As both cycling and swimming are not weight bearing activities, athletes in these fields tend to present with lower bone mineral density than those participating in sports involving impact forces (Nikander et al., 2009). This shows that when assessing bone stimulation, impact forces are a relevant factor.
A seven-year analysis of competitive male cyclists has found that there is a common trend with reduced bone mineral density (Nichols & Rauh, 2011). This will then lead to a higher risk of fractures. Therefore if participating in similar activities, it is important to complement your regime with weight lifting, impact and plyometrics to reduce bone loss, whilst maintaining the aerobic exercises to maintain general health.
Previous assessments of walking have shown that it can have beneficial effects on femoral bone mineral density in postmenopausal women, however it has been found to have no effect of spinal bone mineral density (Martyn-St & Carroll, 2008). This is true at the radius, whilst looking at the entire vertebral body, and includes both postmenopausal and perimenopausal women. However it has been found to have large positive effects on the femoral neck when performed for a long time (at least 6 months) (Ma, Wu & He, 2013). A 2% increase in lumbar bone mineral density and a 6.8% increase in femoral neck bone mineral density has been found when introducing a combination of treadmill walking (for 30 minutes per day) and step climbing (for 10 minutes per day) (Chien et al., 2000).
Running has been found to improve entire body bone mineral density; even in those aged over 65 (Velez et al., 2008).
Studies comparing strength training, high impact and no exercise over a six-month period have found that there was a significant increase in bone mineral density at the lumbar spine and femoral neck in the high impact group compared to the other groups. Along with this, there was also an increase in the bone formation marker Osteocalcin in the high-impact group compared to the other groups (Basat, Esmaeilzadeh & Eskiyurt, 2013). This shows that when looking at bone mineral density, impact forces are an important variable.
References:
Moreira, M.D.F, Oliveria, M.L., Lirani-Galvto, A.P., Marin-Mio, R.V., Santos, R.N., Lazarett-Castro, M. 2014. Physical exercise and osteoporosis: effects of different types of exercises on bone and physical function of postmenopausal women. Arquivos Brasileiros de Endocrinologia & Metabologia. 58(5). [viewed 23 October 2017]. Available from: http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0004-27302014000500514#B12
Nikander R, Kannus P, Dastidar P, Hannula M, Harrison L, Cervinka T, et al. Targeted exercises against hip fragility. Osteoporos Int. 2009;20(8):1321-8.
Nichols JF, Rauh MJ. Longitudinal changes in bone mineral density in male master cyclists and nonathletes. J Strength Cond Res. 2011;25(3):727-34.
Martyn-St JM, Carroll S. Meta-analysis of walking for preservation of bone mineral density in postmenopausal women. Bone. 2008;43(3):521-31.
Ma D, Wu L, He Z. Effects of walking on the preservation of bone mineral density in perimenopausal and postmenopausal women: a systematic review and meta-analysis. Menopause. 2013;20(11):1216-26.
Chien MY, Wu YT, Hsu AT, Yang RS, Lai JS. Efficacy of a 24-week aerobic exercise program for osteopenic postmenopausal women. Calcif Tissue Int. 2000;67(6):443-8.
Velez NF, Zhang A, Stone B, Perera S, Miller M, Greenspan SL. The effect of moderate impact exercise on skeletal integrity in master athletes. Osteoporos Int. 2008;19(10):1457-64
Basat H, Esmaeilzadeh S, Eskiyurt N. The effects of strengthening and high-impact exercises on bone metabolism and quality of life in postmenopausal women: a randomized controlled trial. J Back Musculoskelet Rehabil. 2013;26(4):427-35