DIFFERENCES IN HAMSTRING MECHANICS BETWEEN SHORTENING AND LENGTHENING CONTRACTIONS REVEALED BY DYNAMIC MRI Amy Silder, Christopher Westphal, Scott B. Reeder, and Darryl G. Thelen University of Wisconsin – Madison, Madison, WI, USA,
[email protected] INTRODUCTION Muscle injuries are thought to occur as a result of excessive strain during active lengthening contractions (Lieber and Friden 1993). Given that acute injuries most often occur along the musculotendon junction (DeSmet and Best, 2000), it is possible that regional variations in muscle strain influence injury potential. Dynamic MR imaging is a useful tool for characterizing spatial variations in muscle contraction mechanics (Pappas et al., 2002), and thus could provide insights into injury mechanisms. However, dynamic imaging studies to date have focused primarily on shortening muscle contractions performed against either constant or elastic loads. Our objective was to assess differences in hamstring mechanics between shortening and lengthening contractions using cine phase contrast (CINE PC) MRI. METHODS AND PROCEDURES Three healthy limbs (ages 26-29y) were tested. A MR-compatible device was used to guide the knee through ~30 deg of flexion/extension while simultaneously while imposing either an elastic or inertial load on the limb (Figure 1). Each loading condition required a knee moment of ~15% of the subject’s isokinetic (30°/s) knee flexion strength. Peak loading occurred when the knee was extended in the inertial loading case (lengthening contraction), and when the knee was flexed during the elastic loading (shortening contraction). CINE PC was used to measure pixel velocities within a sagittaloblique imaging plane through the biceps femoris long head (Figure 2). Subjects performed cyclic flexion/extension at a rate of 28 cycles/min to the beat of a metronome. Each scan lasted 1min 39s. Scanning
parameters were: spatial resolution = 1.4x1.4x6mm, VENC=5cm/s, 256×256 matrix, TR/TE=21.6/7.1ms, 2 lines of kspace/cycle, and 40 reconstructed frames/cycle.
Figure 1. The subject lay prone within the scanner. Lengthening contractions were induced using inertial loading (shown), while shortening contractions were induced using an elastic load (see inset). From the magnitude image at the first frame (~5° short of full extension), a set of 11x11mm regions of interest (ROIs) were drawn along the midline of the muscle (Figure 2). The trajectory of each ROI was then computed by integrating vertical and horizontal velocities (Pelc et al., 1995). Through plane velocities were checked to ensure out of plane motion was minimal. RESULTS There were marked differences in velocities and overall muscle motion between the two loading conditions. Comparing the same knee flexion angle, ROI velocities were more vertically oriented in the proximal region under the inertial load, compared to the elastic load (Figure 2). As a result, the inertial load resulted in ~30% less overall change in muscle length. Both loads induced nonuniform shortening along the muscle, with the
greatest shortening occurring in the midproximal region (Figure 3).
refined spatial strain mapping using dynamic MR imaging techniques (Zhong et al. 2007) could provide further insights into this issue.
(a) (b) Figure 2. ROIs were drawn along the midline of the muscle and tracked throughout the knee motion. Arrows represent the velocity of each pixel at the current frame and demonstrate more vertically oriented motion in the proximal region for the (a) elastic load than the (b) inertial load. DISCUSSION Observable differences in muscle mechanics were revealed when loading required a lengthening contraction. In this condition, a more vertically oriented velocity profile during knee flexion resulted in a smaller overall muscle excursion. This difference likely resulted from greater tendon stretch (greater force) when the knee was extended in the inertial loading case. Substantial variations in muscle stretch were observed along the length of the muscle for both contraction types. These variations may arise due to the biceps femoris musculotendon architecture. The large shortening in the midproximal region may reflect a portion of the muscle with the smallest aponeurosis, being distal to the proximal tendon, yet proximal to sheath-like distal aponeurosis of the biceps long head (Woodley et al., 2005). Given the large variations in strain, it is possible that strain gradients within the muscle play a role in the propensity for injury to occur at specific regions within the muscle. More
Figure 3. Average muscle strain measured from four sections of the muscle highlight the variations in local muscle mechanics along the length of the muscle. REFERENCES Desmet, AA and Best, TM (2000). AJR Am J Roentgenol, 174(2):393-9. Lieber, RL and Friden, J (1993). J Appl Physiol, 74:520-6. Pappas, GP, et al., (2002). J Appl Physiol, 92:2381-9. Pelc JN, et al., (1995). J Magn Reson Imaging, 5(3):339-45. Woodley SJ, et al. (2005). Cells Tissues Organs, 179: 125-141. Zhong, X et al. (2008). J Biomech, in press. ACKNOWLEDGEMENTS NIH AR 56201, NSF pre-doctoral fellowship, ASB Student Grant-In-Aid (AS).
