Lately, a common question we have been getting from our software patterns is, why not perform a unilateral test on the force plate? Athletes use (and we prescribe) unilateral movements every day to improve performance, why not use a single-leg test?
The answer is simple, we've tried. We performed thousands of dynamic unilateral testing trials in various planes for almost 2 years ('08-'10). Statisticians combed through the results, examining different genders and sports only to find… poor test-retest reliability. So we decided to focus on the only test (and specific variables from that test) which had high reliability regardless of which day of the week or what time of day it was. In addition, we refined our test protocol until it was a devoid of any learning effect or favored jumping ability.
The major reason for interest in unilateral tests is that people believe that single leg evaluations are better indicators of potential injuries and return to play (after injury). While these are excellent goal for a performance test, we could not tolerate the unreliability of a dynamic single leg test and let it dictate our actions. So we statistically addressed this goal in two ways.
We studied Sparta Signature trajectories which tracked specific training plans that emphasized unilateral strength/stability (i.e. Split Squat vs Squat) with specific force plate variable improvements (correlated by statisticians at Stanford Medical School).
We tracked force plate variables from vertical jump tests (correlated by BYU) with the most reliable measurements of health, how much you play in a season!
Both of these investigations led us to the same answer; the test that our athletes are performing is both reliable and valid, the two keys to a successful test. To abandon this test for a unilateral test which is more unreliable would be foolish. When people inquire about a unilateral test, what they are really after is a test which will tell them if they should prescribe more single leg training movements. Single leg movements can address strength and injury prevention needs that bilateral movements cannot. If you develop and implement a test that can identify those weaknesses, then you can be sure that single leg movements are the correct prescription.