Muscle activation assessment: effects of method, stimulus number, and joint angle.

Activation capacity has traditionally been assessed using the interpolated twitch technique (ITT) and central activation ratio (CAR). However, the quantitative agreement of the two methods and the physiological mechanisms underpinning any possible differences have not been fully elucidated. The aim of this study was to compare and assess the sensitivity of the ITT and CAR to potential errors introduced by (1) evoking inadequate force, by manipulating the number of stimuli, and (2) neglecting differences in series elasticity between conditions, by manipulating joint angle. Ten subjects performed knee extension contractions at 30 degrees and 90 degrees knee-joint angles during which the ITT and CAR methods were applied using 1, 2, 4, and 8 electrical stimuli. Joint angle influenced the ITT outcome with higher values taken at 90 degrees (P < 0.05), while the number of stimuli influenced the CAR outcome with a higher number of stimuli yielding lower values (P < 0.05). For any given joint angle and stimulus number, the CAR method produced higher activation values than the ITT method by 8%-16%. Therefore, in the quantification of voluntary drive with the ITT and CAR methods consideration should be given not only to the number of stimuli applied but also to the effect of series elasticity due to joint-angle differences, since these factors may differently affect the outcome of the calculation, depending on the approach followed.