Loading dose of physical activity is related to muscle strength and bone density in middle-aged women

• First time that loading dose of physical activity was assessed in both different intensity and frequency ranges.
• Positive association between loading dose, muscle strength and bone density only exists above certain intensity level in middle-aged women.
• Loading dose in all frequency ranges is associated with muscle strength and bone density when loading intensity surpasses the threshold.
• With ageing there tended to be a preferential loss of loading dose in high frequency bands of vigorous activity.

The aim of the current study was to investigate the association between loading dose of physical activity, muscle strength and bone density in middle-aged women. Thirty four healthy women (mean age = 49.8 ± 7.5 years) were recruited. They were requested to wear an accelerometer for a period of 10 h (from 9 am to 7 pm) on a day to record the acceleration. On a separate day their knee extension torque (KET) was measured using an isokinetic dynamometer and broadband ultrasound attenuation (BUA) at the heel by an ultrasound bone scanner. The loading dose of physical activity was calculated at four intensity categories — very light, light, moderate, and vigorous (intensities of < 5 BW/s, 5–10 BW/s, 10–15 BW/s and > 15 BW/s) and for three frequency bands — 0.1–2 Hz, 2–4 Hz, and 4–6 Hz. Correlation analysis was used to examine the association between loading dose and age, KET, and BUA. With the increase of age, there tended to be a decrease in the loading dose of vigorous activity in 2–4 and 4–6 Hz frequency bands (Kendall's tau = − .22, p < .1). The increase of loading dose in all three frequency bands in moderate or vigorous activity was associated with higher BUA (Kendall's tau = .27–.41, p < .05). The increase of loading dose in all frequency bands in light, moderate, or vigorous activity was associated with higher KET (Kendall's tau = .30–.45, p < .05). It is concluded that physical activity, especially that at high intensity level and high frequency range, may have beneficial effect on muscle strength and bone density in middle-aged women.