BACKGROUND Ultrasound imaging (USI) is used increasingly for assessing muscle activity during contraction in research and clinical practice [1]. The relationship between changes in cross-sectional shape and linear dimensions that occur during contraction varies in different muscles and cannot be assumed. Both linear and curvilinear relationships have been found for increases in muscle thickness with force, e.g. in calf [2] and abdominal [3,4] muscles. The relationship therefore needs to be characterized in a particular muscle before USI measurements can be used as an indirect indicator of force. This exploratory study aimed to examine the relationship between incremental force production of quadriceps and linear dimensions of the rectus femoris (RF) muscle in a group of young men. METHODS In 14 healthy males, aged 22-27 years (mean 24.8), isometric force of quadriceps was measured using a Biodex dynamometer with the knee flexed to 90 degrees, during maximal voluntary contractions (MVC) and randomly at 10, 20, 30, 50 and 75% of MVC, held for 3 seconds each. An ultrasound scanner (Aquilla, ESAOTE; 6 MHz linear transducer) was used to image RF at mid-thigh at rest and during contractions. Two muscle dimensions (depth or thickness and width) were measured offline. Pearson’s correlation was used to examine the relationship between change in muscle dimension (normalized as percentage) and force. Differences in mean dimensions between force levels were examined by repeated measures analysis of variance and paired samples T-test with Bonferroni correction (p<0.008). RESULTS As force increased, there was a curvilinear decrease in RF width (r= -0.92; p<0.01), with decreases in width greatest at low forces, <30% MVC (Fig 1). Resting width (mean=4.41cm, SD+0.07) was significantly different to that at all contraction levels (p<0.05). Significant differences were also found between contraction levels (p<0.008), except between 20% – 30% MVC, and 75% – MVC (p>0.008). At MVC, width was 3.27cm (SD+0.47), approximately 25% smaller than at rest. Thickness of RF increased with contraction but only changed minimally (p>0.05), from a mean of 2.34cm (SD+0.32) at rest to 2.57cm (SD+0.3) at MVC. CONCLUSIONS Dynamic assessment of RF in young males can be made by measuring decreases in width on USI scans to reflect increases in force, which are most evident a low forces. Muscle thickness cannot be used as a measure of contractile activity of RF. These findings are relevant to healthy young males and larger numbers are needed to calculate predictive regression equations. The relationship also warrants investigation in healthy males and females of different ages and habitual activity, as well as populations with muscle dysfunction.