Introduction and objective of the study:
Physical activity is the most important environmental factor modelling the reconstruction and metabolism of the skeleton. Lack of physical activity may lead to the loss of bone mass. The objective of the study is to determine the relationship between the level of physical activity and bone mineral density of the lumbar spine in middle – aged people.

Material and Methods:
The study covered 357 middle-aged (40–65 yrs) people (142 males and 215 females). A survey form was used to assess the level of physical activity. Evaluation of bone mineral density of the lumbar spine (L2-L4), in g/cm2 (BMD – Bone Mineral Density) was performed by the DEXA densitometric method, using the DPX-L Lunar apparatus, in the Rheumatological Hospital in Sopot. The relationship between the level of physical activity and bone mineral density was determined by the analysis of variance – ANOVA for the system of factors. Results:

Results:
The level of physical activity significantly diversified bone mineral density in the examined males and females. The lowest bone mineral density in L2-L4 was found in inactive persons. Low density was also observed in the oldest age group of women whose physical activity was high. An interaction was observed with gender and self-reported physical activity, regularity, frequency and time devoted by respondents to physical activity.

Conclusions:
Physical activity is a crucial element protecting against premature loss of bone mass, and also very important in osteoporosis prevention and should be performed regularly at every stage of human life.