Repeatability of the stabilometric assessment of body sway

Introduction - Small amounts of sway can be seen during the standing position in human beings. This position involves both voluntary movements and postural reflexes that compensate for minor oscillations of the body. The eyes, the vestibular receptors, and a combination of cutaneous and kinesthetic mechanoreceptors embedded in the skin surface, muscles, joints and tendons, provide the input to the central nervous system to determine such corrections. Some influence is also claimed to be provided by oral proprioceptors. Within clinical contexts, body sway is usually measured using stabilometry, and the modification of the position of the centre of foot pressure (COP) on a force plate for a period of time is assessed (1, 2). The COP is the centre of gravity of the vertical forces that act on a support surface. Literature reports on the intra-day and day-to-day repeatability of COP variations are scanty. Methods - Five men (age 22-29 y, standing height 172-188 cm, body weight 60-78 kg) and five women (age 22-44 y, standing height 160- 170 cm, body weight 53-60 kg), were assessed. Body sway was assessed either with open (EO, looking in a mirror) or closed eyes (EC), in the morning (9:00 AM) and afternoon (2:00 PM) of two week days. A computerized platform (Lizard, Lizarmed, Como, Italy) with two separate left- and right-side sensors was used. Data collection begun approximately 10 s after the correct vision (eyes open or closed) condition had been assumed by the subject, and lasted for 30 s for each test. The variations of COP were analyzed through bivariate analysis, and the area of the 90% standard ellipse was computed. The velocity of COP oscillation was also measured. Data collected in the four repetitions were analyzed by calculating intraclass correlation coefficients (ICC). Results - When considering all four repetitions, COP velocity was more repeatable than the area of the 90% standard ellipse, and the closed eyes condition was more repeatable than the open eyes condition (ICC area EO 0.342, EC 0.443; velocity EO 0.417, EC 0.626). Intra-day repeatability appeared larger in the first day in most occasions (ICC area EO, day 1: 0.22, day 2: 0.471; EC, day 1: 0.579, day 2: 0.413; velocity EO, day 1: 0.627, day 2: 0.433; EC, day 1: 0.744, day 2: 0.66). Day-to-day repeatability appeared larger in the afternoon than in the morning (ICC area EO, AM: 0.005, PM: 0.431; EC, AM: 0.308, PM: 0.59; velocity EO, AM: 0.069, PM: 0.644; EC, AM: 0.546, PM: 0.573). Conclusions - Overall, repeatability in the stabilometric assessment of body sway was limited, and further assessments are warranted within each clinical contest: a particular care should be given to the experimental conditions, and the timing of measurements. The better intra-day repeatability of COP velocity vs area is in accord with previous investigations (3).