Estimating severity of sideways fall using a generic multi linear regression model based on kinematic input variables

Many research groups have studied fall impact mechanics to understand how fall severity can be reduced to prevent hip fractures. Yet, direct impact force measurements with force plates are restricted to a very limited repertoire of experimental falls. The purpose of this study was to develop a generic model for estimating hip impact forces (i.e. fall severity) in in vivo sideways falls without the use of force plates.Twelve experienced judokas performed sideways Martial Arts (MA) and Block (‘natural’) falls on a force plate, both with and without a mat on top. Data were analyzed to determine the hip impact force and to derive 11 selected (subject-specific and kinematic) variables. Falls from kneeling height were used to perform a stepwise regression procedure to assess the effects of these input variables and build the model.The final model includes four input variables, involving one subject-specific measure and three kinematic variables: maximum upper body deceleration, body mass, shoulder angle at the instant of ‘maximum impact’ and maximum hip deceleration. The results showed that estimated and measured hip impact forces were linearly related (explained variances ranging from 46 to 63%). Hip impact forces of MA falls onto the mat from a standing position (3650 ± 916 N) estimated by the final model were comparable with measured values (3698 ± 689 N), even though these data were not used for training the model. In conclusion, a generic linear regression model was developed that enables the assessment of fall severity through kinematic measures of sideways falls, without using force plates.     

The effect of positioning on the biomechanical performance of soft shell hip protectors

Wearable hip protectors represent a promising strategy for reducing risk for hip fracture from a sideways fall. However, small changes in pad positioning may influence their protective benefit. Using a mechanical hip impact simulator, we investigated how three marketed soft shell hip protectors attenuate and redistribute the impact force applied to the hip, and how this depends on displacement from their intended position by 2.5 or 5 cm superiorly, posteriorly, inferiorly or anteriorly. For centrally-placed protectors, peak pressure was reduced 93% below the unpadded value by a 16 mm horseshoe-shaped protector, 93% by a 14 mm horseshoe protector, and 94% by a 16 mm continuous protector. In unpadded trials, 83% of the total force was applied to the skin overlying the proximal femur (danger zone). This was lowered to 19% by the centrally placed 16 mm horseshoe protector, to 34% by the 14 mm horseshoe, and to 40% by the 16 mm continuous protector. Corresponding reductions in peak force delivered to the femoral neck (relative to unpadded) were 45%, 38%, and 20%, respectively. The protective benefit of all three protectors decreased with pad displacement. For example, displacement of protectors by 5 cm anteriorly caused peak femoral neck force to increase 60% above centrally-placed values, and approach unpadded values. These results indicate that soft shell hip protectors provide substantial protective benefits, but decline in performance with small displacements from their intended position. Our findings confirm the need for correct and stable positioning of hip protectors in garment design.     

An injury risk curve for the hip for use in frontal impact crash testing

To facilitate the assessment of hip injury risk in frontal motor-vehicle crashes, an injury risk curve that relates peak force transmitted to the hip to the probability of hip fracture was developed by using survival analysis to fit a lognormal distribution to a recently published dataset of hip fracture forces. This distribution was parameterized to account for the effect of subject stature, which was the only subject characteristic found to significantly affect hip fracture force (X²(1)=6.03, p=0.014). The distribution was further parameterized to account for the effects of hip flexion and abduction from a standard driving posture on hip fracture force using relationships between mean hip fracture force and hip flexion/abduction reported in the literature. The resulting parametric distribution was used to define relationships between force applied to the hip and the risk of hip fracture for the statures associated with the small female, midsize male, and large male crash-test dummies, thus allowing these dummies to assess hip fracture/dislocation risk in frontal crashes, provided that such dummies are sufficiently biofidelic. For the midsize male crash test dummy, a 50% risk of hip fracture was associated with a force of 6.00 kN. For the small female and large male dummies, a 50% risk of hip fracture was associated with forces of 4.46 and 6.73 kN, respectively.     

Kinematics,kinetics and muscle activation patterns of the upper extremity during simulated forward falls

The purpose of this study was to explore the effects of fall type and fall height on the kinematics, kinetics, and muscle activation of the upper extremity during simulated forward falls using a novel fall simulation method.Twenty participants were released in a prone position from a Propelled Upper Limb Fall ARrest Impact System. Impacts occurred to the hands from two fall heights (0.05 m and 0.10 m) and three fall types (straight-arm, bent-arm, self-selected). Muscle activation from six muscles (biceps brachii, brachioradialis, triceps brachii, anconeus, flexor carpi radialis and extensor carpi radialis) was collected and upper extremity joint kinematics were calculated.Peak Fx (medio-lateral), as well as Fx and Fz (inferior–superior) load rate increased between the 0.05 m and 0.10 m heights. With respect to fall type, the straight-arm falls resulted in significantly greater Fy (anterior–posterior) impulse and Fy and Fz load rates. The change in elbow flexion angle was greater during the self-selected and bent-arm falls compared to the straight-arm falls; a pattern also seen in the wrist flexion/extension angles. All muscles experienced peak % MVIC prior to the time of the peak force.The results of this study suggest that, to some extent, individuals are capable of selecting an upper extremity posture that allows them to minimize the effects of an impact and it has confirmed the presence of a preparatory muscle activation response.     

The effect of asymmetrical body orientation during simulated forward falls on the distal upper extremity impact response of healthy people

The occurrence of distal upper extremity injuries resulting from forward falls (approximately 165,000 per year) has remained relatively constant for over 20 years. Previous work has provided valuable insight into fall arrest strategies, but only symmetric falls in body postures that do not represent actual fall scenarios closely have been evaluated. This study quantified the effect of asymmetric loading and body postures on distal upper extremity response to simulated forward falls. Twenty participants were suspended from the Propelled Upper Limb fall ARest Impact System (PULARIS) in different torso and leg postures relative to the ground and to the sagittal plane (0°, 30° and 45°). When released from PULARIS (hands 10 cm above surface, velocity 1 m/s), participants landed on two force platforms, one for each hand. Right forearm impact response was measured with distal (radial styloid) and proximal (olecranon) tri-axial accelerometers and bipolar EMG from seven muscles. Overall, the relative height of the torso and legs had little effect on the forces, or forearm response variables. Muscle activation patterns consistently increased from the start to the peak activation levels after impact for all muscles, followed by a rapid decline after peak. The impact forces and accelerations suggest that the distal upper extremity is loaded more medial-laterally during asymmetric falls than symmetric falls. Altering the direction of the impact force in this way (volar-dorsal to medial-lateral) may help reduce distal extremity injuries caused when landing occurs symmetrically in the sagittal plane as it has been shown that volar-dorsal forces increase the risk of injury.     

Tolerance of the skull to blunt ballistic temporo-parietal impact

Less-lethal ballistic projectiles are used by police personnel to temporarily incapacitate suspects. While the frequency of these impacts to the head is low, they account for more serious injuries than impacts to any other body region. As a result, there is an urgent need to assess the tolerance of the head to such impacts. The focus of this study was to investigate the tolerance of the temporo-parietal skull to blunt ballistic impact and establish injury criteria for risk assessment. Seven unembalmed isolated cadaver heads were subjected to fourteen impacts. Specimens were instrumented with a nine-accelerometer array as well as strain gages surrounding the impact site. Impacts were performed with a 38 mm instrumented projectile at velocities ranging from 18 to 37 m/s. CT images and autopsies were performed to document resulting fractures. Peak fracture force for the seven resulting fractures was 5633±2095 N. Peak deformation for fracture-producing impacts was 7.8±3.2 mm. The blunt criterion (BC), peak force and principal strain were determined to be the best predictors of depressed comminuted fractures. Temporo-parietal tolerance levels were consistent with previous studies. An initial force tolerance level of 2346 N is established for the temporo-parietal region for blunt ballistic impact with a 38 mm diameter impactor.     

Kinematics and kinetics of unanticipated misstep conditions: Femoral fracture implications in the elderly

Most hip fractures are thought to occur after falling during everyday activities. We speculated that hip fractures might also occur because of excessive loading of the hip joint during an unexpected misstep consequently leading to a fall. The aims of this study were to explore the kinematics and kinetics of the lower extremity joints during missteps as compared with regular stepping, as well as to compare the magnitude of forces acting upon the hip joint with the threshold forces expected to fracture the hip. Fourteen healthy adults performed two forward steps on a 17.8 cm high platform under the following four conditions: forward with and without vision, as well as a misstep (the box for the final step unexpectedly removed without participant awareness), and regular stepping down with eyes open. The results revealed no differences between stepping forward with and without vision. When compared with both stepping forward and regular stepping down, the misstep revealed altered joint positions accompanied by increased forces and moments acting upon the hip joint. For example, the peak vertical proximal thigh segment force was 3.05±0.55 BW vs. 1.23±0.14 BW and 0.91±0.09 BW (p<.001; misstep vs. regular stepping down and stepping forward, respectively), while the proximal thigh segment moment in frontal plane was 1.39±0.70 Nm/kg vs. 0.18±0.32 Nm/kg of adduction and 0.16±0.19 Nm/kg of abduction (p<.001). When compared with the literature data, the forces during misstep were within the range of those forces that could result in hip fractures in the elderly. Therefore, it may be possible for the elderly to experience hip/proximal femur fractures during missteps prior to falling.     

An ex vivo experiment to reproduce a forward fall leading to fractured and non-fractured radii

Forward falls represent a risk of injury for the elderly. The risk is increased in elderly persons with bone diseases, such as osteoporosis. However, half of the patients with fracture were not considered at risk based on bone density measurement (current clinical technique). We assume that loading conditions are of high importance and should be considered. Real loading conditions in a fall can reach a loading speed of 2 m/s on average. The current study aimed to apply more realistic loading conditions that simulate a forward fall on the radius ex vivo. Thirty radii from elderly donors (79 y.o. ± 12 y.o., 15 males, 15 females) were loaded at 2 m/s using a servo-hydraulic testing machine to mimic impact that corresponds to a fall. Among the 30 radii, 14 had a fracture after the impact, leading to two groups (fractured and non-fractured). Surfacic strain fields were measured using stereovision and allow for visualization of fracture patterns. The average maximum load was 2963 ± 1274 N. These experimental data will be useful for assessing the predictive capability of fracture risk prediction methods such as finite element models.     

Pelvis and femur geometry: Relationships with impact characteristics during sideways falls on the hip

While metrics of pelvis and femur geometry have been demonstrated to influence hip fracture risk, attempts at linking geometry to underlying mechanisms have focused on fracture strength. We investigated the potential effects of femur and pelvis geometry on applied loads during lateral falls on the hip. Fifteen female volunteers underwent DXA imaging to characterize two pelvis and six femur geometric features. Additionally, participants completed low-energy sideways falls on the hip; peak impact force and pressure, contact area, and moment of force applied to the proximal femur were extracted. No geometric feature was significantly associated with peak impact force. Peak moment of force was significantly associated with femur moment arm (p = 0.005). Peak pressure was positively correlated with pelvis width and femur moment arm (p < 0.05), while contact area was negatively correlated with metrics of pelvis width and femur neck length (p < 0.05). This is the first study to link experimental measures of impact loads during sideways falls with image-based skeletal geometry from human volunteers. The results suggest that while skeletal geometry has limited effects on overall peak impact force during sideways falls, it does influence how impact loads are distributed at the skin surface, in addition to the bending moment applied to the proximal femur. These findings have implications for the design of protective interventions (e.g. wearable hip protectors), and for models of fall-related lateral impacts that could incorporate the relationships between skeletal geometry, external load magnitude/distribution, and tissue-level femur loads.     

Biomechanics of simulated versus natural cross-country sit skiing

The purpose of this study was to investigate the biomechanics of cross-country sit-skiing in simulated and natural skiing. Thirteen international level athletes participated in a ski ergometer test (simulated conditions) and a test on snow in a ski-tunnel (natural conditions) using their personal sit-ski. Tests in both conditions were performed at individual maximal speed. When comparing the two conditions the main results were: (1) maximal speed in simulated conditions was lower (p < 0.05) but correlated well with the natural condition (r = 0.79, p < 0.001); (2) no differences in pole force variables were found; peak force (r = 0.77, p < 0.01) and average force (r = 0.78, p < 0.01) correlated well; (3) recovery time and time to peak did not differ and time to impact correlated with each other (r = 0.88, p < 0.01); (4) no differences were found in peak electromyography (EMG) and average EMG for Triceps, Pectoralis, and Erector Spinae; Rectus Abdominis did not differ in peak. EMG peak and average EMG of all muscles were correlated between the two conditions (r = 0.65–0.94; p < 0.05–0.01). Although some differences were observed, this study demonstrated that technical skill proficiency in natural and simulated cross-country skiing is comparable from a force production and muscle activation perspective.     

Impact of ankle foot orthosis stiffness on Achilles tendon and gastrocnemius function during unimpaired gait

Ankle foot orthoses (AFOs) are designed to improve gait for individuals with neuromuscular conditions and have also been used to reduce energy costs of walking for unimpaired individuals. AFOs influence joint motion and metabolic cost, but how they impact muscle function remains unclear. This study investigated the impact of different stiffness AFOs on medial gastrocnemius muscle (MG) and Achilles tendon (AT) function during two walking speeds. We performed gait analyses for eight unimpaired individuals. Each individual walked at slow and very slow speeds with a 3D printed AFO with no resistance (free hinge condition) and four levels of ankle dorsiflexion stiffness: 0.25 Nm/°, 1 Nm/°, 2 Nm/°, and 3.7 Nm/°. Motion capture, ultrasound, and musculoskeletal modeling were used to quantify MG and AT lengths with each AFO condition. Increasing AFO stiffness increased peak AFO dorsiflexion moment with decreased peak knee extension and peak ankle dorsiflexion angles. Overall musculotendon length and peak AT length decreased, while peak MG length increased with increasing AFO stiffness. Peak MG activity, length, and velocity significantly decreased with slower walking speed. This study provides experimental evidence of the impact of AFO stiffness and walking speed on joint kinematics and musculotendon function. These methods can provide insight to improve AFO designs and optimize musculotendon function for rehabilitation, performance, or other goals.     

Leg stiffness adjustment for a range of hopping frequencies in humans

The purpose of the present study was to determine how humans adjust leg stiffness over a range of hopping frequencies. Ten male subjects performed in place hopping on two legs, at three frequencies (1.5, 2.2, and 3.0 Hz). Leg stiffness, joint stiffness and touchdown joint angles were calculated from kinetic and/or kinematics data. Electromyographic activity (EMG) was recorded from six leg muscles. Leg stiffness increased with an increase in hopping frequency. Hip and knee stiffnesses were significantly greater at 3.0 Hz than at 1.5 Hz. There was no significant difference in ankle stiffness among the three hopping frequencies. Although there were significant differences in EMG activity among the three hopping frequencies, the largest was the 1.5 Hz, followed by the 2.2 Hz and then 3.0 Hz. The subjects landed with a straighter leg (both hip and knee were extended more) with increased hopping frequency. These results suggest that over the range of hopping frequencies we evaluated, humans adjust leg stiffness by altering hip and knee stiffness. This is accomplished by extending the touchdown joint angles rather than by altering neural activity.     

How symmetric are metal-on-metal hip resurfacing patients during gait? Insights for the rehabilitation

Metal-on-metal hip resurfacing patients demonstrate hip biomechanics closer to normal in comparison to total hip arthroplasty during gait. However, it is not clear how symmetric is the gait of hip resurfacing patients. Biomechanical data of 12 unilateral metal-on-metal hip resurfacing participants were collected during gait at a mean time of 45 months (SD 24) after surgery. Ankle, knee, hip, pelvis and trunk kinematics and kinetics of both sides were measured with a motion and force-capture system. Principal component analysis and mean hypothesis’ tests were used to compare the operated and healthy sides. The operated side had prolonged ankle eversion angle during late stance and delayed increased ankle inversion angle during early swing (p = 0.008; effect size = 0.70), increased ankle inversion moment during late stance (p = 0.001; effect size = 0.78), increased knee adduction angle during swing (p = 0.044; effect size = 0.57), decreased knee abduction moment during stance (p = 0.05; effect size = 0.40), decreased hip range of motion in the sagittal plane (p = 0.046; effect size = 0.56), decreased range of hip abduction moment during stance (p = 0.02; effect size = 0.63), increased hip range of motion in the transverse plane (p = 0.02; effect size = 0.62), decreased hip internal rotation moment during the transition from loading response to midstance (p = 0.001; effect size = 0.81) and increased trunk ipsilateral lean (p = 0.03; effect size = 0.60). Therefore, hip resurfacing patients have some degree of asymmetry in long term, which may be related to hip weakness and decreased range of motion, to foot misalignments and to strategies implemented to reduce loading on the operated hip. Interventions such as muscle strengthening and stretching, insoles and gait feedback training may help improving symmetry following hip resurfacing.     

Active leg stiffness and energy stored in the muscles during maximal counter movement jump in the aged

The purpose of this study was to examine the effects of age on active leg stiffness adjustment, electromyogram (EMG) activities and energy stored during eccentric and concentric phases in performing a maximal functional task involving stretch-shorten cycle. Ten young (24.3 ± 2 years) and 10 old (68.6 ± 5 years) healthy male subjects were filmed during maximal performance of counter movement jump (CMJ) and squat jump (SJ) on force plate. Integrated EMG (IEMG), ground reaction force (GRF), active leg stiffness, energy stored/returned and active work done by the muscles were compared between two groups on eccentric (ECC) and concentric (CON) phases of CMJ. The GRF, leg stiffness and energy stored in ECC and GRF, IEMG, energy returned and active work in CON were less in the elderly (p < 0.05). These results demonstrate that the neuromuscular function of adjusting active stiffness, storing elastic energy and optimizing the performance may decrease with age during CMJ.     

Intra-operatively measured spastic semimembranosus forces of children with cerebral palsy

The knee kept forcibly in a flexed position is typical in cerebral palsy. Using a benchmark, we investigate intra-operatively if peak spastic hamstring force is measured in flexed knee positions. This tests the assumed shift of optimal length due to adaptation of spastic muscle and a decreasing force trend towards extension. Previously we measured spastic gracilis (GRA) and semitendinosus (ST) forces. Presently, we studied spastic semimembranosus (SM) and tested the following hypotheses: spastic SM forces are (1) high in flexed and (2) low in extended positions. We compared the data to those of GRA and ST to test (3) if percentages of peak force produced in flexed positions are different. During muscle lengthening surgery of 8 CP patients (9 years, 4 months; GMFCS levels = II–IV; limbs tested = 13) isometric SM forces were measured from flexion (120°) to full extension (0°). Spastic SM forces were low in flexed knee positions (only 4.2% (3.4%) and 10.7% (9.7%) of peak force at KA = 120° and KA = 90° respectively, indicating less force production compared to the GRA or ST) and high in extended knee positions (even 100% of peak force at KA = 0°). This indicates an absence of strong evidence for a shift of optimal muscle length of SM towards flexion.     

The mechanics of activated semitendinosus are not representative of the pathological knee joint condition of children with cerebral palsy

Characteristic cerebral palsy effects in the knee include a restricted joint range of motion and forcefully kept joint in a flexed position. To show whether the mechanics of activated spastic semitendinosus muscle are contributing to these effects, we tested the hypothesis that the muscle’s joint range of force exertion is narrow and force production capacity in flexed positions is high. The isometric semitendinosus forces of children with cerebral palsy (n = 7, mean (SD) = 7 years (8 months), GMFCS levels III–IV, 12 limbs tested) were measured intra-operatively as a function of knee angle, from flexion (120°) to full extension (0°). Peak force measured in the most flexed position was considered as the benchmark. However, peak force (mean (SD) = 112.4 N (54.3 N)) was measured either at intermediate or even full knee extension (three limbs) indicating no narrow joint range of force exertion. Lack of high force production capacity in flexed knee positions (e.g., at 120° negligible or below 22% of the peak force) was shown except for one limb. Therefore, our hypothesis was rejected for a vast majority of the limbs. These findings and those reported for spastic gracilis agree, indicating that the patients’ pathological joint condition must rely on a more complex mechanism than the mechanics of individual spastic muscles.     

Capacidad predictiva de mortalidad del índice Frágil-VIG (IF-VIG) en pacientes ancianos con fractura de fémur

IntroductionFrailty and hip fracture are closely related and are associated with high risk of functional decline and mortality. The objective of this study is to analyze whether the Frail-VIG index [IF-VIG] (fragility index validated in the geriatric population) maintains its predictive capacity for mortality in old patients with hip fracture.MethodsObservational, cohort, longitudinal and ambispective study on patients admitted to an acute geriatric unit with a hip fracture. Patients were classified according to their degree of frailty into three groups by the IF-VIG: no frailty/initial frailty (≤ 0.35), moderate frailty (0.36-0.50) and advanced frailty (> 0.50). The follow-up period was 24 months. The three groups were compared using survival curves and ROC curves were analyzed to assess the prognostic capacity of IF-VIG.ResultsA total of 103 patients were included; 73.8% were women, with a mean age of 87 years. There were no differences between groups in relation to the type of fracture, the kind of surgery, the waiting time until surgery and the mobilization time. Overall, in-hospital mortality was 7.76%, significantly higher in the advanced frailty group (23.3%). We also found significant differences in mortality at 24 months of follow-up according to the IF-VIG. The under the ROC curve area at 3, 6, 12 and 24 months was 0.90 (0.83-0.97), 0.90 (0.82-0.97), 0.91 (0.86-0.97) and 0.88 (0.81-0.94), respectively.ConclusionThe IF-VIG appears to be a good tool in predicting mortality in old patients with hip fracture.     

Prediction of femoral impact forces in falls on the hip.

A major determinant of the risk of hip fracture in a fall from standing height is the force applied to the femur at impact. This force is determined by the impact velocity of the hip and the effective mass, stiffness, and damping of the body at the moment of contact. We have developed a simple experiment (the pelvis release experiment) to measure the effective stiffness and damping of the body when a step change in force is applied to the lateral aspect of the hip. Results from pelvis release experiments with 14 human subjects suggest that both increased soft tissue thickness over the hip and impacting the ground in a relaxed state can decrease the effective stiffness of the body, and subsequently reduce peak impact forces. Comparison between our fall impact force predictions and in-vitro measures of femoral fracture strength suggest that any fall from standing height producing direct, lateral impact on the greater trochanter can fracture the elderly hip.     

Incidencia y factores explicativos de las caídas en ancianos que viven en la comunidad

Introductionfalls in the elderly are a significant health problem that can be prevented once risk factors have been identified. The objective of this study was to determine the rate of falls among elderly persons living in the community and their risk factors.Patients and methodstwo hundred ambulatory individuals aged 75 years or older attending an outpatient clinic were evaluated by personal interview. Demographic and social variables, health characteristics, use of healthcare resources and falls were studied. Bivariate analysis was performed and significant variables were entered into a conditional logistic regression analysis.Resultsthe rate of falls per year was 56.5% and rate of recurrent falls was 10.5%. The factors associated with falls in the logistic regression model were living with a spouse (OR = 0.37), taking three or more drugs (OR = 5.30), cardiovascular diseases (OR = 0.29), and taking nitrates (OR = 0.21). The factors associated with recurrent falls were change of home (OR = 11.10), living with a spouse (OR = 0.20), respiratory (OR = 6.44) and cardiovascular diseases (OR = 0.27), and taking laxatives (OR = 9.55).Conclusionsin our study the rate of falls and recurrent falls was similar to that described in other studies in western countries. Our results suggest that taking at least three medications and the use of laxatives were potentially preventable risk factors. Living with a spouse seemed to protect from both falls and recurrent falls in elderly people living in the community.     

Effect of mental fatigue on induced tremor in human knee extensors

In this study, the effects of mental fatigue on mechanically induced tremor at both a low (3–6 Hz) and high (8–12 Hz) frequency were investigated. The two distinct tremor frequencies were evoked using two springs of different stiffness, during 20 s sustained contractions of the knee extensor muscles at 30% maximum voluntary contraction (MVC) before and after 100 min of a mental fatigue task, in 12 healthy (29 ± 3.7 years) participants. Mental fatigue resulted in a 6.9% decrease in MVC and in a 9.4% decrease in the amplitude of the agonist muscle EMG during sustained 30% MVC contractions in the induced high frequency only. Following the mental fatigue task, the coefficient of variation and standard deviation of the force signal decreased at 8–12 Hz induced tremor by 31.7% and 35.2% respectively, but not at 3–6 Hz induced tremor. Similarly, the maximum value and area underneath the peak in the power spectrum of the force signal decreased by 55.5% and 53.1% respectively in the 8–12 Hz range only. In conclusion, mental fatigue decreased mechanically induced 8–12 Hz tremor and had no effect on induced 3–6 Hz tremor. We suggest that the reduction could be attributed to the decreased activation of the agonist muscles.