Objective      To determine the effect of different durations of 50 Hz 0.1 mT sinusoidal electromagnetic fields (SEMFs) on the peak bone mass (PBM) of rats.       Methods      Fifty female SD rats were randomly and equally divided into 5 groups

that

is

control group

45 min group

90 min group

180 min group and 270 min group. In addition to control group

the other experimental groups were exposed to 50 Hz 0.1 mT magnetic field every day for corresponding time periods. After 8 weeks

their bone mineral densities (BMD) of whole body were examined by DEXA. The effects were evaluated by measurements of bone biomechanical properties of femurs and vertebrae

serum levels of osteocalcin (OC) and tartaric acid phosphatase 5b (TRACP 5b)

and histomorphometry.       Results      Compared with the control group

the body BMD

femoral

vertebral bone mineral density

vertebral biomechanical parameters and serum OC level of 90 min group were increased

the differences were statistically significant (

P

0.01). Femur

vertebral body bone mineral density and serum OC of 180 min group were increased

with significant differences (

P

0.05). Femur bone density and vertebral bone mineral density of 270 min group were increased with statistical differences (

P

0.01)

but no significant difference was seen in the rest of the indicators (

P

0.05). There was no significant difference in each index between the 45 min group and the control (

P

0.05).  Serum TRACP 5b in each group showed no significant change (

P

0.05). The trabecular number

thickness and area were all increased

and the trabecular space were narrowed

with statistically differences compared with 90 min group

180 min group

and 270 min group (

P

0.01)

but not with 45 min group (

P

0.05).       Conclusion      Magnetic field of 50 Hz 0.1 mT strengthes the peak bone mass in rats. But there is a time “window” effect

and 90 min is best. Different times  show different significances in increasing peak bone mass.