ABSTRACT

The purpose of this study was to determine the effects of spinal cord injury (SCI) on the passive properties (viscosity and stiffness) of the lower extremity joints. Isokinetic tests were performed to measure the passive moments at the ankle, knee, and hip joints in both the sagittal and coronal planes. Joint angle, joint velocity, and adjacent joint angles were varied to account for their influences. Using the acquired data, an 11-parameter nonlinear viscoelastic model was fitted to the measurements. Comparisons between the moment curves of the two subject populations showed some differences and many similarities in shape and magnitudes. Future work will include incorporating these modeled passive moments into a larger musculoskeletal model of the lower extremities to examine the impact of passive properties on the sit-to-stand transition and standing balance.

BACKGROUND

Figure 1. Biodex appa-ratus and JR3 trans-ducer used to measure passive moments at each joint

As a joint rotates, passive moments develop due to the elongation or compression of muscle and connective tissue crossing the joint, and due to the contact between articular surfaces. Also, as an adjacent joint rotates it can influence the passive moments at the primary joint through biarticular muscles. The magnitudes of these moments and their influence on movement have been the topics of several investigations [1-3]. However, the passive joint moments of individuals with SCI may differ from able-bodied individuals due to prolonged sitting in wheelchairs, muscle disuse atrophy, and other physical changes that occur after injury [2, 4]. Therefore, to understand the impediments to movement after paraplegia and plan appropriate interventions it is important to have this information and not assume that SCI passive joint properties are the same as able-bodied.

RESEARCH QUESTION

The goal of this study was to determine whether lower extremity passive properties differed between SCI and able-bodied populations, and whether or not those differences were functionally significant. We measured and compared passive moments in 4 individuals with SCI and 5 able-bodied individuals and investigated the influence of neighboring joint position. Specifically, passive moments were measured for ankle inversion/eversion and hip abduction/adduction, in addition to the more typical measurements of ankle, knee, and hip flexion/extension. The MetroHealth Medical Center and Cleveland VA Medical Center Institutional Review Boards approved the experimental protocol, and each subject provided informed consent to participate.

METHODS

Data were recorded during 7 isokinetic trials performed on a Biodex 3® dynamometer [www.Biodex.com]. In addition, a JR3® [www.JR3.com] force and moment transducer was mounted to the spindle of the dynamometer to obtain more accurate and higher resolution signals for all six axes (Figure 1). The primary joint angle, the angular velocity applied to the joint, and the positions of the proximal and distal joints were varied systematically throughout the trials. The proximal and distal joint angles were fixed during each trial by external braces. For all of the joints tested, the angular velocities and adjacent joint angles were chosen to cover those encountered during a sit-to-stand transition [5, 6]. As in previous studies [3], the joint was preconditioned through 20 cycles of its range of motion (ROM) so that the moment measurements would not be history dependent. During each trial, the joint was cycled through its ROM 15 times. The last 5 cycles were utilized for the data analysis. With the obtained data, a nonlinear viscoelastic model was fitted to allow comparisons to be made between the two subject populations.

RESULTS

Figure 2. Passive knee moments for an able-bodied subject and a SCI subject. (a)Velocity fixed at 10 deg/s and ankle plantarflexion/extension set at 12 deg. (b) Velocity fixed at 5 deg/s and ankle plantarflexion/extension set at 23 deg. Filled symbols represent measured data during flexion to extension movements. Open symbols represent data for the re-verse movement. Dashed lines represent model fit.

For all subjects and joints, imposed movements away from the neutral position resulted in the generation of a resistive moment that increased as the joint approached its end of ROM. The data from able-bodied and SCI subjects were very similar in shape and magnitude, and the nonlinear model provided an accurate description of the joint properties under all conditions tested. The shape and magnitudes of the passive moments measured for the ankle, knee, and hip joints in flexion and extension were consistent with those of previous studies [1, 2, 7]. Any differences may be due to natural inter-subject variability and how the limb weights were calculated.

For ankle dorsiflexion, the knee angle had an influence on the ankle moment due to the biarticular gastrocnemius muscle. However, at the knee joint the ankle plantarflexion/dorsiflexion had little influence on the knee passive moment. At the knee joint, the hip flexion/extension angle did affect the knee moments of the able-bodied subjects but not the SCI subjects (Figure 2). Since the hip and the knee joints are coupled through the hamstring and the rectus femoris muscles, this suggests a diminished hamstring effect in the SCI group. Yet the coupling between the knee and hip did result in the knee angle having an influence on the hip flexion/extension passive moment in both subject populations. Hip abduction/adduction was expected to have an influence on the hip flexion/extension moment. However, this did not occur. Similarly ankle flexion/extension was expected to affect the ankle inversion/eversion moment because of the numerous muscles that cross both joints. However, no interaction was observed.

The amount of hysteresis (viscosity) at the inflection point of the passive moment curves was utilized to compare the viscoelastic properties between subject populations[8]. The amount of hysteresis exhibited was not significantly different between the two populations. The hysteresis magnitude changed very little for both subject populations for angular velocities above 10 deg/s.

DISCUSSION

Figure 3. Passive ankle inver-sion/eversion moments for an able-bodied subject and a SCI subject. Ve-locity fixed at 30 deg/s. Symbols rep-resent measured data. Dashed lines represent model fit.

In this study, we investigated how passive joint moments differ between individuals with and without paraplegia. Comparing the able-bodied and SCI populations, the overall shapes of the passive moment curves were very similar. The magnitudes of those curves however differed for some joints. Specifically at the knee joint of subjects with SCI, passive moments at flexed hip positions were smaller than those for the able-bodied subjects. This suggests that the hamstrings are not coupling these joints as in the able-bodied case. For hip abduction/adduction and ankle inversion/eversion (Figure 3), the SCI group had a higher overall slope than the able-bodied group. This would indicate this joint was gen-erally stiffer for the SCI group. Possible mechanisms that could result in these differences between subject populations are changes in the moment arms at the joint that the tissues are acting over, changes in the fascia that hold these tissues in place, or stretching of the musculotendon units during prolonged sitting.

The passive moment measurements obtained in this study will be incorporated into a bipedal musculoskeletal model of the lower extremities, pelvis and trunk. With the musculoskeletal model, we will simulate how the passive properties of individuals can affect their stability during standing. Also methods of changing passive properties to increase one's stability during standing will be examined. The knowledge gained through these studies will enable us to develop better control systems for users of FES standing and walking systems.

REFERENCES

Kofi Amankwah
2500 MetroHealth Dr, H601
Cleveland, OH 44109
Office: 216-778-1424
Email:kaa2@cwru.edu