New Insights about the Relationship between Bone Strength and Muscle

H.Schießl, J. Willnecker Ph.D. Stratec Medizintechnik GmbH Durlacher Str. 35 D-751 72 Pforzheim, Germany According to the mechanostat theory, the mechanical loads on bones help to determine bone strength, and the largest loads come from muscle forces. Indeed, recent studies found that muscle strength and the bending strength of bone are highly correlated (r>0.93), and bone ,,mass“ and muscle mass likewise (r>0.93). Two strain thresholds seem to help to control bone strength. If the mean voluntary loads on bone do not exceed the minimum threshold for remodeling (MESr), remodeling removes bone until this threshold is exceeded. Between 800 and 1600 uStrain bone is preserved. If mean loads on bone regularly exceed 1600 μtrain Strain ( minimum threshold for modeling MESm) bone is added to make it stronger. This suggests that any physical training that does not exceed 1600 μtrain will not increase bone strength. Growing muscle strength and body weight in children can cause bone strains that exceed the modeling threshold. This could help to explain the active bone modeling and increases in bone ,,mass“ and strength that occur in growing children. Data in an Argentine absorptiometric study show that in children bone and muscle mass both increase linearly until puberty, but in girls at 12 years of age bone ,,mass“ begins increasing faster than muscle mass. A similar but smaller increase occurs in boys at age 15. This suggests, estrogen may make girls store more bone than needed for strictly mechanical reasons, possibly to provide calcium stores during lactation after pregnancy. Two separate strain thresholds control bone loss or bone gain [4]. Below the threshold for remodeling (MESr) centered at about 800 μE remodeling removes bone where it touches marrow. In this disuse state bone strength is reduced until MESr is exceeded again. Where strains exceed the modeling threshold MESm, modeling adds bone to increase bone strength and ,,mass“. This threshold ranges from 1500 to 2000 μE [2]. Here bone strength is increases until mean strains stay below MESm. For comparison, the fracture limit of bone is around 17,000 μE. Between MESr and (MESm) bone is preserved. (Figure 1) If estrogen tends to lower MESr, after menopause that threshold would increase again. If so, loss of bone next to marrow should begin at menopause. This would make bone weaker and identical forces will now result in a higher strain When they increased to the level of the new remodeling threshold, bone loss should be reduced and tend to plateau. This suggests the opposite effect should occur at puberty in girls: With the start of estrogen production MESr should be reduced and bone resorption next to marrow should be slowed down. Therefore women from puberty until menopause should acquire more bone than needed for pure mechanical reasons A study from Zanchetta et al. 1995 [5] supplies information that could test that idea. Explanation of the results from the Zanchetta study In 1995 Zanchetta et al [5] reported measurements of total body bone ,,mass“ and whole body lean mass with dual energy X-ray absorption in 778 children and adolescents from 2 to 20 years of age. (Table 1) Since total body bone ,,mass“ is a surrogate for bone strength and whole body lean mass as a surrogate for muscle strength, such data might show the relationship between muscle and bone strength, and also any changes that might occur during puberty. Figure 2 plots those data, with lean body mass on the x axis and total body bone mineral content on the y axis. Each point represents the mean value of the age groups in one year intervals. Until