Quadriceps activation normative values and the affect of subcutaneous tissue thickness

Calculation of the central activation ratio (CAR) using the superimposed burst technique (SIB) is widely used. 0.95 is considered a normal value of the CAR in healthy subjects, but it has not been objectively examined. Since an electrical stimulation penetrates the subcutaneous tissue, the intensity of electrical stimulation may vary depending on the subcutaneous tissue thickness. Subjects performed a maximal voluntary isometric contraction (MVIC) of the quadriceps with the knee at 90°. Once the MVIC reached a plateau, an electrical stimulation was manually delivered to the quadriceps. Quadriceps CAR was quantified using the equation: CAR = MVIC/MVIC + SIB torque. Quadriceps subcutaneous thickness was measured using ultrasound imaging and skinfolder pinch calipers. CAR values were estimated at 0.95 ± 0.04 for dominant and 0.93 ± 0.05 for non-dominant limbs. Pearson's correlation coefficient revealed that there were negative correlations for quadriceps torque output (N m/kg) and subcutaneous tissue thickness measured by ultrasound imaging (dominant: r = −0.54, p < 0.001; non-dominant: r = −0.53, p < 0.001). We found no relationship between CAR and subcutaneous tissue thickness. A CAR of 0.95 may be considered full activation in healthy young adults, and CAR in healthy adults generated by the SIB technique may not be dependent on subcutaneous thickness.