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Rising from a seated to a standing position occurs repeatedly in the everyday lives of healthy individuals. The sit-to-stand movement, one of the most common functional tasks of daily living, is also one of the most mechanically demanding maneuvers. Studies have shown that rising from a chair creates greater torques about the knee joint and higher pressures at the hip joint than activities such as walking and stair-climbing (Riley, Schenkman, Mann, & Hodge, 1991; Gross, Stevenson, Charette, Pyka, & Marcus, 1998). Rising from a chair requires specific sequential coordination and positioning of joints and body segments so that the balance needed to successfully complete the movement is maintained (Bahrami, Riener, Jabedar-Maralani, & Schmidt, 2000). Inability to rise from a seated position may negatively influence health, since standing is a precursor to engagement in other physical activities (Kerr, White, Barr, & Mollan, 1997).
Several variables, including type of chair, seat height, use of arms, and initial positioning of the feet, affect the chain of events that occurs within the muscles and joints during the sit-to-stand movement (Jeng, Schenkman, Riley, & Lin, 1990). Age, joint range of motion, and muscle strength also influence chair rise technique (Landers, Hunter, Wetzstein, Bamman, & Weinsier, 2001). Studies that examine the effects of different variables on the sit-to-stand task allow researchers to determine favorable conditions for successfully rising from a chair. An understanding of the biomechanics of the human body during the sit-to-stand task and consideration of outside variables provides the insight needed to design more user-friendly chairs, to diagnose factors that contribute to unsuccessful sit-to-stand transfers, and to offer effective therapy to patients who have endured physiological setbacks (Schultz, Alexander, & Ashton-Miller, 1992).
This study observes the effect of chair type on the kinematics of the sit-to-stand transfer. Differences in rising from a standard chair and a rocking chair were analyzed in a young male subject (21 years). The following review on the topic of the sit-to-stand task highlights the strengths and weaknesses of existing research, and provides the theoretical framework for the hypotheses concerning the kinematics of the sit-to-stand movement in the study at hand.
In order to fully understand the process of rising from a seated position, researchers have identified distinct phases that transpire during the progression of the movement. Rodosky, Andriacchi, & Andersson (1989) broke down the sit-to-stand task into two main phases, the forward-thrust phase and the extension phase. The forward-thrust phase involves forward flexion of the trunk, and forward and upward thigh displacement. The hip and ankle joints reach maximum flexion and dorsiflexion, respectively, for the sit-to-stand activity, and the greatest torques arise at the hip and knee joints. At this point, the extension phase begins. During the extension phase, extension at the hip and knee joints, and plantar flexion of the ankle joints work together to bring the body to its upright position (Rodosky et al., 1989).
Schenkman, Berger, Riley, Mann, & Hodge (1990) further delineated phases for the sit-to-stand movement by observing changes in the subject’s momentum. The flexion momentum phase consists solely of forward trunk flexion and attainment of upper body momentum. The second phase, the momentum-transfer phase, starts when the buttocks begin to lose contact with the chair and ends with maximal dorsiflexion of the ankles. Transfer of momentum from the forward movement of the upper body to the forward and upward movement of the whole body takes place during the second phase. The time of lift-off, that point at which the buttocks lose contact with the chair, represents the unstable portion of the sit-to-stand task since the subject’s center of mass is located most posterior to the base of support. During the final phase of movement, the extension phase, the subject’s center of mass moves upwards over the base of support. The extension phase terminates when hip angular velocity reaches zero. The subject regains stabilization and balance in the fourth phase (Schenkman et al., 1990). The four-phase approach offers more insight regarding changes in the subject’s momentum. Riley et al. (1991) found that the initial forward trunk movement of the chair rise task was used to generate forward momentum and to position the center of mass for lift-off. After lift-off, forward momentum was transferred to vertical momentum.
Research that examines the phases of the sit-to-stand movement often includes kinematic and kinetic data. Kinematic data consists of body segment displacement and joint angular displacement measurements. Oftentimes, body segment and joint angular velocities are also calculated. Kinetic data measures ground reaction forces and the torques acting at the joints throughout the course of the movement.
Researchers commonly opt for the video analysis technique in order to determine kinematic data. Markers are placed on critical landmarks on the subject’s body, and the movement of the markers is analyzed using digitization or goniometry. Spatial information regarding joints and body segments can then be determined over time. Jeng et al. (1990) found that videotaping and goniometric techniques served as reliable methods for investigation of the sit-to-stand movement. Identification of key events from videotape footage, such as the beginning of lift-off, consistently yielded reproducible results within and between individuals taking measurements. Goniometric measurements of joint angles displayed by videotape also showed high reliability within and between raters (Jeng et al., 1990). Some researchers have collected kinetic data associated with the chair rise independently of video analysis through the use of force platforms and electromyography. In general, methodologies for kinematic and kinetic measurements are fairly consistent from one sit-to-stand study to the next.
One key variable that differentiates research studies is the type of population recruited for the analysis. Different studies have examined the sit-to-stand task in a broad variety of subjects. A popular sample includes healthy older adults, since aging is associated with a decline in physical function (Landers et al., 2001). Comparing elderly subjects to young subjects provides valuable evidence regarding the most functionally demanding portions of the sit-to-stand task. Healthy male and female young and middle-aged adults comprise many chair rise studies that aim to establish norms for kinematic and kinetic data.
Several studies have recorded kinematic data for various healthy populations, revealing trends in the sequence of the sit-to-stand motion. The following kinematic standards reflect data from studies in which subjects rose from chairs of standard height without using arms for support. Kerr et al. (1997) observed the sit-stand-sit cycle in 50 healthy male and female subjects. The subjects ranged from 20 to 78 years of age. The mean time taken to complete the rising stage of the sit-stand-sit cycle was 1.907 + 0.057 s. Elderly females took the most time to complete the action (2.539 s), while young males performed the action most quickly (1.788 s). Schenkman et al. (1990) found that when healthy women (mean age 28.9 years) rose from a chair, the flexion-momentum phase, the momentum-transfer phase, and the extension phase occupied 28%, 18%, and 54%, respectively, of the total time required to complete the three phases.
The phases of the sit-to-stand movement correspond to particular patterns of joint and body segment movement. During the flexion-momentum phase, the trunk and pelvis rotate forward, causing hip flexion. Kerr et al. (1997) reported that subjects moved their trunks forward an average of 0.490 + 0.117 m at a mean velocity of 0.49 + 0.1 m/s. Maximal hip flexion angular velocity occurs during the flexion-momentum phase (Schenkman et al., 1990). Ikeda, Schenkman, Riley, & Hodge (1991) reported maximum hip flexion angular velocities as being 74.33 + 16.3 °/s for young, adult females.
During the momentum-transfer phase, maximal hip flexion and maximal ankle dorsiflexion take place. Gross et al. (1998) examined mean data from young and old subjects, and found the relative angle corresponding to peak hip flexion to be 83.6 + 9.5° and to be 108.3 + 6.5° for peak ankle dorsiflexion. Furthermore, joint angles for young subjects at lift-off included a hip angle of 87.7 + 18.8°, a knee angle of 105.1 + 9.9°, and an ankle angle of 107.8 + 4.4°.
The final phase of movement, the extension phase, is characterized by maximal hip and knee extension angular velocities. Ikeda et al. (1991) reported maximal hip extension velocity values and maximal knee extension velocity values of 161.94 + 27.2 °/s and 150.22 + 27.9 °/s, respectively.
Like joint angle and body segment kinematics, forces acting at joints exhibit certain characteristics throughout the course of the sit-to-stand movement. Maximum hip and knee torques occur during the momentum-transfer phase. The maximal torques at these joints corresponded to the shift of the subject’s base of support from the chair and feet to just the feet (Schenkman et al., 1990). Rodosky et al. (1989) found that the hip joint consistently displayed the largest torques compared to the knee and ankle joints. The large hip torque at lift-off requires a great amount of force production by the extensor muscles, which, in turn, creates high contact forces at the hip joint (Rodosky et al., 1989). Generating large torques in a small amount of time during the sit-to-stand action may become more difficult with age due to a selective loss of fast-twitch muscle fibers. While the elderly are often able to generate the torques needed for the sit-to-stand movement, they use a greater percentage of maximum muscular activity than younger subjects and likely endure greater stress on knee extensor muscles (Landers et al., 2001).
Different types of chairs can vastly alter the typical kinematic and kinetic data displayed during rising from a standard chair. Studies analyzing the sit-to-stand movement using standard chairs generally use a chair characterized by limited cushioning, the presence of a backrest, and no arm rests. Height of the chair is often adjusted to 80% of the subject’s knee height (Riley et al., 1991). Decreasing chair height increases the average maximum hip, knee, and ankle flexion angles, in addition to increasing maximum hip, knee, and ankle torques. Knee stress is greatly reduced with higher seating, but to significantly reduce hip stress, further measures need to be taken, such as the addition of arm support (Rodosky et al., 1991). Using arm support to assist with rising from a chair helps to carry some of the subject’s body weight and decreases lower extremity joint torques (Bahrami et al., 2000). Burdett, Habasevich, Pisciotta, & Simon (1985) found that a specially designed higher chair with armrests decreased muscle and joint stress of the hip and knees, and decreased hip and knee range of motion needed to rise from a chair.
Another study examined the differences between rising from a standard chair and a chair designed for the elderly (Wheeler, Woodward, Ucovich, Perry, & Walker, 1984). The seat of the specially designed chair was more posteriorly slanted than the seat of the standard chair. The posterior slant tilts the subject’s center of mass farther back, and likely contributed to the increased hip flexion exhibited by subjects rising from the specially designed chair. Additionally, more knee extensor muscle activation occurred within older subjects while rising from the special chair. Overall, the sit-to-stand task was more difficult in the special chair condition. The specially designed chair was constructed with the intention of providing optimal comfort during sitting, and ease of rising from the chair was not thoroughly considered (Wheeler et al., 1984).
Studies examining the sit-to-stand transfer are limited by several factors. Many of the studies use small, homogeneous subject samples. Small sample sizes fail to provide representative data, so one cannot accurately generalize data to apply to a population outside of the study sample. Studies often fail to establish whether or not differences in chair-rise strategies exist between men and women. Body composition of subjects is also rarely mentioned, which could be problematic since heavier subjects most likely use different techniques of standing up than subjects of normal weight. In addition, people who are extremely sedentary and unwilling to stand up are not apt to participate in sit-to-stand studies. Performing more sit-to-stand research with sedentary, overweight individuals could offer valuable information regarding ways to help facilitate the chair rise motion for this population.
More current research studies examining the effects of various types of chairs would also be useful. Designing a chair that provides comfort while sitting, yet still allows the individual to stand up easily would be very beneficial for the elderly or for those who have limited joint range of motion or other lower extremity afflictions. Studies that clearly define optimal conditions for the sit-to-stand movement would provide a basis for designing such a chair.
The purpose of this study is to examine the differences in the kinematics of a subject rising from a rocking chair and a standard chair. The experiment will evaluate whether or not more forward flexion at the hip joint will occur in the rocking chair condition due to the lower height and more posterior tilt in the seat of the rocking chair. The study will also test the hypothesis that the subject will experience more forward travel of the knees in the horizontal plane due to greater forward velocity of the trunk. Since the rocking motion of the rocking chair will generate more linear momentum, it is hypothesized that transfer of momentum will allow the subject to complete the sit-to-stand movement more quickly in the rocking chair condition. If the second hypothesis is supported, then the subject would also likely experience greater maximal knee flexion and ankle dorsiflexion during the rising motion from the rocking chair. Finally, this analysis will determine whether or not angular velocities of the knee and hip joints will peak around the midpoint of the extension phase since a large amount of angular displacement will be occurring over a short period of time.
Two-dimensional video analysis was used for this study. The SVHS home video camera was placed on a tripod and leveled from side to side and front to back. The camera was placed 7-10 meters away from the subject so that the entire range of motion would be captured on film. In addition, it was aligned perpendicular to the center of the action. A shutter speed of 1/60 was selected.
Two chairs were selected for the experiment. The standard chair had limited seat cushioning, a back support, and no armrests. The seat was 0.54 m high and was horizontal with respect to the ground. The rocking chair also had back support and limited seat cushioning. The untilted seat was 0.49 m high and the chair did have armrests. The subject was a 21-year-old male (177 cm; 63.5kg). He was barefoot and dressed only in shorts. Colored adhesive markers were applied to the subject’s skin using tufskin. All markers were placed on the right side of the subject’s body, and joint landmarks were found using palpation. Colored dots were placed on the earlobe, the head of the humerus, the olecranon process, the greater trochanter of the femur, the lateral tibial condyle, the lateral malleolus, and the distal fifth metatarsal. The greater trochanter marker was placed on the subject’s shorts. Two markers were also placed at each end of a yardstick, which was placed in the picture to create a reference scale.
The subject first completed the sit-to-stand movement from the standard chair. Several practice trials were performed. The subject folded his hands in front of him, placed his feet at a desired location, and rose at a self-selected speed. The subject then performed the same task in a rocking chair, except he was asked to first rock backwards before initiating forward movement. Several trials of each condition were recorded, with rest in-between.
A Video Capture program was used to convert the video recording of the sit-to-stand movements into computer video files. The computer video was then edited using Adobe Premiere 4.0 to show the movement from beginning to end. Finally, Video Expert was used to digitize a stick figure representing the subject of the study. Connections from one marker to the next were made to form body segments for each frame. Data were imported into Microsoft Excel for analysis. Joint angles were calculated using adjacent body segments. Results were converted from pixels to meters using a conversion of 147 pixels/meter, and y-values of the joint coordinates were subtracted from the height of the pixel frame (374 pixels) in order to reposition them with respect to the bottom of the frame.
On all charts time was normalized for the phases of movement, so that the extension phase has positive time values, the forward thrust phase has negative time values, and the transition frame is the reference point, having a time value of 0.0 seconds. The charts were normalized to provide better comparison of data trends. The exceptions were Chart 3, which related to lift-off, and Chart 4, which does not concern both phases of the movement.
It was found that more forward flexion at the hip joint occurred in the rocking chair condition. The maximum hip flexion created a relative angle of 48.6°, whereas the maximum hip flexion for the standard chair resulted in a relative angle of 63.3° (Table 1). While the rocking chair condition began with less hip flexion than the standard condition, flexion in the rocking chair condition exceeded that of the standard chair at 0.50 seconds before extension phase. This difference in flexion increased slightly, and then remained relatively constant until full extension (Chart 1).
The second hypothesis, that there would be more forward travel of the knees in the horizontal plane in the rocking chair condition, was supported. The rocking chair condition demonstrated a forward displacement of 0.23 m, which was greater than the forward displacement of 0.060 m in the standard condition (Chart 2). At the transition point between phases, the subject experienced no forward displacement of the knee in the standard chair condition, indicating that the knees were in the same location they had been at the beginning of the movement. In the rocking chair condition, however, the subject’s knees had traveled forward 0.12 m at the transition time point. The forward travel of the knees was consistently larger in the rocking chair condition than the standard condition throughout the extension phase.
The second hypothesis stated that the greater forward travel of the knees would be due to the greater forward velocity of the upper trunk at lift-off, indicated by the shoulder velocity, in the rocking chair condition. This idea also supported by the data. The horizontal velocity of the shoulder at lift-off in the rocking chair condition was 1.3 m/s, which was greater than the 0.72 m/s horizontal velocity of the shoulder in the standard chair condition (Chart 3). This difference in horizontal velocities decreased shortly into the extension phase. The velocities then remained similar in both conditions until the end of the extension phase.
The next hypothesis predicted that the rocking chair condition would lead to a quicker completion of the sit-to-stand movement. This hypothesis was supported since the rocking chair condition lasted a duration of 1.07 seconds compared to 1.23 seconds for the standard condition (Chart 4).
The fourth hypothesis stated that if hypothesis two were supported, then it would be expected that higher maximum values for knee flexion and ankle dorsiflexion would occur in the rocking chair condition. This hypothesis was also supported. The minimum relative angle of the knee was slightly less in the rocking chair condition at 91.41°, compared to 92.20° in the standard condition (Table 1). The maximal dorsiflexion of the ankle in the rocking chair condition, 91.39°, was also lower than peak ankle dorsiflexion in the standard chair, 98.80°.
The final hypothesis, stating that the angular velocities of the hip and knee will peak around the midpoint of the extension phase, was supported. The peak extension angular velocity of the hip in the standard condition occurred very closely after the midpoint of the phase, with a difference of 0.051 s. The peak velocity of the hip in the rocking chair condition had a nearly identical result, peaking 0.050 s after midpoint. The angular velocity of the hip increased and decreased in a parabolic manner over the length of the extension phase in both of the conditions (Chart 5). The peak angular velocity of the knee in the rocking chair condition was also very close to the midpoint of the phase, peaking 0.017 s after midpoint. In the standard condition, the peak velocity of the knee was not as close to the midpoint, occurring 0.217 s after midpoint (Table 2). The angular velocity of the knee also increased and decreased in a parabolic manner during the extension phase (Chart 6). However, the angular velocity data points for the knee had less consistency than those for the hip.
This study aimed to determine whether or not significant kinematic differences existed between rising from a standard chair and rising from a rocking chair. Past studies examining different chair types found that slight variations in chair architecture could influence whether or not an individual experienced difficulty when attempting to rise from a chair (Ikeda et al., 1991; Burdett et al., 1985; Rodosky et al., 1989). The current study was conducted under the following assumptions: the subject would display a consistent chair-rise technique with other individuals similar in age and body type, the subject would rise in a sagitally symmetric manner since two-dimensional analysis was used, and the joint markers would accurately represent the corresponding joint landmarks. The methodology of the study remained constant between the two chair conditions, so that data could be compared with confidence.
As predicted the rocking chair condition showed greater forward flexion at the hip joint than the standard chair condition. The subject flexed 14.7° farther forward at the hip joint in the rocking chair condition. Wheeler et al. (1984) found that elderly subjects experienced greater trunk forward lean (11.4°) when rising from a chair with larger posterior seat slant in order to compensate for the more posterior initial positioning of the center of mass. The subject in this study began lift-off from the rocking chair while the seat was tilted more posteriorly than the seat of the standard chair, so the greater hip flexion agrees with that experienced by the subjects in the study by Wheeler et al. (1984). Additionally, the rocking chair was slightly lower than the standard chair. Lower chair height is associated with higher hip flexion (Rodosky et al., 1989; Burdett et al., 1985).
The maximal values for forward hip flexion experienced by the study participant were significantly higher than those reported as standards in the literature. Average hip flexion was reported to be 83.6 + 9.5 degrees when rising from a standard chair (Gross et al., 1998), but the subject of this study experienced a maximal hip flexion of 63.6° for the standard chair condition and 48.6° for the rocking chair condition. The high hip flexion values could be due to discrepancies in placement of joint markers on adjoining body segments. The subject also refrained from placing the feet underneath the seats of both chairs before standing. Placing the feet farther back behind the knee assists with repositioning the center of gravity over the new base of support (Wheeler et al., 1985). The participant in this study may have experienced high forward flexion of the trunk in an attempt to move the center of mass over the more forward base of support.
The greater forward travel of the knees in the rocking chair condition likely resulted from the linear momentum generated during the beginning of the forward-thrust phase of the sit-to-stand movement. Schenkman et al. (1990) hold that forward linear momentum during the beginning of the movement transfers into forward and upward momentum of the entire body as lift-off occurs. In this study, the velocity of the subject’s trunk at lift-off, as indicated by forward shoulder movement, was 0.60 m/s greater in the rocking chair condition than in the standard chair condition. This higher velocity would create more transferable momentum, which would result in more forward knee travel. The trunk velocities experienced by the subject at lift-off were higher than the average forward lean velocities calculated by Kerr et al. (1996). Since the subject begins to reach peak velocity shortly after lift-off, trunk velocity at lift-off would likely be higher than standard average forward lean velocity.
Because the subject was shown to have higher horizontal velocity at lift-off in the rocking chair condition, quicker completion of the extension phase of the chair rise movement would be expected if momentum-transfer was accomplished. The subject completed the extension movement 0.17s more quickly in the rocking chair condition. The extension phases right after lift-off for the standard chair and rocking occupy 63% and 57%, respectively, of the total time taken to complete the movement. This data is fairly close to that found by Schenkman et al. (1990), which stated that the subjects’ period of extension occupied 54% of the total time for the movement. The slight discrepancy could be the result of subject variability and self-selected speed preferences.
The higher knee and ankle flexion in the rocking chair condition agrees with conclusions of Rodosky et al. (1989), which state that lower chair heights create greater hip and knee flexion within subjects rising from a chair. Wheeler et al. (1984) also found that greater knee flexion occurs when the chair seat has more posterior slant. Both the lower seat height and the more slanted seat probably contributed to the subject’s higher knee and ankle flexion in the rocking chair condition. Furthermore, greater ankle flexion would be required to accommodate the farther horizontal travel of the knees, which was shown to occur in the rocking chair condition. Peak ankle dorsiflexion for the subject in this study, 98.8°, under standard conditions, is slightly lower than the peak dorsiflexion, 108.3 + 6.5 ° established in the study by Gross et al. (1998). Once again, subject variability, slight differences in joint marker placement, and initial positioning of the subject could account for these differences.
The subject’s peak hip and knee angular velocity occurred slightly after the midpoint of the extension phase for both the standard chair and rocking chair conditions. Ikeda et al. (1991) also found that maximal hip and knee angular velocities were reached during the middle portion of the extension phase. Hip flexion angular velocity reaches zero during the momentum-transfer phase, and begins to gain extension angular velocity (Schenkman et al., 1990). Therefore, during the extension phase, the subject’s movement is initially speeding up from lift-off position to extend the body and then slowing down to stop in the upright position. Speeding up and slowing down angular motion at the hip and knee joints would suggest that the most displacement would occur over a short period of time during the midpoint of the extension activity, as is seen in the subject of this study. Little difference between attainment of peak hip and knee angular velocities in the two chair conditions was noted. This shows that once the subject reaches the extension phase of the sit-to-stand transfer, patterns of angular displacement were similar over time. In the standard chair condition, the subject’s peak knee extension angular velocity agreed with that reported by Ikeda et al. (1991). Hip extension velocity, however, was higher in the subject of this study than in the subjects of the study by Ikeda et al. (1991). This discrepancy is expected due to the greater maximal hip flexion exhibited by the subject in this study.
Overall, the data exhibited by the subject in this study reflected patterns and trends typical of the sit-to-stand movement. The subject progressed through the various phases of movement depicted in earlier studies. The different chair conditions altered the kinematics of the individual, as was predicted since various chairs designs have been shown to affect chair-rise technique (Burdett et al., 1985; Rodosky et al., 1989; Wheeler et al., 1985). Based on displacement and velocity data of the study participant, one could speculate that rising from the rocking chair was a more difficult task than rising from the standard chair. The lower and more posteriorly slanted seat of the rocking chair and the more forward movement of the knees during the rocking chair condition contributed to greater hip, knee, and ankle flexion (Wheeler et al., 1984; Rodosky et al., 1991). Larger torques at lower extremity joints likely occurred as a result, and more knee extensor muscle activation would be necessary during the extension phase (Wheeler et al., 1984). Using arm supports on rocking chairs would help to decrease joint torques (Bahrami et al., 2000). While greater momentum is generated in the rocking chair, quicker completion of the sit-to-stand transfer does not imply less stress on the joints. While rocking chairs may be comfortable for sitting, they do not provide the best conditions for standing up. For those who lead sedentary lifestyles, the elderly, or those with limited lower extremity range of motion, chair types other than rocking chairs would help to facilitate the sit-to-stand movement.
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