The Journals of Gerontology Series A: Biological Sciences and Medical Sciences 60:910-914 (2005)
© 2005 The Gerontological Society of America
Age-Related Differences in Peak Joint Torques During the Support Phase of Single-Step Recovery From a Forward Fall
Michael L. Madigan1, and
Emily M. Lloyd2
1 Department of Engineering Science and Mechanics, Center for Gerontology
2 Department of Mechanical Engineering, Virginia Polytechnic Institute and State University, Blacksburg.
Address correspondence to Dr. Michael L. Madigan, Department of Engineering Science and Mechanics (0219), Blacksburg, VA, 24061. E-mail: mlm{at}vt.edu
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Abstract
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Background. Previous studies have reported that older adults have a reduced ability to recover balance with a single step after a forward-induced fall. To better understand the reasons for this reduced ability, this study investigated any age-related differences in peak joint torques during the support phase of a single-step balance recovery from a forward fall.
Methods. Ten young (19–23 years old) and 10 older (65–83 years old) men were released from forward-leaning positions and attempted to recover their balance with a single step. Lean was increased until they failed to recover their balance with a single step. Peak extensor torques were calculated for the support phase of balance recovery and were compared across age groups.
Results. A consistent pattern of joint torques emerged during the support phase of balance recovery, suggesting a similar strategy across young and older participants. Despite this similarity, older participants exhibited smaller peak knee extensor torques during the support phase of single-step balance recoveries, and trends toward larger peak extensor torques at the hip and ankle.
Conclusions. The age-related differences found are believed to be the combined result of an age-related reduction in muscle strength and an age-related neuromuscular adaptation to mitigate the effects of muscle strength loss on physical performance capabilities.
FALLS are among the most common and costly medical problems to society. An estimated 14 million falls resulted in injury in 1995 with an overall cost of $64 billion (1). By the year 2020, these numbers are projected to rise to 17 million falls per year and $85 billion (1). Falls are a particular problem in older adults due to higher fall rates and a greater likelihood of injury and death from a fall. An estimated 30%–40% of community-dwelling adults over 65 years old fall each year (2,3). This high rate of falls among older adults, coupled with the expected growth in the older adult population (4), warn of an onerous public health problem.
Previous research has shown that, compared to young adults, older adults have a reduced ability to recover from a forward fall with a single step (5,6). In an attempt to determine the causes behind this reduced ability, researchers have identified differences in the performance of single-step recoveries between young and older adults. These differences include spatiotemporal characteristics of stepping (5,6), joint kinematics (7), joint kinetics (8), and muscle activations (9). With regard to joint kinetics, Wojcik and colleagues (8) reported that older adults exhibited significantly lower peak hip flexion and hip extension torques compared to young adults during single-step recoveries after being released from their maximum achieved lean angle. Their study, however, only investigated joint torques during the time between the initiation of the balance perturbation and the instant that the stepping leg contacted the ground. Joint torques after the stepping leg contacts the ground may also be important for balance recovery. At least two studies support the importance of joint extensor torques during this "support phase" of balance recovery. Grabiner and colleagues (10) concluded that recovery from an anteriorly directed stumble is dependent on arresting the body's forward rotation during the support phase of balance recovery (SPBR). Pavol and colleagues (11) reported that older adults fell after being tripped during gait due to an inability to simultaneously restabilize the trunk and resist buckling of the stepping leg during SPBR. We are unaware of any studies that have quantified joint torques during the SPBR. Therefore, the purpose of this study was to investigate potential age-related differences in lower extremity joint torques during SPBR. If found, these differences could be a contributing factor toward the reduced ability of older adults to recover from a forward fall with a single step.
An age-related redistribution of lower extremity joint torques may also contribute to the reduced fall recovery ability of older adults. A redistribution of joint torques in older adults, compared to young adults, has been reported during various activities of daily living (ADLs). DeVita and Hortobagyi (12), for example, reported an age-related redistribution of joint torques and powers during the stance phase of gait whereby extensor impulse increased at the hip and decreased at the knee and ankle. A similar distal-to-proximal redistribution of extensor torques in older adults has been reported during stair ascent, stair descent, and rising from a chair (13,14). DeVita and Hortobagyi (12) proposed that this redistribution of joint torques in older adults results from an age-related neuromuscular adaptation whose purpose is to mitigate the effects of age-related strength loss on physical performance. The existence of a torque redistribution during tasks more physically demanding than the three above-reported ADLs, such as fall recovery, has not been investigated. Based on these studies, it was hypothesized that, compared to young adults, older adults would exhibit larger hip extensor torques, and smaller knee extensor and ankle plantar flexor torques in the stepping leg during SPBR.
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METHODS
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Participants
Twenty men participated, including 10 young men (19–23 years old) and 10 older men (65–83 years old). Both groups were similar in height and mass (young: 1.76 ± 0.07 m, 71.3 ± 14.0 kg; older: 1.71 ± 0.06 m, 77.0 ± 14.0 kg). Inclusion criteria required that all participants be free of musculoskeletal injury and that older participants pass a medical screening. This medical screening was performed by an internist to rule out individuals with any cardiac, respiratory, neurological, otological, or musculoskeletal disorders, or a history of repeated falls. All participants identified themselves as being right hand and right foot dominant. The study was approved by the Institutional Review Board at Virginia Polytechnic Institute and State University, and all participants provide written informed consent prior to the start of the study.
Protocol
The experimental protocol was adapted from Wojcik and colleagues (8), and has been reported (7). Aspects unique to the present study will be described in more detail. Forward falls were induced by releasing participants from a forward-leaning posture. After release, participants attempted to recover their balance by using a single step of the right foot. Successful recoveries were followed by another trial at a larger lean, and failed recoveries were followed by a second trial at the same lean. This process was repeated until participants failed to recover their balance with a single step for two consecutive trials at the same lean. The ability to recover from a fall was quantified by the maximum lean from which participants could recover their balance with a single step after being released (LeanMAX).
To start each trial, participants stood with their feet shoulders-width apart at a toe line and were leaned forward. Participants were held in this forward-leaning posture using a lean support rope spanning from the back of a belt worn by the participant to a releasable clasp affixed to a stable wooden structure. In this position, participants were asked to equally distribute their weight across both feet while maintaining heel contact with the ground. Equal (within 10%) weight distribution across both feet was verified post hoc using data from two separate force plates, one under each foot. Participants were asked to keep their arms folded across their chest throughout each trial. After the participants were in position at the correct lean, they were verbally reminded to take a single step with their right foot for recovery. Participants were released without warning 0–10 seconds after this verbal reminder. The initial lean corresponded to 12% body weight (BW) in the lean support rope, and lean was increased by 4% BW after each successful recovery. In the event of an unsuccessful recovery, falls to the ground were prevented using a full-torso harness tethered to a ceiling-mounted support track with a fall-prevention lanyard. Three criteria were used to define a failed recovery: 1) when more than one step was taken with the right foot, 2) when more than 30% BW force was applied to the harness at any point during trip recovery, and 3) when the left foot took a step longer than 30% of the participant's body height. All participants practiced the single-step balance recovery prior to the start of the experiment.
Data Collection and Analysis
Body segment positions were sampled at 200 Hz using an Optotrak optoelectronic motion analysis system (NDI, Waterloo, Ontario). Infrared markers were placed on the right side of the body at the fifth metatarsal head, heel, lateral malleolus, lateral femoral epicondyle, greater trochanter, and acromion. Marker data were filtered with a fourth order, 7 Hz low-pass, zero-phase-shift Butterworth filter. In the initial forward-leaning posture, a force plate was under each foot (AMTI, Watertown, MA). Force plate and harness load cell data were sampled at 1000 Hz, and load cell data were subsequently filtered with a fourth order, 10 Hz low-pass, zero-phase-shift Butterworth filter. To estimate joint torques in the stepping leg during SPBR, the body was modeled as a two-dimensional system of four rigid body segments, connected by frictionless pin joints, that included the right foot, right shank, right thigh, and a head/arms/trunk (HAT) segment. The mass and inertial characteristics of the body segments were defined using an anthropometric model (15). Sagittal plane joint torques were estimated for the right ankle, knee, and hip using the governing Newton–Euler equations as described by Winter (16). Peak extensor torques were determined for SPBR, and the SPBR was operationally defined to be the time interval between foot contact and the instant when both knee flexion velocity and HAT forward angular velocity had reached zero (11). Lean (units of % BW in the lean support rope) was converted to lean angle (units of degrees) using the method reported by Thelen and colleagues (5).
Statistical Analyses
Age-related differences in LeanMAX were investigated using an independent t test. Results were significant when p 
.05. Age-related differences in peak extensor torques during SPBR were investigated using a repeated measures analysis, which included a second degree polynomial for lean angle and a class variable for age (equal to 0 for young participants and 1 for older participants). Possible covariates were height and weight. Interactions between age and the polynomial were also included, but were not significant for all dependent measures. A significant age effect indicated a significant difference in peak torque values between age groups while holding lean angle (and all other independent variables) constant. Directional hypotheses were used to improve the statistical power of the tests, and were based on the observed age-related differences in joint torques in previous studies (peak extensor torques in older adults were expected to be higher at the hip and lower at the knee and ankle compared to young adults). Statistical analyses were conducted using SAS (version 9; SAS Institute Inc., Cary, NC).
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RESULTS
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Maximum Lean Angles
Older participants achieved smaller LeanMAX compared to young participants (20.5 ± 4.0° vs 29.9 ± 4.0°, p <.001). One older participant was unable to recover from the smallest lean angle, and was not included in the analysis. As a result, the final sample size included 10 young participants and 9 older participants.
Joint Torques During SPBR
A consistent pattern of joint torques emerged for all participants (Figure 1). Hip, knee, and ankle torques during SPBR were predominantly extensor (or plantar flexor), except for the first 
60 ms after stepping foot contact. During this initial part of the support phase, hip and knee torques tended to oscillate out of phase with each other. The consistent pattern of joint torques during the support phase resulted in a consistent pattern in the sagittal plane angular orientation of the ground reaction force (GRF
). Peak torque values tended to correspond temporally with the maximum and minimum values of GRF (not including the GRF value at impact). The largest hip extensor torque and knee flexor torque occurred simultaneously, were the first peaks after foot contact, and corresponded to the GRF local minimum. The largest knee extensor torque occurred second, and corresponded to the GRF local maximum. The largest ankle plantar flexor torque occurred third, after the GRF local maximum.
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Figure 1. Representative body position, ground reaction force, and joint torques during the support phase of a single-step balance recovery (SPBR). Top: Body position (except for the non-stepping leg) and a graphical representation of the GRF during SPBR. Middle: Angular orientation of the GRF with respect to a horizontal line to the right. Bottom: Joint torques during SPBR. Time zero corresponds to foot impact, or the start of the SPBR. Positive torque values correspond to extensor (or plantar flexor) dominance
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Effects of Age and Lean Angle on Peak Extensor Torques During SPBR
Peak extensor (plantar flexor) torque increased at the hip (p <.001), knee (p =.003), and ankle (p <.001) with increasing lean angle. Peak knee extensor torque during SPBR was lower in older participants compared to young participants (p =.037). There was no age-related increase in peak hip extensor torque (p =.084), or decrease in peak ankle plantar flexor torque (p =.909). These data are summarized in Figure 2. The time duration of SPBR was not affected significantly by lean angle, age, height, or weight.
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Figure 2. Peak joint extensor torques during the support phase of a single-step balance recovery with respect to lean angle. Positive torque values correspond to extensor (or plantar flexor) dominance
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DISCUSSION
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The purpose of this study was to investigate potential age-related differences in peak extensor torques during the support phase of a single-step recovery from a forward fall. Our main findings were that: 1) a consistent pattern of joint torques during SPBR emerged for both young and older participants, and 2) older participants produced smaller peak extensor torques at the knee during SPBR compared to young participants.
The consistent patterns of joint torques and GRF suggest a consistent strategy across age groups during the support phase of single-step balance recoveries. This consistent pattern likely reflects the biomechanical tasks necessary for successful balance recovery. Grabiner and colleagues (10) and Pavol and colleagues (11) reported that two tasks are critical for successful balance recovery: 1) reduce forward rotation about the hips and/or lumbar spine and about the obstacle that induced the perturbation, and 2) resist buckling (i.e., knee flexion) of the stepping leg. Interpreting joint torques and GRF with these tasks in mind can provide insight into the consistent patterns measured. At the GRF local minimum (Figure 1), joint torques caused a nearly vertical GRF orientation that assisted in decelerating the body's forward rotation about the hips and/or lumbar spine and the obstacle that induced the perturbation (assumed to be the initial stepping foot placement in this study). This is consistent with Grabiner and colleagues (10) who reported that the GRF of stepping foot after stepping was a primary contributor to retarding forward rotation of the body. At the GRF local maximum, joint torques caused a more anterior-to-posterior GRF orientation that assisted in decelerating forward movement of the body. The extensor torques during the remainder of the SPBR assisted in resisting buckling of the stepping leg (i.e., decelerating hip flexion, knee flexion, and ankle dorsiflexion), in addition to decelerating body rotation about the obstacle. The predominant order in which these two tasks mentioned above were accomplished during successful recoveries was: 1) decelerate knee flexion velocity to zero, then 2) decelerate hip and lumbar flexion velocity to zero (our biomechanical model included a single HAT segment that precluded the differentiation between hip flexion and lumbar flexion).
Although an age-related difference in peak torque was found only at the knee, our data indicated interesting trends at the hip and ankle (Figure 2). Peak hip extensor torque tended to be higher in older adults, as hypothesized, and a post hoc power analysis indicated that an additional five participants in each age group would likely have resulted in a statistically significant difference. Peak ankle plantar flexor torque also tended to be higher in older adults, which is a trend opposite to that hypothesized. A post hoc power analysis, with a reformulated hypothesis testing for an increase in peak ankle plantar flexor torque, indicated that 16 participants in each age group (compared to the 10 used here) would likely have resulted in a significant difference. Based on the modest increase in sample size that we estimate to be required to achieve statistical significance, we suggest an increased emphasis on the trends at the hip and ankle than the insignificant results would imply.
The age-related differences and/or trends during SPBR may be the combined result of an age-related strength loss and neuromuscular adaptation to mitigate the effects of muscle strength loss on physical performance capabilities (12). However, this adaptation does not appear to completely compensate for strength loss as evidenced by the lower LeanMAX in older participants. Similar age-related differences at the hip and knee have been reported during gait (12), stair negotiation (13,14), and rising from a chair (13). In contrast, the larger ankle torques that we found in older adults were inconsistent with the smaller torques in older adults during gait (12) and stair negotiation (17). The reason for these differences may be due to the differences in performance demands of the tasks themselves. During SPBR, for example, ankle plantar flexor torque is particularly helpful when attempting to decelerate the forward rotation of the body around an obstacle by decreasing GRF. It may not be as critical during these other tasks. The higher ankle torques in older adults were particularly interesting because, compared to other muscle groups, ankle plantar flexors exhibit larger strength reductions with age (18,19). A post hoc analysis of stepping leg kinematics during SPBR showed no age-related difference in joint positions at impact, peak joint velocities, and peak joint flexions that could have contributed to these age-related differences in peak joint torques.
Comparing peak joint torques during SPBR with peak joint torques prior to stepping foot contact with the ground [reported by Wojcik and colleagues (8)] reveals some notable differences (Table 1). Larger hip extensor torques and knee extensor torques were observed during the support phase. In contrast, larger hip flexor torques were observed prior to the support phase. Other joint torques did not vary substantially between the two studies. These trends suggest that different phases of single-step balance recovery have different torque requirements, and that loss of strength in a selected muscle may lead to a specific failure mechanism during recovery. Anthropometric differences between these studies were not significant, so these general trends do not seem to be attributed to differences in participant size.
This study had several limitations. First, this investigation was conducted under the premise that successful balance recovery after stepping requires sufficient joint torques in the sagittal plane to restabilize the body. Successful balance recovery also requires sufficient joint torques in other anatomical planes as well as balance control systems outside of our focus. Second, unlike many falls outside of the laboratory, our participants were expecting a fall during the experimental protocol. We feel, however, that balance recovery ability in our study was mostly a function of participant physical performance capabilities which would not be expected to differ whether falls were induced expectedly or unexpectedly. Third, joint torques in the nonstepping leg might play an important role in balance recovery, but equipment limitations precluded their calculation. Fourth, only healthy male participants participated in this study, so the results may not apply to other population groups. Fifth, differences in psychological factors, such as motivation and fear levels, could account for some differences in balance recovery performance.
Conclusion
A consistent pattern of joint torques emerged during SPBR, suggesting a similar strategy between young and older participants. Despite this similarity, older adults exhibited: 1) smaller peak knee extensor torques during the support phase of single-step balance recoveries, and 2) trends toward larger peak extensor torques at the hip and ankle. These are believed to be the combined result of an age-related reduction in muscle strength and an age-related neuromuscular adaptation to mitigate the effects of muscle strength loss on physical performance capabilities.
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Acknowledgments
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We thank Matthew Runion for his assistance in the data collection for this project.
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Footnotes
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Decision Editor: John E. Morley, MB, BCh
Received February 13, 2004
Accepted April 21, 2004
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References
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Abstract
Methods
