Comparison of Center of Force Trajectory during Sit-to-stand Movements Performed by Elderly and Old-old Elderly Subjects

Abstract

[Purpose] This study compared the center of force (COF) trajectory during sit-to-stand (STS) movements performed by elderly and old-old elderly subjects. [Subjects] We recruited 9 elderly and 10 old-old elderly subjects with no knee joint pain or lower limb orthopedic problems. [Methods] The CONFORMat system was used to measure the length of the COF trajectory as the subjects performed the STS task. [Results] The total length of the COF trajectory during the STS movement was significantly greater for the old-old elderly group than for the elderly group. [Conclusion] Our results highlight the need for novel strategies or the development of assistive devices to reduce the COF trajectory during the STS movement for old-old elderly individuals.

INTRODUCTION

The sit-to-stand (STS) test is a practical and valid measure of the balance ability of older adults. The STS movement is destabilizing because the body rapidly moves from a stable seated position to one with a less stable base of support and higher center of force (COF)¹⁾. Healthy people aged 65 years or older experience an age-related reduction in strength that limits, to varying degrees, their ability to perform tasks such as the STS²⁾. The STS movement is performed several times a day as people change from the sitting to the standing position, and impairment in this movement increases the risk of falling³⁾. Thus, the ability of an elderly individual to rise from a chair is considered an important indicator of functional independence. Decreasing muscle strength and power often cause the elderly to lose of postural control. Moreover, the declining ability to rise from a chair may become a major factor limiting quality of life¹⁾. A previous study found that as people age, they lose muscle strength, particularly after 75 years of age. However, the old-old elderly have little opportunity to participate in regular physical activities, which makes it difficult for them to maintain their strength⁴⁾. The COF trajectory is an indicator of body sway in the sagittal plane during an activity. Effective postural control during STS movements requires the generation of sufficient joint torque to stand, attain stability by moving the COF from one base of support to another using the feet, and the ability to modify these movement strategies depending on environmental constraints⁵⁾. Postural control during STS movements has not been compared in elderly and old-old elderly adults. Thus, we measured the COF trajectory of elderly and old-old elderly subjects while they performed the STS task.

SUBJECTS AND METHODS

We recruited 9 elderly and 10 old-old elderly volunteers from the local community. The subjects in the elderly group had a mean (±SD) age of 69.9 ± 3.3 (range, 60–75) years, an average height of 147.6 ± 4.2 cm, and a mean body weight of 50.0 ± 5.4 kg. Subjects in the old-old elderly group had a mean age of 81.9 ± 3.6 (range, +75) years, an average height of 146.9 ± 6.3 cm, and a mean body weight of 45.1 ± 6.0 kg. Subjects were included in the study if they were able to follow directions, had no lower-extremity amputations, and had no restrictions placed on lower-extremity movement or weight bearing by a physician. The study participants had no muscular pathology or any gait or balance disorders, no chronic deficiencies associated with a neurological, rheumatological or orthopedic affliction, and no chronic or acute illness with an inflammatory syndrome. Our study was approved by the Yonsei University Faculty of Health Science Human Ethics Committee, and all subjects provided their written informed consent prior to participating in the study. COF during STS movement was measured using the CONFORMat system (Model #5330, Tekscan, Boston, MA, USA), a portable interface pressure mapping system that captures seat pressure distribution and contact area. Before performing the measurements, the CONFORMat system was conditioned by loading and unloading it several times, and by equilibrating and calibrating the sensor using the instrumentation provided by the manufacturer. After the sensor was conditioned, it was placed on the floor for foot contact. The subjects’ hip, knee, and ankle joints were flexed 90°, and their feet were positioned shoulder-width apart on the CONFORMat sensor with their toes pointing directly forward. The subjects were instructed to maintain an upright trunk. After one practice trial, the subjects performed three measurement trials. Then subjects were instructed to stand from a sitting position at a self-determined speed and remain standing for 2 s. The length of the COF trajectory was recorded, and the mean value was calculated using all but the first and last values recorded at a frequency of 60 Hz using I-Scan version 6.20 (Tekscan, Boston, MA, USA). All statistical tests were conducted using the Statistical Package for the Social Sciences version 18.0 for Windows, (SPSS, Chicago, IL, USA). The differences in the length of the COF trajectories were analyzed using the independent t-test for comparisons between the groups. P values <0.05 were deemed to indicate statistical significance.

RESULTS

The length of the COF trajectory during the STS task was significantly greater in the old-old elderly group (151.5 ± 25.8 mm) compared with the elderly group (124.2 ± 14.6 mm; p < 0.05).

DISCUSSION

The STS movement requires optimal neuromuscular coordination and postural adjustment to control changes in motion and to prevent the loss of balance⁶⁾. The postural adjustments necessary to modify the task or respond to altered environmental conditions must be controlled appropriately to maintain postural balance⁷⁾. The frequency of falls increases in elderly people over the age of 75 years⁸⁾. For this reason, we compared the COF trajectories of elderly and old-old elderly subjects performing the STS task. The mean age of the subjects in our old-old elderly group was 81.9 years and that of the subjects in the elderly group was 69.9 years. Our results showed that the length of the trajectory of the COF during the STS task was significantly greater in the old-old elderly group than in the elderly group. The COF trajectory has been shown to be a sensitive measure for distinguishing between healthy older adults and those with a balance disorder⁸⁾. A longer COF trajectory means less stable postural control⁵⁾. Typically, postural sway is measured as the COF trajectory when both feet are on the ground⁵⁾. Proprioceptive sensory information from the ankle and compression sensory information from the plantar surface of the foot are important factors for controlling sway⁹⁾. The aging process causes various degrees of deterioration in the neuromuscular, bony, and central nervous systems, leading to loss of strength and impaired balance control. Rapidly occurring physiological changes in the old-old elderly place them at higher risk of falling than other age groups⁸⁾. In the clinical setting, physical therapists working with elderly patients focus on exercises that strengthen lower extremity muscles or the use of devices that assist balance to help prevent falls and bone fractures. A previous study found that postural sway was significantly diminished in elderly people when they used shoes with a rigid insole as opposed to standing barefoot¹⁰⁾. The authors noted that the insoles increased pressure on the plantar foot surface, and thus postural sway was reduced via stimulation of the mechanoreceptors¹⁰⁾. Our results highlight the need for novel strategies or the development of assistive devices to reduce the COF trajectory during the STS movement of old-old elderly individuals.