Biomedical subjects

Richard K Shields

Publications and source records attributed to Richard K Shields.

At least 19 recordsLinked to original sources

PubMed2006

Low frequency fatigue in human quadriceps is fatigue dependent and not task dependent.

It is well accepted that a low intensity/long duration isometric contraction induces more low frequency fatigue (LFF) compared to a high-intensity/short-duration contraction. However, previous reports examined the intensity/duration of the contraction but did not control the level of fatigue when concluding fatigue is task dependent. The purpose of this study was to determine whether a long duration/low intensity fatiguing contraction would induce greater LFF than a short duration/high-intensity contraction when the quadriceps muscle was fatigued to similar levels. Eighteen healthy male subjects performed quadriceps contractions sustained at 35% and 65% of maximal voluntary contraction (MVC) on separate days, until the tasks induced a similar amount of fatigue (force generating capacity=45% MVC). Double pulse torque to single pulse torque ratio (D/S ratio) was obtained before, immediately and 5min after fatigue along with the electromyographic (EMG) signal from vastus medialis (VM) and rectus femoris (RF). The D/S ratio significantly (p<0.05) increased by 8.7+/-8.5% (mean+/-SD) and 10.2+/-9.2% after 35% and 65% tasks, respectively, and remained elevated 5min into recovery; however, there was no significant difference in ratio between the two sessions immediately or 5min post-fatigue (p>0.05) even though the endurance time for the 35% fatigue task (124+/-39.68s) was significantly longer (p=0.05) than that of the 65% task (63+/-17.73s). EMG amplitude and median power frequency (MPF) analysis also did not reveal any significant differences between these two sessions after fatigue. These findings indicate that LFF fatigue is fatigue dependent as well as task intensity/duration dependent. These findings assist us in understanding task dependency and muscle fatigue.

Adult↗

PubMed2006

Feedback-controlled stimulation enhances human paralyzed muscle performance.

Chronically paralyzed muscle requires extensive training before it can deliver a therapeutic dose of repetitive stress to the musculoskeletal system. Neuromuscular electrical stimulation, under feedback control, may subvert the effects of fatigue, yielding more rapid and extensive adaptations to training. The purposes of this investigation were to 1) compare the effectiveness of torque feedback-controlled (FDBCK) electrical stimulation with classic open-loop constant-frequency (CONST) stimulation, and 2) ascertain which of three stimulation strategies best maintains soleus torque during repetitive stimulation. When torque declined by 10%, the FDBCK protocol modulated the base stimulation frequency in three ways: by a fixed increase, by a paired pulse (doublet) at the beginning of the stimulation train, and by a fixed decrease. The stimulation strategy that most effectively restored torque continued for successive contractions. This process repeated each time torque declined by 10%. In fresh muscle, FDBCK stimulation offered minimal advantage in maintaining peak torque or mean torque over CONST stimulation. As long-duration fatigue developed in subsequent bouts, FDBCK stimulation became most effective ( approximately 40% higher final normalized torque than CONST). The high-frequency strategy was selected approximately 90% of the time, supporting that excitation-contraction coupling compromise and not neuromuscular transmission failure contributed to fatigue of paralyzed muscle. Ideal stimulation strategies may vary according to the site of fatigue; this stimulation approach offered the advantage of online modulation of stimulation strategies in response to fatigue conditions. Based on stress-adaptation principles, FDBCK-controlled stimulation may enhance training effects in chronically paralyzed muscle.

Adaptation, Physiological↗

PubMed2006

Postfatigue potentiation of the paralyzed soleus muscle: evidence for adaptation with long-term electrical stimulation training.

Understanding the torque output behavior of paralyzed muscle has important implications for the use of functional neuromuscular electrical stimulation systems. Postfatigue potentiation is an augmentation of peak muscle torque during repetitive activation after a fatigue protocol. The purposes of this study were 1) to quantify postfatigue potentiation in the acutely and chronically paralyzed soleus and 2) to determine the effect of long-term soleus electrical stimulation training on the potentiation characteristics of recently paralyzed soleus muscle. Five subjects with chronic paralysis (>2 yr) demonstrated significant postfatigue potentiation during a repetitive soleus activation protocol that induced low-frequency fatigue. Ten subjects with acute paralysis (<6 mo) demonstrated no torque potentiation in response to repetitive stimulation. Seven of these acute subjects completed 2 yr of home-based isometric soleus electrical stimulation training of one limb (compliance = 83%; 8,300 contractions/wk). With the early implementation of electrically stimulated training, potentiation characteristics of trained soleus muscles were preserved as in the acute postinjury state. In contrast, untrained limbs showed marked postfatigue potentiation at 2 yr after spinal cord injury (SCI). A single acute SCI subject who was followed longitudinally developed potentiation characteristics very similar to the untrained limbs of the training subjects. The results of the present investigation support that postfatigue potentiation is a characteristic of fast-fatigable muscle and can be prevented by timely neuromuscular electrical stimulation training. Potentiation is an important consideration in the design of functional electrical stimulation control systems for people with SCI.

Adaptation, Physiological↗

PubMed2006

Electrically induced muscle contractions influence bone density decline after spinal cord injury.

STUDY DESIGN: Longitudinal repeated-measures; within-subject control. OBJECTIVE: We examined the extent to which an isometric plantar flexion training protocol attenuates bone loss longitudinally after SCI. SUMMARY OF BACKGROUND DATA: After spinal cord injury (SCI), bone mineral density (BMD) of paralyzed extremities rapidly declines, likely because of loss of mechanical loading of bone via muscle contractions. METHODS: Six individuals with complete paralysis began a 3-year unilateral plantar flexor muscle activation program within 4.5 months after SCI. The opposite limb served as a control. Compliance with recommended dose was > 80%. Tibia compressive force was > 140% of body weight. RESULTS: Bilateral hip and untrained tibia BMD declined significantly over the course of the training. Lumbar spine BMD showed minimal change. Percent decline in BMD (from the baseline condition) for the trained tibia (approximately 10%) was significantly less than the untrained tibia (approximately 25%) (P < 0.05). Trained limb percent decline in BMD remained steady over the first 1.5 years of the study (P < 0.05). CONCLUSIONS: Compressive loads of approximately 1 to 2 times body weight, induced by muscle contractions, partially prevent the loss of BMD after SCI. Future studies should establish dose-response curves for attenuation of bone loss after SCI.

Adult↗

PubMed2006

Assessment of physical function and secondary complications after complete spinal cord injury.

PURPOSE: Two physical function questions on the widely used SF-12 Health Related Quality of Life questionnaire appear less than optimal for people with complete spinal cord injury (SCI). Physical function questions typically receive the lowest score unless the individual is ambulatory. Additionally, the influence of secondary complications of SCI on quality of life is largely unknown. The purposes of this report are: (1) to determine whether two SF-12 physical function follow-up questions provide increased information in subjects with complete SCI; and (2) to describe the secondary complications of SCI in this group. METHOD: Ten subjects with complete SCI completed two survey instruments (the SF-12 and a Secondary Complications survey) every 3 months. The SF-12 included two physical function follow-up questions designed to improve the sensitivity and appropriateness of the SF-12 for a population with complete SCI. RESULTS: The SF-12 follow-up questions revealed differences in physical function that did not appear with the original SF-12 items. With the new questions, subject scores approximated population normative values. The most common secondary complications (back pain, shoulder pain, leg spasms, leg joint stiffness, and difficulty coughing) were the most likely to be rated as moderately or greatly bothersome. CONCLUSION: We advocate the use of follow-up questions for the SF-12 in complete SCI populations that are not ambulating to better discriminate changes in physical function. Secondly, we advocate further investigations to better understand the incidence and the severity of secondary complications after SCI.

Adolescent↗

PubMed2006

Musculoskeletal plasticity after acute spinal cord injury: effects of long-term neuromuscular electrical stimulation training.

Maintaining the physiologic integrity of paralyzed limbs may be critical for those with spinal cord injury (SCI) to be viable candidates for a future cure. No long-term intervention has been tested to attempt to prevent the severe musculoskeletal deterioration that occurs after SCI. The purposes of this study were to determine whether a long-term neuromuscular electrical stimulation training program can preserve the physiological properties of the plantar flexor muscles (peak torque, fatigue index, torque-time integral, and contractile speed) as well as influence distal tibia trabecular bone mineral density (BMD). Subjects began unilateral plantar flexion electrical stimulation training within 6 wk after SCI while the untrained leg served as a control. Mean compliance for the 2-yr training program was 83%. Mean estimated compressive loads delivered to the tibia were approximately 1-1.5 times body weight. The training protocol yielded significant trained versus untrained limb differences for torque (+24%), torque-time integral (+27%), fatigue index (+50%), torque rise time (+45%), and between-twitch fusion (+15%). These between-limb differences were even greater when measured at the end of a repetitive stimulation protocol (125 contractions). Peripheral quantitative computed tomography revealed 31% higher distal tibia trabecular BMD in trained limbs than in untrained limbs. The intervention used in this study was sufficient to limit many of the deleterious muscular and skeletal adaptations that normally occur after SCI. Importantly, this method of load delivery was feasible and may serve as the basis for an intervention to preserve the musculoskeletal properties of individuals with SCI.

Adult↗

PubMed2006

Predictive model of muscle fatigue after spinal cord injury in humans.

The fatigability of paralyzed muscle limits its ability to deliver physiological loads to paralyzed extremities during repetitive electrical stimulation. The purposes of this study were to determine the reliability of measuring paralyzed muscle fatigue and to develop a model to predict the temporal changes in muscle fatigue that occur after spinal cord injury (SCI). Thirty-four subjects underwent soleus fatigue testing with a modified Burke electrical stimulation fatigue protocol. The between-day reliability of this protocol was high (intraclass correlation, 0.96). We fit the fatigue index (FI) data to a quadratic-linear segmental polynomial model. FI declined rapidly (0.3854 per year) for the first 1.7 years, and more slowly (0.01 per year) thereafter. The rapid decline of FI immediately after SCI implies that a "window of opportunity" exists for the clinician if the goal is to prevent these changes. Understanding the timing of change in muscle endurance properties (and, therefore, load-generating capacity) after SCI may assist clinicians when developing therapeutic interventions to maintain musculoskeletal integrity.

Adolescent↗

PubMed2006

Peripheral quantitative computed tomography: measurement sensitivity in persons with and without spinal cord injury.

OBJECTIVES: To determine (1) the error attributable to external tibia-length measurements by using peripheral quantitative computed tomography (pQCT) and (2) the effect these errors have on scan location and tibia trabecular bone mineral density (BMD) after spinal cord injury (SCI). DESIGN: Blinded comparison and criterion standard in matched cohorts. SETTING: Primary care university hospital. PARTICIPANTS: Eight able-bodied subjects underwent tibia length measurement. A separate cohort of 7 men with SCI and 7 able-bodied age-matched male controls underwent pQCT analysis. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: The projected worst-case tibia-length-measurement error translated into a pQCT slice placement error of +/-3 mm. We collected pQCT slices at the distal 4% tibia site, 3 mm proximal and 3 mm distal to that site, and then quantified BMD error attributable to slice placement. RESULTS: Absolute BMD error was greater for able-bodied than for SCI subjects (5.87 mg/cm(3) vs 4.5 mg/cm(3)). However, the percentage error in BMD was larger for SCI than able-bodied subjects (4.56% vs 2.23%). CONCLUSIONS: During cross-sectional studies of various populations, BMD differences up to 5% may be attributable to variation in limb-length-measurement error.

Adult↗

PubMed2005

Predicting human chronically paralyzed muscle force: a comparison of three mathematical models.

Chronic spinal cord injury (SCI) induces detrimental musculoskeletal adaptations that adversely affect health status, ranging from muscle paralysis and skin ulcerations to osteoporosis. SCI rehabilitative efforts may increasingly focus on preserving the integrity of paralyzed extremities to maximize health quality using electrical stimulation for isometric training and/or functional activities. Subject-specific mathematical muscle models could prove valuable for predicting the forces necessary to achieve therapeutic loading conditions in individuals with paralyzed limbs. Although numerous muscle models are available, three modeling approaches were chosen that can accommodate a variety of stimulation input patterns. To our knowledge, no direct comparisons between models using paralyzed muscle have been reported. The three models include 1) a simple second-order linear model with three parameters and 2) two six-parameter nonlinear models (a second-order nonlinear model and a Hill-derived nonlinear model). Soleus muscle forces from four individuals with complete, chronic SCI were used to optimize each model's parameters (using an increasing and decreasing frequency ramp) and to assess the models' predictive accuracies for constant and variable (doublet) stimulation trains at 5, 10, and 20 Hz in each individual. Despite the large differences in modeling approaches, the mean predicted force errors differed only moderately (8-15% error; P=0.0042), suggesting physiological force can be adequately represented by multiple mathematical constructs. The two nonlinear models predicted specific force characteristics better than the linear model in nearly all stimulation conditions, with minimal differences between the two nonlinear models. Either nonlinear mathematical model can provide reasonable force estimates; individual application needs may dictate the preferred modeling strategy.

Adult↗

PubMed2005

Neuromuscular control of the knee during a resisted single-limb squat exercise.

BACKGROUND: Closed kinetic chain exercises such as single-limb squats are preferred for knee rehabilitation. A complete understanding of the neuromuscular control of the knee during the single-limb squat is essential to increase the efficiency of rehabilitation programs. HYPOTHESIS: Performing a controlled single-limb squat with resistance to knee flexion and extension will increase the coactivation of the hamstring muscle group, thus reducing the quadriceps/hamstrings ratio. STUDY DESIGN: Descriptive laboratory study. METHODS: A total of 15 healthy human subjects (7 women, 8 men) performed controlled single-limb squats in a custom mechanical device that provided resistance to both flexion and extension. Subjects performed the task at 3 levels of resistance, set as a percentage of body weight. Surface electromyographic recordings from 7 muscles (gluteus medius, rectus femoris, vastus medialis oblique, vastus lateralis, biceps femoris, semitendinosus, and medial gastrocnemius) were collected during the task. RESULTS: Biceps femoris activity during knee flexion increased from approximately 12% maximum voluntary isometric contractions during low resistance (0% body weight) to approximately 27% maximum voluntary isometric contractions during high resistance (8% body weight). Although the quadriceps had greater activity than the hamstrings at all levels of resistance, the quadriceps/hamstrings ratio declined significantly with resistance (F2,27 = 29.05; P = .012) from 3.0 at low resistance to 2.32 at the highest resistance. CONCLUSIONS: Performing controlled resisted single-limb squats may help to simultaneously strengthen the quadriceps and facilitate coactivation of the hamstrings, thus reducing anterior tibial shear forces. The coactivation may also increase the dynamic control of the knee joint. CLINICAL RELEVANCE: The typical single-limb squat exercise performed in the clinic does not usually control for bidirectional resistance and knee joint excursion. As seen in this study, controlled single-limb squats at increased levels of resistance help to increase the coactivation of the hamstring muscles, which is essential to optimize neuromuscular control of the knee.

Adult↗

PubMed2005

Mathematical models use varying parameter strategies to represent paralyzed muscle force properties: a sensitivity analysis.

BACKGROUND: Mathematical muscle models may be useful for the determination of appropriate musculoskeletal stresses that will safely maintain the integrity of muscle and bone following spinal cord injury. Several models have been proposed to represent paralyzed muscle, but there have not been any systematic comparisons of modelling approaches to better understand the relationships between model parameters and muscle contractile properties. This sensitivity analysis of simulated muscle forces using three currently available mathematical models provides insight into the differences in modelling strategies as well as any direct parameter associations with simulated muscle force properties. METHODS: Three mathematical muscle models were compared: a traditional linear model with 3 parameters and two contemporary nonlinear models each with 6 parameters. Simulated muscle forces were calculated for two stimulation patterns (constant frequency and initial doublet trains) at three frequencies (5, 10, and 20 Hz). A sensitivity analysis of each model was performed by altering a single parameter through a range of 8 values, while the remaining parameters were kept at baseline values. Specific simulated force characteristics were determined for each stimulation pattern and each parameter increment. Significant parameter influences for each simulated force property were determined using ANOVA and Tukey's follow-up tests (alpha <or= 0.05), and compared to previously reported parameter definitions. RESULTS: Each of the 3 linear model's parameters most clearly influence either simulated force magnitude or speed properties, consistent with previous parameter definitions. The nonlinear models' parameters displayed greater redundancy between force magnitude and speed properties. Further, previous parameter definitions for one of the nonlinear models were consistently supported, while the other was only partially supported by this analysis. CONCLUSION: These three mathematical models use substantially different strategies to represent simulated muscle force. The two contemporary nonlinear models' parameters have the least distinct associations with simulated muscle force properties, and the greatest parameter role redundancy compared to the traditional linear model.

Journal Article↗

PubMed2005

Influence of age on dynamic position sense: evidence using a sequential movement task.

Age related changes to the nervous system are well documented. The main objectives of this study were to examine age-associated changes in dynamic position sense and relate these changes to measures of balance and physical function. Two groups of individuals (young <30 years; elderly >60 years) performed an upper extremity movement sequence triggered by a pre-determined target angle during passive rotations of the ankle joint at ten random velocities (10-90 degrees s(-1)). Balance was assessed with a series of timed standing tests. Physical function was assessed with the SF 36 questionnaire. Muscle activity was recorded from the ankle dorsiflexors and plantarflexors during the dynamic position tests. Increased error in the elderly group suggested that dynamic position sense declines with age. Moreover, this decline in dynamic position sense was associated with decreased balance and an impaired perception of physical function. The elderly also co-contracted the ankle plantarflexors and dorsiflexors during the proprioceptive testing, perhaps as a strategy to "gain up" spindle sensitivity. These findings suggest that impaired dynamic position sense of the ankle contributes to alterations in the overall physical function and balance in the elderly. Rehabilitative training methods that improve dynamic position sense of the ankles may improve physical function and balance in the elderly.

Adult↗

PubMed2005

Monitoring standing wheelchair use after spinal cord injury: a case report.

PURPOSE: An important issue in spinal cord injury (SCI) research is whether standing can yield positive health benefits. However, quantifying dose of standing and establishing subject compliance with a standing protocol is difficult. This case report describes a method to monitor dose of standing outside the laboratory, describes the standing patterns of one subject, and describes this subject's satisfaction with the standing protocol. METHOD: A man with T-10 complete paraplegia agreed to have his commercially available standing wheelchair instrumented with a custom-designed logging device for a 2-year period. The micro-controller-based logger, under custom software control, was mounted to the standing wheelchair. The logger recorded date, duration, angle of standing, and start/stop times. RESULTS: The client exceeded a suggested minimum dosage of standing per month (130.4% of goal), choosing to stand for short bouts (mean = 11.57 min) at an average angle of 61 degrees, on an average 3.86 days per calendar week. He was generally very satisfied with the standing device and provided subjective reports of improved spasticity and bowel motility. CONCLUSION: This case report describes a standing and surveillance system that allow quantification of standing dose. Future controlled studies are needed to evaluate whether standing can be beneficially affect secondary complications after SCI.

Adult↗

PubMed2005

Bone mineral density after spinal cord injury: a reliable method for knee measurement.

OBJECTIVES: To test the interrater reliability of a standardized method to analyze knee bone mineral density (BMD) using dual-energy x-ray absorptiometry (DXA); to compare spine, hip, and knee BMD of people with spinal cord injury (SCI) with able-bodied controls; and to determine the relation between hip BMD and knee BMD in SCI and able-bodied subjects. DESIGN: Criterion standard and masked comparison. SETTING: Primary care university hospital. PARTICIPANTS: A convenience sample of 11 subjects with complete SCI was age and sex matched with 11 able-bodied control subjects. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Four raters analyzed regions of interest according to operational definitions recently developed to standardize the analysis of BMD of the knee. Subjects with chronic SCI and matched controls underwent conventional DXA scans of the spine and hips and "less conventional" scans of the distal femurs and proximal tibias. The relation between hip and knee BMD was analyzed. RESULTS: The knee measurements were highly reliable (femur intraclass correlation coefficient model 2,1 [ICC(2,1)]=.98; tibia ICC(2,1)=.89). Subjects with SCI had lower BMD values than controls at all hip and knee sites (P<.05). Lumbar spine BMD did not differ between groups. Hip BMD was moderately predictive of distal femur BMD (R2=.67), but less correlated with the proximal tibia (R2=.38). CONCLUSIONS: Knee BMD can be reliably analyzed using DXA with this protocol. Subjects with SCI have diminished knee and hip BMD. Low hip BMD is associated with low distal femur BMD.

Absorptiometry, Photon↗

PubMed2005

Proprioceptive coordination of movement sequences in humans.

OBJECTIVE: To estimate the processing time and neuromuscular delay required to extract and process sensory information from the ankle in order to coordinate an upper extremity movement sequence. METHODS: Nineteen able-bodied subjects were tested on their ability to perform a motor task that involved extension of their left index finger when their left ankle was passively plantar flexed at random velocities through a predetermined target angle. RESULTS: We found that the able-bodied subjects were able to adjust their finger responses up to ankle velocities of 70 degrees /s (300 ms). Reaction time, defined as the delay between onset of ankle rotation and how quickly the index finger could be extended, was 215 ms. The processing time and conduction delay was estimated to be approximately 85 ms. CONCLUSIONS: These results indicate that the nervous system processes kinesthetic input related to joint rotation of the ankle with the central mechanisms to execute a planned coordinated task with the upper extremity. SIGNIFICANCE: The time required to process proprioceptive information from the leg to perform a coordinated task with the upper extremity may vary throughout the lifespan. Understanding the effects of age, exercise, or injury on proprioceptive processing time may have important clinical implications.

Adult↗

PubMed2005

Sustained muscle activity minimally influences dynamic position sense of the ankle.

STUDY DESIGN: Cross-sectional study. OBJECTIVE: The purpose of this study was to determine if a sustained fatiguing contraction of the dorsiflexor muscles alters the dynamic position sense (proprioception) and the associated central nervous system processing time of information from the ankle. BACKGROUND: Ankle injury has been hypothesized to be related to altered proprioception as a consequence of fatiguing exercise. Previous reports assessing proprioception include tests of motor performance (balance and limb repositioning) or tests of a joint under static conditions. This study used a novel experimental approach to test the effects of exercise on the somatosensory system of the ankle. METHODS AND MEASURES: Nineteen healthy subjects were tested on their ability to extend the metacarpophalangeal joint of their left index finger when their left ankle was passively plantar flexed (0 degrees-40 degrees, 10 velocities) through a predetermined target angle (20 degrees). Testing occurred before and after a fatiguing contraction of the dorsiflexor muscles. RESULTS: Subjects accurately indicated the ankle target angle up to ankle velocities of 70 degrees/s (300 ms) both before and after the sustained fatiguing contraction. At velocities above 70 degrees/s all subjects could no longer scale to accurately indicate the target angle with the index finger and consequently overshot the target. The central nervous system processing time was estimated to be approximately 85 milliseconds before and after the sustained contraction. CONCLUSIONS: These results indicate that a sustained activity of the dorsiflexion muscles of the ankle minimally affects dynamic position sense and the ability to process dynamic position sense information. Understanding the impact of exercise on sensory system processing will be integral to establishing the scientific basis for rehabilitation programs that purport to train proprioception.

Adult↗

PubMed2004

Soleus H-reflex recruitment is not altered in persons with chronic spinal cord injury.

OBJECTIVE: To determine whether spasticity in persons with spinal cord injury (SCI) is associated with elevated monosynaptic reflex excitability. DESIGN: One-way experimental. SETTING: Research laboratory. PARTICIPANTS: Convenience sample of 9 subjects (8 men, 1 woman) with chronic and complete SCI and 20 persons (14 men, 6 women) with no neurologic impairment. Subjects with SCI exhibited lower-extremity spasticity as indicated by velocity-dependent increased resistance to passive muscle stretch, abnormally brisk deep tendon reflexes, involuntary lower-extremity flexion and/or extension spasms, and clonus. INTERVENTION: Soleus H-reflex recruitment curves were elicited in all subjects. MAIN OUTCOME MEASURES: Soleus H-reflex threshold (HTH), gain (HGN), and amplitude (HPP). RESULTS: There was no difference between subjects with and without SCI in HTH, HGN, or HPP. CONCLUSIONS: Spasticity in people with chronic and complete SCI was not associated with increased excitability of the connections between Ia afferent projections and motoneurons. Factors extrinsic to these connections may have a role in spasticity caused by SCI.

Adult↗

PubMed2004

Femoral loads during passive, active, and active-resistive stance after spinal cord injury: a mathematical model.

OBJECTIVE: The purpose of this study was to estimate the loading environment for the distal femur during a novel standing exercise paradigm for people with spinal cord injury. DESIGN: A mathematical model based on experimentally derived parameters. BACKGROUND: Musculoskeletal deterioration is common after spinal cord injury, often resulting in osteoporotic bone and increased risk of lower extremity fracture. Potential mechanical treatments have yet to be shown to be efficacious; however, no previous attempts have been made to quantify the lower extremity loading during passive, active, and active-resistive stance. METHODS: A static, 2-D model was developed to estimate the external forces; the activated quadriceps forces; and the overall bone compression and shear forces in the distal femur during passive (total support of frame), active (quadriceps activated minimally), and active-resistive (quadriceps activated against a resistance) stance. RESULTS: Passive, active, and active-resistive stance resulted in maximal distal femur compression estimates of approximately 45%, approximately 75%, and approximately 240% of body weight, respectively. Quadriceps force estimates peaked at 190% of body weight with active-resistive stance. The distal femur shear force estimates never exceeded 24% of body weight with any form of stance. CONCLUSIONS: These results support our hypothesis that active-resistive stance induces the highest lower extremity loads of the three stance paradigms, while keeping shear to a minimum. RELEVANCE: This model allows clinicians to better understand the lower extremity forces resulting from passive, active, and active-resistive stance in individuals with spinal cord injury.

Ankle Joint↗