Physics in motion: force production and tissue mechanics

"Remember, it's not how fast you go in a Formula 1 car that injures or kills you

—It's how hard you stop." - Nigel Mansell

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Introduction

Every time we move, whether we’re walking across the room or accelerating into a max velocity sprint, we’re expressing the laws of physics biologically. Muscles pull, tendons recoil, ligaments stabilize, bones bear load, and the entire system experiences forces that must be absorbed, transmitted, and redirected. But when the physical demands exceed what tissues can tolerate, the same mechanics that enable motion become the catalysts for injury.

Human movement is a collaboration between physics and engineering—only the “materials” involved happen to be living tissue. And just like engineered materials, biological structures can withstand tremendous elements and also fail when compressed, stretched, twisted, or loaded faster than they can respond or adapt. Understanding this interplay between motion and mechanical stress is not just philosophical, it’s fundamentally how we can improve performance and conceptualize many mechanisms of injury.

Human motion is the product of force. Muscles contract and generate tension, pulling on tendons, which in turn rotate bones around joints. The result is torque—rotational force—that accelerates segments of the body. Every leap, turn, and stride is a negotiation between internal forces created by muscles and external forces pushing back on the body.

That being said, motion is never just a matter of producing force; it’s about how that force transcends and travels. Tissues deform under load. Some stretch, some compress, some twist, and some absorb force like springs. Biological materials are viscoelastic, meaning their response depends not only on how much force they experience, but also how quickly that force is applied.

The dual nature of being rigid enough to transmit energy and elastic enough to absorb it makes the human body a masterclass in vivo engineering.

What is Force?

Force is essentially influence that changes the motion of an object. Any time an object speeds up, slows down, or moves at all, forces have acted upon it. When you pull a barbell off the floor, jump on bilateral force plates, or throw a medicine ball overhead, you’re negotiating with forces that are acting on you, any implement you may be using, and forces you’re producing. Our interaction with these forces is largely due to mechanotransduction, where we can convert a mechanical stimulus into electrochemical activity creating a feedback loop.

This is relationship was observed and theorized by Sir Isaac Newton who formulated universal gravitation, connecting gravity and motion. Later formalized into what became his three laws of motion; the law of inertia, acceleration, thirdly action and reaction. He realized that force wasn’t some mystical invisible hand but something measurable, predictable, and entirely governed by the variables of mass and acceleration.

The second law equation, F = m*a, is what explains everything from a falling apple to a heavy deadlift. The more mass an object has, and/or the more you accelerate it, the greater the force involved. But even though Newton gave us the recipe, modern strength and conditioning gives us the ingredients. The equation subtly hints at an entire performance continuum. We can generate more force by increasing mass (i.e. lifting heavy loads) or by accelerating lighter loads more rapidly. This opens the door to very different training outcomes—high force low velocity and lower force higher velocity.

When considering the other laws, our movement execution is the interplay between acceleration and deceleration to manipulate inertia. Additionally, ground reaction forces are a foundational example of every action having a reaction, which allows us to propel ourselves in any direction in just one step. As you start to understand the marionette-like game being played, other global considerations to manipulate force production include vectors, neuromechanical matching, and cellular fatigue. Because force has both magnitude and direction, this can be manipulated to become more task or movement specific (think sport positions or muscle activation). How we then sustain these movements and fend off fatigue through neuromechanical matching and the effect fatigue has on adaptation (e.g. calcium ion accumulation) is where S&C jobs are created.

The Structural Stress Beneath the Surface

The laws of motion provide us with a performance continuum and framework to build strength, power, and speed. In biomechanics, every tissue has a stress–strain relationship which is a curve that describes how much it can stretch, compress, or deform before it begins to fail. Tendons, muscles, ligaments, cartilage, discs, and bone each have their own curves (i.e. tensile strength), shaped by both biology and the physics of loading. Felix and colleagues in 2018 found when muscle tension rises there’s a subsequent decrease in energy absorption. This demonstrates the teetertotter relationship across the performance continuum and further the capacity of our muscle to respond in an attempt to reduce injury¹.

When we move, these tissues cycle through the following in some fashion:

Compression: muscle contracting as the agonist, bones absorbing ground impact, cartilage cushioning joints

Expansion: muscle tissue stretching (“lengthening”) as the antagonist

Tension: tendons stretching to store and later transmit kinetic energy (i.e. elastic recoil)

Shear: tissues sliding past one another through rotational motion

These mechanical behaviors are crucial for movement, but problems arise when the deformation begins to reach or exceed the upper-limit of the stress-strain relationship. Too much compression in too short a time, too much stretch with too little control, too much twist without sufficient stability, is where tissues begin to fail respective to their relationship to the applied stress. Engineers know that materials fail for predictable reasons: excessive load, repeated loading, high strain rates, or structural flaws. Biological tissues respond the very same way.

Excessive tension (strain injuries)

Muscles and tendons can stretch (often referred to as “lengthen”) and deform within reason when they absorb force. This creates necessary tension, but if the stretch exceeds the tissue’s yield point, microscopic fibers begin to tear. A strain is often times not random; it’s the physical consequence of threshold tensile load.

Excessive compression (cartilage, meniscus, and bone injuries)

Impact forces travel through the body when we land, cut, sprint, or are even on the receiving end of a punch, kick, or blast wave. Cartilage, menisci, and vertebral discs are natural shock absorbers, but they have thresholds. Too much compression can deform them beyond recovery (i.e. fracture or degenerative changes).

High strain rates (forces delivered too quickly)

This is where physics becomes unforgiving. Tissues are much weaker when forces are applied faster than they can deform. A slow stretch might be harmless, yet a rapid, violent one can cause catastrophic failure. An example of this would be a hamstring tear occurring during a maximal sprint effort where tension is not necessarily high, but increases faster than the tissue can respond.

Material fatigue (repetitive stress injuries)

Even when loads are small, repeated cycles can cause microdamage. Tendonitis to -opathy, cartilage thins, stress lines form in bone. Over time these small failures accumulate until the tissue can no longer keep up. These can be more predictable consequences of physics interacting with biological material.

"The laws of motion provide us with a performance continuum and framework to build strength, power, and speed."

Engineering Principles Written into the Body

Illustrated now is the dynamic dance between movement, performance, and injury. What makes human movement remarkable is that despite the physical demands we place on our tissues, most of the time they do not fail. They actually respond or adapt. It was even found in an animal study that predictive tensile strength when extrapolated was equivalent to the maximum isometric stress of a muscle contraction². Tendons thicken, bones remodel, and muscles grow stronger or improve their ability to create mechanical tension repeatedly. The body is both resilient and fragile—a living structure that strives for balance under constant mechanical stress.

Three engineering concepts govern this dance and practically involve the laws of motion and consequential physical stress:

1. Load vs. Capacity

In simple terms, injury occurs when the mechanical load exceeds the tissue’s mechanical capacity as seen above. Motion as a byproduct generates load, everything from torque to ground reaction forces, and tissues have their own capacities based on structure, hydration, blood supply, age, and training status. If load rises faster than capacity adapts, failure occurs.

2. Stress Concentration

Just like bridges and aircrafts experience higher stress at weak points, human tissues experience amplified stress at anatomical locations. These include tendon insertions, cartilage edges, spinal discs, and ligament attachment sites.

3. Rate of loading

The speed at which force is applied dramatically changes how tissue responds. Slow deformation allows tissue to distribute load, while rapid deformation causes brittle failure.

The Biological Truth, Brother

The truth at the heart of biomechanics is the body is biological, but the rules it operates under are mechanical. Performance is physics in motion. And force—the quiet, constant companion of every athlete—is where it all begins.

Tissues can heal, they can adapt, they can remodel, but they cannot escape the laws of physics. Expansion, compression, friction, impact, torque, strain—these forces shape us every day. When injury occurs, the cause can typically be connected.

Understanding motion through the lens of physics doesn’t make movement less human. Doing so also does not discredit muscle physiology and complexity of injury. It reveals the astonishing engineering embedded within our biology and reminds us that every step, jump, and lift is a conversation between the laws of physics and the remarkable tissues that allow us to express them. When you lift it, you’re not just getting stronger: you’re becoming an active participant in one of the most fundamental interactions in the universe. And whether you’re a competitive athlete or someone who just loves the feel of iron in your hands, understanding force deepens the meaning behind every rep.

Curiosity is advantageous until it becomes a distraction. Ask questions, be creative, explore new methods.

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References

1.       Tsui, F., & Pain, MTG. (2018). Muscle tension increases impact force but decrease energy absorption and pain during visco-elastic impacts to human thighs. Journal of Biomechanics, 23(67), 123-128. doi: 10.1016/j.biomech.2017.11.032.

2.       Tamura, A., Hongu, J., & Matsumoto, T. (2019). Theoretical elastic tensile behavior of muscle fiber bundles in traumatic loading events. Clinical Biomechanics, 69, 184-190. doi: 10.1016/j.clinbiomech.2019.07.021.

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