Strengthening Knee Extensor Muscles In Healthy Individuals: Single-joint Or Multi-joint Exercises?

Abstract

INTRODUCTION: Increases in muscle strength from resistance training are maximized when the spatiotemporal recruitment of the muscle is full and the peripheral perturbation is high. During maximal single-joint knee extension and multi-joint leg extension movements, activation of the vastii knee extensor muscles is similar, however, little is known of the magnitude and etiology of fatigue development when these movements are repeated. PURPOSE: To compare the magnitude and etiology of fatigue in the vastii knee extensor muscles following repeated maximal contractions performed through single-joint (knee extension) or multi-joint (leg extension) exercises. METHOD: On separate days, 16 participants completed 60 maximal unilateral: i) knee extensions on a dynamometer (KEXT) or ii) leg extensions on a cycle ergometer (LEXT). Knee range of motion (~120° - 30° flexion) and angular velocity (~80°.s-1) were matched. Maximal torque, vastii muscle EMG and M-wave amplitude (Mmax) were calculated during the first and last three contractions of both exercises. Knee extensor isometric maximal voluntary force (IMVF), voluntary activation (VA) and resting twitch force (RT10:100 HZ) were measured pre-exercise and 40-s post-exercise. RESULTS: Similar torque (KEXT: 152 ± 33 N·m vs. LEXT: 165 ± 30 N·m, P > 0.05), EMG (KEXT: 95 ± 6 % vs. LEXT: 96 ± 8 %, P > 0.05) and Mmax (KEXT: 95 ± 5 % vs. LEXT: 97 ± 5 %, P > 0.05) were measured at the start of the exercises. Larger reductions in torque (KEXT: -60 ± 10% vs. LEXT: -38 ± 14%) and EMG (KEXT: -21 ± 16% vs. LEXT: -13 ± 16%) were seen for KEXT during the final part of the exercise (P ≤ 0.05), whereas no differences were reported in Mmax (P > 0.05). Larger reductions in VA were seen after KEXT whereas greater reductions in RT10:100 HZ were seen after LEXT (both P ≤ 0.05). Ultimately, similar reductions in IMVF were seen following KEXT (-32 ± 10%) and LEXT (-35 ± 13%) (P > 0.05). CONCLUSION: A lower-limb resistance training program which adopts multi-joint exercises may induce superior strength gains in vastii knee extensor muscles compared to single-joint exercises, as it is possible to induce larger levels of peripheral fatigue with a smaller reduction in voluntary activation.