Håkan Lanshammar

Biomechanics of Slips

The biomechanics of slips are an important component in the prevention of fall-related injuries. The purpose of this paper is to review the available literature on the biomechanics of gait relevant to slips. This knowledge can be used to develop slip resistance testing methodologies and to determine critical differences in human behaviour between slips leading to recovery and those resulting in falls. Ground reaction forces at the shoe-floor interface have been extensively studied and are probably the most critical biomechanical factor in slips. The ratio of the shear to normal foot forces generated during gait, known as the required coefficient of friction (RCOF) during normal locomotion on dry surfaces or ‘friction used/achievable’ during slips, has been one biomechanical variable most closely associated with the measured frictional properties of the shoe/floor interface (usually the coefficient of friction or COF). Other biomechanical factors that also play an important role are the kinematics of the foot at heel contact and human responses to slipping perturbations, often evident in the moments generated at the lower extremity joints and postural adaptations. In addition, it must be realized that the biomechanics are dependent upon the capabilities of the postural control system, the mental set of the individual, and the perception of the environment, particularly, the danger of slipping. The focus of this paper is to review what is known regarding the kinematics and kinetics of walking on surfaces under a variety of environmental conditions. Finally, we discuss future biomechanical research needs to help to improve walkway-friction measurements and safety.

The dynamics of slipping accidents

Abstract Official statistics indicate that slipping is one of the most common causes of accidents. Falls contribute to about 40% of the 4000 fatal accidents that occur annually in Sweden. In fact, falls are more common than motor vehicles as a registered cause of accidental deaths. During 1975 occupational injuries caused approximately 3 million sickness-days and 26% of them were due to falling accidents. The actual involvement of skidding cannot be evaluated, because slip-ups have been registered only as a subgroup of “falls on the same level”. However, skidding may initiate other types of accidents as well. This is supported by preliminary data from a new occupational injury information system. The computer based system allows selective retrievals. In one of the first outputs slip-ups are involved in at least 25 accidents out of 102 including “fall to a lower level” during house construction work. Thus it is urgently necessary to improve the slipping resistance of shoes, floors and walking surfaces. Development is guided by friction measurement with different kinds of apparatus. Unfortunately, many of these are based on an oversimplified theory of static friction, which seems to be quite irrelevant due to the viscoelastic properties of shoe soles and heels. However, even if the apparatus measures dynamic friction, tests must be performed with forces and motions closely resembling a real human skid. Otherwise, friction measurements and real slipping resistance will be poorly correlated. Recently, a gait analysis system was developed which now has been used to evaluate force and motion data from walking and skidding subjects in the laboratory. The system includes piezoelectric and optoelectronic sensors connected on-line to a computer with substantial software for gait analysis. This paper presents the method and results such as: the average critical slip motion started 0.05 s after heel strike. At this point the vertical load was about 60% of body weight and acting at the heel rear edge, the experiments usually resulting in a fall if the sliding exceeded 0.5 m s−1 in velocity or 0.1 m in distance.

Gait abnormalities in diabetic neuropathy

OBJECTIVE To investigate the effect of peripheral neuropathy on gait in diabetic patients. RESEARCH DESIGN AND METHODS Gait analysis was performed in the following groups matched for age, sex, and BMI: 20 normal healthy control subjects (NC), 20 non-neuropathic diabetic control subjects (DC), 20 neuropathic diabetic subjects (DN), and 20 neuropathic diabetic subjects with a history of foot ulceration (DNU). All subjects with orthopedic foot problems were excluded from the study. The following gait parameters were investigated: 1) walking speed; 2) stance phase duration; 3) joint angles and moment arms for the ankle, knee, and hip joints in both sagittal and frontal planes; 4) the components of the ground reaction force (GRF) vector; and 5) the ankle, knee, and hip joint moments originating from the GRF vector in both planes. RESULTS There were no statistical differences in any of the parameters studied between the NC and DC groups. Walking speed was significantly slower in the DNU group compared with the two control groups (P < 0.02). The maximum knee joint angle was smaller in the sagittal plane for the DNU group compared with the DC group values (P < 0.05). The maximum value of the vertical component of GRF was found to be higher (P < 0.03) in the two control groups compared with the DNU group. The maximum value of the anteroposterior forces was also found to be higher (P < 0.001) in the DC group compared with the DNU group. The maximum frontal plane ankle joint moment was significantly higher (P < 0.05) in the DN compared with the NC group. CONCLUSIONS Diabetic subjects with peripheral neuropathy demonstrate alterations in some gait parameters during walking. These alterations could facilitate foot injuries, thus contributing to frequent foot ulceration.

Relationships between fore- and hindlimb ground reaction force and hoof deceleration patterns in trotting horses.

REASONS FOR PERFORMING STUDY The transmission of shockwaves following hoof impact is proposed to be one major source of stress to the limb. In the forelimb, there are indications that the period of horizontal deceleration of the hoof is related to the attenuation of shockwaves. In the hindlimb, information about the hoof deceleration has been lacking. OBJECTIVE To compare hoof deceleration patterns between the fore- and hindlimbs. METHODS Seven Standardbreds were trotted by hand over a force plate covered with sand, with triaxial accelerometers mounted on the fore and hind hooves. Variables representative of decelerations (first 2 main vertical deceleration peaks; characteristic minimum and maximum values in the craniocaudal deceleration; hoof braking time) and ground reaction forces (vertical loading rates; maximum and the following local minimum of the craniocaudal force) of the initial part of the stance phase, and the differences between individual fore- and hindlimb time and amplitude variables were used for statistical analyses. RESULTS Force plate data showed significantly greater vertical loading rate (mean +/- s.d. 6.5 +/- 5.9 N/sec) and horizontal loads (190.4 +/- 110.2 N) in the forelimb than the hindlimb, but the parameters from accelerometer data showed no significant differences. CONCLUSIONS No significant difference was found in the hoof deceleration, but the deceleration curves displayed a common pattern that described in detail the kinematics of the fore and hind hooves during the initial period of hoof braking. POTENTIAL RELEVANCE These results contribute to further knowledge about the characteristics of these potential risk factors in the development of subchondral bone damage in the horse. Further studies are required on the influence of hoof braking pattern at higher speed, different shoeing and ground surfaces with different properties.

On practical evaluation of differentiation techniques for human gait analysis

Numerical differentiation of noisy measurement data represents a problem frequently encountered in the field of gait analysis. There are two major determinants of the quality in calculated derivatives, namely the quality of the measurement data and the quality of the used differentiation technique. The quality of the measurement data, with respect to the maximum precision that can be obtained in calculated derivatives, is discussed with the help of an error formula valid for all differentiating techniques. It is verified that high precision can be obtained in the calculated second derivatives even with crude techniques, provided that the quality of the measurement data are good enough. This point is illustrated by the differentiation film data from Pezzack et al. (1977), using a least squares polynomial fitting. For the evaluation and comparison of different techniques for numerical differentiation it is recommended that measurement data with a considerable amount of noise is used, and that the quality of calculated derivatives are evaluated not only by visual inspection of graphical displays, but also with the use of a quantitative criteria, such as the root mean squares error.

Analysis of postural sway strategies using an inverted pendulum model and force plate data

The purpose with this study was to develop a simple method for analysis of postural movement strategies in the sagittal plane. The main idea was to utilise force components measured with a force plate in order to estimate both the input and output of an inverted pendulum model, and thereby determine the used balance strategy. The acceleration of the centre of mass is estimated with the model and compared with the true one (the ground reaction force divided by the body mass). The two accelerations must be in agreement if the body behaves like an inverted pendulum (the ankle strategy). Practical experiments demonstrated that tests where only the ankle strategy was used could clearly be separated from tests where non-ankle strategies were used.

Dominant pattern extraction from 3-D kinematic data

A new method for the extraction of a repeating pattern in cyclic biomechanical data is proposed-singular value decomposition pattern analysis (SVDPA). This method is based on the recent work of Kanjilal and Palit (1994, 1995) and can be applied to both contiguous and repeated trials without being constrained to be strictly periodic. SVDPA is a data-driven approach that does not use a preselected set of basis functions; but instead utilizes a data matrix with a special structure to identify repeating patterns. Several important features of SVDPA are described including its close relationship to the Kahunen-Loeve transform. The dominant pattern is defined as the average energy component (AEC). The AEC is obtained from the SVD of the data matrix and is equivalent to the optimal [maximal signal-to-noise ratio (SNR)] ensemble average pattern. The degree of periodicity and SNR for the AEC are defined explicitly from the singular values of the data matrix. The authors illustrate the usefulness of SVDPA for dominant pattern extraction by applying it to the quasiperiodic three-dimensional trajectory of a marker attached to the trunk during treadmill locomotion. The AEC obtained for the normalized trajectory and error estimates at each point suggests that SVDPA could be a useful tool for the extraction of the fine details from cyclic biomechanical data.

Amplitude and frequency analysis of force plate data in sitting children with and without MMC

OBJECTIVE To investigate if force plate measurements can be used to detect postural sway differences in sitting children with and without myelomeningocele (spina bifida).BACKGROUND. The postural sway has not been investigated in children with myelomeningocele previously. Since many of these children are not able to stand independently, force plate measurements during sitting could be one way to detect differences in their postural sway compared to normal children. However, there is very little published regarding assessment of seated postural sway.METHODS. Force plate measurements on 15 six years old children with myelomeningocele and 20 age-matched normal children were analysed. The standard deviation and the median frequency of the horizontal ground reaction force were used to characterise the body sway.RESULTS. The standard deviation of the force was larger only for some of the children with myelomeningocele as compared to the control group. The median frequency was significantly lower in the myelomeningocele group as compared to the control group. Visual input and seat base inclination did not influence the postural sway significantly. CONCLUSIONS The results show that frequency analysis can be used to detect fundamental differences in postural sway that can not be observed visually. RelevanceIn this paper a new method for analysis of seated postural sway is described. The lack of relatively high spectral frequencies for children with myelomeningocele shows that the output from the postural control system differs as compared to the control group.

Measuring Slipperiness - Human Locomotion and Surface Factors

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An investigation of kinematic and kinetic variables for the description of prosthetic gait using the ENOCH system

Gait patterns, joint angles, floor reaction forces and joint moments during walking were investigated for normal subjects and above-knee and below-knee amputees. The investigation showed that the hip-knee angle diagram as well as different symmetry diagrams (e.g. left knee angle versus right knee angle) provide an easily interpreted means of evaluating abnormalities in the gait pattern. It was further concluded that a combined gait pattern-force vector diagram is valuable for the evaluation of the joint moments. Floor reaction forces and muscular moments at the joints were also included in the analysis. The joint moments at the knee were quite different for both above-knee and below-knee amputees as compared to the normal subjects. Some interesting trends were also found concerning the knee stability of the amputees. A system called ENOCH was used for the measurement and analysis. This system consists of a minicomputer connected on-line to equipment for measurement of displacement (Selspot) and floor reaction forces (Kistler). A graphic computer terminal (Tektronix) was used for the result presentation.