Returning to Play: How to Objectively Assess Stability

July 6, 2015

stabilityStability can be defined as resistance to change. For athletes, particularly small sudden changes require more sensitivity than the human eye. Stability is often separated into static (standing) and dynamic (maintaining a stable position while undertaking a prescribed movement). In addition, most research has identified these as separate qualities with little correlation. Stability is an important characteristic, especially when identifying joint health in return to play (RTP) athletes. But static stability can only tell us so much, especially when used to evaluate a population defined by their dynamic abilities...athletes.

Evaluating static stability has one major advantage; it is the only assessment that can be performed early in return to play when dynamic movement is prohibited (early post surgery, concussion, etc.). A static stability assessment is usually done by the sports medicine team (of our software partners) as both baseline for incoming physicals, and subsequent benchmarks when the normal scan (vertical jumps) is unable to be performed.

In the case of static stability, we are measuring the center of pressure (COP) during various standing conditions such as single or double limb stance by using a force plate. A force plate provides accurate information about postural control through calculation of COP, or the point of force application, distributed under the athletes' feet in all directions (left-right, forward-back). The Balance Profile™ evaluates athletes on the scale below, comparing both the stability (greater overall value) versus mobility (lower value), as well as the left limb (59) to the right limb (55). Being either too stiff, or too mobile can put an athlete at risk for injury similar to having too great of a discrepancy between limbs.

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When assessing dynamic stability, we use a similar protocol, measuring the time required to achieve a reasonable COP upon landing from a 1 leg jump (a prescribed movement). Typically, researchers will use a similar protocol, but instead measure the height of a box (that the athlete steps down from) and/or distance they cover.  We have designed the Landing Profile™ which looks at the peak impact forces (as opposed to heights or distances traveled), preventing the athlete from "beating the test". Using peak force rewards hopping onto the plate rather than merely stepping because athletes who generate greater forces upon landing (yet similar COP as lesser forces) will produce better quantitative results (a greater value).

The stabilization assessments provide other variables such as anterior-posterior (weight shifting to the toes) and medial-lateral, which tell a more complete picture. Ultimately we are all comforted by additional data to point to in case of negative outcomes. Like the Sparta signature™, we are focusing on the most reliable variables, as validity (meaningfulness to injury) is worthless without sufficient data consistency. The other advantage of using these peak forces is that they are resultant— a combination of all 3 planes of movement — which includes all of these smaller joint perturbations without jeopardizing the reliability of the overall metric.

In the video below, linebacker James Vaughters completes one of our dynamic stability scans while preparing for the NFL Draft.

As always, the most important thing to consider is context; how to use the data. Like we do with vertical jump, we have built a database of stability scans, so we know when specific values should cause action. Our rapidly growing database is built on T score values, where a proximity of within 10 to certain values is predictive of injury or re-injury. Predictions are also based on other objective risk factors such as gender, sport/position, medical history. Using technology integration built by our engineers we're able to use the same assessment to evaluate female soccer players with prior injury history and male football players with no injury history, and provide different alerts to coaches based on the data.

These stability assessments are used as a baseline and subsequently a Pass/Fail RTP protocol. These objective assessments are unique because not only are they reliable, but they are practical, taking less than 5 minutes to complete.

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