Mechanical loads and cortical bone geometry in healthy children and young adults

Abstract

Muscle and bone form a functional unit. While muscle size is a useful surrogate of mechanical load on bone, the independent contributions to bone strength of muscle force, muscle size, gravitational load (body weight), and physical activity have not been assessed. 321 healthy participants (32% black, 47% male), age 5 to 35 years were assessed. Peak dorsiflexion muscle torque (ft-lbs) of the ankle was assessed using isometric dynamometry. Tibia peripheral quantitative computed tomography measures included polar section modulus (Zp, mm3), periosteal and endosteal circumference (mm), cortical area (mm2), and volumetric bone mineral density (vBMD, mg/cm3) at the 38% site, and muscle cross-sectional area (CSA, mm2) at the 66% site. Physical activity (average hours /week) was assessed by questionnaire. Log linear regression was used to assess determinants of muscle specific force (MSF; torque relative to muscle CSA) and Zp adjusted for age and tibia length. MSF was greater in blacks than whites (p < 0.05) and lower in females than males (p < 0.001). Zp was greater in blacks than whites (p = 0.002) in Tanner stages 1-4, but the difference was attenuated in Tanner 5 (interaction, p = 0.02), R2 = 0.87. Muscle CSA, muscle torque, body weight, and physical activity were added to the model and each load covariate was independently and significantly (all p < 0.02) associated with Zp (R2 = 0.92), periosteal circumference, and cortical area. Inclusion of these measures attenuated but did not eliminate the significant race differences. Only muscle CSA was positively associated with endosteal circumference, while none of the load covariates were associated with vBMD. In conclusion, bone geometry is associated with several factors that define the mechanical load on bone, independent of age, tibia length, maturation, race, and sex. Race differences in Zp were not explained by these measures of mechanical load. Given that inclusion of muscle torque, body weight and physical activity resulted in a nominal increase in the R2, muscle size is an adequate surrogate for the mechanical load on bone in healthy participants.

Bone Density and Cortical Structure after Pediatric Renal Transplantation

“…We recently reported that total physical activity was positively associated with section modulus independent of age, sex, race, Tanner stage, tibia length, and muscle area. 21 …”

“…45 Peak isometric torque (foot-pounds) was measured in triplicate at 20°dorsiflexion, and the highest value was recorded, as previously described in the reference participants. 21 This measurement was used because the tibialis anterior attaches directly to the tibia (the bone of interest in this study) and causes dorsiflexion of the ankle. The coefficient of variation in our laboratory was 4.3%.…”

“…pQCT results were compared with reference data generated in more than 700 concurrent healthy participants. 1,[19][20][21] The hypotheses were that glucocorticoid exposure and resolution of secondary hyperparathyroidism after renal transplantation would be associated with declines in trabecular BMD and increases in cortical BMD. 12,22 We also hypothesized that increases in muscle area after renal transplantation would be associated with increases in cortical dimensions.…”

Skeletal muscle and pediatric bone development

“…This theory posits that bone adapts to the mechanical loads that are placed upon it during muscle contraction. Total body lean mass and muscle cross-sectional area (MCSA) are common surrogates of the muscular contractile forces of which bone is particularly responsive as muscle size and strength are closely related [6]. As a second example, pathogenic fat depots, for example, adiposity stored within the skeletal muscle, have piqued recent interest, as muscle fat in adults has been shown to be predictive of suboptimal bone outcomes and hip fracture [7,8].…”

Relationships between bone mineral density and new indices of body composition in young, sedentary men and women

“…Among many factors affecting bone mass, body weight is a strong and positive predictor acting through mechanical stress, which in turn increases osteogenesis and bone mineral density (BMD) [1]. In turn, it has been suggested that the reduction in body weight brings about bone loss [2].…”