The capability of moving from a sitting to a standing position is considered as an essential marker for functional independence (Gill et al., 1995) as well as a determinant for risk of falls (Campbell et al., 1989). Specific balance examinations have been designed to determine the agility of an individual in standing up from a sitting position. The score system employed in these balance tests is based on the time it takes to achieve the sit-to-stand action and an individual receiving a low score translates to a difficulty in standing up from a sitting position.
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The action of standing up from a seated position is an intricate action that is associated with the shift from one stable position to another through the engagement of technically all the parts of the body except the feet. Hence the simultaneous action of sitting and standing involves two major aspects in musculoskeletal control. Firstly, voluntary movement of several parts of the body facilitate in the transformation of a posture from sitting to standing. Secondly, control of equilibrium is also involved in change in posture because there is a change in the body’s centre of gravity (Cacciatore et al., 2005). Such displacement is associated with the center of gravity moving forward and backward alongside vertical motion. Biomedical science has considered the entire mechanism of the coupled sitting and standing postures as a valuable standard in studying the principles behind the synchronization of posture and movement.
Several reports have suggested the posture and movement are associated with elderly individuals hence investigators have studied anticipatory postural reactions during actions of the arms which are generally more slowly performed. The biomechanics of these postural reactions have also been investigated using a platform that was in constant motion and these studies shows variations in magnitude and timing of movement of body segments between healthy older individuals and young control subjects. Research regarding the action of standing up among young subjects revealed the different stages of movement using kinetic and kinematic data. Other research efforts described the influence of the trunk to the change in the center of gravity.
It has been reported that the employment of a high seat and the facilitative use of hands for pushing results in a lowering of the torque in the hips and knee joints. The standing position involves balance control as well as variations in the speed in performing the act and termination restrictions with changes in the standing position. It was observed that the center of gravity was controlled in a horizontal direction and this represents an essential factor in maintaining the dynamic balance while performing the movement. Investigations regarding the succession of stages from the standing to the sitting position using force platform data revealed that the time involved in sitting down is longer than the time involved in standing up.
Majority of research efforts on the biomechanics of sitting down and standing up aimed to analyze and evaluate the kinetics of the entire action in order to provide a better understanding of the phases of movement in relation to gravity. It has been observed that each phase of the movement is distinct from each other. Research studies were also designed to determine the effect of age of an individual on the movement kinetics. Such research efforts on age-related kinetics reported that critical role of foot position and the centre of velocity in regulating the stability once an individual gets off the seated position. It has been suggested that the main kinetic mechanism involved in sitting and standing actions that differentiates young individuals from the elderly is the first step of standing up or the rising up from the seated position. There are differences in the kinetics of standing up among elderly individuals. The two stages of movement composed of standing up and sitting down have been well studied.
Angular displacement of the trunk is an essential component of the actions of standing up and sitting down. Also known as the to-and-fro action, the first position and mechanical settings of this action is influenced by the effect of gravity. The trajectories of the acromion in the sagittal plane have provided the proof the similar forms are observed in both the seated and standing positions. The trajectories of both positions actually do not change during the movement. However, the trajectory of the trochanter varies with regards to the function of the movements, which in turn are influenced by gravity and the posture of the individual during each stage. It has been observed that the angular displacement of the trunk during the seated and the standing positions remain the same, when analyzed with respect to the vertical axis.
The trunk of the body controls the action of standing up from a seated position and it is strongly related to maintaining an upright position during such voluntary movement. At each stage, the displacement of the trunk in the forward direction has a great possibility of causing disequilibrium to the entire body. With regards to the action of standing up, the rotation of the trunk in the forward direction at the initial stage of the action greatly influences the horizontal momentum of the body’s center of gravity. When sitting down, the rotation of the trunk in the forward direction facilitates control of the body, resulting in stability in terms of the anterior-posterior axis. It should be noted, though, the during the entire transition from the seated to the standing position, the trunk which is active in space, should not be perceived as a body part that provides stability and posture to an individual. Instead, the head serves as the reference frame for stability during stages of sitting down and standing up. Research has shown a minimization of the rotation of the head during the transition from the seated position to the standing position, confirming that a proper angle of the head stabilizes the rest of the limbs of the body with respect to the sagittal plane during body movement (Luomajoki et al., 2007). In addition, the head angle also stabilizes the frontal plans during actions that are related with dynamic equilibrium.
An advantage that head stabilization provides during actions with dynamic equilibrium is that restrictions are provided in relation to the extrinsic or vertical and intrinsic or position of the head with respect to the position of the trunk factors of movement. This condition makes the transformation of sensory and motor actions easier than what is expected. The movement from a seated to a standing position is related to different execution times, wherein it takes a longer duration to sit down than to stand up. However, other investigators have attempted to explain the disparity between these two actions using the concept of braking which is necessary to resist the pull of gravity as well as the absence of visual information during the action of sitting down. Such concept suggests that when performing such action in the dark, it takes a longer duration to stand up.
It has been shown that adjustments arise during the last phase of the action in order to focus on the target. In the absence of visual feedback, the action of standing up which takes place before the action of sitting down provides memorization of the position and the height of the chair. Thus, the precision required in the action of sitting down could explain some differences
observed in duration for the two actions. In addition, the absence of visual information during standing up and sitting down does not correlated with an individual’s performance, suggesting that the two tasks are mainly dependent upon somatosensory information (Blouin et al., 2007). The trajectories among young and elderly individuals during standing up and sitting down were the similar, suggesting that the geometrical parameters of the action are not affected by normal ageing. On the other hand, elderly individuals need more time than young individuals to perform the action. Different notions have been suggested according to which action was been analyzed, wherein an increase in reaction time, the modification of latencies of initial postural adjustments or a lowering in muscular strength.
Regardless of execution time, there are differences between the young and elderly individuals in terms of control of posture. Sitting down, which necessitates fine postural control and precision in the final part of the movement, is strongly influenced by ageing. The comparative analysis of the two phases among two different generations suggests that elderly individuals have less similarities between standing up and sitting down than younger individuals. Among elderly individuals, head rotation follows a non-significant trend of decreasing stabilization in space. The disparity between young and elderly individuals became critical when the action was performed rapidly in darkness. In elderly individuals, changes in kinematic settings may be perceived as modifications in the control of posture. Head stability has been observed to be unreliable among elderly individuals suggesting that the actions are difficult. Clinical implications of these reports are potentially interesting, particularly in elderly subjects showing alterations of posture and gait after falls. These individuals are typically incapable of moving their trunk correctly during standing up and sitting down. Such incapacity both makes it difficult for them to stand up and can causes them to fall backwards when sitting down.
Studies on the biomechanics of the sitting to the standing position have been of great interest because these provide information on the understanding of posture, falls and body stability. One of methods in investigating such musculoskeletal movement involves stabilograms, which calculates risks of falls, as well as motor movements that play a major role in maintaining balance (Amoud et al., 2007).
More than two million falls are recorded among the elderly each year, leading to more than 9000 deaths. Most prospective studies have attempted to identify risk factors, particularly in groups at high risk of falling. The factors identified in these studies have often varied, mainly due to differences in methodology, diagnosis, and the study population. Nevertheless, several factors are regularly cited, such as muscular weakness, a previous fall, or balance problems. In addition, several factors that augment the risk of falling, such as visual, vestibular, or proprioceptive problems, will adversely affect balance. In order to measure postural sway, the movement of the centre of pressure (COP) over the support base of the subject can be evaluated, with the resulting stabilogram displaying the movement of the COP over time for anteroposterior (AP), mediolateral (ML), and resultant (R) directions. Such methods have been used as the stabilogram has been shown to be a nonstationary time series that displays fractal characteristics. The advantage of such methods is that information related to the underlying motor control strategies governing postural stability could be extracted.
- Amoud H, Abadi M, Hewson DJ, Michel-Pellegrino V, Doussot M and Duchêne J (2007): Fractal time series analysis of postural stability in elderly and control subjects. J. of Neuroeng. Rehab. 4:12.
- Blouin J, Teasdale N and Mouchnino L (2007): Vestibular signal processing in a proprioceptively deafferented subject: The case of sitting posture. BMC Neurology 7:25
- Cacciatore TW, Horak FB and Henry SM (2005): Improvement in automatic postural coordination following Alexander technique lessons in a person with low back pain. Phys. Therap. 85(6):565.
- Campbell AJ, Borrie MJ and Spears GF (1989): Risk factors for falls in a community-based prospective study of people 70 years and older. J. Gerontol. 4:12-17.
- Gill TM, Williams CS and Tinetti ME (1995): Assessing risk for the onset of functional dependence among older adults: The role of physical performance. J. Am. Geriatr. Soc. 43:603-9.
- Luomajoki H, Kool J, De Bruin ED and Airaksinen O (2007): Reliability of movement control tests in the lumbar spine. BMC Musculoskeletal Disorders 8:90.