ESTABLISHING A NEW INDEX OF MUSCLE CROSS-SECTIONAL AREA AND ITS RELATIONSHIP WITH ISOMETRIC MUSCLE STRENGTH RYOTA AKAGI, 1 HIROAKI KANEHISA, 2 YASUO KAWAKAMI, 3 AND TETSUO FUKUNAGA 3 1 Graduate School of Sport Sciences, Waseda University, Saitama, Japan; 2 Department of Life Sciences (Sports Sciences), Graduate School of Arts and Sciences, University of Tokyo, Tokyo, Japan; 3 Faculty of Sport Sciences, Waseda University, Saitama, Japan ABSTRACT The present study aimed i) to establish an index of muscle cross- sectional area (CSA) based on muscle thickness and circum- ference through a comparison with muscle CSA determined by magnetic resonance imaging (MRI), and ii) to examine the relationships between muscle strength and the index determined at rest and during the maximal isometric contraction. The muscle CSA of elbow flexors at 60% of the upper arm length (CSA 60 ) and the maximal CSA of elbow flexors (CSA max ) were measured using MRI in 26 men and 8 women. The muscle thickness (MT) of elbow flexors and the circumference (C) of upper arm at 60% of the upper arm length were measured using ultrasonography and anthropometry, respectively, in 29 men and 9 women. The measurements of MT and C were performed in the resting (MT r and C r ) and contracted condition (MT m and C m ), where the subjects performed maximal voluntary contraction (MVC) of isometric elbow joint flexion. The torque developed during MVC was converted into the muscle force (F) of elbow flexors. The MT r 3 C r was significantly correlated both with CSA 60 and CSA max (P , 0.001). The F was significantly correlated with MT m 3 C m (r = 0.847, P , 0.001) and MT r 3 C r (r = 0.839, P , 0.001). However, stepwise multiple regression analysis selected only MT m 3 C m as a significant contributor for estimating F. The present study indicates that MT 3 C reflects muscle CSA, and can be an index for assessing muscle CSA. In addition, the findings obtained here showed a possibility that MT 3 C during MVC is more closely related to F than that at rest. KEY WORDS elbow flexors, muscle thickness, circumference, maximal isometric contraction, ultrasonography INTRODUCTION T he force generation capability of a muscle is related to its cross-sectional area (CSA) (13). Many studies have examined this subject through CSA measure- ments and/or estimates using imaging techniques such as computerized tomography (CT) (17,18,22), ultra- sonography (13) and magnetic resonance imaging (MRI) (3,8,21), or using anthropometry (7). Among these techni- ques, MRI and CT provide precise muscle dimensions, but have poor applicability to practical use in filed studies. On the other hand, ultrasonography and anthropometry are more easily applicable in field studies on a large number of subjects. In particular, ultrasonography has the same merit as those of MRI or CT in directly visualizing fat and muscle tissues, but the major drawback of this technique is the limitation to image size which in many cases cannot cover the whole section of the muscle of interest. Anthropometry overcomes the latter concern, but it cannot evaluate distributions of tissues under the skin. The forte of ultrasonography and anthropometry is that they make it possible to evaluate muscle dimensional changes during contraction owing to their real-time measurement. Hodges et al. (12) reported that an isometric contraction resulted in an increase in muscle thickness of parallel-fibered muscles such as the biceps brachii and brachialis. This implies that the CSA of parallel-fibered muscles increases during contraction and that it might affect muscle force production. One would therefore expect that if proper methodology is developed by combining ultrasonography and anthropometry to estimate muscle dimensions, it is valuable for not only quantitative assessment of muscle size in field studies, but also for examination on the relation- ship between muscle dimensions and force under a con- tracted condition. However, little attention has been paid to contraction-induced changes in muscle dimensions and their relations to the force exerted. Muscle thickness (MT) measured by ultrasonography correlates closely with muscle CSA (1,25) and its squared value has been used as an index of muscle CSA (8,19,20). This index was derived from the idea that the cross-section of the Address correspondence to Ryota Akagi, carp_crespo_0522@ruri. waseda.jp. 1533-4287/22(1)/82–87 Journal of Strength and Conditioning Research Ó 2008, National Strength and Conditioning Association 82 Journal of Strength and Conditioning Research the TM