Musculotendon modeling of the elbow joint for unimpaired subjects and persons after stroke using noninvasive and in vivo information

Abstract

Neuromusculoskeletal modeling provides insights into the muscular system which are not always obtained through experiment or observation alone. One of the major challenges in neuromusculoskeletal modeling is to accurately estimate the musculotendon parameters on a subject-specific basis. Latest medical imaging techniques such as ultrasound for the estimation of musculotendon parameters would provide an alternative method to obtain the muscle architecture parameters noninvasively. In this study, the feasibility of using ultrasonography to measure the musculotendon parameters of elbow muscles is validated. These parameters help to build a subject-specific musculokeletal model for both unimpaired subjects and persons after stroke, and then enhance our understanding of the muscle architectural changes after the onset of stroke. First, ultrasound imaging technology was applied to evaluate the joint angle dependence of brachialis muscle architecture at rest and the changes during isometric voluntary contractions in persons after stroke. The brachialis muscle is one of the elbow flexors and it has the largest contribution to the elbow flexion torque in those elbow flexors and it is important for the elbow functions of the upper limb. The pennation angle and fascicle length of the brachialis muscle were measured on the affected and unaffected sides of people after stroke at 9 different elbow angles ranging form 10o to 90o in the rest condition. Measurements were also carried out at a fixed joint angle of 90o while the subjects were performing isometric muscle contractions at 5 incremental levels from 20% to 100% maximal voluntary contraction (MVC). The data obtained from the affected and unaffected sides of the subjects were compared. The measured pennation angles and fascicle lengths were found to be joint-angle-dependent in both the affected and unaffected groups in the rest condition. Further comparisons found that the pennation angles of the affected brachialis muscle were significantly larger than those of the unaffected muscle in the extended positions (<50o), whereas the affected fascicle lengths were significantly shorter than those of the unaffected muscle in the flexed positions (>20o). As the level of isometric voluntary contraction was increased incrementally from 20% to 100% of MVC, the results showed that the pennation angle increased significantly while the fascicle length descreased significantly in the unaffected muscle. However, the contraction level has a significant effect only on the pennation angle but not on the fascicle length on the affected side. In addition, the measured fascicle lengths in the unaffected group were significantly shorter than those in the affected group for isometric contractions above 40% MVC. We suggest that immobilization and contracture might cause a shortening of the fascicle and an increase in the pennation angle on the affected side. Smaller pennation angle and fascicle length changes on the affected side during isometric contraction might be due to a weakness in the muscle after the onset of stroke. Our findings will be important for the modeling study of persons after stroke since the difference found in the affected muscle may also affect the accuracy of the model. Next, a subject-specific isometric contraction model of the elbow joint was built, which incorporated an anthropometrically scaled graphics-based geometrical model, a Hill-type musculotendon model and used an optimization process, to obtain muscle parameters and to evaluate the muscle properties in persons after stroke. The ultrasound technique was employed to measure the muscle optimal length and pennation angle of each prime elbow flexors (Biceps Brachii, Brachialis Brachioradialis) and extensors (Three heads of Triceps brachii), and these architectural parameters were inputted into the model to reduce the number of unknown parameters to be optimized. The optimizations were conducted to allow changes of individual maximum isometric muscle force by minimizing the root mean square difference between the predicted and measured isometric torque-angle curves. The results showed that the prediction of joint torque fits quite well with the measured one. Moreover, our results revealed that the maximum isometric muscle stress value from the hemiparetic group was significantly smaller than that found in the unimpaired group in flexors and extensors respectively. The subject-specific musculotendon modeling could also be used to validate the assumption of the same maximum isometric muscle stress among prime elbow flexors, which is adopted by previous musculoskeletal modeling studies. In addition, the parameters estimated in this model could be used for the development of a voluntary movement model. The voluntary movement model is an EMG-driven musculoskeletal model, which could predict the individual muscle force and elbow voluntary movement trajectory using the input of EMG signal without any trajectory fitting procedure involved. Our findings demonstrated the feasibility of using EMG-driven neuromusculoskeletal modeling with ultrasound-measured data for prediction of voluntary elbow movement for both unimpaired subjects and persons after stroke. In addition, the results revealed that the prediction of voluntary flexion in the hemiparetic group using ultrasound measured parameters was better than that of using the cadaver data from literature.