1.3 Biomechanics of Human Movement

‍

The short version

‍

Coaches shout things like “drive through the ground” and “get your hips through.” Most athletes nod and carry on doing exactly what they were doing. Biomechanics is what turns those phrases into something you can actually picture and change.

This article walks through the mechanics behind human movement: forces, torque, levers, the push you get back from the ground, and how force travels through the body from one segment to the next. It is for anyone who wants to know why a cue works rather than just being told that it does.

‍

Experiment you can do right now

‍

Grab something a bit heavy — a water bottle, a dumbbell, a thick book. Hug it to your chest. Easy.

Now hold it out at arm’s length and wait ten seconds. Your shoulder starts complaining fast, yet the weight hasn’t gained a single gram.

The weight didn’t change. The lever did. That little trick is biomechanics in miniature, and it’s about to make the whole gym make sense.

Because your body gets no exemption from physics. Every jump, sprint, throw and swing follows the same rules as any other object in motion — falling apples, rolling balls, you.

‍

The floor pushes back

‍

Almost everything starts at the ground, so start there.

Push into the floor and the floor pushes back with exactly as much force, in the opposite direction. That return push is what actually moves you.

You don’t run by pulling yourself along. You run because you shoved the ground backwards and it returned the favour.

So sprinting faster isn’t magic. It’s putting more force into the ground in less time — and that single idea explains a large slice of speed training.

And “less time” isn’t a figure of speech. At top speed a sprinter’s foot is on the ground for roughly 80-100 milliseconds, so the force has to arrive fast or it doesn’t count.

In the gym: this is why leg strength isn’t just for show. More force available on each push is the direct physical basis for faster acceleration — and heavy lifting raises that ceiling, while explosive work like jumps trains you to use it quickly.

‍

One-line recap: the ground pushes you exactly as hard as you push it. Push harder, quicker.

‍

Your bones are levers

‍

Your muscles don’t lift weights directly. They pull on bones, and bones behave like levers — so the angle of a joint changes how hard a muscle has to work for the same result.

That’s the water bottle experiment from the top. Held close it feels light; held at arm’s length it feels brutal. Same load, longer lever.

It’s also why moving a foot an inch, or changing a knee angle slightly, can make a lift feel completely different. Tiny technical adjustments matter far more than they look like they should.

In the gym: when a rep suddenly feels savage halfway up, the bar didn’t get heavier. Your lever got longer at that joint angle.

So before you strip plates off, check your position. Coaches have a line for this: fix the position, and the force often follows.

‍

Power travels like a whip crack

‍

A whip doesn’t crack because the handle moves fast. It cracks because energy travels down the cord in sequence, each section moving faster than the one before it, until the tip is moving at an absurd speed.

A pitch, a punch, a golf swing and a tennis serve all work this way.

Force starts in the legs and hips, passes up through the trunk, then the shoulder, then the arm — and each handover speeds it up.

Your hand is the tip of the whip. It is not the source of anything.

‍

The order is the whole secret

‍

This is why sequencing beats strength here. A common fault in throwing athletes is leading with the arm — starting the motion before the hips and trunk have finished passing their force on (Putnam, 1993).

The whip effect breaks, and the throw slows down.

Worse, the arm now has to make the force by itself rather than simply passing along force that big, robust body parts already built. Shoulders and elbows are not designed for that job.

That’s one plausible route from a technique fault to an overuse problem, though it is rarely the only one. An “arm punch” or an arm-led throw isn’t just weaker — it asks more of joints that were never meant to be the engine.

In the gym: film your throw, swing or jump in slow motion on your phone. Sequencing faults that are invisible at full speed — like the arm firing before the hips — jump straight out of the footage.

One-line recap: legs, hips, trunk, then arm. Sequence, don’t skip.

‍

Force leaks: where strong people lose

‍

Every joint the force passes through is a chance to lose some of it. A link can go wrong in three ways:

• Too stiff. The force gets blocked and can’t pass through.
• Too loose or unbraced. The force gets absorbed instead of transmitted.
• Badly timed. The handover happens too early or too late, and the wave never builds.

Here’s the humbling part. A strong athlete who leaks force at the hips or the trunk can be out-thrown and out-jumped by someone weaker who does not leak anywhere.

Fixing the leak is usually cheaper than adding more engine.

In the gym: the usual suspects are a trunk that wobbles under load, a hip that never fully straightens, and a shoulder that can’t get into a good position. Check those three before you blame the arm.

And remember why the hips matter so much. They’re typically the biggest, strongest link in the chain and the closest to the ground, where the force is born — when they fail, small parts inherit a big job.

‍

The four-question fault finder

‍

When a movement goes wrong, there are only four possible reasons:

‍

1. Not enough force.
2. Force pointed in the wrong direction.
3. Force arriving at the wrong time.
4. Force lost on the way through.

‍

That short list is a diagnostic tool. A jump that will not improve is one of those four. A serve that has stopped working is one of those four.

It stops the guessing — and it stops you adding strength to a problem that was never about strength.

Worked example: your jump has stalled, but your gym numbers keep climbing. Then question one probably isn’t your problem, so start checking direction, timing and leaks instead.

Strength and technique aren’t rivals, by the way. Strength decides how much force you have available; technique decides how much of it actually reaches the throw, jump or strike instead of being lost along the way.

‍

Want the mechanism? Keep reading

‍

The rest of the article goes into the detail: Newton’s laws in a sporting context, ground reaction force measured in three directions, how joint angles change the torque a muscle has to produce, and how to spot the common leaks in a kinetic chain.

Those sections are more technical, so treat them like a reference. Come back to them when you want to know exactly why a cue works.

‍

Advanced Section: The Mechanics of Athletic Movement

Newton's Laws in a Sporting Context

‍

• First law (inertia): A body at rest stays at rest, and a body in motion stays in motion, unless acted on by a force. This is why deceleration and change of direction are so demanding — an athlete's mass, once moving fast, requires large forces to stop or redirect.
• Second law (F = ma): Force equals mass times acceleration. To accelerate a given mass (an athlete's body, a ball, an opponent) more quickly, more force must be applied. This is the direct physical basis for why strength training improves acceleration and striking power.
• Third law (action-reaction): For every force applied, an equal and opposite force is returned. This is the basis of ground reaction force (GRF) — when a sprinter pushes backward and downward into the ground, the ground pushes the sprinter forward and upward with equal force.

‍

Ground Reaction Force in Depth

‍

Ground reaction force is typically measured in three directions: Vertical (supporting body weight and enabling jumping), horizontal/anterior-posterior (propelling the body forward or braking it), and medial-lateral (enabling cutting and change of direction) (Bartlett, 2007). Force plates and specialized treadmills allow sports scientists to measure ground reaction force (GRF) directly, revealing metrics such as peak force, rate of force development, and impulse (force applied over time) — all of which correlate strongly with sprint and jump performance.

‍

Torque, Levers, and Joint Mechanics

‍

Muscles do not simply pull — they create torque, a rotational force, around a joint. Torque is the product of the force applied and the perpendicular distance from the joint's axis of rotation to the line of that force (the "moment arm"). This is why joint angle matters so much: The same muscular force can produce very different amounts of usable torque depending on the joint angle, because the moment arm changes throughout the range of motion (Knudson, 2007).

The body's joints act as one of three classes of lever:

• First-class lever: The axis (joint) sits between the force and the load (e.g., the neck extensors balancing the head atop the spine).
• Second-class lever: The load sits between the axis and the force, allowing mechanical advantage (e.g., the calf raise, where the ball of the foot is the axis, the body weight is the load in the middle, and the calf muscle applies force at the heel).
• Third-class lever: The force sits between the axis and the load — the most common lever arrangement in the human body (e.g., the biceps curl), which prioritizes speed and range of motion over mechanical force advantage.

‍

Most human limbs act as third-class levers, which is a key biomechanical reason humans are comparatively fast and dexterous but mechanically inefficient force generators compared to many animals of similar mass (Knudson, 2007).

‍

Kinetic Chains and Energy Transfer

‍

A kinetic chain is a sequence of connected joints and segments that work together to produce or absorb force. Force generated at one end of the chain (usually the ground, via the legs) must pass through every link to reach the working end (a hand releasing a ball, a foot striking through a target) (Hay, 1993). Any weak or poorly timed link in that chain causes an "energy leak" — a loss of transferable force.

Common energy leaks include: A core that cannot resist unwanted rotation or extension (losing force to unwanted trunk motion), a hip that cannot fully extend (shortening the distance over which force is applied), or a shoulder that lacks the mobility to get into an optimal position for force transfer (forcing compensatory movement elsewhere in the chain).

‍

‍

Impulse and Rate of Force Development

‍

Impulse is the product of force and the time over which it is applied, and it directly determines the change in momentum (and therefore velocity) an athlete or object experiences (Zatsiorsky, 2002). This explains why both maximum force and the speed of producing that force matter: A sprinter's foot is only in contact with the ground for roughly 80-100 milliseconds at top speed, meaning force has to be produced extremely quickly to be useful — a concept known as rate of force development (RFD), which is covered in detail in the Section 3 article on reactive strength.

‍

Practical Section: Applying Biomechanics to Coaching

‍

• Coach positions, not just effort: Since torque depends on joint angle, cueing an athlete into a better position (e.g., a more vertical shin angle at sprint push-off) can improve force output more than simply cueing them to "try harder."
• Train the whole chain, not just the end point: A weak throwing shoulder may reflect a hip or trunk that is not taking its share of the work, so assess the full chain rather than only the sore or weak site. Treat that as one hypothesis to test, not a diagnosis — the local tissue is often genuinely the problem.
• Use video and, where possible, force plates: Even simple slow-motion smartphone video can reveal sequencing faults (e.g., the arm firing before the hips) that are invisible at full speed.
• Train both force and speed of force: Heavy strength training raises the ceiling on force production, while explosive/plyometric training improves how quickly that force can be applied — both matter for impulse (Zatsiorsky, 2002).

A note on pain. Chain thinking is a way to plan training, not a way to diagnose an injury, and the idea that one region is driving symptoms at another is a hypothesis to test rather than a finding. Pain that persists, follows a specific trauma, or comes with swelling, giving way, loss of function, or numbness, tingling or weakness should be assessed by a qualified clinician rather than explained away as a problem further up the chain.

Sport Applications

‍

• mixed martial arts (MMA)/Boxing: Striking power is a direct product of ground reaction force and kinetic chain sequencing from the legs through the hips and trunk to the fist — "arm punches" that skip the lower body chain are mechanically weak (Hay, 1993).
• Football/Rugby: Tackling and blocking rely on producing high force quickly (impulse) while maintaining a stable base, and absorbing the opposing player's ground reaction force without collapsing the kinetic chain.
• Baseball/Tennis/Volleyball: Throwing, serving, and hitting all rely on proximal-to-distal sequencing (legs → hips → trunk → arm → hand/racquet/ball) for maximum velocity at the end point (Putnam, 1993).
• Sprinting: Almost entirely defined by the magnitude, direction, and rate of ground reaction force applied during each brief ground contact.
• Olympic weightlifting/Powerlifting: Technical mastery is essentially the optimization of lever arms and force application angles to move maximum load efficiently.

Common Mistakes

• Cueing "more effort" when the actual limiter is poor positioning or sequencing.
• Training the end of the kinetic chain (e.g., the arm in a throwing athlete) while neglecting the legs, hips, and trunk that generate most of the force.
• Ignoring joint angle and moment arm changes when interpreting strength test results.

Coaching Cues

• "Force comes from the ground up — nothing starts at the hand."
• "Sequence, don't skip — legs, hips, trunk, then arm."
• "Fix the position, and the force often follows."

‍

FAQs

‍

Why do coaches care so much about "hip drive"?
Because the hips are typically the largest, strongest link in the kinetic chain and are closest to the ground, where force is initially generated. A breakdown at the hips leaves smaller, weaker distal segments (the arms, for example) to compensate, which is mechanically inefficient and is one plausible contributor to overload downstream — though pain at the arm still needs assessing on its own merits rather than being attributed to the hips by default.

Does biomechanics matter more than strength?
They are inseparable — strength determines how much force is available, and biomechanics/technique determines how much of that available force is actually transferred to the intended outcome (a throw, a jump, a strike) rather than lost along the way.

Can biomechanics analysis prevent injury?
It can meaningfully reduce risk by identifying inefficient or high-stress movement patterns (such as poor landing mechanics, covered in a dedicated Section 3 article), but it cannot eliminate injury risk entirely, since injury is multifactorial.

Recommended Videos

The Real Science of The Squat — Squat University
Watch on YouTube
A clear, physical-therapist-led breakdown of torque and lever mechanics using the squat as the teaching example, directly illustrating the torque and moment arm concepts covered above.

3 Biomechanics Concepts Every Coach Should Know (But Most Don’t) — The Movement System. Covers levers, moment arms and force application in coaching language rather than textbook notation.

The Rotator Cuff Force Couples — Muscle & Motion
Watch on YouTube
A visual 3D breakdown of how multiple muscles create coordinated "force couples" around a single joint, a useful real-world example of applied torque and joint mechanics.

References

Bartlett, R. (2007). Introduction to sports biomechanics: Analysing human movement patterns (2nd ed.). Routledge.

Force Velocity Curve Explained — The Movement System. Explains the force-velocity curve, the map used later to place strength, power and speed work.

Hay, J. G. (1993). The biomechanics of sports techniques (4th ed.). Prentice Hall.

Knudson, D. (2007). Fundamentals of biomechanics (2nd ed.). Springer.

Kraan, G. A., van Veen, J., Snijders, C. J., & Storm, J. (2001). Starting from standing; why step backwards? Journal of Biomechanics, 34(2), 211-215.

Putnam, C. A. (1993). Sequential motions of body segments in striking and throwing skills: Descriptions and explanations. Journal of Biomechanics, 26(Suppl. 1), 125-135.

Zatsiorsky, V. M. (2002). Kinetics of human motion. Human Kinetics.

‍