Journal of 01 thopaedic Research 10.92C914 Raven Press, Ltd , New York 0 1992 Orthopaedrc Rebearch Society

Physiological Cross-Sectional Area of Human Leg Muscles Based on Magnetic Resonance Imaging T. Fukunaga, TR. R. Roy, $§F. G. Shellock, tJ. A. Hodgson, *M. K. Day, ‘IP. L. Lee, IIH. Kwong-Fu, and *iV. R. Edgerton Departtnenl of Sport F Sciences, University of Tokyo, Tokyo, Japan; *Department of Plzysiologicul Sciences, fBmin Research Institute, and A!kpartment oj’ Radiological Sciences, University of Californici, Los Angeles; §Section of Magnetic Resonance Imaging, Tow3erMusculoskeletul Imaging Center, Cedars-Sinai Medical Center, Los Angeles; and ‘pet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.

Summary: Magnetic resonance imaging techniques were used to determine the physiological cross-sectional areas (PCSAs) of the major muscles or muscle groups of the lower leg. For 12 healthy subjects, the boundaries of each muscle or muscle group were digitized from images taken at I-cm intervals along the length of the leg. Muscle volumes were calculated from the summation of each anatomical CSA (ACSA) and the distance between each section. Muscle length was determined as the distance between the most proximal and distal images in which the muscle was visible. The PCSA of each muscle was calculated as muscle volume times the cosine of the angle of fiber pinnation divided by fiber length, where published fiber 1ength:muscle length ratios were used to estimate fiber lengths. The mean volumes of the major plantarflexors were 489,245, and 140 cm3 for the soleus and medial (MG) and lateral (LG) heads of the gastrocnemius. The mean PCSA of the soleus was 230 cm2, about three and eight times larger than the MG (68 cm2)and LG (28 cm2), respectively. These PCSA values were eight (soleus), four (MG), and three (LG) times larger than their respective maximum ACSA. The major dorsiflexor, the tibialis anterior (TA), had a muscle volume of 143 cm2, a PCSA of 19 cm?, and an ACSA of 9 cm2. With the exception of the soleus, the mean fiber length of all subjects was closely related to muscle volume across muscles. The soleus fibers were unusually short relative to the muscle volume, thus potentiating its force potential. Using the relationship between PCSA and fiber length to represent the maximum force-velocity potential of a muscle and assuming a similar moment arm, the soleus, MG, and LG would be expected to produce -71, 22, and 7% of the force and 54, 30, and 16% of the power of the major plantarflexors. These data illustrate some of the major limitations in the use of ACSA measurements to predict the functional properties of a muscle. Key Words: Magnetic r e s o n a n c e imaging-Muscle volume-Muscle architecturePhysiological cross-sectional area-Human.

The force-velocity characteristics of a muscle reflect its architectural design - as well as the phvsio~logical properties within each of its sarcomeric the maximum force exstructures. For erted by a muscle is closely related to the total

Received April 13, 1990; accepted April 29. 1992. Addre\\ correspondence and reprint requests to Dr. R. R. Roy at Brain Research Institute, UCLA School of Medicine, Center for the Health Sciences, 10833 Le Conte Avenue, Los Angeles, CA 90024-1761, U . S . A .

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ANATOMY OF H U M A N LEG MUSCLES cross-sectional area of all fibers, and the maximum rate of shortening is closely related to the length of the longest fibers within the muscle (4,5,10,32,35). These relationships have been verified in correlative studies involving the in situ testing of contractile properties in one hindlimb and the measurement of the architectural properties in a contralateral limb in a variety of laboratory animals (1,4,6,23.27, 32). In human subjects, there has been a problem in obtaining accurate and sufficient muscle architectural data. The data available are limited because muscle volumes or masses are subject to morphological changes due to fixation and other treatment artefacts ( 8 , l l ) . Also, assessments of size based on a measure from a single cross-section of muscles are known to be too inaccurate to be physiologically meaningful. In addition, these materials usually have been obtained from elderly subjects in which muscle atrophy and myopathies are likely to have been present before death (3,5.7,10,34). Improved imaging techniques, such as magnetic resonance imaging (18,24), computed tomography (14,20,22.25,30), and ultrasound (17,37), have been used to estimate the output potential of muscles or muscle groups. However, most studies have relied on a single cross-sectional image of a muscle group as a measure of its functional potential rather than multiple cross-sectional scans along the length of the muscle or muscle group. Even in those studies in which more than one scan were obtained (19,24), muscle or muscle group volumes were not determined and the functional data were expressed relative to the area of a single cross-section. This approach has severe limitations (10). Magnetic resonance imaging is considered to be the most useful and safest noninvasive imaging device to estimate in vivo human muscle volumes (31). Not only docs it have excellent resolving power for differentiating muscle, fat, connective tissue? and bone without pain and other adverse biological effects (31), but multiple scans can be obtained without moving the subject, thus improving the precision of the three-dimensional reconstruction of the data. In the present study, muscle volumes of human legs were estimated in vivo in healthy subjects using magnetic resonance imaging. Based on the assumption that the published data on relative muscle fiber lengths (i.e.? fiber length to muscle length ratios) from cadavers are representative of human subjects (5,11,34), the physiological cross-sectional areas of individual leg muscles were

also calculated and compared with the volume determinations. Some preliminary results have been published (9,12,28). METHODS Twelve healthy adults (11 men and one woman) volunteered for the present study and followed human consent procedures of Cedars Sinai Medical Center. The physical characteristics of the subjects (mean ? SD) were as follows: age (32.6 k 8.2 years, range 20-459, height (176.4 k 6.2 cm), and weight (73.5 i 9.4 kg). Magnetic resonance imaging was performed with a 1.5-T164 MHz scanner (Signa MR Systems, General Electric Medical Systems, Milwaukee, WI, U.S.A.) with a transmit and receive quadrature body coil. T I-weighted spin-echo, axial-plane imaging was performed with the following variables: TR 600 ms; TE 20 ms; number of excitations. two; matrix 256 x 192; field of v ~ e w18 cm; slice thickness 10 mm; and interslice gap 0 mm. These variables were selected to optimize image quality in order to clearly delineate the border of each muscle and bone and to identify fat or connective tissue. T1weighted images are typically used for the determination of anatomy, as well as to provide good soft tissue contrast between fat/muscle interfaces. The magnetic resonance scanner was checked for proper spatial calibration (in Houndsfield units) every 8 h using a standardized imaging sequence and a saline-filled plexiglass quality assurance phantom. There were no significant deviations (