During our daily lives, our bodies are met with a multitude of challenges to overcome. The most frequent of which is the challenge of keeping the body upright in perfect equilibrium. In my previous article The Three Bodily Balance Systems, I discussed how the visual, vestibular, and somatosensory systems adapt and react constantly to stimuli, in conjunction with the brain, to control our balance. Today, I am taking those ideas a step further, and discussing the motor strategies we use to maintain balance in static stance.

Before we begin, I want to delve into a few definitions so we are all on the same page.

Helpful Definitions

Equilibrium: A state in which opposing forces or influences are balanced.

Perturbation: Any force, internal or external to the body, that disrupts the body’s equilibrium and requires a counter-force to re-establish said equilibrium.

Base of Support (BOS): The perimeter of the contact area between the body and its support surfaces. Can be made up of the feet, hands, or any part of the body in contact with the ground.

Limits of Stability (LOS) (What I refer to as the “Base of Support Circle”* to patients): The area around the base of support, that a person can sway within without losing their balance. Loss of balance will occur when swaying outside of the LOS.

*I refer to this as the “Base of Support Circle” to patients to keep language simple and uniform throughout our conversation. Limits of Stability is the correct term when discussing on academic levels. You can learn more about this in my Base of Support Model video.

Center of Mass (COM): The point of an object which corresponds to the center of the total body mass, and when placed over the center of the BOS, allows the object to be in perfect equilibrium.

Intro to Motor Strategies for Balance Control

When it comes to static balance, there are approximately five different strategies we use to maintain our equilibrium. These strategies include ankle strategies, hip strategies, stepping strategies, weight-shift strategies, and the suspension strategy. The strategy used depends on a few variables including: current base of support, type of support surface, magnitude and speed of the perturbation, where the perturbation occurs in relation to the body, the subjects awareness of the perturbation, posture at time of the perturbation, and a person’s prior experience with this perturbation. Let us dive into the differing strategies, when they occur, and what muscles are used to counteract perturbations.

Ankle Strategies

Ankle strategies are used primarily in the sagittal plane, and occur secondary to low amplitude perturbations that cause the body to move forward or backward. Once the body is perturbed by a low amplitude force, the body reacts by activating musculature in a distal to proximal fashion, in order to re-establish equilibrium over the existing BOS. Muscular activation patterns are found below:

Forward Perturbation: Gastrocnemius —> Hamstrings —> Paraspinals

Backward Perturbation: Anterior Tibialis —> Quadriceps —> Abdominals

Hip Strategies

Hip strategies can be thought of as the second line of defense if ankle strategies are not enough to maintain equilibrium. These strategies occur when our COG is near the LOS, and we sustain a large or high amplitude perturbation. Unlike the ankle strategies, these strategies employ a proximal to distal muscle activation pattern. Muscular activation patterns are found below:

Forward Perturbation: Abdominals —> Quadriceps

Backward Perturbation: Paraspinals —> Hamstrings

Sideways Perturbation: Contralateral Lateral Trunk Flexors —> Contralateral Hip Abductors (Gluteus Medius)

Stepping Strategies

Stepping strategies are our last line of defense when it comes to preventing a fall. Stepping strategies are used when a large and high amplitude perturbation, moves the COM beyond the LOS. When this occurs, a step in the direction opposite of the perturbing force, is utilized to widen and regain our BOS. For more on stepping strategies check out my Stepping Strategies Tutorial.

Weight Shift Strategies

Weight shift strategies are sometimes lumped in with ankle or hip strategies; they are gray area strategies that encompass elements of both in the frontal plane. These strategies are used to counteract laterally directed perturbations, of small amplitude. During these strategies, a person shifts their body weight laterally from one foot to the other. Although hip abductors/adductors and ankle invertors/evertors are used to perform these strategies, most of the work is done at the hips to pull the COM in the appropriate direction. The goal is to regain equilibrium by bringing the COM over the contralateral leg, opposite of where the perturbation occurred.

Suspension Strategy

The suspension strategy is the least known and most uncommon strategy when looked at in static standing. These strategies are used when a person quickly lowers their COM, leading to subsequent hip flexion, knee flexion, and ankle dorsiflexion. Although this suspension strategy is seen a lot in dynamic activities, it is seldom seen statically. One place where this strategy does occur statically, is during the initial ascent in an elevator, as the body reacts to a superiorly directed force of the floor against the body. During this momentary mismatch of elevator and body velocities, the lower extremities act as a spring to absorb the superiorly directed perturbation and maintain balance. The descent in an elevator would do the opposite, essentially lengthening the metaphorical lower extremity spring.

Conclusion

Although all of these strategies are innate to the human body, we can improve upon them by working in situations which challenge each muscular activation pattern. The learning adaptation that occurs subsequently, will significantly reduce the bodies reactivity to perturbations. Such reduced reactivity can be seen in extreme situations like learning to slack-line. During a person’s first trials with a slack-line, the body moves about dramatically with large body movements. As a person learns to stabilize the slack line, the body also learns how to become less reactive, and create intentional low amplitude movements to maintain equilibrium. Finally, although these strategies are split up for academic purposes, they often will occur together or in sequence. For best understanding, I suggest grabbing a friend and playing around with different forces and angles of pushes to see what muscles activate and when.

Thanks for reading! Have you ever explained these strategies to a patient? Did you know about the weight shift and suspension strategies? Need a deeper explanation on this topic? Comment below and let me know.

Happy Falling!