Introduction to Biomechanics

Biomechanics is the study of how our bodies move and the forces that affect them. Two important parts of biomechanics are gait and balance. Gait is the way we walk or run. It includes how our joints and muscles move and how smoothly we take each step. Balance is our ability to stay steady, whether we’re standing still or moving. Together, gait and balance show how the body controls movement and how we respond to forces such as gravity, friction, and uneven ground.

Gait is the way a person walks or moves from place to place. It includes the steps we take, how our joints and muscles work together, and how we stay steady while moving. Understanding gait helps us see how well someone walks, identify movement issues, and create ways to improve walking for health, recovery, or better performance.

Balance is the ability to stay steady and keep your body’s weight centered so you don’t fall. It depends on information from your senses - like what you see, how your body feels its position, and signals from your inner ear - as well as how your muscles work together to keep you upright. Good balance is important for moving safely and avoiding falls.

Studying gait and balance helps scientists and health professionals learn how people move, improve sports performance, prevent injuries, and create better rehabilitation tools and devices that help people walk safely.

There are many ways researchers can study gait and balance in a laboratory. In a lab, biomechanists, physical therapists, and other researchers use high-tech tools to watch, measure, and analyze how people move. The table below shows some common research methods, and the sections that follow explain each one in more detail.

Testing Gait & Balance at Home

1. Overall Safety Concerns

   The methods described above are very useful in a laboratory, but not all clinicians or patients have access to a lab. To make balance testing easier, there are ways to test gait and balance outside of the lab. These methods can also be adapted for students who want to do independent science projects in biomechanics.

   Even though students won’t be using high-tech cameras, force plates, or special software in a lab, they can do similar tests at home or at school and still get useful results. Even outside of a lab, students doing biomechanics projects need to follow important steps for testing people, like keeping the testing conditions consistent and repeating trials. This helps make the data more reliable and allows students to draw accurate conclusions about how the human body moves.

   Safety is the most important thing when working with people in experiments. Biomechanics tests often involve movement, so it’s important to make sure the activity is safe for the participant’s age, fitness, and health. The testing area should be free of obstacles, and all equipment should be set up correctly and used safely. Participants should wear comfortable clothes and supportive shoes, if needed, to prevent injuries. It’s also important to explain the test clearly, get permission from participants, and stop the test right away if anyone feels pain or discomfort. Following these steps keeps participants safe and ensures the research is responsible and ethical.

   As student researchers working with participants, it is important to think about safety at every step. Here are some tips for doing your own biomechanics projects:

   • Choose a safe testing area: Make sure the space is flat, dry, and free of clutter, rugs, cords, or other trip hazards. A hallway or open living room area often works well.
   • Have a spotter nearby: Someone should stand nearby and be ready to help if the person starts to lose their balance, especially during balance tests or when walking on uneven ground.
   • Avoid risky movements: Don’t include tests that involve jumping, running, or tasks that could cause falling. Keep movements slow and controlled.
   • Explain instructions clearly: Make sure participants understand what they need to do before each trial so they don’t get confused or move in a way that could be unsafe.
   • Know when to stop: If anyone feels dizzy, unsteady, or uncomfortable, stop the test immediately and allow rest.
   • Use soft or padded surroundings if needed: When testing balance, do the activity near a wall, sturdy chair, or couch so there is quick support available if the person starts to lose balance.
   • Get permission: If under 18, make sure an adult is aware of the experiment and gives permission to participate or supervise. Please refer to the Science Buddies documents Projects Involving Human Subjects and Scientific Review Committee (SRC) for additional important requirements.

   These steps help make biomechanics testing safe, responsible, and appropriate for student researchers at home, while still allowing for useful and reliable collection.

2. At-Home Gait

   There are many gait tests that students can do outside of a lab using just a stopwatch and something to measure distance:

   1. Timed Walk Test - used to measure walking speed. A common timed walking test is the 10-meter walk test. Mark the floor at 0 meters, 2 meters, 8 meters, and 10 meters.

      1. The individual walks without assistance 10 meters (32.8 feet), and the time is measured for the intermediate 6 meters (19.7 feet) to allow for acceleration and deceleration.
         • Start timing when the toes of the leading foot cross the 2-meter mark.
         • Stop timing when the toes of the leading foot cross the 8-meter mark.
      2. Calculate the walking speed by dividing the distance traveled (6 meters or 19.7 feet) by the time (in seconds) to complete the test.
      3. Collect 3 trials per participant and calculate the average of the three trials.

   2. Step Counting and Cadence - Cadence is measured by counting how many steps a person takes over a set distance or period of time. It’s an important measure because it can help establish a personal baseline, predict health outcomes, and show how well a person controls their movements.

      1. Start the stopwatch or timer.
      2. Count the steps: Begin walking and count the number of times your feet hit the ground for 60 seconds. To make counting easier, you can count how many times just one foot (for example, your left foot) hits the ground.
      3. Calculate: Count the number of steps in that minute. If you counted one foot, multiply your final number by two to get your total steps per minute.
      4. Repeat for accuracy: For a more accurate average, repeat the test three more times, then add your results and divide by three to determine the average steps per minute.
      To explore a student’s independent project involving step counting - check out this project: Keeping Up

   3. Stride Length - Stride length is the distance traveled in one full step cycle - from when one foot first touches the ground to when that same foot touches the ground again. In motion capture analysis, stride length can be measured by looking at the horizontal distance between reflective markers placed on key body points, like the heels or ankles, during a heel strike. If you don’t have motion capture, you can still estimate stride length on your own:

      1. Pour baby powder in a thin layer of a plastic tub, large enough to step into.
      2. Roll out black kraft paper to cover the distance of the walking surface.
      3. Step bare feet in baby powder to cover the surface of the bottom of each foot.
         • This process could also be completed using brown kraft paper and stepping feet into a bucket of water.
      4. Step out of the baby powder and onto the black paper.
      5. Walk a specified distance.
      6. Measure the distance between the heel of two consecutive foot prints from the same foot.

3. At-Home Balance Experiments

   In biomechanics research and clinics, the force plate is the best tool for testing balance because it can measure the exact forces and pressure changes under the feet, showing how much someone sways and how stable they are. Still, students can test balance at home using simpler methods that measure similar ideas of control and stability.

   1. Single-leg stance: This tests your ability to stabilize your body using one leg and challenges the muscles and sensory systems that help you stay upright.

      1. Ask the participant to stand on one leg without holding onto anything.
      2. Use a timer to time how long the participant can maintain balance.
      3. Stop the timer when the participant sets their feet/foot on the ground.
      4. Have the participant do all trials on the same leg.

   2. Tandem stance (heel-to-toe): This test reduces your base of support and challenges your balance control in a narrow stance.

      1. Ask your participant to place one foot directly in front of the other so that the heel of one foot touches the toes of the other.
      2. Use a timer to time how long the participant can maintain balance.
      3. Stop the timer when any part of either foot moves or adjusts.
      4. Make sure that the participant does all trials with the same foot in front.

   3. Quiet stance (eyes open/closed): Taking away visual input makes balancing harder because the body has to rely more on signals from the inner ear and muscles/joints (proprioception). During this test, you will look for signs of stability, like how steady the person is (small amounts of swaying), whether they can keep their balance for the full time without stepping or holding on, and the difference between standing with eyes open versus eyes closed. If the person wobbles more or loses balance with their eyes closed, it shows they depend more on vision to stay balanced.

      1. Ask the participant to stand naturally with their feet hip-width apart and ask that they try to remain as still as possible.
      2. Set a stopwatch for the chosen time period.
      3. Stop the stopwatch if the participant needs to steady themselves in any way.
      4. Repeat the test with the participant’s eyes closed.

   4. Functional reach test: This test evaluates balance, stability limits, and how well you can control your center of gravity during movement.

      1. Have participants stand with one shoulder adjacent to a wall.
      2. Have the participant stand with their feet shoulder-width apart.
      3. Have the participant reach forward as far as possible without stepping or losing balance.
      4. Measure how far the participant can reach by using tape to mark on the wall. Alternatively, you can tape paper on the wall and mark the participant’s reach with a pen on the paper.

4. Smartphone as IMU

   Instead of using expensive sensors found in biomechanics laboratories, researchers have shown that a smartphone’s built-in accelerometers and gyroscopes can measure movement and orientation for balance and gait tests, much like an IMU. This makes smartphones a low-cost and accessible tool for learning basic biomechanics. With apps like Google’s Physics Toolbox Accelerometer, phyphox, or other IM-style apps, students can collect and export accelerometer data for their own balance and gait analysis. Students can also explore the Arduino Science Journal and use an Arduino as an IMU for similar measurements.

   To do balance testing outside of a lab, you can place a smartphone on the body - usually on the lower back - to measure changes in acceleration and body position. This works for tasks like quiet standing, single-leg stance, or tandem stance. The data you collect can be used to calculate measures of stability, such as how much a person sways or how long it takes before they lose balance.

   For gait testing, the phone can be attached to the waist or hip while the participant walks a set distance. As they walk, the accelerometer records their steps, stride timing, and patterns of movement. Many smartphone apps let you collect data such as cadence, step time, stride variability, and how smooth the person’s gait is. By repeating trials and comparing results under different conditions - like changing walking speed, footwear, or whether the eyes are open or closed - students can study how balance and gait change with the environment or task difficulty.

Current Research Methods

1. Instrumented Laboratory Measures

   Even though it does not fully match real-life movement, researchers often use marker-based optical motion capture to study gait and balance. In this method, small reflective markers (or LED markers) are placed on key points of the body, such as the ankle, knee, and hip. Several calibrated cameras around the lab record each marker from different angles at the same time. The software then combines these 2-D views to calculate accurate 3-D coordinates for each marker over time. These marker paths are used to figure out body segment positions and joint motions, including angles and speeds. Marker-based systems are still considered the “gold standard” for collecting detailed 3-D movement data in controlled settings. You can also watch behind-the-scenes videos from movies like Avatar to see how motion capture helps create realistic animation.

   

   https://www.youtube.com/watch?v=uGerIQIjuqg

   A force plate is a special platform that measures the forces between your foot (or body) and the ground. A force plate used for research usually measures forces in three directions (Fx, Fy, Fz) and moments in three directions (Mx, My, Mz). Using this information, the device can calculate the center of pressure (CoP), which is the point where the overall force is applied. Because they measure these forces directly, force plates are very important for studying balance and body movement. They give very accurate data, but they are usually fixed in a lab, so the person being tested has to step exactly on the plate. This can sometimes change how they walk. To measure several steps in a row, labs need multiple force plates or an instrumented treadmill. Force places can also be used during quiet standing to study how the center of pressure moves, which helps measure things like postural sway and sway speed.

   

   https://www.youtube.com/watch?v=SDU7fckXUYI

   Wearable sensors called IMUs (Inertial Measurement Units) are an easy and popular way to measure how people move. An IMU has accelerometers and gyroscopes that detect how the body moves, tilts, and changes speed. When worn on the body - like on the lower back, ankle, or wrist - IMUs can record data during activities such as standing still to test balance or walking to study gait. This data can then be used to calculate things like step timing, stride length, sway while standing, and walking speed. IMUs are small, light, and can connect to phones or computers, making it possible to study movement in real-world settings outside of a lab.

   

   https://www.youtube.com/watch?v=nGW3Pteu0uM

   Pressure-sensitive mats/plates measure how pressure is spread across the bottom of the foot. They use many small sensors, and having more sensors gives greater detail, which can even show pressure under a single toe. Unlike force plates, which measure overall forces and moments, pressure mats create a map showing where the foot presses the most, the contact area, and how pressure changes over time. They are often used in labs as instrumented walkways to record multiple steps, measuring things like foot pressure, stride, and step width. Pressure mats are useful for studying footwear, checking the effects of nerve problems, or analyzing sports performance.

   

   https://www.youtube.com/watch?v=arDZ7C3g-b4

   To study gait in more detail, researchers can combine kinetics, like ground reaction forces measured by a force plate, with kinematics, such as body angles measured by motion capture. This lets them calculate joint movements, power, and forces during activities like walking or jumping. By combining motion capture with pressure mats, researchers can track joint angles, limb movements, and foot pressure at the same time. This shows where and when pressure is applied relative to foot position and is helpful for analyzing multiple steps or larger areas. Using both force plates and pressure mats can also help with injury prevention, improving performance, and rehabilitation. Check out this video to see how baseball pitchers can use a combination of kinetics and kinematics to improve their pitch.

   

   https://www.youtube.com/watch?v=S7eF_uuDIM4

2. Clinical Functional Assessments

   Clinical functional movement assessments are standardized tests used by researchers, clinicians, and rehabilitation specialists to see how well a person moves in everyday situations. These tests are often used when a lab is not available or practical. These tests measure things like balance, stability, walking ability, and the risk of falling. They also show how different body systems - muscles, bones, and nerves - work together during movement. These assessments help connect movement patterns to performance, injury prevention, and recovery.

   The Timed Up and Go (TUG) test is a fast and common way to measure how well someone moves and their risk of falling. In this test, a person starts seated in a chair, stands up when told, walks three meters (about 10 feet), turns around, walks back, and sits down again. The total time it takes is recorded. While it is often used with older adults to check fall risk, the TUG test also shows lower-body strength, coordination, balance, and how easily a person can move between actions. Clinicians and researchers can also look at things like stride length, walking speed, and turning to understand body control and balance.

   

   https://www.youtube.com/watch?v=tNay64Mab78

   The Berg Balance Scale (BBS) measures how well a person can keep their balance during everyday activities. It includes 14 tasks, like standing on one leg, reaching forward, or moving from sitting to standing. Each task is scored from 0 to 4, with higher scores showing better balance. The BBS is used to find people who might be at risk of falling, especially older adults or those with neurological conditions. From a biomechanics view, the BBS looks at both standing still and moving balance, helping researchers connect lab measurements - like how the body sways or where the feet are placed - with real-life movement abilities.

   

   https://www.youtube.com/watch?v=mqr6nUb9X4k

   The Dynamic Gait Index (DGI) measures how well someone can adjust their walking when the environment changes. It has eight walking tasks, like changing speed, turning the head, stepping over obstacles, or pivoting quickly. Each task is scored based on balance, smoothness of walking, and safety. The DGI helps show a person’s dynamic balance and how well they can keep steady while walking and multitasking. Researchers and biomechanists often use motion capture or wearable sensors during the DGI to study how steps, coordination, and movement patterns change under challenging walking conditions.

   

   https://www.youtube.com/watch?v=eKDQEu5QOsc

   The Mini-BESTest (Mini Balance Evaluation Systems Test) is a shorter version of the full BESTest, designed to efficiently assess dynamic balance as well as being used as a diagnostic tool. It looks at four main systems that help control balance: anticipatory postural adjustments, reactive postural control, sensory orientation, and dynamic gait. The assessment includes 14 simple tasks, such as rising onto your toes, taking a quick step to catch your balance, and walking while turning your head. Each task is scored from 0 to 2, with higher scores showing better balance. The Mini-BESTest is helpful in clinics and research because it gives detailed information without taking much time. It allows researchers to see how people adjust their movements when something challenges their balance or when their senses change, making it easier to spot problems in how they control their posture.

   

   https://www.youtube.com/watch?v=jK43dcXrhWM

3. Standardized Conditions and Number of Trials

   In human biomechanics research, it is important to use the same testing conditions and repeat each test several times. This helps make sure the results are accurate, reliable, and easy to compare across different people and studies. Keeping conditions the same - like lighting, flooring, shoes, equipment setup, and instructions - helps reduce outside factors that might affect how someone moves or how the measurements are recorded. Doing multiple trials also helps researchers account for normal differences in movement, learning from practice, and random errors, which leads to more stable averages and clearer data.

   To make a study more reliable, researchers often ask participants to repeat a movement several times. These repeated attempts are called trials. People do not move exactly the same way every time, even during simple tasks like walking or standing up, so multiple trials help researchers see the real pattern of movement. The number of trials needed can change depending on what is being measured. For example, a short walk or a standing balance test might only need 3–5 trials per person, while more complex movements, like jumping, might need 5–10 trials to capture the larger differences. Researchers then look at all the results and calculate an average.

   In human biomechanics studies, the number of people needed depends on what the researchers want to measure and how much people naturally differ from one another. This number is called the sample size. Small studies, like pilot studies, may only include 5–10 participants. Most biomechanics studies need about 15–30 participants to find meaningful differences. Larger studies, especially those with more complicated comparisons, may need more than 50 participants to make sure the results are scientifically reliable.