PURPOSE: The purpose of the study was to examine the relationships between muscle cross-sectional area (CSA) measured by either ultrasonography or biopsy and measures of isometric peak force (IPF) and rate of force development (RFD). METHODS: Fifteen well-trained males (IPF=4445.467 + 613.690 N) participated in the study. Prior to testing and following an overnight fast, each subject’s vastus lateralis (VL) was measured for anatomical CSA (ACSA) using ultrasonography. A muscle biopsy was also sampled from the VL of each subject immediately after the ACSA measurement and at the same site on the VL. Fiber CSA (FCSA) was assessed using immunofluorescent techniques. IPF and RFD at 0-50ms, 0-100ms, 0-150ms, and 0-200ms were assessed during an isometric mid-thigh pull test on force platforms sampling at 1000Hz. Pearson product-moment correlations were calculated between muscle sizes and performance. Statistical significance was set as p≤0.050. RESULTS: Statistically significant correlations were observed between Type II FCSA and IPF (r=0.774, p<0.001), RFD150 (r=0.661, p=0.007), and RFD200 (r=0.725, p=0.002). ACSA was only significantly associated with IPF (r=0.596, p=0.019). Type I FCSA did not relate to any performance measures. CONCLUSIONS: It is clear from our results and previous literature that ultrasonography and muscle biopsy data provide relevant information regarding muscle size and performance. However, the discrepancies observed in the current study between the two suggest each is providing slightly different information to researchers. Specifically, Type II FCSA might be superior when considering more sensitive performance measurements such as RFD.