title: Unique activation of the quadriceps femoris during single- and multi-joint exercises.
abstract: OBJECTIVE
  This study aimed to examine whether muscle activation of the quadriceps femoris differs between single- and multi-joint exercises, and to explore the factors resulting in muscle and exercise specificity in activation.
  
  
  METHODS
  Eleven adults developed isometric hip extension torque gradually while maintaining submaximal isometric knee extension torque (Experiment 1). In Experiment 2, 15 men performed knee extension and leg press separately at intensities of 20, 40, 60 and 80 % of their one repetition maximum (1RM) load, and 14 men conducted leg press at intensities of 40 and 80 % of 1RM until exhaustion (Experiment 3). Muscle activation during exercises was measured using surface electromyography from the rectus femoris, vastus lateralis and medialis.
  
  
  RESULTS
  The addition of isometric hip extension torque significantly decreased rectus femoris activation (Experiment 1). In Experiment 2, the rectus femoris activation was significantly higher during knee extension than during leg press, whereas no differences were observed in the vasti. The rectus femoris activation was not significantly different between leg press at 80 % and knee extension at 20 % of 1RM. The results of Experiment 3 showed significant increases in vasti activation at both intensities, whereas rectus femoris activation did not change at 80 % of 1RM.
  
  
  CONCLUSIONS
  The results revealed that even at high intensity, the rectus femoris activation during multi-joint exercise is low and does not increase with fatigue, unlike the vasti, and that the inter-muscle and inter-exercise differences in activation depend on whether hip extension torque is exerted in the exercise.
authors.affiliation: Graduate School of Engineering and Science, Shibaura Institute of Technology, 307 Fukasaku, Minuma-ku, Saitama-shi, 337-8570, Saitama, Japan. i039491@sic.shibaura-it.ac.jp.|Research Fellow of Japan Society for the Promotion of Science, Chiyoda-ku, Tokyo, Japan. i039491@sic.shibaura-it.ac.jp.
authors.forename: Ryoichi
authors.initials: R
authors.lastname: Ema
authors.affiliation: Human Performance Lab, Faculty of Kinesiology, University of Calgary, Calgary, Alberta, Canada.
authors.forename: Masanori
authors.initials: M
authors.lastname: Sakaguchi
authors.affiliation: College of Systems Engineering and Science, Shibaura Institute of Technology, Saitama-shi, Saitama, Japan.
authors.forename: Ryota
authors.initials: R
authors.lastname: Akagi
authors.affiliation: Faculty of Sport Sciences, Waseda University, Tokorozawa, Saitama, Japan.
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Kawakami
mesh_terms: D000328:Adult; D004576:Electromyography / Q000379:methods; D015444:Exercise / Q000502:physiology*; D005260:Female; D006615:Hip / Q000502:physiology; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology; D007719:Knee Joint / Q000502:physiology; D007866:Leg / Q000502:physiology; D008297:Male; D052097:Quadriceps Muscle / Q000502:physiology*; D019415:Torque; D055815:Young Adult
keywords: Biarticular muscle; Electromyography; Hip extension; Knee extension; Leg extension