Biomechanical factors in human movement are the physical and mechanical properties that determine how your body produces, controls, and sustains motion. They include how your muscles generate force, how your tendons store and release energy, how your joints create leverage, how your body interacts with the ground or water, and how your nervous system coordinates all of it in real time. Understanding these factors explains everything from why you walk the way you do to why certain movements cause injury and others don’t.
How Muscles Generate and Regulate Force
Muscle is not a simple motor that switches on and off. The force a muscle produces depends on how fast it’s shortening and how stretched or compressed it is at the moment of contraction. This relationship between force and velocity was first described in 1938 and remains one of the foundational principles of movement science: the faster a muscle shortens, the less force it can produce, and vice versa.1PubMed Central. Physiological Significance of the Force-Velocity Relation in Skeletal Muscle and Muscle Fibers That tradeoff is why you can hold a heavy suitcase still but can’t throw it across the room, and why a sprinter’s legs produce enormous force at push-off when the muscles are barely changing length.
This matters practically during running. Research on the soleus muscle, a major calf muscle used in every stride, shows that the body actively manages how fast the muscle fibers shorten during the stance phase of running. The fascicles operate at much lower velocities than the overall muscle-tendon unit, and runners whose soleus fibers stayed at slower shortening velocities used less energy.2Proceedings of the Royal Society B: Biological Sciences. The force–length–velocity potential of the human soleus muscle is related to the energetic cost of running Your body, in effect, keeps muscle fibers operating in a sweet spot where they can produce high force without burning through energy too quickly.
Tendons as Elastic Partners
Muscles don’t work alone. Tendons connect muscles to bones, and they do far more than just transmit force. Tendons are elastic, and during movements that involve a stretch followed by a rapid shortening, like jumping or sprinting, tendons store energy during the stretching phase and snap it back during the push-off. Research on stretch-shortening cycle exercises found that tendon dynamics, specifically how much a tendon lengthens during the loading phase and how fast it shortens during push-off, strongly contribute to performance gains in explosive movements.3PubMed Central. Effects of External Load on the Contribution of Tendon Dynamics During Stretch-Shortening Cycle Exercises
This elastic action also explains how the soleus fascicles can operate at a fraction of the speed of the whole muscle-tendon unit during running. The tendon takes up much of the length change, allowing the muscle fibers to contract almost isometrically, producing high force at low metabolic cost. It’s one of the human body’s most elegant energy-saving tricks, and it has implications for everything from shoe design to rehabilitation protocols.
Joint Mechanics and Leverage
Even if two people have identical muscle strength, they won’t necessarily produce the same force at a joint. The moment arm, the perpendicular distance from the muscle’s line of pull to the joint’s center of rotation, acts as a lever that multiplies or diminishes muscle force into joint torque. Research on the ankle joint found that the length of the plantar flexor moment arm was at least as important as muscle size in determining how much torque someone could produce.4PubMed Central. Plantar flexor moment arm and muscle volume predict torque-generating capacity in young men Interestingly, people with shorter moment arms experienced smaller drops in torque at high speeds, suggesting that having a mechanically “weaker” lever can actually be an advantage for fast movements.
Calculating the torques at each joint during complex movements is one of the core tasks in biomechanics, but it’s harder than it sounds. The standard approach, called inverse dynamics, works backward from the motion captured by cameras and force plates to compute the forces acting at each joint. However, errors in tracking body segments through small shifts in skin-mounted markers introduce significant noise. Optimization-based methods that correct for these errors have cut the discrepancy between predicted and measured ground reaction forces by roughly two-thirds compared to older approaches.5PubMed. Improving joint torque calculations: optimization-based inverse dynamics to reduce the effect of motion errors
Ground Reaction Forces and Acceleration
Every time your foot contacts the ground, the ground pushes back with a force equal and opposite to what you applied. These ground reaction forces are broken into components: vertical, propulsive (forward), and braking (backward). How you manage these forces has a huge effect on speed. During sprint acceleration, the propulsive impulse, the total forward-directed push summed over the contact period, accounted for about 57% of the variation in sprint velocity across athletes. Braking impulse, by contrast, explained only about 7%.6PubMed. Relationships between ground reaction force impulse and kinematics of sprint-running acceleration Faster sprinters also tended to land with their foot closer to their body and with a more “active” touchdown, meaning the leg was already sweeping backward at contact rather than reaching forward and decelerating.
Ground reaction forces also change with age and terrain. When older adults walk on uneven surfaces, the collision impulse at heel strike increases more steeply with walking speed compared to young adults, while push-off impulse declines significantly with age regardless of surface.7bioRxiv. Analysis of Ground Reaction Force Impulses During Uneven Walking for Young and Older Adults That combination of harder landings and weaker push-offs helps explain why older walkers are more vulnerable to falls on rough ground.
The Pendulum Trick of Walking
Normal walking is surprisingly efficient because the body behaves partly like an inverted pendulum. As you vault over your stance leg, your center of mass rises, converting kinetic energy into potential energy, and then falls, recovering that energy as speed. This passive exchange means the muscles don’t have to do all the work of accelerating and decelerating the body. When researchers asked people to walk while deliberately keeping their center of mass flat, eliminating the natural up-and-down bounce, metabolic cost roughly doubled across a range of speeds, even though the center of mass traveled the same horizontal distance.8PubMed. Minimizing center of mass vertical movement increases metabolic cost in walking The subjects had to keep their legs more bent, which forced their muscles to work harder to support their weight without the benefit of a stiff-legged pendulum swing.
A related finding shows up in people with diabetic neuropathy, who walk with higher energy costs than healthy controls. Researchers initially wondered whether abnormal vertical bounce explained the difference, but the vertical displacement of the center of mass turned out to be similar across groups. Instead, the higher energy cost was linked to shorter step lengths, higher cadence, and the increased internal work from walking with more flexed joints.9PubMed. Vertical displacement of the centre of mass during walking in people with diabetes and diabetic neuropathy does not explain their higher metabolic cost of walking The takeaway: it’s not just how much your body bounces, but whether your joints stay extended enough to let the pendulum mechanism do its job.
Balance During Walking
Staying upright while walking is a surprisingly active process, not something that happens automatically once you’ve learned to walk. Your brain continuously processes visual, vestibular, and proprioceptive information and makes rapid corrections. Research using a virtual reality cave to simulate the perception of a lateral fall found that ankle muscles responded to visual perturbations within a timeframe suggestive of a direct reflexive pathway from the visual system to the spinal cord, coordinating a sequence of balance adjustments across the gait cycle.10PubMed Central. Neural Control of Balance During Walking
Lateral balance is the bigger challenge. When researchers applied low-frequency visual perturbations during treadmill walking, step variability was roughly ten times more sensitive to side-to-side disturbances than to front-to-back ones.11PubMed Central. Direction-dependent control of balance during walking and standing Standing balance requires continuous integrative control in all directions, but walking balance primarily demands step-by-step control in the lateral direction. This helps explain why older adults and people recovering from stroke are especially vulnerable to sideways falls.
People post-stroke show particular difficulty with proactive balance, the ability to anticipate and prepare for predictable disturbances. When healthy individuals walked on a treadmill with regular, predictable accelerations, they learned to reduce their center-of-mass speed spikes and adjust their leg work in advance. Stroke survivors, by contrast, showed little such anticipatory adjustment, which may make them more susceptible to falls during everyday disturbances like stepping onto a moving walkway or catching a bus that lurches forward.12PubMed Central. Stroke impairs the proactive control of dynamic balance during predictable treadmill accelerations
Neuromuscular Coordination and Co-Contraction
Producing movement isn’t just about activating the right muscles. It’s also about managing the ones that oppose the movement. Co-contraction, the simultaneous activation of muscles on opposite sides of a joint, stiffens that joint and improves stability, but at the cost of extra energy. Computational modeling has shown that the optimal level of co-contraction increases steeply with even small amounts of task uncertainty. When you’re unsure what external force might hit you next, your nervous system ramps up joint stiffness as a kind of insurance policy.13PLOS Computational Biology. Co-contraction embodies uncertainty: An optimal feedforward strategy for robust motor control The relationship is logarithmic: a little uncertainty triggers a large increase in co-contraction, while further increases in uncertainty add less additional stiffness.
The nervous system also has to solve the problem of redundancy. The human arm, for example, has far more muscles and degrees of freedom than are strictly needed to reach for a cup of coffee. Computational models of motor control have shown that the brain resolves this redundancy by generating smooth, stereotyped velocity profiles during pointing and grasping, regardless of which specific combination of muscles and joint angles it selects to reach the target.14PubMed. Computational motor control: redundancy and invariance The movement looks the same on the outside even though the underlying muscle activity patterns can vary.
How Aging Changes Movement Mechanics
Aging degrades nearly every biomechanical factor discussed so far, but the ankle plantarflexors, the muscles and tendons that push you off the ground with each step, bear a disproportionate burden. A meta-analysis found significant age-related declines in plantarflexor strength, stiffness of the muscles and Achilles tendon, and muscle volume.15PubMed. Impact of age-related changes in the mechanical properties of ankle plantarflexor muscles on gait biomechanics and energetics: A systematic review and meta-analysis These changes were linked to slower walking speed, reduced ankle power generation, shorter step lengths, worse stability, and higher energy cost. Lower tendon stiffness, which might sound like a minor mechanical detail, was specifically associated with reduced gait economy, meaning older adults spend more energy per meter walked partly because their tendons aren’t returning elastic energy as effectively.
These changes also differ by sex. Research on incline walking has confirmed that the decline in ankle push-off power and flexibility with aging is not uniform between men and women.16PubMed. Age- and sex-related differences in muscle mechanics and gait biomechanics during incline walking This has practical implications for fall prevention programs, which often treat older adults as a single group when the biomechanical challenges may vary considerably.
When Repetitive Loading Causes Injury
Biomechanical factors don’t only influence performance. They determine where and when injuries occur. Patellar tendinopathy, common in sports that involve repeated jumping and landing, results from the accumulation of microdamage in the tendon under cyclic loading. In laboratory testing of human patellar tendons, the rate at which strain increased from cycle to cycle and the rate of stiffness loss were both highly predictive of when the tendon would fail, with the rate of strain change alone explaining about 96% of the variation in cycles to failure.17PubMed. Effects of cyclic loading on the mechanical properties and failure of human patellar tendon The implication is that monitoring tendon strain in real time, perhaps through wearable sensors, could eventually provide meaningful early warnings of overuse injury.
Joint loading patterns matter for pain too. In a study of repetitive single-leg landings, the peak force at the patellofemoral joint, where the kneecap meets the thigh bone, was the strongest predictor of how rapidly knee pain developed.18PubMed. Changes in patellofemoral pain resulting from repetitive impact landings are associated with the magnitude and rate of patellofemoral joint loading Cartilage itself manages high stresses through a combination of elasticity, fluid flow through its porous structure, and swelling pressure, all of which must be considered in any approach to repairing damaged joints.19PubMed. Cartilage biomechanics: From the basic facts to the challenges of tissue engineering
Footwear, Surfaces, and Spinal Loading
External factors reshape biomechanics in ways that can be either protective or harmful. Shoe design is a well-studied example. A systematic review of footwear constructions found that softer midsoles reduce impact forces and loading rates during running, while thicker midsoles provide better cushioning but can decrease the foot’s ability to sense the ground.20PubMed Central. Systematic Review of the Role of Footwear Constructions in Running Biomechanics: Implications for Running-Related Injury and Performance Ground stiffness matters too. Modeling work has shown that impact forces during running are strongly affected by the stiffness of the surface, making ground properties an essential parameter in footwear design rather than an afterthought.21PubMed. Towards a footwear design tool: influence of shoe midsole properties and ground stiffness on the impact force during running
Lifting mechanics demonstrate how much technique shapes the loads on your spine. Heavy deadlifts can produce compressive spinal loads ranging from 5 to 18 kilonewtons and shearing loads of 1.3 to 3.2 kilonewtons.22PubMed Central. Low Back Biomechanics during Repetitive Deadlifts: A Narrative Review For context, that upper range of compression is roughly equivalent to stacking a couple of small cars on your lower spine. But the way the load is held changes things: lifting a barbell, which keeps the weight close to the body’s midline, produced substantially lower lumbar compression and bending moment than lifting a crate of the same weight.23PubMed. A comparative analysis of lumbar spine mechanics during barbell- and crate-lifting: implications for occupational lifting task assessments This has direct implications for workplace ergonomics, where the shape and position of the object can matter as much as its weight.
Biomechanics in Water
Movement through water introduces an entirely different set of biomechanical factors. Instead of ground reaction forces, the dominant external forces are drag and lift. Drag increases with the square of speed, so doubling your swimming velocity roughly quadruples the resistance. Research on competitive swimmers found that while absolute passive drag differed enormously across towing speeds, the drag coefficient, which accounts for size and speed, stayed relatively stable.24PubMed Central. Understanding the Importance of Drag Coefficient Assessment for a Deeper Insight into the Hydrodynamic Profile of Swimmers That makes the drag coefficient a better tool for tracking changes in a swimmer’s body position and streamlining over time.
Propulsion in swimming depends on how the hand and forearm interact with the water. Towing-tank experiments with hand and forearm models showed that drag force peaked when the palm faced nearly perpendicular to the flow, while lift forces peaked at intermediate angles, with the hand pitched at about 30 to 50 degrees. The hand’s orientation was critical for generating lift, whereas velocity and hand size had comparatively little effect on the drag and lift coefficients.25Journal of Biomechanics. Hydrodynamic drag and lift forces on human hand/arm models Skilled swimmers use a combination of drag and lift propulsion, sweeping their hands through curved paths that take advantage of both forces.
Measuring Movement Outside the Lab
Traditionally, biomechanical research relied on optical motion capture systems with reflective markers tracked by infrared cameras. These remain the gold standard, but they’re expensive, confined to a lab, and require time-consuming setup. Wearable inertial measurement units, small sensors containing accelerometers and gyroscopes, have opened up the possibility of measuring movement anywhere. However, accuracy is still a concern. A study comparing IMU-based gait analysis to optical motion capture found that differences exceeded five degrees for most joint motions, with the biggest discrepancies outside the forward-backward plane.26Measurement: Sensors. Inertial measurement unit-based motion capture to replace camera-based systems for assessing gait in healthy young adults: Proceed with caution
For some applications, wearable sensors are already good enough. When researchers compared IMU-derived and camera-derived estimates of center-of-mass acceleration during walking, the agreement was moderate for direct comparisons but improved when both methods were benchmarked against a lumbar-mounted marker cluster.27PubMed Central. Center of mass acceleration during walking: comparison between IMU and camera-based motion capture methodologies In practical terms, wearable sensors can detect broad trends and gross abnormalities during walking, but fine-grained measurements like small changes in knee rotation still need optical systems.
Assistive Devices and Rehabilitation
Biomechanical factors are central to the design of assistive technology. For people with spinal cord injury, walking aids have traditionally meant rigid braces that lock the knee and force the user to swing each leg around in a wide arc. Powered exoskeletons aim to restore a more natural gait pattern, and early results are encouraging. In a crossover trial comparing a knee-powered exoskeleton to conventional braces, the exoskeleton reduced circumduction by about 38%, increased step length by roughly a quarter, and brought the body’s center of mass closer to the leading foot, indicating better weight shifting and stability.28Scientific Reports. Comparing walking with knee-ankle-foot orthoses and a knee-powered exoskeleton after spinal cord injury: a randomized, crossover clinical trial These improvements go beyond aesthetics; a more physiological gait reduces compensatory strain on the hips and lower back.
Evolutionary Roots of Human Movement
Many of the biomechanical factors that shape modern human movement evolved under selection pressures that have nothing to do with gym performance or clinical gait analysis. Humans are unique among primates in their capacity for sustained endurance running, a capability that emerged roughly two million years ago. The structural adaptations include long spring-like tendons that store and return elastic energy, a plantar arch that recovers up to about 20% of the energy generated during the weight-bearing phase of a stride, a large gluteus maximus for trunk stabilization, long legs relative to body mass, and modifications to the hips and shoulders that generate counter-rotating forces for smooth stride transitions.29PubMed Central. Evolutionary Aspects of Human Exercise – Born to Run Purposefully Heat dissipation mattered too: sweat glands, reduced body hair, and an elongated body form all helped early humans run in warm conditions that would overheat most quadrupeds.
Even at the molecular level, there are running-related adaptations. Compared to other primates, humans carry a mutation that inactivates the CMAH gene. When researchers introduced the same mutation in mice, the animals showed increased running endurance and reduced muscle fatigue, suggesting a link between this molecular change and the endurance running capability that defines our lineage.30PubMed Central. Human-like Cmah inactivation in mice increases running endurance and decreases muscle fatigability: implications for human evolution The biomechanical factors we measure in a lab today, tendon elasticity, force-velocity relationships, pendulum-like energy exchange, are the functional expressions of millions of years of evolutionary tinkering with the human body’s structure and chemistry.