Study design and participants
This study was semi-experimental in design. Study participants consisted of 42 older adults (21 males and 21 females) aged 60 years and over. The required sample size was calculated based on the main outcomes (e.g., dynamic and static balance measures) and factors (e.g., physical and cognitive demands) evaluated in the study, considering a 95% confidence level and a power of 80%. The study participants were selected through random sampling from the elderly who were under the coverage of an aging friendly centre located in Tabriz, Iran. The Mini-Mental Status Examination (MMSE) cognitive test was applied for screening of cognitively impaired cases and those volunteers scoring < 23 on this test were excluded [14]. Other inclusion criteria were: being 60 years or older and not using alcohol and drugs (particularly psychedelic, hypnotic, anticonvulsant, antidepressant, sedative consciousness, and impaired balance drugs) and an assistive device to walk. Participation in the study was on a voluntary basis and the participants were told that they were free to withdraw at any stage of the study. Each participant signed a written consent form before participation. The study protocol was reviewed and approved by the ethical review committee of the Tabriz University of Medical Sciences.
Study variables
Two dependent variables evaluated in the study were: 1) dynamic balance (by using the Timed Up and Go [TUG] test) and 2) static balance (by using a stabilometer). Independent variables included cognitive and physical loads. To perform the cognitive task (CT), each participant was instructed to speak out the name of the weekdays in a reverse order as accurately as possible, starting from any random weekday specified by the investigator, similar to the protocol used in previous research [15]. The physical task (PT) had three levels and consisted of handling weights of 1kg (PT1), 2kg (PT2), and 3kg (PT3) in each hand (2kg, 4kg, and 6kg in total).
Data collection
Data were collected by using questionnaire, personal interview and experimental tests (for evaluation of dynamic and static balances). Participants were asked to complete a personal information questionnaire before undertaking the experimental test. The questionnaire included demographic details including the participants’ age, gender (male, female), education level (primary/secondary, diploma, undergraduate, postgraduate), marital status (single, married), living condition (living with family or alone) and having chronic disease as well as other information such as illness history, sleep condition (good, not good) and history of falling during the past few months (no, yes). The participants were asked not to have any physical activity before undertaking the test.
The TUG test was used to measure the dynamic balance of participants. The TUG is a commonly used test in older adults as it is easy to administer and can be completed by the majority of participants [5]. It uses the time required that a participant takes to rise from a chair, walk 3 meters, turn around, walk back to the chair, and sit down. During the tests, the participants were instructed to wear their regular footwear and use any mobility aids that they would normally require, following the procedure described in the literature [5]. The time taken to complete this test was measured using a stopwatch.
A purpose-built stabilometer (Danesh Salar Iranian Co., Iran) was used to measure the static balance. This type of stabilometer has been shown to be valid and reliable in measuring static balance [16]. Measurements were taken while the participants were standing on both feet on the stabilometer platform (40 × 40 cm). Participants were instructed to stand in an upright standing posture on the stabilometer platform for 30 s with eyes open. They were asked to stand flat, without moving and look at a point that was in front of their eyes during each trial. The stabilometer sway index (SM.SI in % and cm), which is the numerical value of the standard deviation of the distance the participant moved away from the centre of balance [17], was used as a measure static balance in this study. Three repetitions of static balance measurements were made for each test condition (with a 20 s rest break between successive measurements), and their average value was used for subsequent analysis.
Procedure
On arrival, participants were given instructions about the aims and procedures of the experiment. The test equipment was then introduced and any questions were answered by the investigator. Each participant performed 16 different experimental conditions as follows: two single tasks (TUG and stabilometer tests without cognitive/physical task), eight dual-task conditions (TUG test with CT, TUG test with 3 levels of PT, stabilometer test with CT, and stabilometer test with 3 levels of PT), and 6 multi-task conditions (TUG test with simultaneous CT and 3 levels of PT, and stabilometer test with simultaneous CT and 3 levels of PT). The order of presentation of the experimental conditions was randomised between the participants. Data were collected between 9:00 and 12:00 am in a lab environment. The experimental tests were carried out during a single session, and took approximately one hour to complete for each participant.
Statistical analysis
Data analysis was performed using the SPSS v.16. Independent sample t-test and one-way ANOVA analyses were used to evaluate gender and age differences. For parameter comparison in different experimental conditions, considering the single task test condition as the control group, mixed model analysis of variance (mixed model ANOVA) with repeated measures design was used. In this model, the parameters were estimated by the Restricted Maximal Likelihood (REML) method and the covariance structure was selected as first order autoregressive (AR1) based on Akaike Information Criteria (AIC). This was followed by Sidak’s post hoc tests on adjusted means to explore the effects in more detail. P < 0.05 was considered as significant level for all tests.