Rick N. Robertson

Pushrim Forces and Joint Kinetics During Wheelchair Propulsion

OBJECTIVE To investigate pushrim forces and joint kinetics during wheelchair propulsion and to discuss the differences between inexperienced and experienced wheelchair users. DESIGN Cohort study. SETTING Human engineering laboratory at a state university. SUBJECTS Four men who use manual wheelchairs for mobility and four nondisabled men who did not have extensive experience pushing a wheelchair; all subjects were asymptomatic for upper extremity pain or injury. METHODS Subjects pushed a commonly used wheelchair fitted with a force-sensing pushrim on a stationary wheelchair dynamometer. Video and force data were collected for 5 strokes at one speed of propulsion. Pushrim forces and net joint forces and moments were analyzed. MAIN OUTCOME MEASURES Pushrim forces, radial (Fr) and tangential (Ft), were analyzed and compared for both groups in relation to peak values and time to peak values and as ratios of overall forces generated. Net joint forces and moments were analyzed in a similar fashion. RESULTS Pushrim forces and joint moments were similar to those previously reported, with radial forces averaging between 34 and 39N and tangential forces ranging on average between 66 and 95N. Tangential forces were higher than radial forces, and mean ratios of tangential forces to the resultant force were approximately 75%, whereas mean radial force ratios were approximately 22%. All subjects showed higher joint moments at the shoulder than at the elbow or wrist. A large component of vertical reaction force was seen at the shoulder. Significant differences (p < .05) were found between groups for peak tangential force and time to peak tangential and peak vertical forces, with wheelchair users having lower values and longer times to reach the peak values. CONCLUSIONS Discrete variables from the force-time curves can be used to distinguish between wheelchair users and nonusers. The experienced users tended to push longer, used forces with lower peaks, and took longer time to reach peak values. This propulsive pattern may have been developed to reduce the chance of injury by minimizing the forces at the joints, as a means of maximizing efficiency or as a combination of these factors. More work investigating 3-dimensional forces and the influence of seating position and various conditions of propulsion such as speed changes, ramps, and directional changes on injury mechanisms needs to be completed.

Three-dimensional pushrim forces during two speeds of wheelchair propulsion.

Upper limb pain frequently occurs in manual wheelchair users. Analyzing the pushrim forces and hub moments occurring during wheelchair propulsion is a first step in gaining insight into the cause of this pain. The objectives of this study were as follows: to describe the forces and moments occurring during wheelchair propulsion; to obtain variables that characterize pushrim forces and are statistically stable; and to determine how these variables change with speed. Convenience samples (n = 6) of paralympic athletes who use manual wheelchairs for mobility and have unimpaired arm function were tested. Each subject propelled a standard wheelchair on a dynamometer at 1.3 and 2.2 m/s. Biomechanical data were obtained using a force- and moment-sensing pushrim and a motion analysis system. A number of variables that describe the force and moment curves were evaluated for stability using Cronbachs alpha. Those measures found to be stable (alpha > 0.8) at each speed were then examined for differences associated with speed. The tangential, radial, and medial-lateral forces were found to comprise approximately 55, 35, and 10% of the resultant force, respectively. In addition to duration of stroke and propulsion, the following variables were found to be stable and to differ with speed (1.3 m/s +/- SD; 2.2 m/s +/- SD): peak force tangential to the pushrim (45.9 +/- 17.9 N; 62.1 +/- 30 N), peak moment radial to the hub (9.8 +/- 4.5 N x m 13.3 +/- 6 N x m), maximum rate of rise of the tangential force (911.7 +/- 631.7 N/sec; 1262.3 +/- 570.7 N/sec), and maximum rate of rise of the moment about the hub (161.9 +/- 78.3 N x m/s; 255.2 +/- 115.4 N x m/s). This study found stable parameters that characterize pushrim forces during wheelchair propulsion and varied with speed. Almost 50% of the forces exerted at the pushrim are not directed toward forward motion and, therefore, either apply friction to the pushrim or are wasted. Ultimately, this type of investigation may provide insight into the cause and prevention of upper limb injuries in manual wheelchair users.

Wrist biomechanics during two speeds of wheelchair propulsion: An analysis using a local coordinate system

OBJECTIVE To describe motion, forces, and moments occurring at the wrist in anatomic terms during wheelchair propulsion; to obtain variables that characterize wrist function during propulsion and are statistically stable; and to determine how these variables change with speed. DESIGN Case series. SETTING Biomechanics laboratory. PARTICIPANTS Convenience sample of Paralympic athletes (n = 6) who use manual wheelchairs for mobility and have unimpaired arm function. INTERVENTION Subjects propelled a standard wheelchair on a dynamometer at 1.3m/sec and 2.2m/sec. Biomechanical data were obtained using a force and moment sensing pushrim and a motion analysis system. MAIN OUTCOME MEASURES Maximum angles, forces, and moments in a local, wrist coordinate system. Each variable was evaluated for stability using Cronbachs alpha. Measures found to be stable (infinity > .8) at each speed were then compared to look for differences associated with speed. RESULTS The following measures were stable at both speeds: maximum wrist flexion, ulnar deviation, and radial deviation angles, peak moments acting to cause wrist flexion, extension, and ulnar deviation, peak shear forces acting between the radial and ulnar styloids, and peak axial force acting at the wrist. Of these measures, the following measures differed (p < .05) between speeds (+/-SD): maximum radial deviation (1.3m/sec, 25.1 degrees +/- 9.0; 2.2m/sec, 21.4 degrees +/- 6.9), peak flexion moment (1.3m/ sec, 3.4N.m +/- 3.0; 2.2m/sec, 5.2N.m +/- 3.7), peak extension moment (1.3m/sec, 10.4N.m +/- 4.8; 2.2m/sec, 13.6N.m +/- 5.1), peak shear acting from the ulnar styloid to the radial styloid (1.3m/sec, 2.3N +/- 2.7, 2.2m/sec, 8.3N +/- 7.5) and maximum axial force (1.3m/sec, 50.9N +/- 18.2; 2.2m/sec, 65.9N +/- 27.6). CONCLUSION This study found stable parameters that characterize wrist biomechanics during wheelchair propulsion and varied with speed. Ultimately these parameters may provide insight into the cause and prevention of wrist injuries in manual wheelchair users.

Force sensing control for electric powered wheelchairs

People with many types of disabilities use electric powered wheelchairs (EPWs) for mobility. Excessive intention tremor, limited range of motion, athetoid motions, and spastic rigidity can reduce or prohibit the control over an EPW. This study focused on the design and testing of a newly developed isometric joystick (IJ) for EPW control. The IJ was tested against existing performance standards and compared to a commercial position sensing joystick (PSJ). When averaged across all subjects (both controls and impaired subjects) the root-mean-square tracking error and time to complete the driving course were not significantly different when using the rule (p<0.05). Results within a subject, however, did show significance. Across all subjects, nevertheless, the IJ was superior to the PSJ for two tasks: driving straight and driving in a circle. Further development of control algorithms, especially the implementation of digital control, could lead to greater improvement in performance with the IJ.

Upper limb nerve entrapments in elite wheelchair racers.

The prevalence of upper limb nerve injuries has been reported to be as high as 73% in individuals who rely on manual wheelchairs for mobility. Many authors hypothesize that the repetitive trauma to carpal canal structures caused by propelling a wheelchair is the reason for this high prevalence. The purpose of this study was to determine the prevalence of nerve conduction abnormalities in a group of elite wheelchair racers whose wrists are exposed to additional propulsion-related trauma during training and competition. We performed bilateral upper limb nerve conduction studies on each athlete (n = 12). The racers pushed their chairs an average of 56 miles a week for training purposes. Fifty percent of the athletes (n = 6) had evidence of median mononeuropathy by nerve conduction. Of these 6 racers, 5 had evidence of mononeuropathy bilaterally, making a total of 11 positive hands of the 23 tested. Twenty-five percent of the athletes had evidence of ulnar mononeuropathy at the wrist, and 25% had evidence of ulnar mononeuropathy at the elbow. Seventeen percent of athletes had evidence of radial nerve injury. Years with a disability accounted for a significant amount of the variance in the mean median sensory amplitude (R2 = 0.511; P = 0.020) and the mean ulnar palmar amplitude (R2 = 0.605; P = 0.008). Variables not correlated with nerve conduction studies include age, hours per day in a wheelchair not spent training, years competing, and number of miles pushed in training. Despite the amount of time spent training these wheelchair athletes have a similar or lower prevalence of median mononeuropathy then reported in the general wheelchair-using population.

Model reference adaptive control of heart rate during wheelchair ergometry

This study used parametric methods to identify a model of the wheelchair users heart rate response to changes in physical workload while on a wheelchair dynamometer. A model reference adaptive control (MRAC) algorithm was developed based upon the results of the system identification process. Based upon subjects maximum speed, ten random levels from zero to maximal speed were selected to form a tracking input vector. Two autoregressive models with exogenous inputs were developed using a stepwise regression-based algorithm. Analysis of variance results imply that the model development and model validation groups were different. Linear regression was used to compare the model predicted heart rates with recorded heart rates for the validation group. The application of indirect MRAC appears to be feasible for controlling heart rate kinetics of people with paraplegia or lower limb impairments during wheelchair propulsion on a computer-controlled wheelchair dynamometer.

Projection of the point of force application onto a palmar plane of the hand during wheelchair propulsion

The objective of this study was to develop and test a method for projecting the pushrim point of force application (PFA) onto a palmar plane model of the hand. Repetitive wheelchair use often leads to hand and wrist pain or injury. The manner by which the hands grasp the pushrim and how the forces and moments applied to the pushrim are directed may contribute to the high incidence of pain and injury. The projections of the PFA onto the palmar surface model of the hand reside primarily within zone II. These results are in agreement with previous studies which have assumed the PFA to be coincident with one of the metacarpophalangeal (MP) joints. However, the results from three subjects show different PFA patterns within the palmar surface of the hand which can be related to each subjects propulsion pattern, and the PFA is not focused at a single MP joint. Projection of the world coordinates of the four hand marker system onto the palmar plane show the resolution to be within 3 mm, or one half the diameter of the passive reflective markers. The errors in the planar model assumption were greatest for the second and fifth MP markers. This was expected because as the hand grasp changes these markers do not remain coplanar. The results of this study indicate that new knowledge about how forces are applied by the hand onto the pushrim can be obtained using this method. This technical note provides insight into understanding the details within the kinetics of wheelchair propulsion and describes a technique for estimation of the PFA on the palmar surface of the hand. This technical note provides initial results from three different wheelchair users.

A method for analyzing center of pressure during manual wheelchair propulsion

Little published information is available on joint kinetics during wheelchair propulsion. This is partially due to the lack of appropriate instrumentation and techniques. Biomechanical-mechanical techniques may be developed to assist in the amelioration of upper extremity pain among wheelchair users, Elbow, wrist, and hand pain have been reported to exist among 16, 13, and 11% of manual wheelchair users, respectively. This paper focuses on methods for determining the location of the pushrim center of pressure during wheelchair propulsion. Wheelchair propulsion is accomplished by bilateral simultaneous repetitive motion of the upper extremities. The pushrim is grasped or struck and pushed downward and forward, in turn, rotating the wheels. During the propulsive phase, the hand is capable of exerting a three-dimensional moment against the pushrim. The moments and forces exerted on the pushrim were measured by a specialized wheelchair wheel, the SMART/sup wheel/. The center of pressure (COP) for the upper extremities is found in three planes parallel to the frontal, sagittal, and transverse anatomical planes, This calculation of the COP is analogous to the calculation of the COP for lower extremity gait analysis using a force plate. One difference is that the hand has the ability to pull on the pushrim and, therefore, the upper extremity COP does not necessarily reside within the projection of the hand. Another difference is that with a force plate, there is only one plane of interest (the plane of the force plate), and three are used for the complete analysis of the upper extremity. Kinetic data were collected using the SMART/sup Wheel/ from three subjects who are wheelchair users. Kinematic data were also collected concurrently using a PEEKS video analysis system. Graphs of the COP from the sagittal plane show great variability, which is probably due to the low medial-lateral forces exerted against the pushrim. Frontal COP graphs show less variability and indicate that with these three subjects the line of action for the anterior-posterior force component is located between 10 and 15 cm lateral to the pushrim and a variable distance above or below the location of the 2nd metacarpal-phalangeal joint. Future studies with more subjects may show force offset to be a modality in the cause of carpal-tunnel syndrome.

Uncertainty analysis for wheelchair propulsion dynamics

Wheelchair propulsion kinetic measurements require the use of custom pushrim force/moment measuring instruments which are not currently commercially available. With the ability to measure pushrim forces and moments has come the development of several dynamic metrics derived for analyzing key aspects of wheelchair propulsion. This paper presents several of the equations used to calculate or derive the primary variables used in the study of wheelchair propulsion biomechanics. The uncertainties for these variables were derived, and then numerically calculated for a current version of the SMARTWheel. The uncertainty results indicate that the SMARTWheel provides data which has better than 5 to 10% uncertainty, depending upon the variable concerned, at the maximum, and during most of the propulsion phase the uncertainty is considerably smaller (i.e., approximately 1%). The uncertainty analysis provides a more complete picture of the attainable accuracy of the SMARTWheel and of the degree of confidence with which the data can be recorded. The derivations and results indicate where improvements in measurement of wheelchair propulsion biomechanical variables are likely to originate. The most efficient approach is to address those variables in the design of the system which make the greatest contribution to the uncertainty. Future research will focus on the point of force application and examination of nonlinear effects.

A unified method for calculating the center of pressure during wheelchair propulsion

AbstractThe measurement of the center of pressure (COP) has been and continues to be a successful tool for gait analysis. The definition of a similar COP for wheelchair propulsion, however, is not straightforward. Previously, a COP definition similar to that used in force plate analysis had been proposed. Unfortunately, this solution has the disadvantage of requiring a separate COP definition for each plane of analysis. A definition of the generalized center of pressure (GCOP) which is consistent in all planes of analysis is derived here. This definition is based on the placement of a force-moment system, equivalent to the force-moment system at the hub, on a line in space where the moment vector (wrench moment) is parallel to the force vector. The parallel force-moment system is then intersected with three planes defined by anatomical landmarks on the hand. Data were collected using eight subjects at propulsion speeds of 1.34 m/s and 2.24 m/s (1.34 m/s only for subject 1, 0.894 m/s and 1.79 m/s for subject 8). Each subject propelled a wheelchair instrumented with a SMARTWheel. A PEAK 5 video system was used to determine the position of anatomical markers attached to each subject’s upper extremity. The GCOP in the transverse plane of the wrist formed clusters for all subject’s except subject 2 at 1.34 m/s. The clustering of the GCOP indicates that the line of action for the force applied by the hand is approximately perpendicular to the transverse plane through the wrist. When comparing the magnitude of the moment vector part of the wrench with the moment of the force vector of the wrench about the hub, the wrench moment is approximately an order of magnitude smaller. This indicates that the role of the wrist for wheelchair propulsion is primarily to stabilize the force applied by the arm and shoulder.