Tag: balance

Slipping or Tripping? Researchers Find Best Way to Regain Your Balance

Everyone has slipped or tripped at some point in their lives. Whether it is walking on an icy road to get to your car or tripping over the Lego set your kid refused to put away, everyday obstacles can cause us to lose our balance. Often this results in a brief moment of panic followed by the uneasy relief of regaining your footing, but for those who aren’t lucky enough to avoid falling, the results can be devastating. This is especially prevalent in populations more susceptible to falling. Falling in the workplace accounts for 16.8% of all non-fatal injuries leading to days taken off work. It is thought that this is due to the high volume of slipping or tripping obstacles encountered in some occupations. Additionally, 36 million falls resulting in 32,000 deaths were reported for the 65+ year old population of the US (Bruijn et. al 2022). Elderly individuals may lack the strength and reflexes necessary to recover their balance quickly. This is especially worrisome because the elderly are also the most at risk for the major health complications that can be caused by fall related injuries.

While it may not always be possible to avoid the obstacles that cause us to slip or trip, it may be possible to decrease your likelihood of falling as a result. Biomechanics researchers Daniel Marigold, Sjoerd Bruijn, and Rakié Cham attempt to determine what human responses allow us to recover our balance after slipping or tripping. To do this, these researchers subjected volunteers to a slip or trip course where they would be caused to periodically slip or trip. When the subject slipped or tripped, the researchers would use a special camera to map out the subject’s movements throughout the response and sensors placed on the subject and course to determine the muscles and forces acting throughout. As a result, the researchers were able to find various patterns that may explain how some subjects were better able to recover their balance.

Example Testing Course Diagram Showing Cameras and Sensors (Marigold 2003)

When discussing slip and trip responses, researchers distinguish between a subject’s legs by defining the leg that first encounters the slippery surface or hits the tripping obstacle as the “perturbed limb.” On the other hand, the leg that does not encounter the obstacle first is called the “unperturbed limb.” 

Though each researcher devoted their attention to a different limb response, there were similar patterns seen throughout all of the research. Firstly, it was found that the unperturbed limb is used within slipping or tripping primarily to catch the body. For example, under slipping, the unperturbed limb would help catch the body by touching the ground briefly near the perturbed limb before taking its subsequent step. Similarly, when tripping on an obstacle, the unperturbed limb would step over the obstacle to catch the subject. Additionally, it was found that subjects were able to recover better when their arms were extended as opposed to abruptly brought into the body. Researchers struggled to come to a consensus on why this is the case, but Marigold believed this was the body’s way of balancing its offset mass (like when someone extends their arms while walking on a balance beam). On the other hand, Bruijn thought that it was important to keep one’s arms extended and moving to delay the transfer of their momentum (arm mass movement) to the rest of the body, like when a spinning figure skater keeps their arms extended to spin slowly. Finally, it was found that the perturbed limb responded by attempting to pull back towards the neutral standing position. When you slip, your perturbed limb tends to slide forward and away from your body, so in response, the muscles in this limb try to pull it backwards and towards your body.

So your body responds by catching itself with the unperturbed limb, extending your arms, and pulling the perturbed limb closer to your body, but what makes some people more successful at this than others? Researchers emphasize acting quickly but not abruptly. Though reaction time is important, subjects who panicked would rush the stabilizing step with their unperturbed limb, pull their arms in suddenly, and pull their perturbed limb inwards too harshly. It seems that the best thing to do when slipping or tripping is to remain calm and allow your body to react at its own measured pace.


Bruijn, Sjoerd M., et al. “Contribution of Arm Movements to Balance Recovery after Tripping in Older Adults.” Journal of Biomechanics, vol. 133, Mar. 2022, p. 110981. ScienceDirect, https://doi.org/10.1016/j.jbiomech.2022.110981.

Cham, Rakié, and Mark S. Redfern. “Lower Extremity Corrective Reactions to Slip Events.” Journal of Biomechanics, vol. 34, no. 11, Nov. 2001, pp. 1439–45. ScienceDirect, https://doi.org/10.1016/S0021-9290(01)00116-6.

Marigold, Daniel S., et al. “Role of the Unperturbed Limb and Arms in the Reactive Recovery Response to an Unexpected Slip During Locomotion.” Journal of Neurophysiology, vol. 89, no. 4, Apr. 2003, pp. 1727–37. journals.physiology.org (Atypon), https://doi.org/10.1152/jn.00683.2002.

Slip, Trip, and Fall Prevention – Tokio Marine America – Property and Casualty Insurance Provider. https://tmamerica.com/insights/blog/Loss-Control-Slip-Trip-Fall.aspx. Accessed 4 May 2022.

Stein, Jeff. “Man Slipping.” Pixabay, 23 Jan. 2020, https://pixabay.com/vectors/man-slipping-4785896/. —. “Man Tripping on Debris.” Pixabay, 23 Jan. 2020, https://pixabay.com/vectors/man-tripping-on-debris-4785967/.

Living Off Balance

Person walking in woods, balancing on a fallen tree

Imagine yourself walking at a normal pace down the sidewalk. Maybe you are on your way to class. The sidewalk has a little bit of a tilt causing your left foot to be higher than the right as it plants on the ground. Imagine how your body may compensate after a few minutes of walking on this path. We have all walked on uneven ground and began to feel the effects with sore knees or hips. But what if you felt this same way all the time even on perfectly flat terrain? This is the reality for those with leg length discrepancies.

Leg Length Discrepancy

A leg length discrepancy (LLD) is any difference in your legs compared to one another. This can be as small as a few millimeters or as large as a few centimeters. Leg length discrepancies can be caused by a number of things including genetics, trauma, or disease. Leg length discrepancies can be categorized in two ways; real and apparent LLD. Real leg length discrepancies are one in which the bony structures are measured to be two different lengths. Apparent leg length discrepancies are caused by other factors such as muscle or joint tightness making the limbs appear two different lengths.

Image depicting pelvic tilt when leg length discrepancy is present
Pelvic tilt caused by real leg length discrepancy

Hopping Along

The actual significance of a LLD on posture and gait depends heavily on the magnitude of the discrepancy. It is highly debated by researchers if a LLD of less than 2-3cm has physical effects on the body and if symptoms a patient is experiencing are due to another cause. R.K. Mahar and R.L. Kirby at Dalhousie University performed a study in which people without a LLD, asking them to stand on blocks simulating a real leg length discrepancy, the researchers saw a misalignment of the hips, an increase in knee flexion and a shift in the center of gravity.

In contrast D.C. Reid conducted a study for those with actual LLD and many did not complain of pain or feeling off balance and chose to not use corrective devices. The body is able to compensate for the difference over time to minimize the displacement of the center of mass of the body. It was also seen in a study done by Gross that athletes are more likely to correct smaller LLD than the average person due to the increased loads experienced during their activity.

Lift is placed in the sole of the shoe to correct moderate LLD
Shoe lift place in sole used to correct LLD

Fix it

For people that are experiencing pain because of the difference there are several ways to reduce the pain. For small discrepancies (less than 1cm) inserts can be placed into the shoe to even out the hips. For differences between 1cm and about 5cm a lift can be placed in the sole of the shoe for the same reason as the inserts. For some special cases or discrepancies larger than 5cm corrective surgery to lengthen or shorten the limb can be performed, but this is often used as a last resort.

Put One Foot in Front of the Other? It’s Not that Easy

From Christmas movies to pop songs to motivational posters, we are encouraged to keep putting “one foot in front of the other.” While the sentiment is inspiring, recent studies show that there is a lot more to the seemingly simple task of walking than this phrase would suggest. Understanding this is especially important for balance and mobility after an injury or as people age.

The steps that make up the human walking cycle. Order of steps: heel-strike right, toe-off left, midstance right, heel-strike left, toe-off right, midstance left, hell-strike right. The body spends the time between heel-strike and toe-off with double support and the midstances are single-leg support.

Image from Wikimedia Commons

The human gait has a set structure that switches the weight between each leg, with only 20% of the typical walking motion distributing the weight across both feet. Maintaining balance throughout this process requires coordination in the muscles controlling the hips, knees, ankles, and feet. Mechanically, these adjustments keep the body’s center of mass (also known as center of gravity) over the base formed by feet positioning.

Obstacles and challenges to balance require a body’s quick response to mitigate shifts in the acceleration and momentum at the center of mass. Lack of efficient control over these parameters results in a fall. Many conditions, as well as age, can affect a person’s ability to respond to mobility challenges.

One specific study looked at how people who had had a stroke and subsequent partial paralysis on one side (paresis) faced mobility challenges compared with healthy folks. This condition effects approximately 400,000-500,000 people in the United States annually. It presents a unique opportunity to compare an individual’s non-damaged stride with their deficient stride at the point in the gait at which only one leg is on the ground (SLS, or single-leg-stride). The timing of the gait, the body’s momentum in all three planes of the body, and the location of the center of mass were recorded in this study.

Anatomical planes of the body. The sagittal plane splits the body left and right. The coronal plane splits the body forward and back. The transverse plane splits the body top and bottom.
Image from Wikimedia Commons

Versus healthy people, stroke survivors had significant trouble regulating momentum in the coronal plane, making falls more likely. Although it makes sense that momentum regulation suffers when muscles are paretic, it is yet unclear why the coronal plane was most affected. Additionally, post-stroke individuals’ centers of gravity were higher, which is also linked to instability. For stroke survivors, the partially paralyzed SLS took longer and extended farther from the center of mass than the regular SLS. While this is not as immediately dangerous as increasing falling risk, it slows mobility, unevenly works muscles (which can lead to injury), and is less efficient.

Going forward, these findings can be used to improve mobility success in people with balance issues or after injuries. This could manifest in better technologies, such as walkers that better help settle a person’s center of mass and partial exoskeletons that would help a person mitigate acceleration and momentum changes, or more targeted and individualistic physical therapies to strengthen weakened muscles and practice patient-specific challenges, such as overcoming obstacles that threaten coronal-plane balance. Understanding more about balance adjustment when walking may make some common phrases trite, but its potential benefits have life-changing impacts for many.

Further Reading and Sources:

Stroke/Paresis Information

Stability of Stepping

Secret Behind Kangaroos’ Tail

Red kangaroos can reach speed of more than 35 miles an hour, they can also cover an area 25 feet long and get up to 6 feet high in one jump using their tail like a spring to give them more power. When kangaroos want to move slowly, they do kind of lean on their tail, to support their

Schematic representation of the tail involved in accelerating. Photograph: Heather More (theguardian.com)

body. When kangaroos are grazing they move their hind pairs of feet together which makes their movement awkward but the power behind them in their tail is keeping them balanced.There was always a question of why Kangaroos are placing their tail on the ground when they are walking slowly.

Most of the researchers believed that the tail is only used for the purpose of balancing. Professor Max Donelan from Simon Fraser University, collaborated with his colleagues Shawn M. O’Connor, Terence J. Dawson, Rodger Kram trained kangaroos to walk on a measuring device called the force plate, what they found was that the tail was doing a lot more than anyone have realized.  They Found that kangaroos actually used their tail like a fifth leg when they are hopping around or walking. For this study, they documented the movement of five red kangaroos in Sydney Australia which are the largest species of kangaroo and the biggest marsupial on the planet. They observed that kangaroos when walking first put their forelimbs on the ground and when it is the time for their hind limbs to move forward, they use their tail to accelerate and push the whole body forward and then they put their hind limbs on the ground.

They have published a paper in Biology Letters which presents that the tail exerted as much force as four other legs combined. By measuring the commonly work in physics called the mechanical force, the kangaroos tail is as important when it walks as one of our legs as we walk. They found that the kangaroos’ tails are involved on their movement in three ways. First of all, most of the propulsive force which is needed for the movement is provided by the tail. Furthermore, the previous belief that the tail is needed to balance the body weight have been examined and turned out to be that although the tail plays an important role in the balancing, it only provides the 13% of the vertical force needed to balance the body. Besides, investigating on the mechanical work that the tail applies to the whole body for pushing forward, it demonstrates a substantial role of the tail in performing positive mechanical work.

The mechanical representation of the Kangaroo’s movement on the force plate (Shawn M. O’Connor et. al. 2014)

Human’s back leg helps to push the body forward when walking (wikihow.fitness)


In simple words, it can be compared with the role of one of human’s leg when walking. You probably are thinking what exactly makes a leg a leg? The answer could be simple, if a leg exists to play a key role in walking, then kangaroo has five legs.

Kangaroos are the only animals that use their tails as a leg, Max Donelan said.


Continue reading “Secret Behind Kangaroos’ Tail”

How do Flamingos Stand on One Leg?

How long can you stand up before you get tired?

This is an important question for animals that sleep standing up, like horses and flamingos.  Our joints are stabilized by muscles, but the constant activation of muscles needed to maintain balance requires energy and induces fatigue.

flamingo standing on one leg while grooming
Photo by Lieselot. Dalle on Unsplash

Flamingos are especially perplexing because they often sleep on only one leg. This requires that single leg to support the entire weight of the animal and maintain balance. Researchers think that this is beneficial because it allows them to switch legs when one gets tired. But does that benefit outweigh the cost of maintaining balance on a single leg?

Researchers Young-Hui Chang at Georgia Tech and Lena Ting at Emory investigated this question in a recent paper by examining the muscle forces required to support body weight and maintain balance in flamingos standing on one leg.

Using dead flamingos (that can’t generate active muscle forces), the researchers clamped one leg and tilted the cadavers forward and backward (video).   They found that the leg remained straight even after rotating it more than 45 degrees in each direction. This only happens when the bird’s foot is right underneath its body, not when it’s off center (like it is when standing on two legs).

This is remarkable, because flamingos’ femurs (the large bone in our thighs) are horizontal. Essentially, a standing flamingo is in a position similar to a human doing a squat! The researchers think that the bird’s bodyweight generates passive joint moments around the hip and knee, keeping the joints into a fixed position in order to support the weight of their body. A similar arrangement, called a stay apparatus, is found in horses for the same purpose, and bat fingers contain a similar lock that helps them stay hanging for long periods of time.

In a second experiment involving live baby flamingos, the researchers used a force plate to measure the center of pressure in their feet as they stood on one leg. (To feel this center of pressure, stand on one leg and feel different parts of your foot press into the ground as you try to keep your balance.)

baby flamingo standing on one leg, with diagrams showing force plate readings
modified from Chang & Ting, Biology Letters 2017

While they were awake and active, the center of pressure moved a lot, but when they rested or fell asleep, they were remarkably stable. This led the researchers to suggest that the birds may have a way to balance without active muscle forces as well, although they do need to work actively to keep their balance when being active, like while grooming.

Flamingos, with their big bodies and long, slender legs, resemble an inverted pendulum. Inverted pendulums are a classic example of an unstable system, which will fall over without active control. But flamingos manage to stay upright for long stretches of time – and if we can figure out how, we might be able to bring stability to other unstable systems! This could be helpful as we try to make robots who can walk on uneven surfaces – and they need all the help they can get with that: