Impact severity in self-initiated sits and falls associates with center-of-gravity excursion during descent

The timing and amplitude of the muscular activity of the arms preceding impact in a forward fall is modulated with fall velocity

Protective arm reactions have been shown to be an important injury avoidance mechanism in unavoidable falls. Protective arm reactions have been shown to be modulated with fall height, however it is not clear if they are modulated with impact velocity. The aim of this study was to determine if protective arm reactions are modulated in response to a forward fall with an initially unpredictable impact velocity. Forward falls were evoked via sudden release of a standing pendulum support frame with adjustable counterweight to control fall acceleration and impact velocity. Thirteen younger adults (1 female) participated in this study. Counterweight load explained more than 89% of the variation of impact velocity. Angular velocity at impact decreased (p < 0.001), drop duration increased from 601 ms to 816 ms (p < 0.001), and the maximum vertical ground reaction force decreased from 64%BW to 46%BW (p < 0.001) between the small and large counterweight. Elbow angle at impact (129 degrees extension), triceps (119 ms) and biceps (98 ms) pre-impact time, and co-activation (57%) were not significantly affected by counterweight load (p-values > 0.08). Average triceps and biceps EMG amplitude decreased from 0.26 V/V to 0.19 V/V (p = 0.004) and 0.24 V/V to 0.11 V/V (p = 0.002) with increasing counterweight respectively. Protective arm reactions were modulated with fall velocity by reducing EMG amplitude with decreasing impact velocity. This demonstrates a neuromotor control strategy for managing evolving fall conditions. Future work is needed to further understand how the CNS deals with additional unpredictability (e.g., fall direction, perturbation magnitude, etc.) when deploying protective arm reactions.

Test-retest reliability of the FALL FIT system for assessing and training protective arm reactions in response to a forward fall

The use of the hands and arms is an important protective mechanism in avoiding fall-related injury. The aim of this study was to evaluate the test-retest reliability of fall dynamics and evokd protective arm response kinematics and kinetics in forward falls simulated using the FALL simulator For Injury prevention Training and assessment system (FALL FIT). Fall FIT allows experimental control of the fall height and acceleration of the body during a forward fall. Two falls were simulated starting from 4 initial lean angles in Experiment 1 and with 4 different fall accelerations in Experiment 2. Fourteen younger adults (25.1±3.5 years) and 13 older adults (71.3±3.7 years) participated in Experiment 1 and 13 younger adults (31.8±5.7 years) participated in Experiment 2. Intraclass correlation coefficients (ICC) were used to the evaluate absolute agreement of single measures at each condition and averages across conditions. Average measures of fall dynamics and evoked kinematics and kinetics exhibited excellent reliability (ICC(A,4)>0.86). The reliability of single measures (ICC(A,1) > 0.59) was good to excellent, although 18% of single measures had a reliability (ICC(A,1)) between 0.00 and 0.57. The FALL FIT was shown to have good to excellent reliability for most measures. FALL FIT can produce a wide range of fall dynamics through modulation of initial lean angle and body acceleration. Additionally, the range of fall velocities and evoked kinematics and kinetics are consistent with previous fall research.•

The FALL FIT can be used to gain further insight into the control of protective arm reactions and may provide a therapeutic tool to assess and train protective arm reactions.

The Role of Recovery Lower Limb Segments in Post-Slip Determination of Falls Due to Instability or Limb Collapse

Slip-related falls can be induced by instability or limb collapse, but the key factors that determine these two fall causations remain unknown. The purpose of this study was to investigate the factors that contribute towards instability-induced and limb-collapse-induced slip-related falls by investigating 114 novel slip trials. The segment angles and moments of the recovery limb after slip-onset from pre-left-touchdown (pre-LTD) to post-left-touchdown (post-LTD) were calculated, and logistic regression was used to detect which variable contributed most to instability-induced and limb-collapse-induced falls. The results showed that recovery from instability was determined by the angle of the thigh at LTD (87.7%), while recovery from limb collapse was determined by the angle of the shank at post-LTD (90.4%). Correspondingly, instability-induced falls were successfully predicted (81.5%) based on the initial thigh angle at pre-LTD and the following peak thigh moment, while limb-collapse-induced falls were successfully predicted (85.5%) based on the initial shank angle at LTD and the following peak shank moment. According to our findings, taking a shorter recovery step and/or increasing the counterclockwise moment of the thigh after pre-LTD would help individuals resist instability-induced falls, while taking a larger recovery step and/or increasing the clockwise moment of the shank post-LTD would help resist limb-collapse-induced falls. The findings of this study are crucial for future clinical applications, because individually tailored reactive balance training could be provided to reduce vulnerability to specific types of falls and improve recovery rates post-slip exposure.

Sideways fall-induced impact force and its effect on hip fracture risk: a review

Osteoporotic hip fracture, mostly induced in falls among the elderly, is a major health burden over the world. The impact force applied to the hip is an important factor in determining the risk of hip fracture. However, biomechanical researches have yielded conflicting conclusions about whether the fall-induced impact force can be accurately predicted by the available models. It also has been debated whether or not the effect of impact force has been considered appropriately in hip fracture risk assessment tools. This study aimed to provide a state-of-the-art review of the available methods for predicting the impact force, investigate their strengths/limitations, and suggest further improvements in modeling of human body falling.

METHODS

We divided the effective parameters on impact force to two categories: (1) the parameters that can be determined subject-specifically and (2) the parameters that may significantly vary from fall to fall for an individual and cannot be considered subject-specifically.

RESULTS

The parameters in the first category can be investigated in human body fall experiments. Video capture of real-life falls was reported as a valuable method to investigate the parameters in the second category that significantly affect the impact force and cannot be determined in human body fall experiments.

CONCLUSIONS

The analysis of the gathered data revealed that there is a need to develop modified biomechanical models for more accurate prediction of the impact force and appropriately adopt them in hip fracture risk assessment tools in order to achieve a better precision in identifying high-risk patients. Graphical abstract Impact force to the hip induced in sideways falls is affected by many parameters and may remarkably vary from subject to subject.

Developing safe fall strategies for lower limb exoskeletons

One of the main challenges in the use of a powered lower limb exoskeleton (LLE) is to ensure that balance is maintained throughout the operation of the device. Since no control strategy has yet been implemented that prevents falls in the case of a loss of balance, head or other serious injuries may occur during independent use of LLEs in the event of a fall. These safety concerns limit LLEs in the community to supervised use only. Using the backward fall as a model, we used optimization techniques to develop safe fall control strategies in order to avoid head impact and mitigate the impact velocity of the hips. From available human biomechanics data, we first developed an optimization methodology to study falls of healthy people. The results showed similar kinematic and dynamic characteristics to findings of previous studies on real-life human falls. Second, we extended the optimization methodology to include characteristics of a hypothetical LLE and to generate optimal joint trajectories and optimal torque profiles for the fall duration. The results revealed that by applying the optimal fall strategy, the severity of a simulated fall was minimized compared to when the device fell with locked joints (i.e., how currently used exoskeletons fall): head impact was avoided and hip impact velocity was reduced by more than 50%.

Risk Factors for Traumatic Brain Injuries During Falls in Older Persons

OBJECTIVE

To identify risk factors for traumatic brain injuries (TBIs) during falls in older Taiwanese people.

PARTICIPANTS

Case patients consisted of 113 patients aged 60 years or older with a moderate/severe TBI due to a fall. Two control groups: (1) 339 older patients with a soft-tissue injury; and (2) 113 with a mild-TBI due to a fall. Proxies were required to provide information for a considerable number of patients.

DESIGN

Matched case-control study.

SETTINGS

The emergency departments of 3 general hospitals.

MEASURES

Sociodemographic, lifestyle behavior, chronic condition, medication use, functional abilities, and fall-related characteristics.

RESULTS

When patients with a soft-tissue injury were assigned to the control group, men were 2.06-fold more likely to have a moderate/severe TBI than women. Subjects who took antiarrhythmics within 4 hours of a fall were 2.59-fold more likely to have a moderate/severe TBI than those who took none. Subjects who were negotiating stairs and getting in/out of the bed/chair were 3.12-fold and 2.97-fold, respectively, more likely to have a moderate/severe TBI than those who fell while walking. Falling backward and sideways was 4.07-fold and 2.30-fold, respectively, more likely to cause a moderate/severe TBI than falling forward. When patients with a mild-TBI were assigned to the control group, results were similar, with the exception that the effect of antiarrhythmic use became nonsignificant and subjects who took 2 or more medications were 3.07-fold more likely to have a moderate/severe TBI than those who took none.

CONCLUSION

Avoiding a head impact during a backward or sideways fall, reducing unnecessary use of polypharmacy and antiarrhythmics, and maintaining safety during stair negotiation and bed/chair transfer may protect an elderly person from a severe brain injury.

Two types of slip-induced falls among community dwelling older adults

Little is known about the landing behavior of the trailing (recovery) foot and ensuing types of falls following a forward slip in walking. The purposes of this study were to (1) determine if community-dwelling older adults experienced bilateral slips at the same rate as had been previously observed for young adults during over-ground walking; (2) determine if fall rate in older adults was dependent on slip type (unilateral vs. bilateral); and (3) identify differences in spatiotemporal variables of the trailing leg step between unilateral and bilateral slips. One-hundred-seventy-four participants experienced an unannounced, unrehearsed slip while walking on a 7-m walkway. Each trial was monitored with a motion capture system and bilateral ground reaction force plates. Although the experimental design, developed with original data from a young adult population, favored bilateral slips, more older adults (35%) than anticipated (10% previously observed in young, p<0.001) displayed a unilateral slip. The probability of fall was equal in the two types of slips. Eighty-two people recovered from the slip, while the remaining 92 (53%) fell. These 92 were classified into two exclusive categories based on the heel distance at the time of fall arrest using cluster analysis: those which resembled a fall into a "splits" position (n=47) or a feet-forward fall (n=45). All (100%) unilateral slips led to splits falls, as expected. Yet, not all bilateral slips (only 83%) resulted in feet-forward falls. A longer forward recovery step with a prolonged step time led to both feet slipping, nearly together, hence a feet-forward fall.

Sandwich type ceramic liner fracture rate with the Atlas III™ socket: A study of 144 primary total hip replacements at a mean 74 months' follow-up

INTRODUCTION

Ceramic friction bearings have been proposed as a means of reducing wear in total hip replacement (THR). A "sandwich" composite concept including a ceramic bearing surface has been proposed as simplifying the modularity while matching metal-back cups with a polyethylene liner. It is not precisely known how frequently abnormal noise would occur during functioning of this type of implant, which moreover entails a risk of ceramic liner fracture.

HYPOTHESIS

Results with sandwich type ceramic liners are comparable to those with polyethylene liners, without risk of side effects (noise, fracture).

PATIENTS AND METHODS

Clinical and radiological results of 144 cementless Atlas III™ cups containing a 28mm-diameter polyethylene-ceramic sandwich type liner coupled to a ceramic Biolox Forte™ head were retrospectively analyzed at a mean 74 months' follow-up. Mean patient age was 59.4 years. Twelve patients were lost to follow-up. Femoral components comprised 61 ESOP™ anatomic stems and 71 BHS™ Corail stems. The radiologic study used Imagika™ software.

RESULTS

Global function scores were satisfactory: PMA score, 17.2±1.2 (range, 9 to 18); global Harris score, 93.6±3.1 (49 to 100). Global survivorship was 91.6% (95% CI: 86.34-96.9). Radioclinical analysis found seven liner fractures (5.3%) at a mean 32 months; all were non-traumatic and asymptomatic. Clinical risk factors for liner fracture were overweight, advanced age, dislocation, prosthetic impingement, increased postoperative offset was a radiologic risk factor.

DISCUSSION AND CONCLUSION

Despite these satisfactory radioclinical results, matching those for metal-backed implants containing a polyethylene liner, close surveillance is mandatory with this type of composite implant. The high fracture rate with ceramic-polyethylene sandwich type liners and relative lack of symptoms warrant caution in their use.

LEVEL OF EVIDENCE

Level IV, retrospective or historic series.

The force attenuation provided by hip protectors depends on impact velocity, pelvic size, and soft tissue stiffness

Wearable hip protectors represent a promising strategy for preventing hip fractures. However, there is lack of agreement on biomechanical testing standards and subsequent uncertainty about the ability of hip protectors to attenuate impact force during a fall. To address this issue, we designed a fall impact simulator that incorporated a "biofidelic" surrogate pelvis, which matched the surface geometry and soft tissue stiffness measured in elderly women (n=15). We then used this system to measure the attenuation in peak femoral neck force provided by two commercially available soft shell protectors (Safehip Soft and Hipsaver) and one rigid shell protector (Safehip Classic). Finally, we examined how the force attenuation provided by each protector was influenced by systematic changes in fall severity (impact velocity), body size (pelvis size), and soft tissue stiffness. With the biofidelic pelvis, the force attenuation averaged over all impact velocities was 27% for Safehip Soft, 17% for Safehip Classic, and 19% for Hipsaver. However, the rank order of hip protectors (and especially the performance of Safehip Classic) varied with the test conditions. Safehip Classic attenuated force by 33% during a low velocity (1 ms) fall, but only by 8% for a high velocity (4 ms) fall. In the latter condition, improved attenuation was provided by the soft shell hip protectors (19% by Safehip Soft and 21% by Hipsaver). As soft tissue stiffness increased from softest to most rigid, the attenuation provided by Safehip Classic increased 2.9-fold (from 26% to 76%), while Safehip Soft increased 1.7-fold (from 36% to 60%) and Hipsaver increased 1.1-fold (from 36% to 38%). As pelvis size decreased from largest to smallest, the attenuation provided by Safehip Classic increased 8-fold, but for a high velocity fall and moderate tissue stiffness, never exceeded that provided by Safehip Soft and Hipsaver. Our results indicate that, under biofidelic testing conditions, the soft shell hip protectors we examined generally provided greater force attenuation (averaging up to 27%) than the hard shell protector. Measured values of force attenuation were highly sensitive to variations in impact velocity, pelvic size, and pelvic soft tissue stiffness. This indicates the need to develop international testing standards to guide market approval, the selection of protectors for clinical trials, and the design of improved hip protectors.

Effect of soft shell hip protectors on pressure distribution to the hip during sideways falls

INTRODUCTION

While hip protectors represent a promising strategy for preventing hip fractures, clinical efficacy has been limited by poor user compliance. Soft shell protectors may be more acceptable to users than traditional hard shell designs. However, before embarking on clinical trials to assess efficacy, laboratory experiments are required to determine how soft shell protectors affect the force applied during impact to the hip. This was the goal of the current study.

METHODS

Fifteen women participated in "pelvis release experiments," which safely simulate the impact stage of a sideways fall. During the trials, we measured total impact force and mean pressure over the greater trochanter with the participant unpadded, and while wearing two commercially available soft shell protectors.

RESULTS

Mean pressure over the greater trochanter was reduced by 76% by a 14-mm thick horseshoe-shaped protector and by 73% by a 16-mm thick continuous protector. Total force was reduced by 9% by the horseshoe and by 19% by the continuous protector.

CONCLUSIONS

Soft shell hip protectors substantially reduce the pressure over the greater trochanter, while only modestly reducing total impact force during simulated sideways falls. These data support the need for clinical trials to determine whether soft shell protectors reduce hip fracture risk in vulnerable populations.

Mechanisms of limb collapse following a slip among young and older adults

Recovery from a large perturbation, such as a slip, can be successful when stability of movement can be reestablished with protective stepping. Nevertheless, one dilemma for executing a protective step is that its liftoff can weaken support against limb collapse. This study investigated whether failures in limb support leading to falls after a protective step result from insufficient joint moment generation, and whether such insufficiency is greater among older fallers. A novel, unexpected slip was induced immediately following seat-off during a sit-to-stand. Joint work and mechanical energy were calculated for 43 young (9 falls, 34 recoveries) and 22 older (13 falls, 9 recoveries) adults who responded with a protective step. Comparisons of the work produced at three joints of the bilateral lower limbs revealed that insufficient concentric knee and hip extensor work prior to step liftoff was a primary differentiating factor between falling and recovery, regardless of age. Also, during stepping, fallers regardless of age failed to limit the eccentric knee extensor work at their stance limb sufficiently to retard rapid knee flexion and the consequent potential energy loss. We concluded that young and older fallers had comparable weak limb support. The greater fall incidence among the older adults likely resulted from a greater proportion of subjects who responded to the slip with insufficient knee extensor support, possibly attributable to age-differences in chair-rising. One strategy to address this dilemma may rely on task-specific training to enhance feedforward control that improves movement stability, and thus lessens the reliance on protective stepping.

Effect of the "squat protective response" on impact velocity during backward falls

Risk for injury during a fall depends on the position and velocity of the body segments at the moment of impact. One technique for reducing impact velocity is to absorb energy in the lower extremity muscles during descent, as occurs during squatting or sitting. However, the protective value of this response may depend on the time during descent when the response is initiated. We tested this hypothesis by conducting backward falling experiments with young women (n = 23; aged 21-29 years), who fell onto a soft gymnasium mattress after being suddenly releasing from an inclined position. In trials where subjects were released from a 5 degrees lean, average impact velocities were reduced by 18% when squatting was utilized as opposed to inhibited. Furthermore, increases in the release angle caused an increase in average impact velocity of 8% between lean angles of 2 degrees and 5 degrees, and 7% between lean angles of 5 degrees and 12 degrees. This was due to declines in peak extensor torques and peak flexion rotations, and corresponding reductions in both joint work and potential energy at impact. These results suggest that squatting during descent reduces impact severity, but the effectiveness of the response depends on the stage during descent when it is initiated, diminishing in benefit as the fall progresses and the state of imbalance grows increasingly severe.

Strategies for avoiding hip impact during sideways falls

During a fall, hip fracture risk increases 30-fold if there is direct impact to the hip. We conducted sideways falling experiments and found that subjects were able to avoid hip impact by rotating forward or by rotating backward during descent. These simple safe-landing strategies should be considered in designing hip fracture prevention programs.

INTRODUCTION

Ninety percent of hip fractures in the elderly are caused by falls. During a fall, hip fracture risk is increased 6-fold by falling sideways (instead of backward or forward) and 30-fold if direct impact occurs to the hip. Previous studies suggest that impact to the hip during a sideways fall can be avoided by rotating forward during descent to land on the outstretched hands. Presumably, an alternative strategy for avoiding hip impact is to rotate backward to land on the buttocks. We conducted sideways falling experiments to test the hypothesis that each of these falling strategies is equally effective in allowing one to avoid hip impact.

MATERIALS AND METHODS

Twenty-two young adult women participated in trials where they were released from an inclined standing position into a sideways fall onto a foam mattress. Subjects were instructed to "land as softly as possible" and to "avoid impacting the hip" by either rotating forward or rotating backward during descent.

RESULTS

We found that absolute values of the hip proximity angle, which described how close the impact site was to the lateral aspect of the pelvis, were not different in forward rotation and backward rotation trials (mean = 55.9 +/- 22.4 degrees versus 61.5 +/- 15.8 degrees, respectively). However, compared with forward rotation trials, backward rotation trials involved greater pelvis impact velocity (2.95 +/- 0.25 versus 2.45 +/- 0.77 m/s; p = 0.001) and greater whole-body kinetic energy at impact (238 +/- 70 versus 156 +/- 90 J; p = 0.001).

CONCLUSIONS

These results suggest that, during a sideways fall, individuals can avoid impact to the hip and thereby lower the risk for hip fracture by rotating forward or by rotating backward during descent. These simple yet effective safe-landing strategies should be considered in designing exercise-based hip fracture prevention programs.

Fall-related upper body injuries in the older adult: a review of the biomechanical issues

Although the epidemiology of fall-related injuries is well established for the elderly population over 65 years of age, the biomechanics of how, when and why injuries do and do not occur when arresting a fall have received relatively little attention. This paper reviews the epidemiological literature in the MEDLINE data base pertinent to the biomechanics of fall-related injuries, including data on fall rates, fall-related injury rates, fall directions and types of injuries available. It also covers primary sources not listed on MEDLINE, along with the pertinent biomechanics literature. Many falls in older adults are in a forward direction, and as a result the upper extremities are one of the most commonly injured structures, presumably in protecting the head and torso. In this review emphasis is placed on what is, and what is not, known of the biomechanical factors that determine the impact forces and injury risk associated with upper extremity injuries in forward falls. While decreased bone mineral density may be contributory, it is not a reliable predictor of fracture risk. Evidence is presented that fall-related impact forces can be reduced by appropriate volitional arrest strategies. Further theoretical and experimental research is needed to identify appropriate fall-arrest strategies for the elderly, as well as the physical capacities and skills required to do so. Inexpensive interventions might then be developed to teach safe fall-arrest techniques to older individuals.

An analysis of the effect of lower extremity strength on impact severity during a backward fall

At least 280 000 hip fractures occur annually in the U.S. at an estimated cost of $9 billion. While over 90 percent of these are caused by falls, only about 2 percent of all falls result in hip fracture. Evidence suggests that the most important determinants of hip fracture risk during a fall are the body's impact velocity and configuration. Accordingly, protective responses for reducing impact velocity and the likelihood for direct impact to the hip, strongly influence fracture risk. One method for reducing the body's impact velocity and kinetic energy during a fall is to absorb energy in the lower extremity muscles during descent, as occurs during sitting and squatting. In the present study, we employed a series of in verted pendulum models to determine: (a) the theoretical effect of this mechanism on impact severity during a backward fall, and (b) the effect on impact severity of age-related declines (or exercise-induced enhancements) in lower extremity strength. Compared to the case of a fall with zero energy absorption in the lower extremity joints, best-case falls (which involved 81 percent activation of ankle and hip muscles, but only 23 percent activation of knees muscles) involved 79 percent attenuation (from 352 J to 74 J) in the body's vertical kinetic energy at impact (KEv), and 48 percent attenuation (from 3.22 to 1.68 m/s) in the downward velocity of the pelvis at impact (v(v)). Among the mechanisms responsible for this were: (1) eccentric contraction of lower extremity muscles during descent, which resulted in up to 150 J of energy absorption; (2) impact with the trunk in an upright configuration, which reduced the change in potential energy associated with the fall by 100 J; and (3) knee extension during the final stage of descent, which "transferred" up to 90 J of impact energy into horizontal (as opposed to vertical) kinetic energy. Declines in joint strength reduced the effectiveness of mechanisms (1) and (3), and thereby increased impact severity However, even with reductions of 80 percent in available torques, KEv was attenuated by 50 percent. This indicates the importance of both technique and strength in reducing impact severity. These results provide motivation for attempts to reduce elderly individuals' risk for fall-related injury through the combination of instruction in safe falling techniques and exercises that enhance lower extremity strength.