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Technical Briefs

# A Magnetic Resonance-Compatible Loading Device for Dynamically Imaging Shortening and Lengthening Muscle Contraction Mechanics

[+] Author and Article Information
Amy Silder

Christopher J. Westphal

Darryl G. Thelen1

Department of Biomedical Engineering, Department of Mechanical Engineering, and Department of Orthopedics and Rehabilitation, University of Wisconsin–Madison, Madison, WI 53706thelen@engr.wisc.edu

1

Corresponding author.

J. Med. Devices 3(3), 034504 (Sep 03, 2009) (5 pages) doi:10.1115/1.3212559 History: Received May 08, 2009; Revised July 29, 2009; Published September 03, 2009

## Abstract

The purpose of this study was to design and test a magnetic resonance (MR)-compatible device to induce either shortening or lengthening muscle contractions for use during dynamic MR imaging. The proposed device guides the knee through cyclic flexion-extension, while either elastic or inertial loads are imposed on the hamstrings. Ten subjects were tested in a motion capture laboratory to evaluate the repeatability of limb motion and imposed loads. Image data were subsequently obtained for all ten subjects using cine phase contrast imaging. Subjects achieved $∼30 deg$ of knee joint motion, with individual subjects remaining within $∼1 deg$ of their average motion across 56 repeated cycles. The maximum hamstring activity and loading occurred when the knee was flexed for the elastic loading condition (shortening contraction), and extended for the inertial loading condition (lengthening contraction). Repeat MR image acquisitions of the same loading condition resulted in similar tissue velocities, while spatial variations in velocity data were clearly different between loading conditions. The proposed device can enable dynamic imaging of the muscle under different types of loads, which has the potential to improve our understanding of basic muscle mechanics, identify potential causes of muscle injury, and provide a basis for quantitatively assessing injury effects at the tissue level. Slight modifications to the device design and/or subject positioning could allow for imaging of the quadriceps or the knee.

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## Figures

Figure 1

(a) The subject lays prone on the base of the device with his/her knee joint aligned to an axis of rotation. A large sprocket and chain connect the externally applied loads to the knee. Further amplification of the loads, due to spur gears within the gear box, result in an overall gear ratio of 1:10. As a result, the equivalent inertia at the knee is 100 times greater than the inertia of the disks mounted on the loading shaft. (b) Alternatively, a torsional spring can be mounted on the loading shaft, thereby inducing an elastic load (i.e., shortening contraction). (c) The device fits securely within the bore of a scanner. The gear box is located at the far end of the base, such that the inertia disks and torsional spring can be interchanged without translating the couch between image sequence acquisitions.

Figure 2

(a) A representative sagittal-oblique image, from which the data were obtained. The imaging plane was prescribed graphically using static localizer images. In this image, the subject is relaxed, with the knee in maximum extension. (b) Velocity images were obtained using cine phase contrast imaging. The gray scale represents the magnitude of the superior-inferior velocity for the elastic loading.

Figure 3

The ensemble averaged net internal knee moments were out of phase between the elastic and inertial loading conditions. The maximum knee flexion moment occurred within 2 deg of peak knee flexion during the elastic loading, indicating that the muscle is undergoing a shortening contraction. In contrast, the maximum knee flexion moment occurred within 1 deg of peak knee extension for the inertial loading, indicating that the muscle is undergoing a lengthening contraction.

Figure 4

Hamstring electromyographic measurements indicated that the device can be used to alter the timing of the hamstring activity between loading conditions. The mean hamstring activity was significantly greater (*p<0.5) when the knee was flexed for the elastic loading, and when the knee was extended during the inertial loading condition.

Figure 5

(a) Shown are the ensemble averaged tissue velocities within the biceps femoris long head during the cyclic knee flexion-extension tasks. Mean tissue velocities within the muscle tissue were similar between loading conditions, with the inertial loading condition inducing slightly greater tissue velocities than the elastic loading condition. (b) Significantly (*p<0.5) greater regional variability was present during the elastic loading condition, suggesting more nonuniform motion throughout the muscle tissue.

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