Postural balance is the result of a complex interaction among the sensory, nervous and musculoskeletal systems. It requires a constant adaptation of the muscular activity, sense of articular positioning and other sensory information in order to maintain the center of mass of the body on the support base [16
Studies involving posture control and balance [12
] have been using the force platform to measure the strength and moments generated by movements and body posture sway [3
]. This assessment method has proven to be adequate to qualify small losses and provides precise data and more efficient results than subjective methods [10
The platforms determine the location and temporal evolution of the center of pressure (COP), the main parameter analyzed in postural balance studies [17
], representing the application point of the vertical resultant forces over the support area . In turn, COP is influenced by the center of gravity (CG). The movement of the body segments causes the CG displacement. And the COP displacement is generated by the ground reaction force, CG acceleration, body inertia and the muscular strength applied in the ankle. In other words, it is the neuromuscular answer to the CG oscillation [17
A significant increase of the plantar area as well as a displacement and average oscillation speed reduction were observed due to the upper limbs elevation, which is an important factor to improve support base and balance maintenance. According to Toledo and Barela [5
], the somatosensory system has an important role maintaining balance in older individuals because the other sensory systems, such as the vestibular and visual systems, are altered by senescence, and cannot provide information with enough quality to control and keep an upright position.
Thereby, it is safe to deduct that the CNS prefers somatosensory afferences to readjust posture, given they inform the CNS about the different segments of the body through proprioceptors (muscle spindles, neurotendinous organs and articular receptors) and mechanoreceptors (Pacinian corpuscles and Merker´s discs) [18
Moreover, some studies point out the importance of the somatosensory component, showing that the decrease of plantar sensibility resulting from the aging process is related to postural unbalance [20
]. Thus, the elevation of the upper limbs stimulated this afferent system (the proprioceptive system), increasing the plantar area and, consequently, reducing postural oscillation.
The postural alterations in the elderly people associated to articular instability, muscular weakness and changes in the support base [22
], moved the CG forward [23
]. The shortening of the flexors and hip adductors and abductors in an attempt to lower the CG closer to the support base to keep body balance and prevent falls corroborate this fact [24
Yiou et al. [25
] suggest the horizontal elevation of the upper limbs can induce internal forces and torques at the shoulder level, which is initially directed downward. Additionally, the center of gravity is relocated when the arms are elevated, disturbing the initial balance conditions.
So, the elevation of the upper limbs in older people moves the center of gravity to a physiological location. Hall [26
] points out the displacement of the center of gravity may occur due a repositioning of the body segments at a given moment. Every time an arm, a leg, a finger, the torso moves, CG shifts towards the weight change. Therefore, it is important to say the weight is equally distributed in all directions around the CG. Thus, the smaller the displacement oscillation the better the body balance.
The anticipatory postural adjustments could also have contributed to reduce body sway in the women evaluated after elevating the upper limbs. For instance, the experimental data obtained using the force platform showed a series of dynamic phenomena occurred right before the upper limb elevation, including the acceleration of the center of gravity forward and upward [27
A decrease in the total average speed oscillation was observed when comparing the upright position to elevation 1, and also in elevation 2.
Wieczorek et al. [17
] mention a previous study that evaluates speed and amplitude of oscillation in a population of elderly people. It showed a significantly higher speed for those who fell at least once in a one-year period, demonstrating postural instability. Based on these data, the best balance achieved was with the upper limbs elevated.
Seniors oscillate more than young adults when standing upright, especially in situations demanding a stronger postural control. And the increase in sway is proportional to age [12
]. This explains the correlation among age, displacement, speed, and amplitude of displacement of the center of pressure in the anterior–posterior direction found in the second upper limb elevation (T3), when a stronger postural control is necessary. This can be attributed to the fact that there is greater muscle activation to maintain postural control, since the elevation of the upper limbs is a postural disturbance.
The amplitude of oscillation in the anterior-posterior direction was the only parameter that showed a correlation with age in the three phases studied (T1,T2, and T3). Júnior Freitas and Barela [13
] investigated in which life period postural control changes occur in a sample composed of youngsters, adults and older people. The authors observed an increase in the amplitude of oscillation in the anterior-posterior direction after the age of 60, inferring the aging process influences the maintenance of postural balance, especially in this direction.