Design and participants
This study was a cross-sectional study that involved four participants that had previously had a stroke. Inclusion criteria for participants were: age ≥65 years; more than 6 months post diagnosis of stroke; capable of walking at least 15 m without the aid of a walker; capable of following verbal instructions and able to stand for >30 seconds without assistance. Exclusion criteria for participants were: people with severe hemi-neglect; history or current diagnosis of other neurological or musculoskeletal impairment or pain. No racial/ethnic-based differences were present.
Ethical approval for the study was granted by the Ethics Committee of the Faculty of Health Sciences, University of Malaga. The study complied with the principles laid out in the Declaration of Helsinki.
Before the SLS test, each participant was given an information sheet and provided informed consent for participation. Participants were informed that participation was voluntary and they could withdraw at any point. They were also assured that their personal data would be treated in accordance with the Organic Law of Protection of Personal Data.
Inertial sensors
The inertial sensors (IS) used in this study were the InertiaCube3 model (InterSense Inc., Bedford, MA, USA) with a sampling frequency of 180 Hz. The InertiaCube3 is a small sensor (26.2 mm × 39.2 mm × 14.8 mm), based on micro-electro-mechanical systems (MEMS) technology and does not incorporate castors, which might generate noise, inertial forces and increase the risk of mechanical failure. The InertiaCube3 measures nine physical properties simultaneously, namely: angular rates, linear accelerations and magnetic field components along the three axes (yaw, pitch and roll). Miniature vibrating elements are used to measure all angular velocities and linear accelerations.
Two inertial sensors were placed in the trunk (T7– T8) and in lumbar (L5–S1) regions to collect kinematic data whilst the patient was performing the test (Figure 1). The IS were fixed to the skin using double adhesive and reinforced with inextensible tape surrounding the entire diameter of the trunk or the lumbar region of each participant. Movement recording started 3 s before the patient started the SLS and finished 3 s after the patient had completed the test to allow the identification of the start and the end of the test in the kinematic record.Sensors were positioned so that the origin of the coordinates was in the postero-inferior left corner (Figure 2).After data collection had been completed we extracted kinematic data (direct and indirect) offline from all the graphs generated during the SLS test by all of the participants (Figure 3).
Procedure: single-leg stance (SLS) test
To perform the SLS test, each participant must remain supported on one leg, arms resting on the hips. The test is timed (in seconds) from the moment the participant gets into the test position, until the other foot touches the ground or until the arms are separated from the hips [21].
Slight modifications to the protocol were made to standardise the implementation of the test for all participants. To ensure the safety of participants (who all had a diminished sense of balance) two researchers stood in front of and behind each patient to ensure safety in the event of serious instability which might result in a fall. Although the patient stood on the floor, he or she was surrounded by mats to minimise the negative effects of any fall. The time record began when the opposite foot of leg support was lifted from the floor until the foot touched the other leg or the ground, losing the balance [22, 23]. At the start of the test the patient was instructed to lift one leg with a knee flexion of 90 degrees, and to retain this static single-leg stance for 30 seconds [22–24].
The SLS test was carried out three times for each condition and in the following order: non-affected leg, eyes open; non-affected leg, eyes closed; affected leg, eyes open and affected leg, eyes closed. The resting time between each repetition was 150 seconds and they were allowed to sit during rest period. Patients were given the opportunity to practise as many times as they wanted to in order to make sure they understood how to perform the SLS test.
Before the test, patients stood in a relaxed position on two legs. We explained and demonstrated the test to help patients understand what was required of them. It has been reported that the reliability of the SLS test is 0.89 and 0.86 with eyes opened and closed, respectively, in elderly people [25].
Each participant performed the same protocol twice on two different days. The raters that supervised the protocols were different on the first and second day.
Variables
The variables measured in the present study were 136 variables in total (72 displacement variables and 64 velocity variables); 9 displacement variables in each condition (4 conditions) in each inertial sensor (lumbar and trunk) and 8 velocity variables in each condition (4 condition) in each inertial sensor (lumbar and trunk).
Displacement: Rotation (left/right side): maximum positive displacement (right) and negative displacement (left) relative to the X-axis. Flexion/extension: maximum positive displacement (flexion) and negative displacement (extension) relative to the Y-axis. Inclination (left/right side): maximum positive displacement (right) and negative (left) relative to the Z-axis. Mean displacement: mean displacement of each axis (X, Y, Z). Velocity: Rotation (left/right side): maximum positive velocity (right) and negative velocity (left) relative to the X-axis. Flexion/Extension: maximum positive velocity (flexion) and negative velocity (extension) relative to the Y-axis. Inclination (left/right side): maximum positive velocity (right) and negative velocity (left) relative to the Z-axis. Resultant velocity: the resultant velocity (V) vector was calculated using the formula:
Data analysis
We analysed anthropometric measurements and the data obtained from various self-report questionnaires designed for use in patients with neurological impairments. We also made a descriptive analysis of the kinematic data from the two inertial sensors (trunk and lumbar).
The Kolmogorov-Smirnov (K-S) test was used to assess the distribution of kinematic data. Following this we compared data from the trunk and lumbar sensors the normally distributed data was analysed using the parametric student t-test and non-normally distributed data was analysed using the non-parametric Wilcoxon’s test. A significance threshold of p ≤ 0.05 was used.
Reliability measures were calculated by analysing the internal consistency (intra-class correlation coefficients (ICC 3,1) were calculated for intra-observer and inter-observer reliability) of the measures with 95% confidence intervals for each outcome variable. Reliability was classified as follows: excellent (ICC > 0.80), good (0.80 > ICC > 0.60), moderate (0.60 > ICC > 0 .40), or poor (ICC < 0.40) [26].
Data analysis was performed with SPSS (version17.0 for Windows; SPSS Inc., Illinois, and USA).