The most important anatomical determinants of in vivo joint moment magnitude have yet to be defined. Relationships between maximal knee extensor moment and quadriceps muscle volume, anatomical (ACSA) and physiological (PCSA) cross-sectional area, muscle architecture and moment arm (MA) were compared. Nineteen untrained men and women performed maximal isokinetic knee extensions under isometric conditions (90 degrees joint angle) and at 30 degrees and 300 degrees s(-1). Magnetic resonance and ultrasound imaging techniques were used to measure vastus lateralis PCSA and fascicle length (FL), quadriceps ACSA, volume and patellar tendon MA. Muscle volume was the best predictor of extensor moment measured isometrically (R(2) = 0.60) and at 30 degrees s(-1)(R (2) = 0.74). PCSA x FL was the best predictor of moment at 300 degrees s(-1) (R(2) = 0.59). MA was not an important predictor. ACSA was the second best predictor at all three speeds and could be recommended as an ideal measure given its relative ease of measurement.