The impact of seatback angle on electromyographical activity of the lower back and quadriceps muscles during bilateral knee extension

Thelma R. Gomez, Fangchao Ma, Jessica B. Adams, Mark Stoutenberg, Joseph Signorile

Research output: Contribution to journalArticle

1 Scopus citations


This study investigated variations in electromyographic (EMG) responses of the erector spinae (ES), vastus medialis (VM), rectus femoris (RF), and vastus lateralis (VL) to different seatback angles during leg extension. Twenty men and women (10 men, 10 women; age 27.49 ± 6.16 years) performed 8 repetitions at 70% of 8 repetition maximum at seatback angles of 1.57, 1.75, and 1.92 radius (rad). Analyses using repeated-measures analysis of variance indicated the greatest root square mean of the EMG (rmsEMG) and integrated EMG (intEMG) for the ES were at 1.92 rad, and the greatest for the VM (concentric) and VL (eccentric) were at 1.57 rad. No differences were observed among seat angles for the RF except for a higher normalized intEMG at 1.92 than 1.75 rad (concentric). Throughout all sets for all conditions and muscles, rmsEMG and intEMG significantly increased and median power frequency significantly decreased. These data indicate that a seatback angle of 1.57 rad is best for a leg extension machine, because this angle maximizes quadriceps activity while minimizing stress on the lower back muscles.

Original languageEnglish
Pages (from-to)908-917
Number of pages10
JournalJournal of Strength and Conditioning Research
Issue number4
StatePublished - Nov 1 2005



  • Lower back
  • Muscular activity
  • Weight-training

ASJC Scopus subject areas

  • Public Health, Environmental and Occupational Health
  • Physical Therapy, Sports Therapy and Rehabilitation
  • Orthopedics and Sports Medicine

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