Tag Archives: balance

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: