in the shoulder and elbow, raising injury risk.
How Biomechanics Is Measured
Traditional biomechanical analysis relies on laboratory tools: motion-capture cameras tracking body segment positions, and force platforms recording ground reaction forces. These systems are highly accurate but confined to a lab setting. Increasingly, wearable inertial measurement units (IMUs) are bringing biomechanical analysis into real-world environments. These small, low-cost sensors contain accelerometers and gyroscopes that estimate movement variables like stride length, stride duration, and cadence during actual training or competition.
Foot-mounted IMU systems, for example, have been validated against lab-based motion capture during sprinting. They measure stride length with a root mean square error of about 0.22 meters and stride duration within roughly 0.04 seconds. For velocity and acceleration, the agreement is less precise, with biases up to 0.67 meters per second for peak velocity. These sensors work well for tracking trends over time and flagging changes in running mechanics, even if they don’t yet match laboratory-grade precision for absolute measurements. Their real value is ecological validity: they capture how an athlete actually moves on the field, not just how they move in a controlled lab corridor.