Sean D. Shimada

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.

Glenohumeral joint kinematics and kinetics for three coordinate system representations during wheelchair propulsion.

The shoulder plays a very important role during manual wheelchair propulsion. Unfortunately, substantial numbers of manual wheelchair users eventually develop shoulder injury or pain. Recently, studies have begun to investigate the etiology of wheelchair user shoulder injuries. This study compared three coordinate systems used to represent the shoulder during wheelchair propulsion. Our results show statistically significant differences between the three shoulder representations analyzed. Differences are seen for individuals and for the subjects as a group. Based upon our results, the fixed-z model appears preferable over the other representations due to its simplicity, low hardware requirements, and the similarity of the results to the free representation. This article also provides some insight into maximal shoulder joint forces and moments recorded during manual wheelchair propulsion. Future work should include more sophisticated models of the shoulder complex.

Shoulder and elbow motion during two speeds of wheelchair propulsion: a description using a local coordinate system.

Individuals who propel wheelchairs have a high prevalence of upper extremity injuries. To better understand the mechanism behind these injuries this study investigates the motion of the shoulder and elbow during wheelchair propulsion. The objectives of this study are: (1) To describe the motion occurring at the shoulder and elbow in anatomical terms during wheelchair propulsion; (2) to obtain variables that characterize shoulder and elbow motion and are statistically stable; (3) to determine how these variables change with speed. The participants in the study were a convenience sample of Paralympic athletes who use manual wheelchairs for mobility and have unimpaired arm function. Each subject propelled an ultralight wheelchair on a dynamometer at 1.3 and 2.2 meters per second (m/s). Biomechanical data was obtained using a force and moment sensing pushrim and a motion analysis system. The main outcome measures investigated were: maximum and minimum angles while in contact with the pushrim, range of motion during the entire stroke and peak accelerations. All of the measures were found to be stable at both speeds (Cronbachs alpha >0.8). The following measures were found to differ with speed (data format: measure at 1.3 m/s±SD; measure at 2.2 m/s±SD): minimum shoulder abduction angle during propulsion (24.5°±6.7, 21.6°±7.2), range of motion during the entire stroke in elbow flexion/extension (54.0°±9.9, 58.1°±10.4) and shoulder sagittal flexion/extension (74.8°±9.4, 82.6°±8.5), and peak acceleration in shoulder sagittal flexion/extension (4044°/s2±946, 7146°/s2±1705), abduction/adduction (2678°/s2±767, 4928°/s2±1311), and elbow flexion/extension (9355°/s2±4120, 12889°/s2±5572). This study described the motion occurring at the shoulder and elbow using a local coordinate system. Stable parameters that characterize the propulsive stroke and differed with speed were found. In the future these same parameters may provide insight into the cause and prevention of shoulder and elbow injuries in manual wheelchair.

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.

Upper limb strength in individuals with spinal cord injury who use manual wheelchairs.

Abstract Introduction: Manual wheelchair users have been found to be at risk for secondary upper extremity injuries. Purpose: The primary goal of this study was to compare shoulder strength and muscle imbalance of individuals with paraplegia to case-wise matched unimpaired controls (UC). A secondary goal was to evaluate the impact of age and neurologic level of injury (NLI) on weight-normalized strength (WNS). Methods: The SCI group (n = 28) and the UC group (n = 28) completed bilateral shoulder isokinetic strength testing in the sagittal, frontal, and horizontal plane at 60 degrees/second using the BioDex system. Strength ratios, an indicator of muscle imbalance, were also calculated. Results: No significant difference was seen in shoulder strength or strength ratios between the SCI group and the UC group. However, NLI was significantly related to WNS on several planes in the SCI group. Therefore, we dichotomized the SCI group into equal groups based on an NLI. The Low-SCI group was significantly stronger than the High-SCI group in most planes (P < 0.05). The High-SCI group was significantly weaker than the UC in extension (P < 0.01) and a trend (P < 0.01) was seen in flexion, abduction, and external rotation. The Low-SCI group was significantly stronger in abduction than the UC. Conclusion: WNS at the shoulder correlated with NLI. It is likely that this is related to contributions of the trunk and abdominal muscles during testing, since proximal trunk strength aids in generating forces distally. This study and others of strength in individuals with paraplegia may overestimate shoulder strength.

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.

Comparison of three different models to represent the wrist during wheelchair propulsion

Due to the high incidence of secondary wrist injury among manual wheelchair users, recent emphasis has been placed on the investigation of wheelchair propulsion biomechanics. Accurate representation of wrist activity during wheelchair propulsion may help to elucidate the mechanisms contributing to the development of wrist injuries. Unfortunately, no consensual wrist biomechanical model has been established. In order to determine if different methodologies obtain similar results, this investigation created and compared three different wrist models: 1) a fixed joint center placed between the styloids (midstyloid joint center); 2) a joint center with 2/spl deg/ of freedom computed from de Levas joint center data; and 3) a floating joint center. Results indicate that wrist flexion and extension angles are highly consistent between models, however, radial and ulnar deviation angles vary considerably. Mean maximum right flexion angles were found to be 3.5/spl deg/, 2.2/spl deg/, and 5.0/spl deg/ for the midstyloid, de Leva, and floating joint center models, respectively. Extension angles were 22.3/spl deg/, 23.6/spl deg/, and 23.6/spl deg/, respectively. Mean maximum right radial deviation angles for the midstyloid, de Leva, and floating joint center models were 26.0/spl deg/, 26.9/spl deg/, and 45.1/spl deg/, respectively, and ulnar deviation angles were found to be 30.5/spl deg/, 38.8/spl deg/, and 10.2/spl deg/, respectively. This information is useful when comparing kinematic studies and further supports the need for consensual methodology.

Computer controlled wheelchair dynamometer

The application of dynamometers, including treadmills, exercise bicycles, and arm-crank ergometers have been used for many years in the field of exercise physiology to conduct biomechanical and physiological analyses of locomotion. The purpose of this research project was to create a computer controlled wheelchair dynamometer that would simulate varying road and track conditions. Two Hewlett Packard (HP) electronic loads and power supplies, along with two direct current (DC) motors were interfaced to the Human Engineering Research Laboratories (HERL) two-drum wheelchair dynamometer. The software, created in Visual BASIC, was written to control the load resistances and power supply voltages, manipulated by scroll bar movements. The computer controlled wheelchair dynamometer allowed the researcher to create a simulation of uphill and downhill training conditions, while simultaneously providing the means to execute valid exercise tests and kinematic analyses.

Effect of start-up kinetics on wheelchair pushrim dynamic analysis

Many investigators have examined pushrim forces and moments and joint kinetics during wheelchair propulsion, but rarely are start-up kinetics performed in the analysis. Kinetic data were collected as nine spinal cord injured experienced wheelchair users propelled their wheelchairs with instrumented pushrims on a stationary dynamometer. The subjects pushed from rest to at least 1.8 meters/second (m/s) for 20 seconds. A paired t-test was used to determine significant differences between steady state propulsion and start-up propulsion. Significant increases (p<0.05) in Fx (horizontal force), Fy (vertical force), Mz (moment about the wheelchair wheel axis), and F (three-dimensional resultant force) were found between start-up and steady state propulsion. Steady state analysis of wheelchair biomechanics will underestimate the magnitudes of forces and moments at the pushrim. Investigation of wheelchair propulsion biomechanics as a risk for injury should include start-up kinetics.