title: The effect of Nordic hamstring strength training on muscle architecture, stiffness, and strength.
abstract: OBJECTIVE
  Hamstring strain injury is a frequent and serious injury in competitive and recreational sports. While Nordic hamstring (NH) eccentric strength training is an effective hamstring injury-prevention method, the protective mechanism of this exercise is not understood. Strength training increases muscle strength, but also alters muscle architecture and stiffness; all three factors may be associated with reducing muscle injuries. The purpose of this study was to examine the effects of NH eccentric strength training on hamstring muscle architecture, stiffness, and strength.
  
  
  METHODS
  Twenty healthy participants were randomly assigned to an eccentric training group or control group. Control participants performed static stretching, while experimental participants performed static stretching and NH training for 6 weeks. Pre- and post-intervention measurements included: hamstring muscle architecture and stiffness using ultrasound imaging and elastography, and maximal hamstring strength measured on a dynamometer.
  
  
  RESULTS
  The experimental group, but not the control group, increased volume (131.5 vs. 145.2 cm3, p < 0.001) and physiological cross-sectional area (16.1 vs. 18.1 cm2, p = 0.032). There were no significant changes to muscle fascicle length, stiffness, or eccentric hamstring strength.
  
  
  CONCLUSIONS
  The NH intervention was an effective training method for muscle hypertrophy, but, contrary to common literature findings for other modes of eccentric training, did not increase fascicle length. The data suggest that the mechanism behind NH eccentric strength training mitigating hamstring injury risk could be increasing volume rather than increasing muscle length. Future research is, therefore, warranted to determine if muscle hypertrophy induced by NH training lowers future hamstring strain injury risk.
authors.affiliation: Kinesiology Department, East Carolina University, Greenville, NC, USA. kaylaseymore@boisestate.edu.|Center for Orthopaedic and Biomechanics Research, Boise State University, Boise, ID, USA. kaylaseymore@boisestate.edu.
authors.forename: Kayla D
authors.initials: KD
authors.lastname: Seymore
authors.affiliation: Kinesiology Department, East Carolina University, Greenville, NC, USA.
authors.forename: Zachary J
authors.initials: ZJ
authors.lastname: Domire
authors.affiliation: Kinesiology Department, East Carolina University, Greenville, NC, USA.
authors.forename: Paul
authors.initials: P
authors.lastname: DeVita
authors.affiliation: Kinesiology Department, East Carolina University, Greenville, NC, USA.
authors.forename: Patrick M
authors.initials: PM
authors.lastname: Rider
authors.affiliation: Department of Health Education and Promotion, East Carolina University, Greenville, NC, USA.
authors.forename: Anthony S
authors.initials: AS
authors.lastname: Kulas
mesh_terms: D000293:Adolescent; D054459:Elasticity Imaging Techniques; D015444:Exercise; D005260:Female; D000070633:Hamstring Muscles / Q000000981:diagnostic imaging / Q000502:physiology*; D006801:Humans; D008297:Male; D053580:Muscle Strength; D055815:Young Adult
keywords: Biomechanics; Dynamometry; Eccentric; Injury prevention; Intervention; Ultrasound