Introduction
Improving health and increasing life expectancy has led to an increase in the elderly population in the world. Holding an external load while standing is an aspect of daily and occupational activities. In the elderly, carrying shopping bags may challenge the stability of the posture during standing and walking and lead to falls. The reason for this, in addition to the negative effects of old age on balance, is the longer shopping process in the elderly. Considering the importance of having an active old age, it is very important to maintain independence during daily activities such as shopping, keeping and manually carrying external loads. This study aims to determine the effect of manual load holding mode and magnitude on postural sway during standing in the community-dwelling older adults.

Methods
This is a quasi-experimental study approved by the Ethics Committee of Urmia University of Medical Sciences. Fifty healthy community-dwelling elderly (7 men and 8 women), with a mean age of 71.53±6.18 years, were selected using a convenience sampling method. The inclusion criteria were age 65 years and older, no history of falling in the last six months, independence in performing daily activities based on the Persian version of the Katz Activities of Daily Living scale, having the minimum mental-cognitive status based on the Persian version of the Mini Mental Status Examination test score based on the educational level (14 for illiterate people, 17 for people with primary school education, and 22 for people with high school education or higher), not using any kind of assistive devices, and not suffering from known neurological, orthopedic and musculoskeletal diseases. The exclusion criteria were unwillingness to continue participation, and feeling any pain or discomfort during or after the study.
The independent variables were holding manual loads with a magnitude of 0%, 5% (3.71±0.54 kg), 10% (7.42 ± 1.08 kg), 15% (11.13±1.62 kg) and 20 % (14.84±2.16 kg) of body weight in symmetrical (both hands) and asymmetrical (dominant or non-dominant hand) conditions, standing still on the force plate for 30 seconds. Dependent variables included the amount and velocity of the displacement of the center of pressure (COP) in two anteroposterior and mediolateral directions and the area of the COP displacement. According to the national health guideline for manual lifting as well as the NIOSH manual lifting guide, the manual loads used in this study were within the permissible limits.

Results
The results of Mauchly’s test on the mode of holding a manual load showed that the assumption of sphericity was met for all study variables except for the mean displacement of COP in the mediolateral direction. In terms of manual loads with different magnitudes and the interaction effect of manual load holding mode and magnitude on all data, the assumption of sphericity was rejected. Therefore, considering that the epsilon value was not >0.75, Greenhouse-Geisser correction was used to interpret the results.
The results showed that the effects of manual load holding mode symmetrically and asymmetrically, as well as the magnitude of the manual load, on the amount and velocity of COP displacement in the anteroposterior direction and the velocity of the overall COP displacement were significant, but the interaction effect of these two factors was not significant on any of the dependent variables. The results of paired t-test showed that the amount and velocity of the COP displacement in the anteroposterior direction in asymmetrical condition was significant compared to the symmetrical condition. In comparing the dominant hand with two hands in amount of COP displacement in anteroposterior direction, P value was 0.009, and in comparing the non-dominant hand with two hands in amount of COP displacement anteroposterior direction, P value was 0.003. In comparing the dominant hand with two hands in velocity of COP displacement in anteroposterior direction, P value was 0.002, and in comparing the non-dominant hand with two hands in velocity of COP displacement in anteroposterior direction, P value was <0.001. In comparing the dominant hand with two hands in velocity of overall COP displacement, P value was 0.015, and in comparing the non-dominant hand with two hands in velocity of overall COP displacement, P value was <0.001. The difference between dominant and non-dominant hand was not significant in any variables.
The amount of COP displacement in anteroposterior direction with manual loads of 5%, 10%, 15% and 20% body weight were higher and statistically significant compared to the load magnitude of 0% (P=0.001, 0.009, 0.001, 0.001, respectively). The effects of the mode of holding and manual load magnitude were not statistically significant on any of the COP variables in the mediolateral direction. Based on Cohen' method for interpretation of the effect size using eta squared (0.01 for small effect size, 0.06 for medium effect size, and 0.14 for large effect size), the effect of manual load holding mode on the COP displacement in anteroposterior direction, on the velocity of COP displacement in anteroposterior direction, and on the velocity of overall COP displacement was large. The effect of manual load magnitude on the amount and velocity of COP displacement in anteroposterior direction was also large.

Discussion
Holding external loads asymmetrically with any magnitudes can increase the amount and velocity of COP displacement in the anteroposterior direction. For preventing falls, it is recommended for the elderly to pay attention to the fact that holding an external load with two hands is less challenging to the balance system than holding with one hand, whether dominant or non-dominant. In terms of the load magnitude, an external load with 5% of body weight can challenge the balance system compared to the situation without external load. With some exceptions, there is no difference between loads of 5%, 10%, 15% and 20% of body weight.

Ethical Considerations

Compliance with ethical guidelines
This study was approved by the Ethics Committee of Urmia University of Medical Sciences (Code: IR.UMSU.1398.141).

Funding
This article was extracted from the thesis of the first author  a master's student in ergonomics, occupational health engineering training group, faculty of health, Urmia University of Medical Sciences and did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Authors' contributions
All authors contributed equally in preparing all parts of the research.

Conflicts of interest
The authors declared no conflict of interest.

Acknowledgements
The research team would like to thank  the older people who participated in the research.
 

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