The Short Version:
Sway and Control are the two main variables measured during the Balance Scan and represent one's ability to balance in an upright stance. These two qualities are represented in all of human movement. A higher Sway and Control for an individual reflects more efficient movement and better Stability.
Sway measures the ability to remain steady and minimize motion while balancing.
Control measures the ability to make complex adjustments to maintain stability in unpredictable conditions.
Collectively, Sway and Control create an individuals unique Balance Signature™, which is the visual representation of their unique balance capabilities. Sway and Control also combine into an individual's Stability Score which is a single measure of relative balance capability.
The Long Version:
Though the Sparta Movement Health Platform calculates many individual features for balance (> 40), two key metrics are chosen that concisely characterize a person’s balance capabilities. Sway and Control are the two main variables measured during the Balance Scan and represent one's ability to balance in an upright stance. These two qualities are represented in all of human movement. A higher Sway and Control for an individual reflects more efficient movement and better Stability.
Sway, Control, and Symmetry
SWAY - How well are you able to remain steady while balancing?
What is Sway?
The Sway score is based on the ‘average postural sway velocity’ which is the average of the instantaneous COP velocity magnitude over a particular period of time. Velocity is converted to Sway score using the statistical properties of the population.
Higher sway velocities (lower Sway scores) correlate with poorer balance and increased fall risk. High Sway most closely matches our intuitive understanding of good balance as the ability to remain steady and minimize motion.
CONTROL - How well are you able to maintain stability in unpredictable conditions?
What is Control?
Measurement of the ‘complexity’ of dynamic physiological signals is an increasingly important assessment tool [2, 3]. Heart rate variability (HRV) is familiar to many people, but other signals like respiration and EEG are also studied. In general, high complexity of response is associated with better health. Signal complexity is typically measured through sample entropy calculations. Control score is based on 'multivariate multiscale entropy in x-y plane.' To calculate we perform a multivariate, multi-scale sample entropy calculation using the COPx, & COPy signals for a given period of time.
Higher Control implies a more well adapted balance control capability. The intuition for Control is that higher complexity of response implies better ability to make fine adjustments at different timescales in order to maintain stability in the face of unpredictable destabilizing perturbations .
In addition to the primary value provided for Sway and Control (the average value across left and right sides of the body), a Symmetry score is produced that measures relative left-right symmetry (where a value of 100% is max symmetry and 0% is minimum).
Small variations in symmetry are common in people subjected to repeated testing and are typically not significant. A symmetry value for Sway or Control of a little over 80% is average across our database, with two-thirds of our users falling between 72% and 95% symmetry for Sway and 69%-95% symmetry for control. Also, some people may exhibit low symmetry due to known non-correctable issues such as pre-existing injuries. There are cases, however, where subjects are unknowingly compensating with a better side, and where low symmetry may reveal opportunities to improve overall balance capabilities. Context knowledge of the subject is therefore important in using symmetry information.
Assessing Balance using Sway and Control
A deficit in either Sway or Control is a potential cause of concern and motivation for balance improvement exercises. Underlying conditions that adversely affect balance can manifest differently in terms of reductions in Sway and Control, e.g.:
- Musculoskeletal issues such as muscle weakness, inflexibility, or joint injuries are most likely to impact Sway more than Control. This is likewise true of vestibular system issues.
- While brain injuries such as concussion have been shown to impact Sway, recent research has shown reduction in entropy measures (like Control) to be a better predictor of the presence of traumatic brain injury (TBI) 
The combination of Sway & Control provides a much better ability to differentiate people’s balance challenges and to stratify risk than approaches that rely solely on sway velocity. As ever greater number of assessments are captured and outcomes are tracked, ever improving models will arrive that make more specific predictions, such as:
- Relative fall risk
- General injury risk
- TBI severity
- Neurological disorder onset
The Stability Score provides a convenient single metric to assess balance capability. Based on combining Sway and Control, this metric captures multiple aspects of balance. A relative Stability Score provides a quick screening tool to identify potential balance issues for further investigation.
Improving Balance using Metric Insights
Some aspects of balance improvement guidance are common sense:
- Better general fitness and muscle tone improve balance
- Repetitive practice of response to balance challenges improves balance
Targeting an individual’s specific balance improvement needs based on their balance assessment and Sway and Control scores, offers a more efficient path to balance improvement.
As mentioned previously, balance is a complex activity that involves many physiological systems. A simplified 3 component electro-mechanical system analogy can be helpful in understanding balance response and paths to improvement:
- the mechanism: The musculoskeletal system that is responsible for interacting with the ground and repositioning of mass.
- the sensors: vision, vestibular, and tactile systems that provide the signal feedback about the state of the body over time.
- the controller: the brain and spinal cord that process feedback and guide responses.
Balance improvement, then, can be characterized in term of these different components , e.g.:
- mechanism improvement: Improvements in mass distribution, muscle tone, core strength, flexibility, muscle actuation, etc. can significantly affect how efficiently the body can transmit force or reposition mass.
- sensor improvement: Improving the quality and quantity of feedback signal data, greatly improves the ability of the controller to efficiently do its job. Likewise any sensor deficit can handicap control.
- controller improvement: Sending more optimal commands to the mechanism is a guaranteed path to balance response improvement. In this case, the controller is executing multiple strategies in parallel for managing coarse and fine-grained responses.
As more data is collected, score-based guidance will become more refined. For now, here is the high-level guidance:
Sway can be enhanced by improving the mechanism of response (better force transmission and mass repositioning). Example exercises types include:
- Weight reduction exercises
- Core Strength Improvement
- Flexibility Improvement
Sway can be enhanced by improving coarse-grained control over balance response Examples exercises include:
- Simple balance practice (e.g. knee lift, beams, ..)
Control can be enhanced through repetitive practice at Sway boundary exploration . E.g. balancing in the face of the following:
- Distracting tasks
- Unexpected perturbations
Role of Symmetry in Guidance
A low symmetry that is deemed correctable can be addressed by choosing exercises that ensure that each side is equally worked - this can avoid the tendency for subjects to compensate with a more developed side over the other.
- Chen,B.;Liu,P.;Xiao,F.; Liu, Z.; Wang, Y. Review of the Upright Balance Assessment Based on the Force Plate. Int. J. Environ. Res. Public Health 2021, 18, 2696. https:// doi.org/10.3390/ijerph18052696
- Stergiou, N. (2016). Nonlinear Analysis for Human Movement Variability (1st ed.). CRC Press. https://doi.org/10.1201/9781315370651
- Costa M, Goldberger AL, Peng CK. Multiscale entropy analysis of complex physiologic time series. Phys Rev Lett. 2002 Aug 5;89(6):068102. doi: 10.1103/PhysRevLett.89.068102. Epub 2002 Jul 19. PMID: 12190613.
- Manor B, Costa MD, Hu K, et al. Physiological complexity and system adaptability: evidence from postural control dynamics of older adults. J Appl Physiol (1985). 2010;109(6):1786-1791. doi:10.1152/japplphysiol.00390.2010
- Purkayastha S, Adair H, Woodruff A, Ryan LJ, Williams B, James E, Bell KR. Balance Testing Following Concussion: Postural Sway versus Complexity Index. PM R. 2019 Nov;11(11):1184-1192. doi: 10.1002/pmrj.12129. Epub 2019 Apr 3. PMID: 30729729.