title: Increased resistance during jump exercise does not enhance cortical bone formation.
abstract: OBJECTIVE
  This study sought to elucidate the effects of a low- and high-load jump resistance exercise (RE) training protocol on cortical bone of the tibia and femur mid-diaphyses.
  
  
  METHODS
  Sprague-Dawley rats (male, 6 months old) were randomly assigned to high-load RE (HRE; n = 16), low-load RE (LRE; n = 15), or cage control (CC; n = 11) groups. Animals in the HRE and LRE groups performed 15 sessions of jump RE for 5 wk. Load in the HRE group was progressively increased from 80 g added to a weighted vest (50 repetitions) to 410 g (16 repetitions). The LRE rats completed the same protocol as the HRE group (same number of repetitions), with only a 30-g vest applied.
  
  
  RESULTS
  Low- and high-load jump RE resulted in 6%-11% higher cortical bone mineral content and cortical bone area compared with controls, as determined by in vivo peripheral quantitative computed tomography measurements. In the femur, however, only LRE demonstrated improvements in cortical volumetric bone mineral density (+11%) and cross-sectional moment of inertia (+20%) versus the CC group. The three-point bending to failure revealed a marked increase in tibial maximum force (25%-29%), stiffness (19%-22%), and energy to maximum force (35%-55%) and a reduction in elastic modulus (-11% to 14%) in both LRE and HRE compared with controls. Dynamic histomorphometry assessed at the tibia mid-diaphysis determined that both LRE and HRE resulted in 20%-30% higher periosteal mineralizing surface versus the CC group. Mineral apposition rate and bone formation rate were significantly greater in animals in the LRE group (27%, 39%) than those in the HRE group.
  
  
  CONCLUSIONS
  These data demonstrate that jump training with minimal loading is equally, and sometimes more, effective at augmenting cortical bone integrity compared with overload training in skeletally mature rats.
authors.affiliation: 1Department of Biomedical Engineering, Texas A&M University, College Station, TX; 2Department of Health and Kinesiology, Texas A&M University, College Station, TX; and 3Department of Mechanical Engineering, Texas A&M University, College Station, TX.
authors.forename: Ramon D
authors.initials: RD
authors.lastname: Boudreaux
authors.forename: Joshua M
authors.initials: JM
authors.lastname: Swift
authors.forename: Heath G
authors.initials: HG
authors.lastname: Gasier
authors.forename: Michael P
authors.initials: MP
authors.lastname: Wiggs
authors.forename: Harry A
authors.initials: HA
authors.lastname: Hogan
authors.forename: James D
authors.initials: JD
authors.lastname: Fluckey
authors.forename: Susan A
authors.initials: SA
authors.lastname: Bloomfield
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D015519:Bone Density; D005269:Femur / Q000502:physiology*; D008297:Male; D010012:Osteogenesis / Q000502:physiology*; D010805:Physical Conditioning, Animal; D011897:Random Allocation; D017207:Rats, Sprague-Dawley; D055070:Resistance Training; D013977:Tibia / Q000502:physiology*
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