Daily impact score in long-term acceleration measurements of exercise

Mechanical loading increases and maintains bone mass and strength. Daily stress stimulus and osteogenic index theories have been suggested to describe the osteogenic potential of exercise, using exponential or logarithmic relationships, respectively, between loading numbers and magnitude. Inspired by these theories, the aim of this study was to develop and test a daily impact score (DIS) using long-term continuous acceleration measurements of exercise. Acceleration data were collected during a previous exercise trial, in which the subjects (healthy women, 35-40 years, N=34 in the high-impact exercise group and N=30 in the control group) wore a body movement monitor on their waist during the 12-month study. DIS was calculated from the 12-month average daily acceleration distributions in two ways: DIS(Exp) adopted from the daily stress stimulus and DIS(Log) simplified from the osteogenic index. Areal bone mineral density (aBMD) at the proximal femur and cortical bone geometry at the mid-femur were measured at baseline and 12 months. DIS calculated in either of the ways was significantly higher in the exercise group than in the control group. DIS(Exp) and DIS(Log) were strongly correlated (R=0.982). Both DIS(Exp) and DIS(Log) were significantly associated with 12-month aBMD changes at the hip (R up to 0.550, p<0.01) and geometry changes at the mid-femur (R up to 0.472, p<0.05) in the exercise group. DIS calculated either from exponential or logarithmic relationship can be used in acceleration-based measurements of daily exercise. DIS was positively related with changes in hip aBMD and mid-femur bone geometry after 12 months of exercise.