Muscle contributions to recovery from forward loss of balance by stepping

The purpose of this study was to determine the muscular contributions to the stepping phase of recovery from forward loss of balance in 5 young and 5 older adults that were able to recover balance in a single step, and 5 older adults that required multiple steps. Forward loss of balance was achieved by releasing participants from a static forward lean angle. All participants were instructed to attempt to recover balance by taking a rapid single step. A scalable anatomical model consisting of 36 degrees-of-freedom was used to compute kinematics and joint moments from motion capture and force plate data. Forces for 94 muscle actuators were computed using static optimisation and induced acceleration analysis was used to compute individual muscle contributions to net lumbar spine joint, and stepping side hip joint and knee joint accelerations during recovery. Older adults that required multiple recovery steps used a significantly shorter and faster initial recovery step and adopted significantly more trunk flexion throughout recovery compared to the older single steppers. Older multiple steppers also produced significantly more force in the stance side hamstrings, which resulted in significantly higher hamstring induced flexion accelerations at the lumbar spine and extension accelerations at the hip. However since the net joint lumbar spine and hip accelerations remained similar between older multiple steppers and older single steppers, we suggest that the recovery strategy adopted by older multiple steppers was less efficient as well as less effective than for older single steppers.     

Decreased lower limb muscle recruitment contributes to the inability of older adults to recover with a single step following a forward loss of balance

In response to a balance disturbance, older individuals often require multiple steps to prevent a fall. Reliance on multiple steps to recover balance is predictive of a future fall, so studies should determine the mechanisms underlying differences between older adults who can and cannot recover balance with a single step. This study compared neural activation parameters of the major leg muscles during balance recovery from a sudden forward loss of balance in older individuals capable of recovering with a single step and those who required multiple steps to regain balance. Eighty-one healthy, community dwelling adults aged 70 ± 3 participated. Loss of balance was induced by releasing participants from a static forward lean. Participants performed four trials at three initial lean magnitudes and were subsequently classified as single or multiple steppers. Although step length was shorter in multiple compared to single steppers (F = 9.64; p = 0.02), no significant differences were found between groups in EMG onset time in the step limb muscles (F = 0.033–0.769; p = 0.478–0.967). However, peak EMG normalised to values obtained during maximal voluntary contraction was significantly higher in single steppers in 6 of the 7 stepping limb muscles (F = 1.054–4.167; p = 0.045–0.024). These data suggest that compared to multiple steppers, single steppers recruit a larger proportion of the available motor unit pool during balance recovery. Thus, modulation of EMG amplitude plays a larger role in balance recovery than EMG timing in this context.     

Adaptive recovery responses to repeated forward loss of balance in older adults

Experiments designed to assess balance recovery in older adults often involve exposing participants to repeated loss of balance. The purpose of this study was to investigate the adaptive balance recovery response exhibited by older adults following repeated exposure to forward loss of balance induced by releasing participants from a static forward lean angle. Fifty-eight healthy, community-dwelling older adults, aged 65-80 years, participated in the study. Participants were instructed to attempt to recover with a single step and performed four trials at each of three lean angles. Adaptive recovery responses at four events (cable release, toe-off of the stepping foot, foot contact and maximum knee flexion angle following landing in the stepping leg) were quantified for trials performed at the intermediate lean angle using the concept of margin of stability. The antero-posterior and medio-lateral margin of stability were computed as the difference between the velocity-adjusted position of the whole body centre of mass and the corresponding anterior or lateral boundary of the base of support. Across repeated trials adaptations in reactive stepping responses were detected that resulted in improved antero-posterior stability at foot contact and maximum knee flexion angle. Improved antero-posterior stability following repeated trials was explained by more effective control of the whole body centre of mass during the reactive stepping response and not by adjustments in step timing or base of support. The observed adaptations occurred within a single testing session and need to be considered in the design of balance recovery experiments.     

Age-related differences in muscle power during single-step balance recovery

The purpose of this study was to investigate age-related differences in muscle power during a surrogate task of trip recovery. Participants included 10 healthy young men (19-23 years old) and 10 healthy older men (65-83). The task involved releasing participants from a forward-leaning posture. After release, participants attempted to recover their balance using a single step of the right foot. Muscle power at the hip, knee, and ankle of the stepping limb were determined from the product of joint angular velocity and joint torque. Muscle powers during balance recovery followed a relatively consistent pattern in both young and older men, and showed effects of both lean and age. Interestingly, the effects of age did not always involve smaller peak power values in the older men as expected from the well-documented loss of muscle power with aging. Older men exhibited smaller peak muscle power at the knee and larger peak muscle power at the ankle and hip compared to young men. The increases in muscle power at the ankle and hip may result from a neuromuscular adaptation aimed at improving balance recovery ability by compensating for the age-related loss of muscle function.     

The concept of margins of stability can be used to better understand a change in obstacle crossing strategy with an increase in age

The purpose of the current study was to use the margins of stability (MoS) to investigate how older adults choose between minimizing the risk of a forward fall when crossing an obstacle and the ease of maintaining forward progression during the steps taken behind the obstacle. In the current study 143 community-dwelling older adults aged between 55 and 83 years old, were divided into three age groups based on tertials of age. All participants were asked to complete five trials of obstacle walking and five trials of normal walking. For the trials of normal walking, the main difference between groups was that MoS at initial contact was lower in the older age groups. For the trials of obstacle crossing the MoS at the instants of obstacle crossing with both the leading and trailing limb became smaller with an increase in age. This result might imply that older people choose to use a strategy during obstacle crossing that results in smaller chance of falling forward if an obstacle was struck. A negative consequence of this more conservative strategy was a smaller MoS at the instants of initial contact after crossing the obstacle, thus a larger chance of a backward fall. These findings provide more insight into the regulation of stability during obstacle crossing and specifically in the differences in strategy between younger and older people, and therefore these results might be used for further research to investigate whether obstacle crossing strategies are trainable in older adults, which could be used as advisory programs aimed at fall prevention and/or engagement in an active lifestyle.     

Age and gender differences in peak lower extremity joint torques and ranges of motion used during single-step balance recovery from a forward fall.

Previous studies have found substantial age and gender group differences in the ability of healthy adults to regain balance with a single step after a forward fall. It was hypothesized that differences in lower extremity joint strengths and ranges of motion (ROM) may have contributed to these observed differences. Kinematic and forceplate data were therefore used with a rigid-link biomechanical model simulating stepped leg dynamics to examine the joint torques and ROM used by subjects during successful single-step balance recoveries after release from a forward lean. The peak ROM and torques used by subjects in the study were compared to published estimates or measured values of the available maxima. No significant age or gender group differences were found in the mean ROM used by the subjects for any given initial lean angle. As initial lean angle increased, larger knee ROM and significantly larger hip ROM were used in the successful recoveries. There were substantial gender differences and some age group differences in peak lower extremity joint torques used in successful recoveries. Both young and older females often used nearly maximal joint torques to recover balance. Subjects' maximum joint strengths in plantarflexion and hip flexion were not good predictors of single-step balance recovery ability, particularly among the female subjects.     

Muscle activities used by young and old adults when stepping to regain balance during a forward fall

The current study was undertaken to determine if age-related differences in muscle activities might relate to older adults being significantly less able than young adults to recover balance during a forward fall. Fourteen young and twelve older healthy males were released from forward leans of various magnitudes and asked to regain standing balance by taking a single forward step. Myoelectric signals were recorded from 12 lower extremity muscles and processed to compare the muscle activation patterns of young and older adults. Young adults successfully recovered from significantly larger leans than older adults using a single step (32.2° vs. 23.5°). Muscular latency times, the time between release and activity onset, ranged from 73 to 114 ms with no significant age-related differences in the shortest muscular latency times. The overall response muscular activation patterns were similar for young and older adults. However older adults were slower to deactivate three stance leg muscles and also demonstrated delays in activating the step leg hip flexors and knee extensors prior to and during the swing phase. In the forward fall paradigm studied, age-differences in balance recovery performance do not seem due to slowness in response onset but may relate to differences in muscle activation timing during the stepping movement.     

Investigation of balance strategy over gait cycle based on margin of stability

This study was conducted to investigate the balance strategy of healthy young adults through a gait cycle using the margin of stability (MoS). Thirty healthy young adults participated in this study. Each performed walking five times at a preferred speed and at a fast speed. The MoS was calculated over a gait cycle by defining the base of support (BoS) changes during a gait cycle. The MoS was divided into medial/lateral and anterior/posterior components (ML MoS and AP MoS). The central values and the values at 12 gait events of the MoS were compared. Positive/negative integration of ML MoS (ML MoS_POS and ML MoS_NEG, respectively) and the average ML/AP MoS over a cycle (ML/AP MoS_mean) were significantly lower at a fast gait than at a preferred gait. ML/AP MoS were lower at a fast speed than at the preferred speed, except for the ML MoS immediately before left heel strike (pre left HS) and right and left heel strike (HS). ML/AP MoS were significantly lower immediately before heel strike (pre-HS) than in other gait events, regardless of walking speed. It was suggested that pre-HS is the most unstable moment in both ML/AP directions and a crucial moment in control of gait stability. The results presented above might be applicable as basic data regarding dynamic stability of healthy young adults through a gait cycle for comparisons with elderly people and patients with orthopedic disorders or neurological disorders.     

Minimal step length necessary for recovery of forward balance loss with a single step

Although the boundary conditions necessary to trigger a step in reaction to a forward balance loss have been predicted in previous research, the relationship between minimal step length needed for balance recovery with this single step and the center of mass (COM) motion state (i.e., its position and velocity) remains unknown. The purpose of this paper was to present a theoretical framework within which the minimal step length needed for balance recovery can be estimated. We therefore developed a simplified four-segment sagittal model of human body stepping for balance recovery. The work-energy principle of the Newtonian mechanics was employed in the simulation to determine the amount of excess mechanical energy that can be absorbed as a function of step length and the corresponding eccentric joint work that can be generated in a single step. We found that an increase in initial forward velocity and a greater forward shift of the COM require a corresponding increase in the minimal step length needed for balance recovery. Furthermore, the minimal step length is also a function of the muscle strength at the ankle: the lower the muscle strength, the greater the minimal step length required. Our theoretical framework reduces the complexity associated with previous studies relying on forward dynamics and iterative optimization processes. This method may also be applied to study aspects of balance control such as the prevention of balance loss in the posterior or mediolateral direction.     

Instructions limiting the number of steps do not affect the kinetics of the threshold of balance recovery in younger adults

To our knowledge, no one has explored the effect of modifications in balance recovery instructions on the kinetics of the threshold of balance recovery. In particular, the effect of instructions limiting the number of steps on joint torques at the maximum lean angle has not been quantified. We determined the joint torques at the ankle, knee and hip of 28 younger adults recovering balance at their maximum lean angle using: (i) only a single step, (ii) no more than two steps and (iii) no limit on the number of steps. Results showed that instructions limiting the number of steps did not affect peak normalized joint torques by more than 0.0083 or 10Nm except for knee and hip flexion torques from first to second heel strike for the first step leg as well as from second toe-off to heel strike for the second step leg. However, these large differences in peak normalized joint torques after the first step were simply caused by the additional steps used when participants could take more than one step compared to when participants were limited to only a single step. Between the three limits on the number of steps, the kinetics of both legs were nearly identical up to the end of the first step and the additional steps did not help to increase the maximum lean angle. Therefore, we have demonstrated that instructions limiting or not limiting the number of steps appear to be equally valid to study falls in younger adults.     

Retention of improvement in gait stability over 14 weeks due to trip-perturbation training is dependent on perturbation dose

Perturbation training is an emerging approach to reduce fall risk in the elderly. This study examined potential differences in retention of improvements in reactive gait stability over 14 weeks resulting from unexpected trip-like gait perturbations. Twenty-four healthy middle-aged adults (41–62 years) were assigned randomly to either a single perturbation group (SINGLE, n = 9) or a group subjected to eight trip-like gait perturbations (MULTIPLE, n = 15). While participants walked on a treadmill a custom-built brake-and-release system was used to unexpectedly apply resistance during swing phase to the lower right limb via an ankle strap. The anteroposterior margin of stability (MoS) was calculated as the difference between the anterior boundary of the base of support and the extrapolated centre of mass at foot touchdown for the perturbed step and the first recovery step during the first and second (MULTIPLE group only) perturbation trials for the initial walking session and retention-test walking 14 weeks later. Group MULTIPLE retained the improvements in reactive gait stability to the perturbations (increased MoS at touchdown for perturbed and first recovery steps; p < 0.01). However, in group SINGLE no differences in MoS were detected after 14 weeks compared to the initial walking session. These findings provide evidence for the requirement of a threshold trip-perturbation dose if adaptive changes in the human neuromotor system over several months, aimed at the improvement in fall-resisting skills, are to occur.     

Maximum allowable force on a safety harness cable to discriminate a successful from a failed balance recovery

A safety harness system is essential to ensure participant safety in experiments at the threshold of balance recovery where avoiding a fall is not always possible. The purpose of this study was to propose a method to determine the maximum allowable force on a safety harness cable to discriminate a successful from a failed balance recovery. Data from 12 younger adults, who participated in experiments to determine the maximum forward lean angles that participants could be suddenly released from and still recover balance using three different limits on the number of steps, were used. For each participant, the coefficients of an asymptotic exponential regression, between the maximum vertical force on the safety harness cable and the initial lean angle at each trial, were evaluated by a least squares method. A proposed threshold for the maximum allowable vertical force of five force constants ensured that the initial lean angle reached 99% of its steady state value with respect to its initial value. It should thus discriminate well a successful (below the threshold) from a failed (above the threshold) balance recovery. Furthermore, although the amplitude of the horizontal forces should not be neglected in safety harness system designs, the contributions of the medial–lateral and anterior–posterior forces can be neglected in experiments at the threshold of balance recovery. Finally, although our five force constants method could be used, the actual value obtained for the maximum allowable vertical force may vary with other safety harness systems and postural perturbations.     

Effects of aging on hip abductor-adductor neuromuscular and mechanical performance during the weight transfer phase of lateral protective stepping

Aging brings about challenges in the ability to recover balance through protective stepping, especially in the lateral direction. Previous work has suggested that lateral protective stepping during weight transfer may be affected by impaired muscle composition and performance of the hip abductors (AB) in older adults. Hence, this study investigated the influence of hip abductor-adductor (AB-AD) neuromuscular performance on the weight transfer phase of lateral protective stepping in younger and older adults. Healthy younger (n = 15) and older adults (n = 15) performed hip AB-AD isometric maximal voluntary contractions (IMVC). Lateral balance perturbations were applied via motorized waist-pulls. Participants were instructed to recover their balance using a single lateral step. Kinetic, kinematic and electromyographic (EMG) data were analyzed during the weight transfer phase. In the hip IMVC task, older adults showed reduced peak AB-AD torque, AB rate of torque development and AB-AD rate of EMG neuromuscular activation (RActv). During the lateral balance perturbations, older individuals had a lower incidence of lateral steps, reduced hip AB-AD RActv and delayed weight transfer. However, several outcomes were larger in the older group, such as, center of mass momentum at step onset, step-side peak rate of vertical force development, hip AB net joint torque, and power. Although older adults had greater hip muscular output during the weight transfer phase, their lateral balance recovery was still impaired. The reduced maximal hip AB-AD capacity, especially RActv, may have been a greater contributor to this impairment, as it affects the ability to generate rapid force, crucial for balance recovery.     

Aging effects of motor prediction on protective balance and startle responses to sudden drop perturbations

This pilot study investigated the effect of age on the ability of motor prediction during self-triggered drop perturbations (SLF) to modulate startle-like first trial response (FTR) magnitude during externally-triggered (EXT) drop perturbations. Ten healthy older (71.4 ± 1.44 years) and younger adults (26.2 ± 1.63 years) stood atop a moveable platform and received blocks of twelve consecutive EXT and SLF drop perturbations. Following the last SLF trial, participants received an additional EXT trial spaced 20 min apart to assess retention (EXT RTN) of any modulation effects. Electromyographic (EMG) activity was recorded bilaterally over the sternocleidomastoid (SCM), vastus lateralis (VL), biceps femoris (BF), medial gastrocnemius (MG), and tibialis anterior (TA). Whole-body kinematics and kinetic data were recorded. Stability in the antero-posterior direction was quantified using the margin of stability (MoS). Compared with EXT trials, both groups reduced SCM peak amplitude responses during SLF and EXT RTN trials. VL/BF and TA/MG coactivation were reduced during SLF FTR compared to EXT FTR (p < 0.05) with reduced peak vertical ground reaction forces (vGRF) in both younger and older adults (p < 0.05). Older adults increased their MoS during SLF FTR compared to EXT FTR (p < 0.05). Both groups performed more eccentric work during SLF trials compared to EXT (p < 0.05). These findings indicate that abnormal startle effects with aging may interfere with balance recovery and increase risk of injury with external balance perturbations. Motor prediction may be used to acutely mitigate abnormal startle/postural responses with aging.     

健康青年静态立位平衡功能的研究

目的:完善健康青年静态立位平衡参数常模和探讨人处于不利站立条件下姿势控制变化规律。方法:从某军校本科学员中随机抽取108名,对8种不同站立条件下的立位平衡功能进行测试,同时比较不同站立姿势下重心晃动的变化。结果:与睁眼站立相比,闭眼站立和闭眼站立于脚垫上,人体重心会不自主地向脚掌移动,同时重心晃动的轨迹长度和面积明显增加,而单位面积轨迹长度明显降低。另外,人体前后方向晃动的程度较大,而左右晃动的程度稍小。结论:健康人姿势控制由视觉、前庭和下肢本体感觉等共同维持的,任何系统受到限制,都会影响平衡功能。当人处于不利站立条件时,人的身体会本能地向前倾斜来维持平衡,同时本研究为涉及人体立位平衡方面的研究提供了新的研究方法和思路。     

Induced limb collapse in a sudden slip during termination of sit-to-stand

Despite repeated demonstration of how balance can be restored with protective stepping after the initiation of an induced fall, little is known about how accidental falling to the ground with the participant's body resting in a non-standing posture can be avoided during balance recovery. This is due to the difficulties inherent in experimentally eliciting such an event. The purpose of this study was, therefore, to determine failure rate and the characterization for balance recovery after young adults exposed to an experimentally induced novel slipping perturbation. Twenty-four healthy young adults first performed three to nine trials of regular sit-to-stand. In the following trial, slipping suddenly occurred during the termination of the sit-to-stand when the low-friction platform on which the participant stood was released. Participants were given no prior practice or knowledge of the experiment design. Slipping was then repeated in the subsequent trials. The results demonstrated for the first time that a high percentage (62%) of participants failed to recover standing balance, despite the fact that 14 of these 15 participants had initiated stepping at their first encounter of a sudden slip. Such failure was avoided immediately after the first encounter. It was postulated that a delay in the step initiation might have contributed to substantial vertical descent of the center-of-mass, leading to failure of balance recovery in limb collapse. To verify this and other hypotheses, a shift in experimental paradigms is warranted to include the study of spontaneous protective responses elicited when individuals first encounter previously unfamiliar balance perturbation as in real-life situations.     

A new measure for upright stability

The control of balance is a primary objective in most human movements. In many cases, research or practice, it is essential to quantitatively know how good the balance is at a body posture or at every moment during a task. In this paper we suggest a new measure for postural upright stability which assigns a value to a body state based on the probability of avoiding a fall initiation from that state. The balance recovery problem is solved for a population sample using a strength database, and the probability of successfully maintaining the balance is found over the population and called the probability of recovery (PoR). It, therefore, describes an attribute of a body state: how possible the control of balance is, or how safe being at that state is. We also show the PoR calculated for a 3-link body model for all states on a plane, compare it to that found using a 2-link model, and compare it to a conventional metric: the margin of stability (MoS). It is shown, for example, that MoS may be very low at a state from which most of the people will be able to easily control their balance.     

Decrease in required coefficient of friction due to smaller lean angle during turning in older adults

We investigated age-related differences in the required coefficient of friction (RCOF) during 90° turning, the difference of RCOF during step and spin turn, and how affects observed differences. Sixteen healthy young and healthy older adults (eight men and eight women in each group) participated. Participants performed 90° step and spin turns to the right at a self-selected normal speed. Older adults turned with lower RCOF than the young adults during both step and spin turns. This was associated with reduced mediolateral (ML) RCOF component (RCOF_ML) for the older adults. Reduced RCOF_ML in older adults was associated with reductions in the ML component of the lean angle of the body during turning. This age-related gait changes during turning can be compensatory mechanisms that allowed older adults to turn while reducing the risk of slipping. Spin turns exhibited lower RCOF, resulting from significantly lower RCOF_ML, than step turns in young and older adults; thus, spin turning is a safer turning strategy for preventing lateral slips. This may suggest that, in older adults, slip prevention may take precedence over balance recovery after slips sustained during turning. These results illustrate a turning gait mechanism that helps prevent slips and falls, and how age affects this mechanism.     

Adaptive feedback potential in dynamic stability during disturbed walking in the elderly

After perturbation of the gait, feedback information may help regaining balance adequately, but it remains unknown whether adaptive feedback responses are possible after repetitive and unexpected perturbations during gait and if there are age-related differences. Prior experience may contribute to improved reactive behavior. Fourteen old (59-73 yrs) and fourteen young (22-31 yrs) males walked on a walkway which included one covered element. By exchanging this element participants either stepped on hard surface or unexpectedly on soft surface which caused a perturbation in gait. The gait protocol contained 5 unexpected soft trials to quantify the reactive adaptation. Each soft trial was followed by 4-8 hard trials to generate a wash-out effect. The dynamic stability was investigated by using the margin of stability (MoS), which was calculated as the difference between the anterior boundary of the base of support and the extrapolated position of the center of mass in the anterior-posterior direction. MoS at recovery leg touchdown were significantly lower in the unexpected soft trials compared to the baseline, indicating a less stable posture. However, MoS increased (p<0.05) in both groups within the disturbed trials, indicating feedback adaptive improvements. Young and old participants showed differences in the handling of the perturbation in the course of several trials. The magnitude of the reactive adaptation after the fifth unexpected perturbation was significantly different compared to the first unexpected perturbation (old: 49±30%; young: 77±40%), showing a tendency (p=0.065) for higher values in the young participants. Old individuals maintain the ability to adapt to feedback controlled perturbations. However, the locomotor behavior is more conservative compared to the young ones, leading to disadvantages in the reactive adaptation during disturbed walking.     

Energetic response to repeated restraint stress in rapidly growing mice

There is a cost of stress that may result in the loss of normal biological function (e.g., growth). Repeated, and even single, applications of stressors have been shown to induce negative energy balance in rodents. However, here we addressed whether this energetic response changes during multiple stress exposure and whether there is complete recovery subsequent to the cessation of stress exposure. These questions were addressed in growing C57Bl/6 mice (31 day) by determining at different times the energetic and endocrine responses after the exposure to restraint (R) stress for 4 h applied once (R1), repeatedly over 3 days (R3), or repeatedly over 7 days (R7). Compared with control values, R elevated (P<0.05) plasma corticosterone and reduced plasma insulin-like growth factor I on all days of exposure to the stressor. Seven days, but not 1 or 3 days of R, decreased the net growth (126%, P<0.05) and deposition of fat (71%, P<0.05) and lean (60%, P<0.05) energy over the 7 days. Only R7 depressed the 7-day metabolizable energy intake (P<0.05), and R7, but not R1 or R3, increased the overall energy expenditure (10%, P<0.05). Our results demonstrate that repeated episodes of stress are energetically costly to the rapidly growing animal, but compensatory mechanisms mitigate this cost of repeated stress exposure and permit complete recovery of energy balance after the cessation of stress application.