Human Movement Science 41 (2015) 230–239
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Gender difference in older adult’s utilization of gravitational and ground reaction force in regulation of angular momentum during stair descent Kunal Singhal ⇑, Jemin Kim, Jeffrey Casebolt, Sangwoo Lee, Ki-Hoon Han, Young-Hoo Kwon Motion Analysis Laboratory, Department of Kinesiology, Texas Woman’s University, 304 Administration Drive, P.O. Box 425589, Denton, TX 76204, United States
a r t i c l e
i n f o
Article history: Available online 2 April 2015
PsycINFO classification: 3380 4010 Keywords: Stair descent transitions Angular momentum changes Gait Gender differences Human factor engineering
a b s t r a c t Angular momentum of the body is a highly controlled quantity signifying stability, therefore, it is essential to understand its regulation during stair descent. The purpose of this study was to investigate how older adults use gravity and ground reaction force to regulate the angular momentum of the body during stair descent. A total of 28 participants (12 male and 16 female; 68.5 years and 69.0 years of mean age respectively) performed stair descent from a level walk in a step-over-step manner at a self-selected speed over a custom made three-step staircase with embedded force plates. Kinematic and force data were used to calculate angular momentum, gravitational moment, and ground reaction force moment about the stance foot center of pressure. Women show a significantly greater change in normalized angular momentum (0.92 Nms/Kgm; p = .004) as compared to men (0.45 Nms/Kgm). Women produce higher normalized GRF (p = .031) during the double support phase. The angular momentum changes show largest backward regulation for Step 0 and forward regulation for Step 2. This greater difference in overall change in the angular momentum in women may explain their increased risk of fall over the stairs. Ó 2015 Elsevier B.V. All rights reserved.
⇑ Corresponding author at: 930 Madison Ave, Memphis, TN 38163, United States. Tel.: +1 901 448 2632. E-mail address: [email protected] (K. Singhal). http://dx.doi.org/10.1016/j.humov.2015.03.004 0167-9457/Ó 2015 Elsevier B.V. All rights reserved.
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1. Introduction Walking is a complicated task involving careful control of movement; executed through various cognitive, musculoskeletal, physiological, and psychological processes. With aging, these functions decline predisposing older adults to increased risk of fall and diminished activities of daily living (ADL). Activities like stair negotiation become highly demanding with age (Brouwer & Olney, 2004; Reid, Graham, & Costigan, 2010; Williamson & Fried, 1996) and are associated with a large number of falls among older adults in the U.S. (Centers for Disease Control., 2010; Lord, Ward, Williams, & Anstey, 1993). Stair descent accounts for 75% of falls making it more unsafe than stair ascent (Startzell, Owens, Mulfinger, & Cavanagh, 2000). Numerous biomechanical studies have been conducted to understand the reasons for increased risk of fall by analyzing the stair walking mechanism in older adults through comparisons of kinematics (Andriacchi, Andersson, Fermier, Stern, & Galante, 1980), ground reaction force (Hamel, Okita, Bus, & Cavanagh, 2005; Novak & Brouwer, 2011; GRF), joint moments and muscle activity (James & Parker, 1989; Spanjaard, Reeves, van Dieen, Baltzopoulos, & Maganaris, 2007, 2008), and center of mass–center of pressure (COM–COP) interactions (Startzell et al., 2000; Zachazewski, Riley, & Krebs, 1993). Though these studies indicated altered walking patterns in older adults, none provided an explanation of how people maintained a stable, fall-free descent. Mechanically, fall occurs due to a failure to control the angular momentum of the body. Whole body angular momentum is considered a tightly controlled variable (Bennett, Russell, Sheth, & Abel, 2010; Neptune & McGowan, 2011) during a variety of human tasks and studies have suggested that it may be important in maintaining balance and stability during ADL such as sit-to-stand (Riley, Krebs, & Popat, 1997) and trip events (Pijnappels, Bobbert, & van Dieen, 2004). It has also been suggested that during walking, the central nervous system and the control synergies help regulate the angular momentum and thus, the altered regulation of the angular momentum may be a sign of an increased risk of fall (Popovic, Hofmann, & Herr, 2004; Rietdyk, McGlothlin, & Knezovich, 2005). Most studies utilize segmental contribution in calculating angular momentum. This approach provides important information about the role of segments in whole body stability; nevertheless, the angular momentum is affected by the movement of the body COM with respect to the COP and the external forces acting on the body (Herr & Popovic, 2008; Popovic et al., 2004; Silverman, Wilken, Sinitski, & Neptune, 2011). However, the effects of external forces (ground reaction force and gravitational force) and COM–COP interactions on changes in the angular momentum have been largely overlooked. During the single support phase, the angular momentum is controlled by the gravitational moment alone as COM of the body moves from behind the COP of the stance leg to in front of the COP. However, during the subsequent double support phase, GRF moment acting on the contralateral foot provides an additional mechanism to control the fall of COM. Analyzing the angular momentum from the stance foot perspective reduces these external forces to gravitational force and GRF acting about the contralateral foot. In addition, Pijnappels et al. (2004) suggested that the stance leg provides maximum control of the angular momentum during fall-related events (Pijnappels et al., 2004). Therefore, utilizing the stance foot perspective is a novel way of understanding regulation of whole body angular momentum about the stance foot. Walking on stairs presents a unique situation where the motion of the body is constrained by step dimension, forcing individuals to walk in a certain pattern. Transition from level walking to stair descent and vice versa would require a careful regulation of angular momentum in order to maintain stability because stair descent and level walking are inherently different. Theoretically, the top most transition step should result in anticlockwise momentum in order to provide control to the descent motion while the step from stairs to level ground should result in clockwise momentum as individuals land on the ground. However, this has not yet been examined and it is important to analyze the mechanism of regulation of the angular momentum of the body during the stair descent transitions. Since most studies do not include a level walk prior to the stair descent provides a more realistic and unique approach to investigate stair descent mechanics as compared to other studies.
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The purpose of this study was to investigate the role of GRF and gravitational force on regulation of angular momentum in healthy older adults during stair descent. It was hypothesized that: (1) There would be a significant gender difference in changes in angular momentum during stair descent over each step in a 3-step stair case; (2) There would be significant gender differences in the factors (external moments and duration of application of these moments) causing changes in the angular momentum during stair descent over each step in a 3-step stair case; (3) The topmost and the last transition step will result in more anti-clockwise and clockwise changes in the angular momentum, respectively. 2. Methods 2.1. Participants A total of 28 healthy and active older adults (12 male and 16 female; mean age: 68.5 ± 7 years and 69 ± 5 years, respectively; mean body mass: 77.0 ± 9.5 kg and 66.2 ± 15.9 kg, respectively) from the community volunteered in this Institutional Review Board approved study. Participants completed a self-reported health history questionnaire as a prescreening for good general health, after which an informed consent was obtained. The exclusion criteria for the study were any physical, mental, or emotional limitations, other than those associated with healthy aging, which may prevent the potential participants from their normal gait under research protocol. In addition, potential participants who had any injuries to any other joints or structures for the last three months prior to data collection or any other conditions which might affect the way they walk were excluded. 2.2. Experimental setup Stair descent was performed on a three step staircase, with each step of 0.18 m height, 0.28 m depth and 1.20 m width to conform to American Disability Act requirements. Four AMTI force plates (OR-6-5, American Mechanical Technologies, Inc., Watertown, MA) were used with one mounted in the middle of each step and one on level ground. The top end of the staircase had a 1.20 m wide and 3.60 m long walkway; no handrails were provided. 2.3. Trial conditions Participants wore their exercise shoes and walked at a self-selected speed throughout the data collection period. Participants started from the end of the walkway and descended on the staircase in a step-over-step manner and continued their walk for approximately 3.60 m after stepping down from the staircase. Participants were allowed to walk on the staircase until they were comfortable with the walking environment. The trials where the stance foot was in contact with its respective force plate only were recorded as ‘‘good’’ trials. Five good trials were collected for each participant. 2.4. Data collection and processing Body was modeled as a 13 segment rigid body model with modified Helen Hayes marker set (Silverman, Wilken, Sinitski, & Neptune, 2012). Stair descent trials were captured using a 10-camera Vicon optical real-time motion capture system (Vicon, Inc., Oxford, UK). GRF and motion data were sampled at 250 Hz. Further processing and analysis was conducted in Kwon3D XP motion analysis software (Visol, Inc., Seoul, Korea). Data reduction was performed using a zero-phase lag 4th-order Butterworth low-pass filter with a cut-off frequency of 6 Hz. Reference frame was set as positive z-axis directed vertically up, positive y-axis directed in the direction of motion, and positive x-axis directed from left to right. Segmental center of mass location was obtained from the body segment parameter data (Chandler, Clauser, McConville, Reynolds, & Young, 1975). The XYZ (mediolateral-anteroposterior-longitudinal) rotation sequence and inverse dynamics were used to calculate the joint moments and powers.
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2.5. Data analysis Stair descent was divided into three steps. The topmost step of the staircase was labeled Step 0, and the subsequent steps were recorded as Step 1 and Step 2. For each step, only the stance phase was analyzed. The stance foot for a particular step was defined as the foot in contact with the force plate on that step. The step cycle analyzed began with toe-off of the contralateral extremity and ended with the toe-off of the stance foot at which point the step cycle of the next step began. The step cycle for each step was divided into a single support (SS) phase and a subsequent double support (DS) phase. The initial DS phase of a typical level walking gait cycle was not considered as it was also the second double support phase of the previous step (Fig. 1). The SS phase began with the toe-off and ended with the foot contact of the contralateral extremity. The SS phase was further subdivided into SS1 and SS2 phases based on the posterior and anterior position of COM with respect to the COP of the stance foot, respectively (Fig. 1). The gravitational moment (MW) about the stance foot (the moment produced by the weight about the stance foot) affects the angular momentum throughout the entire step, while the regulatory effect of the GRF moment (MGRF) is present during the DS phase only. For each step, the COP of the stance foot on that step was used as the point of reference and all the variables, the angular momentum of the body (trunk, upper extremity, lower extremity, and head; HCOP), MW, and MGRF, were computed with respect to that foot:
HCOP ¼ ðr COM r st Þ mv COM þ HCOM
ð1Þ
MW ¼ ðr COM r st Þ mg
ð2Þ
MGRF ¼ ðr sw rst Þ F GRF
ð3Þ
where rsw and rst are the COP positions of the contralateral and stance foot, respectively, rCOM, is the COM position and HCOM is the angular momentum of the body about the COM. During the SS phase, angular momentum of the body was controlled by the Mw only, while during the DS phase it was regulated by the Mw and MGRF acting on the contralateral foot (Fig. 2). A representative trial for each individual was selected using inspection and used to generate normalized patterns of sagittal HCOP, MW and MGRF (Figs. 3 and 4). The sagittal (x-component) angular momentum remained negative (clockwise/forward) throughout the entire step cycle. For interpretation purposes, increase and decrease in negative angular momentum were labeled as forward regulation and backward regulation, respectively. Similarly, the positive values of gravitational and GRF moments were labeled as backward moment and vice versa.
Level
Step0
Step0 Step1
+Z
DS Phase
SS2 Phase
SS1 Phase
Step0
Step0 Step 1
Step1
Step 1 Step 2 Level
+Y +X Fig. 1. Division of stance phase into sub-phases over the steps.
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GRF mg Stance foot
MW MGRF
Swing foot
Fig. 2. Diagrammatic representation of moment produced by GRF and gravity about the stance foot COP.
During the SS1, angular momentum increased (negative momentum decreased) showing a backward regulation (Fig. 4). During the SS2, angular momentum decreased (negative momentum increased) showing a forward regulation which changed to backward regulation again during the DS phase. Gravitational moment during the SS1 was a backward moment but kept decreasing until COM moves in front of COP and became increasingly negative during the SS2 resulting in a forward gravitational moment. The gravitational moment during the DS phase was also in the forward direction. GRF moment was applied only during the DS phase and maintained a backward moment throughout the phase (Figs. 3 and 4). Net change in HCOP within each step and phase were computed as the difference between initial and final values. Since the changes in the DS phase are due to MW and MGRF, the change in angular momentum due to each of these moments (DHW and DHGRF, respectively) was computed as the time integral of the respective moments. As the changes in HCOP during the SS phase were solely from MW (a)
(b)
(c)
Fig. 3. Representative pattern of normalized (a) angular momentum, (b) gravitational torque, (c) GRF torque; in women during steps 0, 1 and 2 of stair descent. Error bars represent standard deviation.
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(a)
(b)
(c)
Fig. 4. Representative pattern of normalized (a) angular momentum, (b) gravitational torque, (c) GRF torque; in men during steps 0, 1 and 2 of stair descent. Error bars represent standard deviation.
only, it was not necessary to compute the time integral. All the angular momentum values were normalized with ‘body mass ⁄ leg length’. For an in-depth understanding of the mechanism of angular momentum change, factors affecting the Mw and MGRF (forces and moment arm) and duration of moment application were also analyzed within each phase. The horizontal deviation of the COM from the stance foot COP at maximum MW in each phase was used as the max MW moment arm (MAw). The perpendicular distance from the stance foot COP to the line of action of the GRF projected to the sagittal plane was used as the moment arm of the GRF. The GRF magnitude and moment arm at the time point of maximum GRF moment were extracted. GRFSag was normalized to body mass; while MAw and MAGRF were normalized to the leg length of the participants. The duration of each phase was calculated from the time difference between each event and was reported in seconds. Mean of each of these variables was computed from the trials and used for statistical analysis. 2.6. Statistical analysis The statistical analysis was conducted using a two-way mixed factor repeated measures design with gender (between-subject) and step (within-subject) factors. For the first hypothesis, the dependent variables, DHCOP (over the entire step and within each phase), DHW and DHGRF, were compared for gender and step. For the second hypothesis, GRFSag, MAw, MAGRF, and duration of each phase were analyzed. Differences in dependent variables among steps and gender were evaluated using two-way repeated measures multivariate analysis of variance (MANOVA). Significant MANOVA was followed up with univariate tests and a post hoc analysis with Bonferroni correction. Alpha level (a) was set at .05 and statistical analysis was conducted with SPSS 14.0 (SPSS, Inc., Chicago, Illinois). 3. Results Gender and steps did not show a significant interaction, however there were significant step and gender effects (p = .001 and .04, respectively).
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3.1. Net changes in angular momentum Significant gender effects were observed in total change in angular momentum (p = .004), change during the DS phase (p = .027), and DHGRF (p = .031) with women showing higher values (Table 1). Significant step effects were observed in all angular momentum variables: DHCOP, angular momentum changes during the SS1, the SS2 and the DS phase, DHGRF, and DHW (p = .001). The greatest change in angular momentum was observed in Step 0. Step 1 showed a very small change while a small forward change was noticed for Step 2 (Table 1). The changes during the SS1 phase were found to be largest during Step 0, followed by Step 1 and then Step 2 (Table 1) while the trends were reversed for the changes during the SS2 phase. Step 0 also showed greatest changes during the DS phase. This was followed by Step 2 and then Step 1 (Table 1). Significantly larger forward regulation was found in DHW during Step 2 as compared to Step 0 and Step 1 (Table 1). DHGRF showed backward regulation with largest changes present during Step 2 as compared to Step 0 and Step 1. 3.2. Moment arm of gravitational moment (MAw) No gender effect was observed for MAw, while a significant step effect was observed for MAw during all the phases (p = .001). The moment arm during the SS1 phase was found to be significantly larger for Step 0 followed by Step 1 and then Step 2 (Table 1). The moment arm during the SS2 was
Table 1 Changes in the angular momentum, phase duration, GRF, moment arm of GRF and gravitational force within each step in men and women. Step 0
Changes in angular momentum (Nms/Kgm)
SS1 SS2 DS
D HW DHGRF Total Phase duration (sec)
SS1 SS2 DS
MAW (m)
SS1 SS2 DS
Step 1
Step 2
Men
Women
Men
Women
Men
Women
0.29⁄ (0.08) 0.03 (0.04) 0.11⁄ (0.07) 0.76 (0.40) 0.89 (0.51) 0.38⁄ (0.09)
0.28⁄ (0.06) 0.05 (0.4) 0.15⁄,n (0.05) 0.89 (0.34) 1.06n (0.42) 0.37⁄,n (0.05)
0.05§ (0.04) 0.12§ (0.05) 0.04 (0.06) 0.80 (0.17) 0.84 (0.26) 0.04§ (0.07)
0.08§ (0.04) 0.13§ (0.04) 0.09n (0.05) 0.81 (0.27) 0.90n (0.32) 0.05§,n (0.03)
0.02 (0.01) 0.31⁄ (0.09) 0.08 (0.04) 1.02⁄ (0.53) 1.10⁄ (0.38) 0.21 (0.09)
0.04 (0.03)
0.27⁄ (0.06) 0.26 (0.08) 0.12⁄ (0.05)
0.27⁄ (0.05) 0.30 (0.07) 0.14⁄,n (0.03)
0.11§ (0.04) 0.36 (0.06) 0.10 (0.03)
0.14§ (0.05) 0.38 (0.07) 0.12 (0.02)
0.06 (0.03) 0.44⁄ (0.09) 0.10 (0.03)
0.09 (0.04)
0.20⁄ (0.03) 0.11 (0.02) 0.03 (0.04)
0.19⁄ (0.02) 0.12 (0.03) 0.04 (0.07)
0.10 (0.04) 0.13 (0.04) 0.01 (0.07)
0.10 (0.03) 0.12 (0.03) 0.02 (0.13)
0.06 (0.04) 0.28⁄ (0.06) 0.06⁄ (0.03)
0.07 (0.03)
0.29⁄ (0.08) 0.10n (0.04) 1.10⁄ (0.35) 1.35⁄,n (0.45) 0.13n (0.05)
0.33⁄ (0.06) 0.12 (0.03)
0.23⁄ (0.04) 0.05⁄ (0.03)
GRF (N/kg)
DS
11.68 (2.38)
12.61n (2.57)
11.03 (3.69)
12.94n (2.59)
12.16⁄ (3.13)
15.36⁄,n (2.30)
MAGRF (m)
DS
0.31 (0.08)
0.33 (0.09)
0.31 (0.07)
0.27 (0.05)
0.36⁄ (0.07)
0.37⁄ (0.06)
Note: values in parenthesis represent standard deviation. n denotes gender effect and ⁄ and § denote post-hoc results for step effect. ⁄ denotes the step with statistically largest value while § signifies second largest step value.
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significantly longer for Step 2 with Step 1 and Step 0 showing similar values (Table 1). During the DS phase, the moment arm was highest for Step 2 as compared to Step 0 and Step 1 (Table 1). 3.3. Sagittal plane GRF (GRFSag) and moment arm of DHGRF (MAGRF) Significant gender and step effects were observed for GRFSag (p = .031 and .003, respectively). Women showed significantly higher values as compared to men. GRFSag values during Step 0 and 1 were similar, while Step 2 showed significantly larger values. MAGRF showed significant step effect (p = .001) with largest value observed during Step 2. MAGRF values for Step 0 and 1 were found to be similar (Table 1). 3.4. Phase duration Significant step effect (p = .002) was found for duration of all the phases, while gender effect was found for the DS phase (p = .038). The SS1 phase for Step 0 was the longest followed by Step 1 and then Step 2 (Table 1). The SS2 phase was longest for Step 2 followed by Step 1 and then Step 0 (Table 1). The DS phase duration was found to be longest for Step 0. Women spent significantly larger time in the DS phase as compared to men (Table 1).
4. Discussion Regulation of angular momentum of the body is essential for maintenance of balance and execution of the motion effectively. This study aimed to better understand biomechanics of stair descent by analyzing the angular momentum of the body and external moments affecting the changes in it. To our knowledge, this study was the first to analyze angular momentum of the body to explain the gender differences and changes within each step while descending stairs. More importantly, including level walking before participants descend stairs has resulted in a more realistic execution of tasks as compared to other studies where participant began stair descent from standing position at the top of the stair case. The study’s methodological approach to understand the control of angular momentum during stair descent is highly applicable as gravitational force and GRF of contralateral foot act about the stance foot to regulate the angular momentum. Thus, this approach provides how external forces interact with COM to create torque about the stance foot COP and help in regulating angular momentum. Men exhibited less change in their angular momentum over the entire stair descent as compared to women (Table 1). Since smaller changes in angular momentum during a given task (descent in our case) imply greater stability (Silverman et al., 2011), results of the study suggest increased risk of falls among older women during stair descent. It has been suggested that women show lower peak knee eccentric power on the upper transition step while lowering as compared to men (Singhal et al., 2014). Though the study only compared upper transition, it provides evidence to support the results of the current study. On analyzing changes within each phase over the steps, it was found that the women showed larger changes during the DS phase as compared to men (Table 1). In DS phase, the gravitational moment performs forward regulation which is countered by backward regulation of GRF moment. Women have been reported to show lower scores on the stair climbing component of Dynamic Gait Index primarily because of using handrails, irrespective of being a faller or a non-faller (Herman, Inbar-Borovsky, Brozgol, Giladi, & Hausdorff, 2009). Evidence of gender differences during gait transition while descending, with women showing greater precaution than men during upper transition (transition from flat surface to stairs) have been reported by previous studies (Hamel & Cavanagh, 2004; Herman et al., 2009). A higher backward regulation of angular momentum provides a quantitative evidence of an altered strategy of controlling balance among women. Analysis of the angular momentum produced by Mw and MGRF indicates that women show a significantly higher DHGRF as compared to men. When the factors affecting the DHGRF were analyzed, it was found that women spent longer time during the DS phase and also showed a greater GRF at the
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instance of maximum MGRF. Thus, women produce a higher GRF and stay in the DS phase for a longer duration of time to bring about greater changes in angular momentum during the stair descent. The angular momentum pattern for each step showed distinct patterns. The results highlight the importance of Step 0, the transition step, in controlling angular momentum during stair descent by providing a backward regulation. Step 1 shows minimal changes suggesting adoption of a more stable pattern. Step 2 was a transition step from stairs to level ground, and thus it is reasonable to expect individuals to show a larger forward regulation. A recent study has reported the tendency of people to show increased control of angular momentum during descent (Silverman, Neptune, Sinitski, & Wilken, 2014); thus this larger change in angular momentum during Step 2 confirms our assertion. As stair descent transitions require higher neuromuscular effort to control the body movements (James & Parker, 1989), large changes in angular momentum during transitions, found in this study, suggest greater challenges to balance and increased neuromuscular effort. These results were corroborated by the changes observed within each phase and DHW and DHGRF. The backward regulation within the SS1 was the largest for Step 0 which subsequently decreased as the steps progressed. This was because the moment arm of MW and the time duration of the SS1 for Step 0 was the highest resulting in greater backward regulation, implying that participants assumed a more vertical stance as the stair descent progressed. The SS2 phase saw a reverse trend with Step 2 showing the largest forward regulation because of increased MAW. When the changes in the DS phase were analyzed it was found that even though the highest values of DHW and DHGRF were observed for Step 2, the changes in DHCOP during the DS phase was largest for Step 0. MAW, MAGRF and GRF values during the DS phase were largest for Step 2. These results along with changes in MAW during the SS2 phase imply that separation between COP of the stance foot and COM of the body was largest for Step 2 establishing a greater forward lean by the participants during the transition from stairs to level walking. This behavior of participants was also reported by Lee and Chou (2007). One limitation of the current study was that the angular momentum was normalized to leg length of the participants though the step dimensions remained constant. This resulted in uniform comparison among the variables but altered the stair descent task as the ratio of step height to leg length was higher in women as compared to men. This may have resulted in overestimation of the changes in angular momentum; however, no gender differences were observed during the SS2 phase when the body is lowered negating any effects of altered step height to leg length ratio in angular momentum. 5. Summary and conclusion The study analyzed the gender differences in regulation of angular momentum through gravitational moment and GRF moment about the stance leg during stair descent. The changes in angular momentum were measured within each step along with GRF, moment arm of external forces and time of application of external moments. The results of the study showed that the women had a greater change in angular momentum through the entire descent. These changes in angular momentum were brought by increasing the GRF and the duration of double support period. These differences highlight a possible instability in stair descent gait in women. Step 0 and Step 2 showed a transition behavior with increased backward and forward regulation respectively. The SS1 phase during Step 0 and the SS2 phase during Step 2 brought about these changes. Similar studies should be conducted on stair ascent since the challenges during ascent are different than descent. Moreover, a staircase with more than 3 steps can be used to analyze the steady stair gait pattern to establish whether angular momentum changes minimize during this period. References Andriacchi, T. P., Andersson, G. B., Fermier, R. W., Stern, D., & Galante, J. O. (1980). A study of lower-limb mechanics during stairclimbing [Research Support, U.S. Gov’t, P.H.S.]. The Journal of Bone and Joint Surgery. American Volume, 62(5), 749–757. Bennett, B. C., Russell, S. D., Sheth, P., & Abel, M. F. (2010). Angular momentum of walking at different speeds. Human Movement Science, 29, 114–124. Brouwer, B., & Olney, S. J. (2004). Aging skeletal muscle and the impact of resistance exercise. Physiotherapy Canada, 56, 80–87. Centers for Disease Control. (2010, December 8). Falls Among Older Adults: An Overview. Retrieved 4th May, 2011, from .
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Chandler, R. F., Clauser, C. E., McConville, J. T., Reynolds, H. M., & Young, J. W. (1975). Investigation of inertial properties of the human body. Wright-Patterson Air Force Base, Ohio: Aerospace Medical Research Laboratories. Hamel, K. A., & Cavanagh, P. R. (2004). Stair performance in people aged 75 and older. Journal of the American Geriatrics Society, 52, 563–567. Hamel, K. A., Okita, N., Bus, S. A., & Cavanagh, P. R. (2005). A comparison of foot/ground interaction during stair negotiation and level walking in young and older women. Ergonomics, 48(8), 1047–1056. Herman, T., Inbar-Borovsky, N., Brozgol, M., Giladi, N., & Hausdorff, J. M. (2009). The Dynamic Gait Index in healthy older adults: The role of stair climbing, fear of falling and gender. Gait & Posture, 29, 237–241. Herr, H., & Popovic, M. (2008). Angular momentum in human walking. The Journal of Experimental Biology, 211, 467–481. James, B., & Parker, A. W. (1989). Electromyography of stair locomotion in elderly men and women. Electromyography and Clinical Neurophysiology, 29, 161–168. Lee, H. J., & Chou, L. S. (2007). Balance control during stair negotiation in older adults. Journal of Biomechanics, 40(11), 2530–2536. Lord, S. R., Ward, J. A., Williams, P., & Anstey, K. J. (1993). An epidemiological study of falls in older community-dwelling women: The Randwick falls and fractures study. [Research Support, Non-U.S. Gov’t]. Australian Journal of Public Health, 17(3), 240–245. Neptune, R. R., & McGowan, C. P. (2011). Muscle contributions to whole body sagittal plane angular momentum during walking. Journal of Biomechanics, 44, 6–12. Novak, A. C., & Brouwer, B. (2011). Sagittal and frontal lower limb joint moments during stair ascent and descent in young and older adults. Gait & Posture, 33(1), 54–60. Pijnappels, M., Bobbert, M. F., & van Dieen, J. H. (2004). Contribution of the support limb in control of angular momentum after tripping. Journal of Biomechanics, 37, 1811–1818. Popovic, M., Hofmann, A., & Herr, H. (2004). Angular momentum regulation during human walking: Biomechanics and control. Paper presented at the IEEE International Conference on Robotics & Automation, New Orleans, LA. Reid, S. M., Graham, R. B., & Costigan, P. A. (2010). Differentiation of young and older adult stair climbing gait using principal component analysis. Gait & Posture, 31, 197–203. Rietdyk, S., McGlothlin, J. D., & Knezovich, M. J. (2005). Work experience mitigated age-related differences in balance and mobility during surface accommodation. [Controlled Clinical Trial Research Support, U.S. Gov’t, P.H.S.]. Clinical biomechanics, 20(10), 1085–1093. Riley, P. O., Krebs, D. E., & Popat, R. (1997). Biomechanical analysis of failed sit-to-stand. IEEE Translational Rehabilitation and Engineering, 5, 353–359. Silverman, A. K., Wilken, J. M., Sinitski, E. H., & Neptune, R. R. (2011). Whole body angular momentum while walking on sloped surfaces. Long Beach, CA: Paper presented at the American Society of Biomechanics. Silverman, A. K., Wilken, J. M., Sinitski, E. H., & Neptune, R. R. (2012). Whole body angular momentum in incline and decline walking. Journal of Biomechanics, 45, 965–971. Silverman, A. K., Neptune, R. R., Sinitski, E. H., & Wilken, J. M. (2014). Whole body angular momentum during stair ascent and descent. Gait & Posture, 39, 1109–1114. Singhal, K., Kim, J., Casebolt, J., Lee, S., Han, K.-H., & Kwon, Y.-H. (2014). Kinetic comparison of older men and women during walk-to-stair descent transition. Gait & Posture, 40, 600–604. Spanjaard, M., Reeves, N. D., van Dieen, J. H., Baltzopoulos, V., & Maganaris, C. N. (2007). Gastrocnemius muscle fascicle behavior during stair negotiation in humans. Journal of Applied Physiology, 102(4), 1618–1623. Spanjaard, M., Reeves, N. D., van Dieen, J. H., Baltzopoulos, V., & Maganaris, C. N. (2008). Lower-limb biomechanics during stair descent: Influence of step-height and body mass. Journal of Experimental Biology, 211(Pt 9), 1368–1375. Startzell, J. K., Owens, D. A., Mulfinger, L. M., & Cavanagh, P. R. (2000). Stair negotiation in older people: A review. Journal of the American Geriatrics Society, 48, 567–580. Williamson, J. D., & Fried, L. P. (1996). Characterization of older adults who attribute functional decrements to ‘‘old age’’. Journal of the American Geriatric Society, 44, 1429–1434. Zachazewski, J. E., Riley, P. O., & Krebs, D. E. (1993). Biomechanical analysis of body mass transfer during stair ascent and descent of healthy subjects. [Research Support, Non-U.S. Gov’t]. Journal of Rehabilitation Research and Development, 30(4), 412–422.
Contents lists available at ScienceDirect
Human Movement Science journal homepage: www.elsevier.com/locate/humov
Gender difference in older adult’s utilization of gravitational and ground reaction force in regulation of angular momentum during stair descent Kunal Singhal ⇑, Jemin Kim, Jeffrey Casebolt, Sangwoo Lee, Ki-Hoon Han, Young-Hoo Kwon Motion Analysis Laboratory, Department of Kinesiology, Texas Woman’s University, 304 Administration Drive, P.O. Box 425589, Denton, TX 76204, United States
a r t i c l e
i n f o
Article history: Available online 2 April 2015
PsycINFO classification: 3380 4010 Keywords: Stair descent transitions Angular momentum changes Gait Gender differences Human factor engineering
a b s t r a c t Angular momentum of the body is a highly controlled quantity signifying stability, therefore, it is essential to understand its regulation during stair descent. The purpose of this study was to investigate how older adults use gravity and ground reaction force to regulate the angular momentum of the body during stair descent. A total of 28 participants (12 male and 16 female; 68.5 years and 69.0 years of mean age respectively) performed stair descent from a level walk in a step-over-step manner at a self-selected speed over a custom made three-step staircase with embedded force plates. Kinematic and force data were used to calculate angular momentum, gravitational moment, and ground reaction force moment about the stance foot center of pressure. Women show a significantly greater change in normalized angular momentum (0.92 Nms/Kgm; p = .004) as compared to men (0.45 Nms/Kgm). Women produce higher normalized GRF (p = .031) during the double support phase. The angular momentum changes show largest backward regulation for Step 0 and forward regulation for Step 2. This greater difference in overall change in the angular momentum in women may explain their increased risk of fall over the stairs. Ó 2015 Elsevier B.V. All rights reserved.
⇑ Corresponding author at: 930 Madison Ave, Memphis, TN 38163, United States. Tel.: +1 901 448 2632. E-mail address: [email protected] (K. Singhal). http://dx.doi.org/10.1016/j.humov.2015.03.004 0167-9457/Ó 2015 Elsevier B.V. All rights reserved.
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1. Introduction Walking is a complicated task involving careful control of movement; executed through various cognitive, musculoskeletal, physiological, and psychological processes. With aging, these functions decline predisposing older adults to increased risk of fall and diminished activities of daily living (ADL). Activities like stair negotiation become highly demanding with age (Brouwer & Olney, 2004; Reid, Graham, & Costigan, 2010; Williamson & Fried, 1996) and are associated with a large number of falls among older adults in the U.S. (Centers for Disease Control., 2010; Lord, Ward, Williams, & Anstey, 1993). Stair descent accounts for 75% of falls making it more unsafe than stair ascent (Startzell, Owens, Mulfinger, & Cavanagh, 2000). Numerous biomechanical studies have been conducted to understand the reasons for increased risk of fall by analyzing the stair walking mechanism in older adults through comparisons of kinematics (Andriacchi, Andersson, Fermier, Stern, & Galante, 1980), ground reaction force (Hamel, Okita, Bus, & Cavanagh, 2005; Novak & Brouwer, 2011; GRF), joint moments and muscle activity (James & Parker, 1989; Spanjaard, Reeves, van Dieen, Baltzopoulos, & Maganaris, 2007, 2008), and center of mass–center of pressure (COM–COP) interactions (Startzell et al., 2000; Zachazewski, Riley, & Krebs, 1993). Though these studies indicated altered walking patterns in older adults, none provided an explanation of how people maintained a stable, fall-free descent. Mechanically, fall occurs due to a failure to control the angular momentum of the body. Whole body angular momentum is considered a tightly controlled variable (Bennett, Russell, Sheth, & Abel, 2010; Neptune & McGowan, 2011) during a variety of human tasks and studies have suggested that it may be important in maintaining balance and stability during ADL such as sit-to-stand (Riley, Krebs, & Popat, 1997) and trip events (Pijnappels, Bobbert, & van Dieen, 2004). It has also been suggested that during walking, the central nervous system and the control synergies help regulate the angular momentum and thus, the altered regulation of the angular momentum may be a sign of an increased risk of fall (Popovic, Hofmann, & Herr, 2004; Rietdyk, McGlothlin, & Knezovich, 2005). Most studies utilize segmental contribution in calculating angular momentum. This approach provides important information about the role of segments in whole body stability; nevertheless, the angular momentum is affected by the movement of the body COM with respect to the COP and the external forces acting on the body (Herr & Popovic, 2008; Popovic et al., 2004; Silverman, Wilken, Sinitski, & Neptune, 2011). However, the effects of external forces (ground reaction force and gravitational force) and COM–COP interactions on changes in the angular momentum have been largely overlooked. During the single support phase, the angular momentum is controlled by the gravitational moment alone as COM of the body moves from behind the COP of the stance leg to in front of the COP. However, during the subsequent double support phase, GRF moment acting on the contralateral foot provides an additional mechanism to control the fall of COM. Analyzing the angular momentum from the stance foot perspective reduces these external forces to gravitational force and GRF acting about the contralateral foot. In addition, Pijnappels et al. (2004) suggested that the stance leg provides maximum control of the angular momentum during fall-related events (Pijnappels et al., 2004). Therefore, utilizing the stance foot perspective is a novel way of understanding regulation of whole body angular momentum about the stance foot. Walking on stairs presents a unique situation where the motion of the body is constrained by step dimension, forcing individuals to walk in a certain pattern. Transition from level walking to stair descent and vice versa would require a careful regulation of angular momentum in order to maintain stability because stair descent and level walking are inherently different. Theoretically, the top most transition step should result in anticlockwise momentum in order to provide control to the descent motion while the step from stairs to level ground should result in clockwise momentum as individuals land on the ground. However, this has not yet been examined and it is important to analyze the mechanism of regulation of the angular momentum of the body during the stair descent transitions. Since most studies do not include a level walk prior to the stair descent provides a more realistic and unique approach to investigate stair descent mechanics as compared to other studies.
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The purpose of this study was to investigate the role of GRF and gravitational force on regulation of angular momentum in healthy older adults during stair descent. It was hypothesized that: (1) There would be a significant gender difference in changes in angular momentum during stair descent over each step in a 3-step stair case; (2) There would be significant gender differences in the factors (external moments and duration of application of these moments) causing changes in the angular momentum during stair descent over each step in a 3-step stair case; (3) The topmost and the last transition step will result in more anti-clockwise and clockwise changes in the angular momentum, respectively. 2. Methods 2.1. Participants A total of 28 healthy and active older adults (12 male and 16 female; mean age: 68.5 ± 7 years and 69 ± 5 years, respectively; mean body mass: 77.0 ± 9.5 kg and 66.2 ± 15.9 kg, respectively) from the community volunteered in this Institutional Review Board approved study. Participants completed a self-reported health history questionnaire as a prescreening for good general health, after which an informed consent was obtained. The exclusion criteria for the study were any physical, mental, or emotional limitations, other than those associated with healthy aging, which may prevent the potential participants from their normal gait under research protocol. In addition, potential participants who had any injuries to any other joints or structures for the last three months prior to data collection or any other conditions which might affect the way they walk were excluded. 2.2. Experimental setup Stair descent was performed on a three step staircase, with each step of 0.18 m height, 0.28 m depth and 1.20 m width to conform to American Disability Act requirements. Four AMTI force plates (OR-6-5, American Mechanical Technologies, Inc., Watertown, MA) were used with one mounted in the middle of each step and one on level ground. The top end of the staircase had a 1.20 m wide and 3.60 m long walkway; no handrails were provided. 2.3. Trial conditions Participants wore their exercise shoes and walked at a self-selected speed throughout the data collection period. Participants started from the end of the walkway and descended on the staircase in a step-over-step manner and continued their walk for approximately 3.60 m after stepping down from the staircase. Participants were allowed to walk on the staircase until they were comfortable with the walking environment. The trials where the stance foot was in contact with its respective force plate only were recorded as ‘‘good’’ trials. Five good trials were collected for each participant. 2.4. Data collection and processing Body was modeled as a 13 segment rigid body model with modified Helen Hayes marker set (Silverman, Wilken, Sinitski, & Neptune, 2012). Stair descent trials were captured using a 10-camera Vicon optical real-time motion capture system (Vicon, Inc., Oxford, UK). GRF and motion data were sampled at 250 Hz. Further processing and analysis was conducted in Kwon3D XP motion analysis software (Visol, Inc., Seoul, Korea). Data reduction was performed using a zero-phase lag 4th-order Butterworth low-pass filter with a cut-off frequency of 6 Hz. Reference frame was set as positive z-axis directed vertically up, positive y-axis directed in the direction of motion, and positive x-axis directed from left to right. Segmental center of mass location was obtained from the body segment parameter data (Chandler, Clauser, McConville, Reynolds, & Young, 1975). The XYZ (mediolateral-anteroposterior-longitudinal) rotation sequence and inverse dynamics were used to calculate the joint moments and powers.
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2.5. Data analysis Stair descent was divided into three steps. The topmost step of the staircase was labeled Step 0, and the subsequent steps were recorded as Step 1 and Step 2. For each step, only the stance phase was analyzed. The stance foot for a particular step was defined as the foot in contact with the force plate on that step. The step cycle analyzed began with toe-off of the contralateral extremity and ended with the toe-off of the stance foot at which point the step cycle of the next step began. The step cycle for each step was divided into a single support (SS) phase and a subsequent double support (DS) phase. The initial DS phase of a typical level walking gait cycle was not considered as it was also the second double support phase of the previous step (Fig. 1). The SS phase began with the toe-off and ended with the foot contact of the contralateral extremity. The SS phase was further subdivided into SS1 and SS2 phases based on the posterior and anterior position of COM with respect to the COP of the stance foot, respectively (Fig. 1). The gravitational moment (MW) about the stance foot (the moment produced by the weight about the stance foot) affects the angular momentum throughout the entire step, while the regulatory effect of the GRF moment (MGRF) is present during the DS phase only. For each step, the COP of the stance foot on that step was used as the point of reference and all the variables, the angular momentum of the body (trunk, upper extremity, lower extremity, and head; HCOP), MW, and MGRF, were computed with respect to that foot:
HCOP ¼ ðr COM r st Þ mv COM þ HCOM
ð1Þ
MW ¼ ðr COM r st Þ mg
ð2Þ
MGRF ¼ ðr sw rst Þ F GRF
ð3Þ
where rsw and rst are the COP positions of the contralateral and stance foot, respectively, rCOM, is the COM position and HCOM is the angular momentum of the body about the COM. During the SS phase, angular momentum of the body was controlled by the Mw only, while during the DS phase it was regulated by the Mw and MGRF acting on the contralateral foot (Fig. 2). A representative trial for each individual was selected using inspection and used to generate normalized patterns of sagittal HCOP, MW and MGRF (Figs. 3 and 4). The sagittal (x-component) angular momentum remained negative (clockwise/forward) throughout the entire step cycle. For interpretation purposes, increase and decrease in negative angular momentum were labeled as forward regulation and backward regulation, respectively. Similarly, the positive values of gravitational and GRF moments were labeled as backward moment and vice versa.
Level
Step0
Step0 Step1
+Z
DS Phase
SS2 Phase
SS1 Phase
Step0
Step0 Step 1
Step1
Step 1 Step 2 Level
+Y +X Fig. 1. Division of stance phase into sub-phases over the steps.
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GRF mg Stance foot
MW MGRF
Swing foot
Fig. 2. Diagrammatic representation of moment produced by GRF and gravity about the stance foot COP.
During the SS1, angular momentum increased (negative momentum decreased) showing a backward regulation (Fig. 4). During the SS2, angular momentum decreased (negative momentum increased) showing a forward regulation which changed to backward regulation again during the DS phase. Gravitational moment during the SS1 was a backward moment but kept decreasing until COM moves in front of COP and became increasingly negative during the SS2 resulting in a forward gravitational moment. The gravitational moment during the DS phase was also in the forward direction. GRF moment was applied only during the DS phase and maintained a backward moment throughout the phase (Figs. 3 and 4). Net change in HCOP within each step and phase were computed as the difference between initial and final values. Since the changes in the DS phase are due to MW and MGRF, the change in angular momentum due to each of these moments (DHW and DHGRF, respectively) was computed as the time integral of the respective moments. As the changes in HCOP during the SS phase were solely from MW (a)
(b)
(c)
Fig. 3. Representative pattern of normalized (a) angular momentum, (b) gravitational torque, (c) GRF torque; in women during steps 0, 1 and 2 of stair descent. Error bars represent standard deviation.
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(a)
(b)
(c)
Fig. 4. Representative pattern of normalized (a) angular momentum, (b) gravitational torque, (c) GRF torque; in men during steps 0, 1 and 2 of stair descent. Error bars represent standard deviation.
only, it was not necessary to compute the time integral. All the angular momentum values were normalized with ‘body mass ⁄ leg length’. For an in-depth understanding of the mechanism of angular momentum change, factors affecting the Mw and MGRF (forces and moment arm) and duration of moment application were also analyzed within each phase. The horizontal deviation of the COM from the stance foot COP at maximum MW in each phase was used as the max MW moment arm (MAw). The perpendicular distance from the stance foot COP to the line of action of the GRF projected to the sagittal plane was used as the moment arm of the GRF. The GRF magnitude and moment arm at the time point of maximum GRF moment were extracted. GRFSag was normalized to body mass; while MAw and MAGRF were normalized to the leg length of the participants. The duration of each phase was calculated from the time difference between each event and was reported in seconds. Mean of each of these variables was computed from the trials and used for statistical analysis. 2.6. Statistical analysis The statistical analysis was conducted using a two-way mixed factor repeated measures design with gender (between-subject) and step (within-subject) factors. For the first hypothesis, the dependent variables, DHCOP (over the entire step and within each phase), DHW and DHGRF, were compared for gender and step. For the second hypothesis, GRFSag, MAw, MAGRF, and duration of each phase were analyzed. Differences in dependent variables among steps and gender were evaluated using two-way repeated measures multivariate analysis of variance (MANOVA). Significant MANOVA was followed up with univariate tests and a post hoc analysis with Bonferroni correction. Alpha level (a) was set at .05 and statistical analysis was conducted with SPSS 14.0 (SPSS, Inc., Chicago, Illinois). 3. Results Gender and steps did not show a significant interaction, however there were significant step and gender effects (p = .001 and .04, respectively).
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3.1. Net changes in angular momentum Significant gender effects were observed in total change in angular momentum (p = .004), change during the DS phase (p = .027), and DHGRF (p = .031) with women showing higher values (Table 1). Significant step effects were observed in all angular momentum variables: DHCOP, angular momentum changes during the SS1, the SS2 and the DS phase, DHGRF, and DHW (p = .001). The greatest change in angular momentum was observed in Step 0. Step 1 showed a very small change while a small forward change was noticed for Step 2 (Table 1). The changes during the SS1 phase were found to be largest during Step 0, followed by Step 1 and then Step 2 (Table 1) while the trends were reversed for the changes during the SS2 phase. Step 0 also showed greatest changes during the DS phase. This was followed by Step 2 and then Step 1 (Table 1). Significantly larger forward regulation was found in DHW during Step 2 as compared to Step 0 and Step 1 (Table 1). DHGRF showed backward regulation with largest changes present during Step 2 as compared to Step 0 and Step 1. 3.2. Moment arm of gravitational moment (MAw) No gender effect was observed for MAw, while a significant step effect was observed for MAw during all the phases (p = .001). The moment arm during the SS1 phase was found to be significantly larger for Step 0 followed by Step 1 and then Step 2 (Table 1). The moment arm during the SS2 was
Table 1 Changes in the angular momentum, phase duration, GRF, moment arm of GRF and gravitational force within each step in men and women. Step 0
Changes in angular momentum (Nms/Kgm)
SS1 SS2 DS
D HW DHGRF Total Phase duration (sec)
SS1 SS2 DS
MAW (m)
SS1 SS2 DS
Step 1
Step 2
Men
Women
Men
Women
Men
Women
0.29⁄ (0.08) 0.03 (0.04) 0.11⁄ (0.07) 0.76 (0.40) 0.89 (0.51) 0.38⁄ (0.09)
0.28⁄ (0.06) 0.05 (0.4) 0.15⁄,n (0.05) 0.89 (0.34) 1.06n (0.42) 0.37⁄,n (0.05)
0.05§ (0.04) 0.12§ (0.05) 0.04 (0.06) 0.80 (0.17) 0.84 (0.26) 0.04§ (0.07)
0.08§ (0.04) 0.13§ (0.04) 0.09n (0.05) 0.81 (0.27) 0.90n (0.32) 0.05§,n (0.03)
0.02 (0.01) 0.31⁄ (0.09) 0.08 (0.04) 1.02⁄ (0.53) 1.10⁄ (0.38) 0.21 (0.09)
0.04 (0.03)
0.27⁄ (0.06) 0.26 (0.08) 0.12⁄ (0.05)
0.27⁄ (0.05) 0.30 (0.07) 0.14⁄,n (0.03)
0.11§ (0.04) 0.36 (0.06) 0.10 (0.03)
0.14§ (0.05) 0.38 (0.07) 0.12 (0.02)
0.06 (0.03) 0.44⁄ (0.09) 0.10 (0.03)
0.09 (0.04)
0.20⁄ (0.03) 0.11 (0.02) 0.03 (0.04)
0.19⁄ (0.02) 0.12 (0.03) 0.04 (0.07)
0.10 (0.04) 0.13 (0.04) 0.01 (0.07)
0.10 (0.03) 0.12 (0.03) 0.02 (0.13)
0.06 (0.04) 0.28⁄ (0.06) 0.06⁄ (0.03)
0.07 (0.03)
0.29⁄ (0.08) 0.10n (0.04) 1.10⁄ (0.35) 1.35⁄,n (0.45) 0.13n (0.05)
0.33⁄ (0.06) 0.12 (0.03)
0.23⁄ (0.04) 0.05⁄ (0.03)
GRF (N/kg)
DS
11.68 (2.38)
12.61n (2.57)
11.03 (3.69)
12.94n (2.59)
12.16⁄ (3.13)
15.36⁄,n (2.30)
MAGRF (m)
DS
0.31 (0.08)
0.33 (0.09)
0.31 (0.07)
0.27 (0.05)
0.36⁄ (0.07)
0.37⁄ (0.06)
Note: values in parenthesis represent standard deviation. n denotes gender effect and ⁄ and § denote post-hoc results for step effect. ⁄ denotes the step with statistically largest value while § signifies second largest step value.
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significantly longer for Step 2 with Step 1 and Step 0 showing similar values (Table 1). During the DS phase, the moment arm was highest for Step 2 as compared to Step 0 and Step 1 (Table 1). 3.3. Sagittal plane GRF (GRFSag) and moment arm of DHGRF (MAGRF) Significant gender and step effects were observed for GRFSag (p = .031 and .003, respectively). Women showed significantly higher values as compared to men. GRFSag values during Step 0 and 1 were similar, while Step 2 showed significantly larger values. MAGRF showed significant step effect (p = .001) with largest value observed during Step 2. MAGRF values for Step 0 and 1 were found to be similar (Table 1). 3.4. Phase duration Significant step effect (p = .002) was found for duration of all the phases, while gender effect was found for the DS phase (p = .038). The SS1 phase for Step 0 was the longest followed by Step 1 and then Step 2 (Table 1). The SS2 phase was longest for Step 2 followed by Step 1 and then Step 0 (Table 1). The DS phase duration was found to be longest for Step 0. Women spent significantly larger time in the DS phase as compared to men (Table 1).
4. Discussion Regulation of angular momentum of the body is essential for maintenance of balance and execution of the motion effectively. This study aimed to better understand biomechanics of stair descent by analyzing the angular momentum of the body and external moments affecting the changes in it. To our knowledge, this study was the first to analyze angular momentum of the body to explain the gender differences and changes within each step while descending stairs. More importantly, including level walking before participants descend stairs has resulted in a more realistic execution of tasks as compared to other studies where participant began stair descent from standing position at the top of the stair case. The study’s methodological approach to understand the control of angular momentum during stair descent is highly applicable as gravitational force and GRF of contralateral foot act about the stance foot to regulate the angular momentum. Thus, this approach provides how external forces interact with COM to create torque about the stance foot COP and help in regulating angular momentum. Men exhibited less change in their angular momentum over the entire stair descent as compared to women (Table 1). Since smaller changes in angular momentum during a given task (descent in our case) imply greater stability (Silverman et al., 2011), results of the study suggest increased risk of falls among older women during stair descent. It has been suggested that women show lower peak knee eccentric power on the upper transition step while lowering as compared to men (Singhal et al., 2014). Though the study only compared upper transition, it provides evidence to support the results of the current study. On analyzing changes within each phase over the steps, it was found that the women showed larger changes during the DS phase as compared to men (Table 1). In DS phase, the gravitational moment performs forward regulation which is countered by backward regulation of GRF moment. Women have been reported to show lower scores on the stair climbing component of Dynamic Gait Index primarily because of using handrails, irrespective of being a faller or a non-faller (Herman, Inbar-Borovsky, Brozgol, Giladi, & Hausdorff, 2009). Evidence of gender differences during gait transition while descending, with women showing greater precaution than men during upper transition (transition from flat surface to stairs) have been reported by previous studies (Hamel & Cavanagh, 2004; Herman et al., 2009). A higher backward regulation of angular momentum provides a quantitative evidence of an altered strategy of controlling balance among women. Analysis of the angular momentum produced by Mw and MGRF indicates that women show a significantly higher DHGRF as compared to men. When the factors affecting the DHGRF were analyzed, it was found that women spent longer time during the DS phase and also showed a greater GRF at the
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instance of maximum MGRF. Thus, women produce a higher GRF and stay in the DS phase for a longer duration of time to bring about greater changes in angular momentum during the stair descent. The angular momentum pattern for each step showed distinct patterns. The results highlight the importance of Step 0, the transition step, in controlling angular momentum during stair descent by providing a backward regulation. Step 1 shows minimal changes suggesting adoption of a more stable pattern. Step 2 was a transition step from stairs to level ground, and thus it is reasonable to expect individuals to show a larger forward regulation. A recent study has reported the tendency of people to show increased control of angular momentum during descent (Silverman, Neptune, Sinitski, & Wilken, 2014); thus this larger change in angular momentum during Step 2 confirms our assertion. As stair descent transitions require higher neuromuscular effort to control the body movements (James & Parker, 1989), large changes in angular momentum during transitions, found in this study, suggest greater challenges to balance and increased neuromuscular effort. These results were corroborated by the changes observed within each phase and DHW and DHGRF. The backward regulation within the SS1 was the largest for Step 0 which subsequently decreased as the steps progressed. This was because the moment arm of MW and the time duration of the SS1 for Step 0 was the highest resulting in greater backward regulation, implying that participants assumed a more vertical stance as the stair descent progressed. The SS2 phase saw a reverse trend with Step 2 showing the largest forward regulation because of increased MAW. When the changes in the DS phase were analyzed it was found that even though the highest values of DHW and DHGRF were observed for Step 2, the changes in DHCOP during the DS phase was largest for Step 0. MAW, MAGRF and GRF values during the DS phase were largest for Step 2. These results along with changes in MAW during the SS2 phase imply that separation between COP of the stance foot and COM of the body was largest for Step 2 establishing a greater forward lean by the participants during the transition from stairs to level walking. This behavior of participants was also reported by Lee and Chou (2007). One limitation of the current study was that the angular momentum was normalized to leg length of the participants though the step dimensions remained constant. This resulted in uniform comparison among the variables but altered the stair descent task as the ratio of step height to leg length was higher in women as compared to men. This may have resulted in overestimation of the changes in angular momentum; however, no gender differences were observed during the SS2 phase when the body is lowered negating any effects of altered step height to leg length ratio in angular momentum. 5. Summary and conclusion The study analyzed the gender differences in regulation of angular momentum through gravitational moment and GRF moment about the stance leg during stair descent. The changes in angular momentum were measured within each step along with GRF, moment arm of external forces and time of application of external moments. The results of the study showed that the women had a greater change in angular momentum through the entire descent. These changes in angular momentum were brought by increasing the GRF and the duration of double support period. These differences highlight a possible instability in stair descent gait in women. Step 0 and Step 2 showed a transition behavior with increased backward and forward regulation respectively. The SS1 phase during Step 0 and the SS2 phase during Step 2 brought about these changes. Similar studies should be conducted on stair ascent since the challenges during ascent are different than descent. Moreover, a staircase with more than 3 steps can be used to analyze the steady stair gait pattern to establish whether angular momentum changes minimize during this period. References Andriacchi, T. P., Andersson, G. B., Fermier, R. W., Stern, D., & Galante, J. O. (1980). A study of lower-limb mechanics during stairclimbing [Research Support, U.S. Gov’t, P.H.S.]. The Journal of Bone and Joint Surgery. American Volume, 62(5), 749–757. Bennett, B. C., Russell, S. D., Sheth, P., & Abel, M. F. (2010). Angular momentum of walking at different speeds. Human Movement Science, 29, 114–124. Brouwer, B., & Olney, S. J. (2004). Aging skeletal muscle and the impact of resistance exercise. Physiotherapy Canada, 56, 80–87. Centers for Disease Control. (2010, December 8). Falls Among Older Adults: An Overview. Retrieved 4th May, 2011, from .
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