The institutional review boards (IRB) of the University of Puerto Rico Medical Science Campus, reviewed and approved this study before any data collection was initiated. An informed consent was provided to each subject to access their medical records, HIV status, CD4+ cell count and as a requirement to participate in the study.

The subjects were recruited from La Perla de Gran Precio (Community wellness center that specializes in individuals with HIV) localized in San Juan, Puerto Rico. Each subject was evaluated by interview and review of their medical record for the inclusion and exclusion criteria assessment. Different tests were performed as a screening tool to identify additional limitations or impairments in the different sensory systems. To rule out severe neuropathy, the subject had to be able to detect a Semmes- Weinstein Monofilament of 5.07 or less in more than two areas of the foot.12,13 The visual system was screened using Snellen chart and the subject had to obtain at least 20/40 of visual acuity. To rule out severe balance impairments, the Romberg Test and Fukuda's Stepping Test were used. For Romberg Test, subjects had to maintain standing position for 30 seconds and Fukuda's Stepping Test required that the subject did not rotate more than 30° from the initial position or deviate more than 0.5 meters from the center of the circle. Functional strength of lower extremities was screened using a Five Times Sit to Stand test, where the subject had to complete the test in 10 seconds or less. Severe proprioception impairments were assessed using a Proprioceptive Awareness test, where the subject was asked to replicate the initial position performed by the evaluator in the contralateral ankle joint in three trials. After the screening of the inclusion and exclusion criteria, a total of 20 subjects were able to participate in the study.

Each subject was instructed to stand in a static bipedal posture on the MatScan® pressure mat and performed eight balance tasks. Data of center of pressure, antero-posterior sways and medial-lateral sways were collected in each of the conditions. Each task took 30 seconds to be performed. The first four tasks were performed with the mat on the hard surface of the floor. These four tasks are as follows:1) eyes open looking at a fixed point to evaluate all systems (EO); 2) eyes closed (with head fixed approximately 90˚) to eliminate visual input (EC); 3) eyes open looking at a fixed point and actively moving head upward and downward to alter vestibular inputs (EO+HUD). The subject was instructed to maintain a movement frequency of 60bpm and amplitude of approximately 45 degrees in each direction: neck flexion and extension; 4) eyes closed (with head fixed approximately 90˚) to eliminate visual input and the head actively moves upward and downward to alter vestibular inputs (EC+HUD). The subject is instructed to maintain the movement frequency and amplitude as described in task 3.

The subjects were asked to perform four more tasks while standing on a thick piece of balance foam that was placed on top of the MatScan®, creating an unstable surface. The remaining four tasks (tasks 5-8) are as follows: 5) eyes open looking at a fixed point while standing on the balance foam to alter somatosensory inputs (EO+BF); 6) eyes closed with head fixed approximately 90˚ while standing on the balance foam to alter somatosensory inputs and eliminate visual inputs (EC+BF); 7) eyes open looking at a fixed point and actively moving the head upward and downward while standing on the balance foam to alter somatosensory and vestibular inputs (EO+HUD+BF). The subject was instructed to maintain the movement frequency and amplitude as described in task 3; 8) eyes closed with head fixed approximately 90˚ and actively move head upward and downward while standing on the balance foam to eliminate visual inputs and alter somatosensory and vestibular inputs (EC+HUD+BF). The subject is instructed to maintain the movement frequency and amplitude as described in task 3.

Data of postural stability was obtained using the MatScan® pressure mat (TekScan, Boston, MA). The data collected from the pressure mat was analyzed with Tekscan Sway Analysis Module™ (SAM) software (TekScan, Boston, MA) designed for this purpose. Mean and standard deviation were used to describe data. The statistical software used was SPSS-20 to perform a Paired-Samples t-test and compare the center of pressure, antero-posterior and right-left body sways displacement. P values \<0.05 were accepted as statistically significant.