Mark D. Grabiner

Variability of step kinematics in young and older adults

Fall-related injuries are the most common and serious medical problems facing older adults. Recent studies of older adults have focused on the variability of step kinematics and the relationship to falling. The accuracy of step variability estimates is proportional to the number of steps that are collected. The use of an instrumented treadmill allows simultaneous collection of spatial and temporal step kinematics for a large number of continuous steps. The current study was conducted to determine the influence of age, walking velocity and handrail use on the variability of step kinematics using a treadmill protocol. Eighteen young adults (average age: 27.7 +/- 3.3 years) and 12 healthy older adults (average age: 73.4 +/- 2.3 years) were recruited from the community. Temporal and spatial gait parameters were quantified using custom designed software from measurements collected during treadmill walking. The primary independent variables were the variability of step length, step width, and step time. Step width variability of older adults was significantly larger than that of young adults. Walking velocity did not influence step kinematic variability. Handrail usage influenced the variability of step length and step width, but not of step time. The present results, and those of previous studies, point to a consistent relationship between age and step width variability. Since step width variability has been implicated in falls, further research is warranted.

Effect of an Ankle Orthosis and Ankle Ligament Anesthesia on Ankle Joint Proprioception

The purposes of this study were to determine the effect of a rigid ankle orthosis (Aircast Air-Stirrup) and lateral ankle ligament anesthesia on ankle joint proprioception. Twelve noninjured subjects attempted to match nine reference ankle joint positions with their eyes closed before and after application of the ankle brace and be fore and after one or two of the lateral ankle ligaments (anterior talofibular and calcaneofibular) were anesthe tized. Three-dimensional ankle joint orientations were recorded with a Motion Analysis system. No significant differences in the constant, variable, or absolute error were seen between subjects in the non- anesthetized and anesthetized conditions (P > 0.05), regardless of whether one or two ligaments were anes thetized. Thus, it appears that ligament mechanorecep tors contributed little to ankle joint proprioception, and that the afferent feedback from skin, muscle, and other joint receptors was adequate for the positioning task of the present study. Both the variable and absolute error in matching the reference positions were significantly less with the orthosis than without (P < 0.05). Applica tion of an orthosis may increase the afferent feedback from cutaneous receptors in the foot and shank, which may in turn lead to an improved ankle joint position sense.

Neuromechanics of the patellofemoral joint

Patellofemoral joint pain is one of the most common ailments associated with visits to sports medicine clinics and can be disabling, although conservative clinical treatment has a reportedly very high success rate. Patellofemoral joint pain is often associated with improper tracking of the patella within the femoral trochlear notch. Improper tracking of the patella can be associated with increased patellofemoral contact pressures that may be a mechanical stimulus underlying patellar cartilage degeneration. In those cases in which anatomic anomalies and trauma may be excluded as the basis for improper tracking, attention is directed toward possible disruptions to the central nervous system control and contractile potential of the knee joint extensor musculature that underlies proper patellofemoral mechanics. This paper presents a review of three seminal components related to the neuromechanics of patellofemoral function; patellofemoral tracking, patellofemoral contact pressures, and neuromotor control of patellofemoral agonists. It is the intent of the authors to illuminate areas requiring further basic and clinical research and provide a point of departure for this work.

Foot displacement but not velocity predicts the outcome of a slip induced in young subjects while walking

The purpose of the present study was to induce slips in healthy subjects as a means to determine if recovery from an induced slip is possible under conditions in which the displacements and velocities of the slipping foot exceed the generally accepted limits of 10cm and 50cm/s, respectively, and to determine if there are gait-related variables that predispose an individual to falling after a slip. Thirty-three young and barefoot adults, protected by an instrumented safety harness, were subjected to a single slipping trial following a series of unperturbed walking trials. The slip was induced when the bare foot contacted a vinyl sheet coated with mineral oil. Lower extremity kinematics were acquired using a video-based motion capture system. Fourteen and 12 subjects could be unambiguously categorized as having fallen or recovered, respectively. Four variables demonstrated significant between-group differences and two were used to compute the probability of the slip outcome using logistic regression. The variables were the displacement of the foot during the slip and the angle of the shank relative to the ground at the instant of ground contact just prior to the slip. Separate univariate logistic regressions using each variable were significant and correctly classified about 70% of the slip outcomes. The results demonstrated that previously published values for the displacement and velocity of the slipping foot, 10cm and 50cm/s, respectively, may not accurately represent the upper limits beyond which recovery is not possible. The results also demonstrated that heel-strike angle, reflective of stride length, exerts a significant influence on the outcome of a slip.

In vivo tracking of the human patella

The purpose of this study was to describe the dynamic, in vivo, three-dimensional tracking pattern of the patella for one normal male subject. Intracortical pins were inserted into the patella, tibia, and femur. The subject performed seated and squatting knee flexion/extension, and maximum voluntary quadriceps contractions. In addition, the vastus medialis oblique was subjected to maximal electrical stimulation. Motions of the markers attached to the intracortical pins were analyzed using an automated video system. Patellar and tibial motions were determined relative to a femoral reference system. While the tibia flexed 50 degrees from full extension (seated condition), the patella flexed 30.3 degrees, tilted laterally 10.3 degrees, and shifted laterally 8.6 mm. In general, these results show qualitative agreement with the data collected from cadaveric specimens [van Kampen and Huiskes, J. orthop. Res. 8, 372-382 (1990)]. The differences present may reflect different passive constraints to patellar motions, and different relative loading of the individual quadriceps components, in our study compared to the cadaveric study. Only small differences were found between patellar motions in the seated and squatting conditions. Differences in patellar displacements produced by (1) maximal electrical stimulation of the vastus medialis oblique, and (2) maximum voluntary quadriceps contraction, at 30 degrees knee flexion and full extension, may reflect the dominant influence of passive constraints, and the vastus lateralis, on normal patellar motions. Further in vivo study of patellar tracking seems warranted to evaluate surgical and conservative interventions for patellofemoral disorders.

Measuring step kinematic variability on an instrumented treadmill: how many steps are enough?

Variability of step kinematics has been associated with falls by older adults. However, between-study differences with regard to the number of steps used to compute variability have varied by an order of magnitude. If the number of steps used to compute variability is too low there is the potential for a statistically spurious outcome. On the other hand, for subjects with mobility impairments a protocol requiring too many steps to estimate variability imposes an unnecessary burden on the subjects. We have determined the minimum number of steps needed to estimate the variability of spatial and temporal step kinematics. More than 700 steps were collected during level walking on an instrumented treadmill. Accurate estimation of step kinematic variability required at least 400 steps. The increased error in estimating the mean and standard deviations of the step kinematic variables with too few steps can impose an experimental cost with regard to statistical design considerations. The extent to which translation of these results can be made to the variability of spatial and temporal step kinematics collected during over-ground walking awaits further research.

Mechanisms of failed recovery following postural perturbations on a motorized treadmill mimic those associated with an actual forward trip.

OBJECTIVE To examine the recovery strategies employed during a treadmill acceleration task, to determine if mechanisms that contributed to failed recoveries on a motorized treadmill are the same general biomechanical mechanisms that contributed to falls from a trip, and to determine if failed recovery responses could be modified to allow for successful recoveries on subsequent trials. DESIGN A motorized treadmill was used to induce postural perturbations in healthy older adults. BACKGROUND Previously, we induced trips in older adults to identify the mechanisms of failed recovery. However, inducing trips is not a clinically practical test for identifying older adults who are predisposed to falling. METHODS Safety-harnessed older adults stood on a treadmill that was accelerated from 0 to 0.89 m/s to impose a postural perturbation. Recoveries were classified as successful (n=42) or failed (n=23). Selected biomechanical variables were calculated using motion analysis methods. RESULTS Initial failed recoveries had slower reaction times, shorter step lengths, and greater trunk flexion angles and velocities. Subjects who failed on the initial attempt modified their recovery strategy to successfully recover. The biomechanics of these recoveries resembled those used by subjects who successfully recovered on their initial attempt. CONCLUSIONS The biomechanical mechanisms involved with a failed treadmill recovery mimic those responsible for failed recoveries from an induced trip. Subjects who failed on their initial recovery response made modifications allowing successful recoveries on subsequent attempts. RELEVANCE This protocol may be useful as a testing and rehabilitation tool for fall recovery.

Measures of Postural Stability Are Not Predictors of Recovery from Large Postural Disturbances in Healthy Older Adults

OBJECTIVES: To determine, in healthy older adults, the relationship between postural steadiness, stability limits, and the ability to recover balance from three postural disturbances requiring anteriorly directed stepping responses.

Influence of lower extremity strength of healthy older adults on the outcome of an induced trip

OBJECTIVES: To determine whether decreased lower extremity strength contributes to trip‐related falls in older adults.

Evaluation of methods to minimize cross talk in surface electromyography

The purposes of this study were to quantify the effectiveness of various methods to minimize cross talk in surface electromyography (EMG) using common recording equipment, and to compare the intra- and interday variabilities of signals recorded with these methods. Comparisons were made for signals recorded with the single differential (SD), double differential (DD) and branched electrode (BE) techniques with large and small electrodes and corresponding interelectrode distances. The amount of cross talk in tibialis anterior EMG signals during maximum voluntary effort (MVE) triceps surae excitation was estimated using a protocol involving electrical stimulation of the triceps surae via the tibial nerve. In SD signals, cross talk averaged 12.2 and 10.2 percent MVE, for the large and small interelectrode distances, respectively. DD and BE signals contained significantly less cross talk (approximately 5 percent MVE for both techniques and interelectrode distances) than SD signals. The intra- and interday variabilities associated with these methods were estimated by recording tibialis anterior EMG signals during maximum voluntary isometric dorsiflexion (3 trials on each of 2 days) and calculating coefficients of variation for average-rectified values and median frequencies. EMG signals recorded with the small interelectrode distance showed greater interday amplitude variability than those recorded with the large interelectrode distance. Intra- and interday amplitude variabilities were similar across SD, DD and BE recording techniques. Intra- and interday frequency variabilities were similar across all experimental conditions. Thus, the DD and BE techniques, in conjunction with the large interelectrode distance (and large electrodes), provide a signal which contains significantly less cross talk than the SD technique without sacrificing intra- and interday amplitude and frequency stability.