Subjects
All subjects were recruited from the student populations at two public universities through research study advertisements. Eighteen males (age: 22.4 ± 5.8 years, height: 179.7 ± 6.8 cm, mass: 84.2 ± 14.9 kg) and fourteen females (age: 20.1 ± 1.9 years, height: 166.1 ± 6.5 cm, mass: 65.6 ± 7.6 kg) with self-reported CAI participated in the study. Inclusion criteria was a history of more than one ankle sprain and self-reported symptoms of disability due to ankle sprains qualified by a score of ≤ 90% on the Foot and Ankle Disability Index (FADI) and ≤ 75% FADI Sport surveys.[18] All CAI subjects had no history of lower extremity injury, including ankle sprain within the past six weeks and no history of lower extremity surgery, balance disorders, neuropathies, diabetes, or other conditions known to affect balance. If a subject reported bilateral ankle instability, the more unstable ankle as reported by the subject was used for analysis. Only the affected or self-reported worse limb of the CAI subjects was tested. CAI subjects reported a mean of 7.8 ± 5.7 ankle sprains with a mean of 10.3 ± 16.4 months since last significant ankle sprain and had a mean FADI score of 86.8 ± 8.2% and a FADI Sport score of 66.5 ± 16.4%.
Eighteen healthy males (age: 23.7 ± 4.6 years, height: 175.6 ± 7.2 cm, mass: 75.4 ± 9.3) and fourteen healthy females (age: 20.8 ± 1.1 years, height: 163.6 ± 6.2 cm, mass: 63.6 ± 10.1 kg) were gender and side matched to the CAI subjects for comparisons. All healthy subjects reported no lower extremity injury in the past year, no history of injury or illness known to affect balance, and no self-reported disability associated with the foot or ankle (100% on the FADI and FADI Sport). Prior to testing, all subjects signed an informed consent form approved by the Institutional Review Board at our institution.
Procedures
Subjects performed three trials of barefoot single limb stance on each leg with eyes open and closed on a forceplate (Accusway Plus, AMTI, Watertown, MA) for 10 seconds. Subjects were instructed to stand as still as possible during testing with arms folded across their chests, holding the opposite limb at approximately 45° of knee flexion and 30° of hip flexion in accordance with a previously established protocol. [19] All subjects were given one practice trial in each condition to familiarize themselves to the task. If a subject touched down with the opposite limb, made contact with the stance limb, or was unable to maintain standing posture during the 10-second trial, the trial was terminated and repeated. All failed trials were recorded and compared between groups.
Instrumentation
Postural control was assessed with the Accusway Plus forceplate (AMTI, Watertown, MA). Three dimensional force and moment signals arising from the foot/forceplate interface were filtered using a fourth-order low zero lag, low-pass filter with a cutoff frequency of 5 Hz. COP was calculated from the force and moment signals through Balance Clinic Software (AMTI, Watertown, MA) and sampled at a rate of 50 Hz.[13, 15]
Data Reduction
TTB measures were computed using previously described methods.[15] The mean of three trials for each measure was used for analysis. In order to calculate TTB, each subject's foot was modeled as a rectangle, based on length and width measurements, in order to separate the anteroposterior (AP) and mediolateral (ML) components of COP.[15] TTB measures estimated the time it would take the COP to reach the boundary of the base of support if the COP were to continue on its trajectory at its instantaneous velocity.[15] TTB was processed with the use of a custom software program in MatLab (MathWorks, Inc., Natick, MA). For each COPML data point, the instantaneous position and velocity were used to calculate TTB. The distance between COPMLi and the previous COPML data point was calculated and divided by the sampling rate (0.02 s) to determine the velocity of COPMLi. If the COPMLi was moving medially, the distance from the COPMLi instantaneous position to the respective (medial) boundary of the foot was determined. By dividing the COPMLi distance to the boundary by its instantaneous velocity, the theoretical time it would take COPMLi to reach the medial border of the foot if it continued on the same trajectory without a change in velocity or direction was calculated.[15] If the COP data point was moving laterally, the distance of the COP data point to the lateral border of the foot was determined. TTB in the AP direction was calculated similarly to TTB in the ML direction using the AP borders of the foot rather than the ML. Each TTB series in the ML and AP directions produced a sequence of peaks and valleys. The valleys represented the TTB minima; the lowest values in the TTB series represent the critical times where the sensorimotor system had the least time to make a postural correction in order to maintain single limb stance over the base of support.[15] From the identification of TTB Minima, the absolute minimum TTB, the mean of the TTB minima, and standard deviation of TTB minima were computed separately for the ML and AP directions.
Statistical Analysis
The means of each dependent measure of the three eyes open and eyes closed trials was used for analysis. Based on analysis of the data using Kolmogorov-Smirnov Z test for normality, all measures were found to be normally distributed, p > 0.05. In order to evaluate whether gender significantly impacted the measures, a series of 2 by 2 analyses of variance were used to determine the effects of group (CAI, control) and gender (male, female) on all TTB variables from the involved limb of the CAI group with eyes open and closed. Because failed trials between groups were not normally distributed, a Mann Whitney U test was used for gender and group comparisons. Alpha level was set a priori at p ≤ 0.05 for all analyses. We opted not to perform any correction for multiple comparisons on the alpha level to protect against type II errors.[20] Instead, effect sizes were generated for each measure by calculating the mean difference between groups and dividing by the standard deviation of the control group. Ninety-five percent confidence intervals (CI) were calculated around each effect size. The strength of effect sizes were classified using Cohen's guidelines.[21] An effect of less than 0.4 was considered small, .41-.7 moderate, and greater than .7 large.