Rest-inserted loading rapidly amplifies the response of bone to small increases in strain and load cycles

• Published in: Journal of applied physiology (1985) Vol. 102; no. 5; pp. 1945 - 1952
• Main Authors: Srinivasan, Sundar, Ausk, Brandon J, Poliachik, Sandra L, Warner, Sarah E, Richardson, Thomas S, Gross, Ted S
• Format: Journal Article
• Language: English
• Published: Bethesda, MD Am Physiological Soc 01-05-2007; American Physiological Society
• Subjects: Adaptation, Physiological; Animals; Biological and medical sciences; Biomechanical Phenomena; Bones; Female; Fundamental and applied biological sciences. Psychology; Linear Models; Mice; Mice, Inbred C57BL; Musculoskeletal system; Orthopedics; Osteogenesis; Rodents; Strain; Stress, Mechanical; Tibia - physiology; Weight-Bearing; Osteoarticular system; Vertebrata; Rest; Mammalia; strain magnitude; Saturation; saturation delay; bone formation; Bone; load cycle numbers; Formation
• ISSN: 8750-7587; 1522-1601
• DOI: 10.1152/japplphysiol.00507.2006

Departments of 1 Orthopaedics and Sports Medicine and 2 Statistics, University of Washington, Seattle, Washington
Submitted 2 May 2006
; accepted in final form 25 January 2007
We hypothesized that a 10-s rest interval (at zero load) inserted between each load cycle would increase the osteogenic effects of mechanical loading near previously identified thresholds for strain magnitude and cycle numbers. We tested our hypothesis by subjecting the right tibiae of female C57BL/6J mice (16 wk, n = 70) to exogenous mechanical loading within a peri-threshold physiological range of strain magnitudes and load cycle numbers using a noninvasive murine tibia loading device. Bone responses to mechanical loading were determined via dynamic histomorphometry. More specifically, we contrasted bone formation induced by cyclic vs. rest-inserted loading (10-s rest at zero load inserted between each load cycle) by first varying peak strains (1,000, 1,250, or 1,600 µ ) at fixed cycle numbers (50 cycles/day, 3 days/wk for 3 wk) and then varying cycle numbers (10, 50, or 250 cycles/day) at a fixed strain magnitude (1,250 µ ). Within the range of strain magnitudes tested, the slope of periosteal bone formation rate (p.BFR/BS) with increasing strain magnitudes was significantly increased by rest-inserted compared with cyclical loading. Within the range of load cycles tested, the slope of p.BFR/BS with increasing load cycles of rest-inserted loading was also significantly increased by rest-inserted compared with cyclical loading. In sum, the data of this study indicate that inserting a 10-s rest interval between each load cycle amplifies bone's response to mechanical loading, even within a peri-threshold range of strain magnitudes and cycle numbers.
bone formation; strain magnitude; load cycle numbers; saturation delay
Address for reprint requests and other correspondence: S. Srinivasan, Dept. of Orthopaedics and Sports Medicine, Univ. of Washington, PO Box 359798, 325 Ninth Ave., Seattle, WA 98104-2499 (e-mail: sundars{at}u.washington.edu )