title: Effect of instruction, surface stability, and load intensity on trunk muscle activity.
abstract: The aim of this study was to assess the effect of verbal instruction, surface stability, and load intensity on trunk muscle activity levels during the free weight squat exercise. Twelve trained males performed a free weight squat under four conditions: (1) standing on stable ground lifting 50% of their 1-repetition maximum (RM), (2) standing on a BOSU balance trainer lifting 50% of their 1-RM, (3) standing on stable ground lifting 75% of their 1-RM, and (4) receiving verbal instructions to activate the trunk muscles followed by lifting 50% of their 1-RM. Surface EMG activity from muscles rectus abdominis (RA), external oblique (EO), transversus abdominis/internal oblique (TA/IO), and erector spinae (ES) were recorded for each condition and normalized for comparisons. Muscles RA, EO, and TA/IO displayed greater peak activity (39-167%) during squats with instructions compared to the other squat conditions (P=0.04-0.007). Peak EMG activity of muscle ES was greater for the 75% 1-RM condition than squats with instructions or lifting 50% of 1-RM (P=0.04-0.02). The results indicate that if the goal is to enhance EMG activity of the abdominal muscles during a multi-joint squat exercise then verbal instructions may be more effective than increasing load intensity or lifting on an unstable surface. However, in light of other research, conscious co-activation of the trunk muscles during the squat exercise may lead to spinal instability and hazardous compression forces in the lumbar spine.
authors.affiliation: Utah State University, Health, Human Movement Sciences, and Recreation Department, 7000 Old Main Hill, Logan, UT 84322, USA. Eadric.bressel@usu.edu
authors.forename: Eadric
authors.initials: E
authors.lastname: Bressel
authors.forename: Jeffrey M
authors.initials: JM
authors.lastname: Willardson
authors.forename: Brennan
authors.initials: B
authors.lastname: Thompson
authors.forename: Fabio E
authors.initials: FE
authors.lastname: Fontana
mesh_terms: D000005:Abdomen / Q000502:physiology*; D000009:Abdominal Muscles / Q000502:physiology; D001415:Back / Q000502:physiology*; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D010806:Physical Education and Training / Q000379:methods; D005082:Physical Exertion / Q000502:physiology*; D013647:Task Performance and Analysis; D014891:Weight Lifting / Q000502:physiology*; D016474:Weight-Bearing / Q000502:physiology*; D055815:Young Adult