Biomedical subjects

Thomas S Buchanan

Publications and source records attributed to Thomas S Buchanan.

At least 19 recordsLinked to original sources

PubMed2007

Do ACL-injured copers exhibit differences in knee kinematics?: An MRI study.

Kinematic changes after anterior cruciate ligament (ACL) injury may play a role in the long-term development of osteoarthritis (OA). Some ACL-injured patients (copers) successfully return to demanding activities without the reconstructive surgery usually recommended for functionally unstable patients (noncopers). We determined whether copers exhibit less disruption to kinematics than noncopers, perhaps because of lower impairment of muscular control as observed in earlier studies. We used dynamic magnetic resonance imaging and model-based tracking to investigate anteroposterior (AP) and internal-external tibial positioning in copers, presurgical noncopers, and uninjured control subjects during dynamic nonloaded knee extension. Copers and control subjects showed similar levels of side-to-side differences in AP tibial positioning (1.1 +/- 4.9 mm and 1.4 +/- 2.7 mm, respectively), whereas noncopers exhibited anterior tibial positioning in their injured knees (2.6 +/- 3 mm) that differed from control subjects. Copers were the most variable of the three groups, and contrary to our hypothesis, tibial positioning in copers was not different from that of noncopers. Differences in tibial positioning did not correlate with side-to-side differences in AP laxity in any of the groups, and we identified no changes to tibial axial rotation patterns associated with ACL deficiency.

Adaptation, Physiological↗

PubMed2006

Altered knee kinematics in ACL-deficient non-copers: a comparison using dynamic MRI.

Kinematics measured during a short arc quadriceps knee extension exercise were compared in the knees of functionally unstable ACL-deficient patients, these patients' uninjured knees, and uninjured control subjects' knees. Cine phase contrast dynamic magnetic resonance imaging, in combination with a model-based tracking algorithm developed by the authors, was used to measure tibiofemoral kinematics as the subjects performed the active, supine posture knee extension exercise in the terminal 30 degrees of motion. Two determinants of tibiofemoral motion were measured: anterior/posterior location of the tibia relative to the femur, and axial rotation of the tibia relative to the femur. We hypothesized that more anterior tibial positioning, as well as differences in axial tibial rotation patterns, would be observed in ACL-deficient (ACL-D) knees when compared to uninjured knees. Multifactor ANOVA analyses were used to determine the dependence of the kinematic variables on (i) side (injured vs. uninjured, matched by subject in the control group), (ii) flexion angle measured at five-degree increments, and (iii) subject group (ACL-injured vs. control). Statistically significant anterior translation and external tibial rotation (screw home motion) accompanying knee extension were found. The ACL-D knees of the injured group exhibited significantly more anterior tibial positioning than the uninjured knees of these subjects (average difference over extension range=3.4+/-2.8 mm, p<0.01 at all angles compared), as well as the matched knees of the control subjects. There was a significant effect of interaction between side and subject group on A/P tibial position. We did not find significant differences in external tibial rotation associated with ACL deficiency. The changes to active joint kinematics documented in this entirely noninvasive study may contribute to cartilage degradation in ACL-D knees, and encourage more extensive investigations using similar methodology in the future.

Adult↗

PubMed2006

Lower extremity muscle morphology in young athletes: an MRI-based analysis.

PURPOSE: This study was conducted to describe lower extremity muscle morphology (volume and peak cross-sectional area (CSA)) in young athletes and compare these with previously reported values. A second aim was to determine if muscle morphological values differ significantly between sides, implying that unilateral measurements cannot represent both limbs accurately. METHODS: Axial spin-echo T1-weighted magnetic resonance (MR) images were obtained between the ankle mortise and iliac crest in 10 athletes (age 18.8 +/- 3.7 yr). Subsequently, each subject's three-dimensional anatomy was digitally reconstructed. Muscle volume, peak CSA, and length were calculated for 13 muscles. RESULTS: The mean volumes and CSA for the current sample of athletes were larger than previously reported values (primarily from cadaver studies of nonathletes). The ratio of total quadriceps volume to total hamstrings volume averaged nearly 3:1 (2.9 +/- 0.2), whereas previous reports have been closer to 2:1 (2.1 +/- 0.2). The relative contribution of each muscle to the muscle group (hamstrings or quadriceps) volume was also different for these athletes. Significant differences in side-to-side muscle morphology were observed in several knee muscles (P < 0.03). Vastus medialis muscle volume was larger in the dominant leg (difference between sides: 59 +/- 25 cm3, representing 15.9 +/- 6.7% of its average volume), whereas vastus lateralis muscle volume was larger in the nondominant leg (difference: 54 +/- 47 cm3 representing 9.6 +/- 8.3% of its average). Despite this, total quadriceps volumes were similar between sides. CONCLUSIONS: Findings suggest that the morphology data presented in this study should be used instead of data from cadavers when studying young athletic people. These data should improve the accuracy of biomechanical modeling in the athletic population.

Adolescent↗

PubMed2006

Tibialis anterior volumes and areas in ACL-injured limbs compared with unimpaired.

PURPOSE: Past research has shown that subjects with ACL injuries show activation differences and atrophy in the muscles that cross the knee, including the gastrocnemii, which predominately act at the ankle. However, it is not known how the other ankle muscles that do not cross the knee are affected. We focused on the two muscles that control the ankle, the soleus and tibialis anterior muscles, to see how they were affected by an ACL injury. We hypothesized that the ankle muscles of subjects with ACL injuries that did not require surgery (copers) would be more like normals and that the muscles of subjects with ACL injuries who required surgery to return to normal activity (noncopers) would atrophy. METHODS: Twenty-seven subjects were divided into three even categories: unimpaired subjects, copers, and noncopers. Axial spin-echo T1-weighted MRI images were used to digitally reconstruct the tibialis anterior and the soleus. We used the digitally reconstructed muscles to determine the peak cross-sectional area and volume of each muscle. RESULTS: The copers' tibialis anterior muscles were similar to the unimpaired subjects, but, surprisingly, the noncoper's tibialis anterior muscles of the injured leg were larger than those of their uninjured legs (P < 0.05). In the soleus, the results showed a trend of not being affected. CONCLUSION: The increase in size of the tibialis anterior in noncopers may have been caused by altered gait patterns in noncopers. We believe this is due to either an ankle-stiffening strategy during heel strike or from the inversion of the foot causing external rotation of the tibia as a stabilizing technique for the knee.

Adaptation, Physiological↗

PubMed2005

Knee height, knee pain, and knee osteoarthritis: the Beijing Osteoarthritis Study.

OBJECTIVE: Few risk factors for knee osteoarthritis (OA) are appreciated, and the discordance between symptoms and the severity of structural disease has not been explained. Knee height contributes to moments around the knee. The longer the leg, the more torque is present. Although this would suggest that having long legs would be related to the occurrence of knee OA and pain, this issue has not been studied. Our aim was to explore the association between knee height, knee pain, and knee OA. METHODS: We recruited a random sample of Beijing residents ages 60 years and older. Subjects answered questions about joint symptoms, and radiographs of their knees were obtained. A knee joint with a Kellgren/Lawrence grade of >/=2 was defined as having radiographic OA. Patellofemoral OA was defined as being present when grade >/=1 osteophytes or grade >/=1 joint space narrowing was observed on skyline views of the patella or anterior femur. Subjects were considered to have symptomatic OA when both radiographic OA and self-reported pain were present in the same joint. Knee height was measured on the right leg using a sliding broad-blade caliper; the subject was seated, and the subject's feet were bare. We used logistic regression analyses to assess whether knee height was associated with prevalent radiographic and symptomatic OA. We then assessed whether knee height was associated with knee symptoms independently of structural change. RESULTS: A total of 1,006 men (mean +/- SD age 68.4 +/- 6.4 years) and 1,500 women (mean +/- SD age 67.5 +/- 6.1 years) participated in this study. Higher knee height was associated with an increasing prevalence of both radiographic and symptomatic OA, especially among women. For radiographic OA, the magnitude of association was similar for the patellofemoral and tibiofemoral compartments. Among women with knee pain, higher knee height was associated with more severe knee pain (P = 0.0004 for the highest quartile versus the lowest quartile of knee height) independently of the severity of radiographic OA. CONCLUSION: Knee height is associated with prevalent radiographic and symptomatic knee OA. It may also play an important role in knee symptoms. This study highlights the importance of mechanical forces in the determination of OA and knee symptoms.

Anthropometry↗

PubMed2005

Quadriceps femoris muscle morphology and function after ACL injury: a differential response in copers versus non-copers.

The morphology (volume and peak cross-sectional area) and voluntary muscle control of 27 athletic people were evaluated with magnetic resonance imaging (MRI) and an established method of testing neuromuscular control in order to explain why some people are able to cope with anterior cruciate ligament (ACL) injury (copers), whereas most cannot (non-copers). Axial spin-echo T1 weighted MRI images were acquired from the level of the ankle mortise to the iliac crest. The subjects' quadriceps, hamstrings, and gastrocnemius muscles were digitally reconstructed from the MRI images. The volume and peak cross-sectional area (CSA) of each muscle were then calculated. Voluntary muscle control was evaluated using an established target-matching protocol that requires subjects to produce and modulate force with control over a range of directions. Electromyographic signals were collected from seven muscles as the subjects performed the experiment. Circular statistics methods were used to calculate a specificity index that describes how focused the activity pattern of each muscle was with respect to its principal direction of action. The results of the non-copers, copers, and uninjured subjects were then compared. The non-copers displayed significantly greater quadriceps atrophy than the copers. The most profound differences were observed in the vastus lateralis muscle. The non-copers also displayed diminished vastus lateralis and lateral gastrocnemius muscle control. Little differences were observed in the results of the copers and uninjured subjects. In general, the copers' results fell between those of the non-copers and uninjured subjects. The results of this study suggest that quadriceps muscle function is a critical factor in the differential response to ACL injury.

Adolescent↗

PubMed2005

Neuromuscular function after anterior cruciate ligament reconstruction with autologous semitendinosus-gracilis graft.

BACKGROUND: The quadrupled autologous semitendinosus-gracilis graft is the first choice of many orthopaedic surgeons when reconstructing the anterior cruciate ligament. The effect that this procedure has on voluntary muscle control remains unclear. The purpose of this study was to evaluate the effect that anterior cruciate ligament reconstruction with autologous semitendinosus-gracilis graft has on voluntary muscle control by assessing subjects' specificity of muscle action. METHODS: The voluntary muscle control of 10 people (seven males, three females) with acute, isolated ACL ruptures was assessed in the days prior to when they underwent anterior cruciate ligament reconstruction with quadrupled autologous semitendinosus-gracilis grafts and after they had returned to play in sports requiring quick changes of direction and jumping (approximately 6 months later). The experimental protocol included the use of an established target-matching protocol that requires subjects to produce and modulate force with fine control, electromyographic recordings from 11 muscles about the knee, and the use of circular statistics to calculate specificity indices that describe the degree of focus (specificity) associated with the activity pattern of each muscle. Data were analyzed by performing pre-surgery and post-return to sports side-to-side comparisons, as well as, pre-surgery to post-surgery ipsilateral comparisons. RESULTS: Diminished specificity of muscle action was observed in the activity patterns of most of the muscles of the subjects' anterior cruciate ligament deficient knees prior to surgery. The quadriceps muscles were particularly affected. Post-return to sports results indicated that voluntary muscle control had improved in most muscles. There was no significant difference in pre-surgery and post-return to sports semitendinosus and gracilis muscle control. The semimembranosus muscle displayed less specific muscle activity patterns following surgery, which may represent a compensation strategy for minor changes in neuromuscular function. CONCLUSIONS: Voluntary muscle control improved in most muscles following ACL reconstruction with semitendinosus-gracilis autografts. Semitendinosus and gracilis muscle control did not appear to be altered significantly by the procedure.

Adult↗

PubMed2005

A method for measurement of joint kinematics in vivo by registration of 3-D geometric models with cine phase contrast magnetic resonance imaging data.

A new method is presented for measuring joint kinematics by optimally matching modeled trajectories of geometric surface models of bones with cine phase contrast (cine-PC) magnetic resonance imaging data. The incorporation of the geometric bone models (GBMs) allows computation of kinematics based on coordinate systems placed relative to full 3-D anatomy, as well as quantification of changes in articular contact locations and relative velocities during dynamic motion. These capabilities are additional to those of cine-PC based techniques that have been used previously to measure joint kinematics during activity. Cine-PC magnitude and velocity data are collected on a fixed image plane prescribed through a repetitively moved skeletal joint. The intersection of each GBM with a simulated image plane is calculated as the model moves along a computed trajectory, and cine-PC velocity data are sampled from the regions of the velocity images within the area of this intersection. From the sampled velocity data, the instantaneous linear and angular velocities of a coordinate system fixed to the GBM are estimated, and integration of the linear and angular velocities is used to predict updated trajectories. A moving validation phantom that produces motions and velocity data similar to those observed in an experiment on human knee kinematics was designed. This phantom was used to assess cine-PC rigid body tracking performance by comparing the kinematics of the phantom measured by this method to similar measurements made using a magnetic tracking system. Average differences between the two methods were measured as 2.82 mm rms for anterior/posterior tibial position, and 2.63 deg rms for axial rotation. An intertrial repeatability study of human knee kinematics using the new method produced rms differences in anterior/posterior tibial position and axial rotation of 1.44 mm and 2.35 deg. The performance of the method is concluded to be sufficient for the effective study of kinematic changes caused to knees by soft tissue injuries.

Biomechanical Phenomena↗

PubMed2005

Quadriceps weakness, atrophy, and activation failure in predicted noncopers after anterior cruciate ligament injury.

BACKGROUND: Quadriceps weakness is common after anterior cruciate ligament injury, especially in those who do not compensate well for the injury ("noncopers"). Both atrophy and activation failure have been demonstrated in this population but have not been directly related to quadriceps weakness. HYPOTHESES: (1) Quadriceps strength, volumes, and cross-sectional areas of the noncopers would be smaller than those of the contralateral muscles, whereas other muscles would not demonstrate atrophy. (2) Quadriceps muscle activation deficits would be observed. (3) Atrophy and activation failure would account for the quadriceps weakness in these patients. STUDY DESIGN: Cross-sectional study, Level of evidence, 3. METHODS: Seventeen noncopers with isolated anterior cruciate ligament injury underwent burst-superimposition strength and activation testing of the quadriceps and magnetic resonance imaging of 12 muscles an average of 2 months after injury. Morphological characteristics was described by digitally reconstructing each muscle from the axial images and calculating muscle volume and peak cross-sectional area. RESULTS: The quadriceps muscles of the anterior cruciate ligament-deficient limb were significantly weaker (average 25%) than those of the uninjured side; activation failure (8%-10%) was observed for the quadriceps muscles of both limbs. The total quadriceps, vastus lateralis, and vastus intermedius volume and cross-sectional area were significantly smaller in the anterior cruciate ligament-deficient limb. There was no significant atrophy of any other muscle or muscle group. Atrophy and activation failure explained more than 60% of the variance in quadriceps weakness (P = .004). CONCLUSION: The quadriceps femoris weakens soon after acute anterior cruciate ligament injury. Activation deficits and atrophy occur and affect quadriceps strength. Rehabilitation techniques that address activation deficits as well as atrophy may be necessary to restore quadriceps strength.

Adolescent↗

PubMed2005

Neuromuscular biomechanical modeling to understand knee ligament loading.

PURPOSE: This article examines our use of EMG-driven neuromuscular biomechanical models to study how muscles stabilize the knee. EMG can be used to establish which activation patterns are used by people for knee stabilization. However, it does not reveal the effectiveness of these patterns. The EMG-driven models provide quantitative comparisons of the effectiveness of the different knee-stabilizing activation patterns. METHODS: Subjects performed static tasks and common sporting maneuvers that challenged knee joint stability. EMG, joint posture and motion, and external forces and moments were measured during these tasks. These data were used to calibrate the EMG-driven neuromuscular biomechanical model. We then used the model to predict the role of muscles in supporting varus and valgus moments at the knee. RESULTS: We found specific muscle activation patterns to support varus and valgus moments. The most potent activation pattern to stabilize the knee is when the hamstrings or quadriceps are required to generate flexion or extension moments, respectively. The next most effective knee-stabilizing pattern is cocontraction of the hamstring and quadriceps. The small biarticular muscles at the knee provided the least support of varus and valgus moments. In the sporting tasks, sidestepping was found to place the anterior cruciate ligament at high risk of injury. We found that the muscles are the main defense against knee ligament injuries in these tasks. CONCLUSION: Traditional biomechanical and neurophysiological methods have shown that there are specific activation patterns used to stabilize the knee. By also using the EMG-driven neuromuscular biomechanical model, we have shown how effective muscles are in stabilizing the knee. This modeling method provides a new tool to understand knee joint stabilization.

Biomechanical Phenomena↗

PubMed2005

Estimation of muscle forces and joint moments using a forward-inverse dynamics model.

PURPOSE: This paper presents a forward dynamic neuromusculoskeletal model that can be used to estimate and predict joint moments and muscle forces. It uses EMG signals as inputs to the model, and joint moments predicted are verified through inverse dynamics. The aim of the model is to estimate or predict muscle forces about a joint, which can be used to estimate the corresponding joint compressive forces, and/or ligament forces in healthy and impaired subjects, based on the way they activate their muscles. METHODS: The estimation of joint moments requires three steps. In the first step, muscle activation dynamics govern the transformation from the EMG signal to a measure of muscle activation--a time-varying parameter between 0 and 1. In the second step, muscle contraction dynamics characterize how muscle activations are transformed into muscle forces. The final step requires a model of the musculoskeletal geometry to transform muscle forces to joint moments. Each of these steps involves complex, nonlinear relationships. RESULTS: An application is provided to demonstrate how this model can be used to study the forces in the healthy ankle during dynamometer trials and during gait. The model-predicted estimates of joint moment were found to match experimentally determined values closely. CONCLUSION: Neuromusculoskeletal models that use EMG as inputs can be employed to accurately estimate joint moments. The muscle forces predicted from these models can be used to better understand tissue loading in joints, and to provide in vivo estimates of tensile ligament forces and compressive cartilage loads during dynamic tasks. This tool has great potential for aiding in the study of injury mechanisms in sports.

Ankle↗

PubMed2005

Use of an EMG-driven biomechanical model to study virtual injuries.

INTRODUCTION: How the CNS activates muscles to produce coordinated movement is a matter of debate and great interest. We are attempting to answer this question, in part, by investigating how individual muscles and groups of muscles are activated under different physiologic and environmental conditions. We have developed an EMG-driven virtual arm to assist in this endeavor. PURPOSE: To demonstrate how the virtual arm can be used to simulate a neuromuscular injury and to examine whether a virtual injury can evoke a change in muscle activation patterns. METHODS: The virtual arm is a three-dimensional graphical representation and biomechanical model of a human arm including the major flexor and extensor muscles crossing the elbow. The muscles are actuated based on experimentally recorded electromyograms. A Hill-type muscle model was used to predict muscle forces, which in turn were used to move the graphical display of the arm on the screen. Two subjects, one considered highly trained and the other a novice, participated in this study. Virtual movements, before and after simulating an injury were evaluated, and model performance was assessed by comparing the virtual arm-predicted moment and the actual moment generated by the subjects. RESULTS: The highly trained subject was proficient at controlling the virtual arm. For this subject, simulating a neuromuscular injury evoked a different pattern of activation compared to the preinjured state. CONCLUSIONS: The virtual arm may be a useful tool for the study of motor coordination and how muscle activation patterns change in response to injury. Future work involving more subjects and experimental conditions is planned to better assess the efficacy of the virtual arm as a research tool for investigating motor control strategies.

Arm↗

PubMed2004

High-arched runners exhibit increased leg stiffness compared to low-arched runners.

Leg stiffness between high-arched (HA) and low-arched (LA) runners was compared. It was hypothesized that high-arched runners would exhibit increased leg stiffness, increased sagittal plane support moment, greater vertical loading rates, decreased knee flexion excursion and increased activation of the knee extensor musculature. Twenty high-arched and 20 low-arched subjects were included in this study. Leg stiffness, knee stiffness, vertical loading rate and lower extremity support moment were compared between groups. Electromyographic data were collected in an attempt to explain differences in leg stiffness between groups. High-arched subjects were found to have increased leg stiffness and vertical loading rate compared to low-arched runners. Support moment at the impact peak of the vertical ground reaction force was related to leg stiffness across all subjects. High-arched subjects demonstrated decreased knee flexion excursion during stance. Finally, high-arched subjects exhibited a significantly earlier onset of the vastus lateralis (VL) than the low-arched runners. Differences exist in leg stiffness and vertical loading rate between runners with different foot types. Differences in lower extremity kinetics in individuals with different foot types may have implications for new treatment strategies or preventative measures.

Adolescent↗

PubMed2004

Neuromusculoskeletal modeling: estimation of muscle forces and joint moments and movements from measurements of neural command.

This paper provides an overview of forward dynamic neuromusculoskeletal modeling. The aim of such models is to estimate or predict muscle forces, joint moments, and/or joint kinematics from neural signals. This is a four-step process. In the first step, muscle activation dynamics govern the transformation from the neural signal to a measure of muscle activation-a time varying parameter between 0 and 1. In the second step, muscle contraction dynamics characterize how muscle activations are transformed into muscle forces. The third step requires a model of the musculoskeletal geometry to transform muscle forces to joint moments. Finally, the equations of motion allow joint moments to be transformed into joint movements. Each step involves complex nonlinear relationships. The focus of this paper is on the details involved in the first two steps, since these are the most challenging to the biomechanician. The global process is then explained through applications to the study of predicting isometric elbow moments and dynamic knee kinetics.

Journal Article↗

PubMed2004

Altered quadriceps control in people with anterior cruciate ligament deficiency.

PURPOSE: The purpose of this study was to determine whether similar patterns of quadriceps dysfunction are observed when people with anterior cruciate ligament (ACL) deficiency perform static and dynamic tasks. METHODS: EMG data were collected from 15 subjects with an ACL deficient knee and 15 uninjured subjects as they performed static and dynamic tasks that were isolated to the knee and presented no threat to joint stability. The dynamic task was cyclic flexion and extension in the terminal 30 degrees of knee extension; the static task was an established isometric target-matching protocol. The muscle activity patterns observed during the tasks were evaluated and compared. RESULTS: The subjects with ACL deficiency exhibited quadriceps muscle control strategies that were significantly different from those of the uninjured subjects. This was true in both the dynamic and the static tasks. The findings were most noteworthy in the vastus lateralis muscle. Good agreement (r = -0.73 to -0.75) was observed in subjects' static and dynamic VL results; more moderate agreement was observed in results of the other quadriceps muscles. CONCLUSION: Diminished quadriceps control was observed when people with ACL deficiency performed static and dynamic tasks. The most striking feature of this impaired control was failure to turn the quadriceps "off" when performing flexion tasks in which the knee extensors are usually "silent." Our findings suggest that quadriceps dyskinesia after ACL injury is relatively global. Changes in neural function and muscle physiology after ACL injury are put forth as the most likely source of the observed dyskinesia.

Adolescent↗

PubMed2004

Muscle and tendon morphology after reconstruction of the anterior cruciate ligament with autologous semitendinosus-gracilis graft.

BACKGROUND: The autologous semitendinosus-gracilis graft is the first choice of many orthopaedic surgeons when reconstructing the anterior cruciate ligament. The effect that graft harvest has on muscle and tendon morphology remains unclear. The purpose of this study was to describe these effects more completely. METHODS: Magnetic resonance images were acquired from eight patients before the anterior cruciate ligament reconstruction with semitendinosus-gracilis autograft and then again postoperatively after they had returned to sports. Muscle and tendon morphology was described by determining the volume and peak cross-sectional area of each structure on digitally reconstructed images. The effects that the procedure had on muscle and tendon length were evaluated separately and then together as a muscle-tendon complex. RESULTS: Anterior cruciate ligament reconstruction with semitendinosus-gracilis autograft resulted in a marked decrease in volume, cross-sectional area, and length of the semitendinosus and gracilis muscles. Tendon regeneration occurred in varying degrees in nearly all subjects. The morphology of the biceps femoris and semimembranosus muscles suggested that they had been compensating for the reduced semitendinosus and gracilis muscle function. Although semitendinosus and gracilis muscle retraction occurred following tendon stripping, nearly all of the subjects displayed evidence of at least partial tendon regeneration. CONCLUSIONS: Anterior cruciate ligament reconstruction with semitendinosus-gracilis autograft had a marked impact on semitendinosus and gracilis muscle morphology. However, this altered muscle morphology did not appear to have a clinically important impact on short-term outcomes. The biceps femoris and semimembranosus muscles appear to compensate for reduced semitendinosus and gracilis function. Tendon regeneration is observed in most people, but it is often incomplete at six months.

Adult↗

PubMed2003

A one-parameter neural activation to muscle activation model: estimating isometric joint moments from electromyograms.

Nonlinearities have been observed in the isometric EMG-force relationship. However, these are generally not included when using EMG-driven Hill-type muscle models that account for muscle activation dynamics. In this paper, we present a formulation for a one-parameter transformation model (i.e., A-model) that accounts for the type of physiological nonlinearities observed at low levels of force. The general shape for the curvilinear portion of the curve was based on phenomenological data reported by Woods and Bigland-Ritchie. The one-parameter A-model is easy to implement, and when used with an EMG-driven Hill-type model, was shown to provide a better fit of the measured joint moment. Optimization methods were used to determine the appropriate curvature of the relationship for each muscle, and thus introduced a degree of "tuning" to each subject.

Action Potentials↗

PubMed2003

Specificity of muscle action after anterior cruciate ligament injury.

Neuromuscular control is believed to be a critical factor in dynamic knee stability. The purpose of this study was to evaluate voluntary muscle control in anterior cruciate ligament deficient (ACL-D) and uninjured people. Twenty athletes of similar age participated in this study. Subjects performed a target-matching protocol that required them to produce isometric moments about the knee with fine control in flexion, extension, varus, and valgus (i.e., loads were generated in the plane perpendicular to the long axis of the shank). Electromyographic data were collected from 10 muscles that span the knee. A specificity index was calculated for each muscle to describe how fine-tuned (specific) its muscle activity pattern was with respect to its principal direction of action in the load plane. Diminished specificity of muscle action was observed in 8 of 10 muscles in the ACL-D subjects' involved knees when compared with the activity patterns from their uninvolved knees and those from the uninjured subjects' knees. The vastus lateralis muscle was especially affected. Increased and more global co-contraction was also observed in the ACL-D limbs. The alterations in muscle firing patterns observed in this study are consistent with diminished neuromuscular control.

Adult↗