Accessory muscle activation during the superimposed burst technique

Quadriceps muscle activation is assessed using the superimposed burst technique. This technique involves percutaneous muscle stimulation superimposed during maximal isometric volitional knee extension. It is unknown whether accessory muscle activation during maximal knee extension influences estimates of quadriceps muscle activation. Our aim was to compare accessory muscle activation while performing the superimposed burst technique using investigator delivered verbal instruction to constrain the system (CS) and a participant preferred (PP) technique. Twenty five healthy, active individuals (13M/12F, age = 23.8 ± 3.35, height = 72.73 ± 14.51 cm, and weight = 175.29 ± 9.59 kg) were recruited for this study. All participants performed superimposed burst testing with (CS) and without (PP) verbal instruction to encourage isolated quadriceps activation during maximal isometric knee extension. The main outcome variables measured were knee extension torque, quadriceps central activation ratio and mean EMG of vastus lateralis, biceps femoris, and lumbar paraspinal muscles. There were significant differences in knee extension torque (CS = 2.87 ± 0.93 Nm/kg, PP = 3.40 ± 1.12 Nm/kg, p < 0.001), superimposed burst torque (CS = 3.40 ± 0.98 Nm/kg, PP = 3.75 ± 1.11 Nm/kg, p = 0.002) and quadriceps CAR (CS = 84.1 ± 12.0%, PP = 90.2 ± 9.9%, p < 0.001) between the techniques. There was also a significant difference in lumbar paraspinal EMG (CS = 6.40 ± 8.52%, PP = 11.86 ± 14.89%, p = 0.043) between the techniques however vastus lateralis EMG was not significantly different. Patient instruction via verbal instruction to constrain proximal structures may help patient minimize confounders to knee extension torque generation while maximizing quadriceps activation.