Faster intrinsic rate of torque development in elbow flexors than knee extensors: Effect of muscle architecture?

We studied the effect of pennate vs. fusiform muscle architecture on the rate of torque development (RTD) by examining the predominately fusiform elbow flexors (EF) and highly-pennate knee extensors (KE). Seventeen male volunteers (28.4 ± 6.2 years) performed explosive isometric EF and KE contractions (MVCs). Biceps brachii and vastus lateralis fascicle angles were measured to confirm their architecture, and both the rate of voluntary muscle activation (root-mean-square EMG in the 50 ms before contraction onset; EMG-50) and electromechanical delay (EMD; depicting muscle-tendon series elasticity) were assessed as control variables to account for their influence on RTD. MVC torque, early (RTD50) and late (RTD200) RTDs were calculated and expressed as absolute and normalized values. Absolute MVC torque (+412%), RTD50 (+215%), and RTD200 (+427%) were significantly (p < 0.001) higher in KE than EF. However, EF RTD50 was faster (+178%) than KE after normalization (p = 0.02). EMG-50 and EMD did not differ between muscle groups. The results suggest that the faster absolute RTD in KE is largely associated with its higher maximal torque capacity, however in the absence of differences in rates of muscle activation, fiber type, and EMD the fusiform architecture of EF may be considered a factor allowing its faster early RTD relative to strength capacity.