Bone health during late adolescence: Effects of an 8-month training program on bone geometry in female athletes

Abstract

Objectives

This study investigated the short term changes in hip structural parameters in high level adolescent soccers and swimmers.

Methods

Twenty-six girl swimmers (15.9 ± 2 years) and 32 girl soccer players (16.2 ± 0.7 years) were investigated before and after 8 months of training. Fifteen non-active age-matched adolescents were enrolled at baseline to serve as a reference group. The geometric contributions to bone strength in the proximal femur were quantified using the hip structural analysis technique.

Results

Eight months of training had increased the bone density in soccer players but not in swimmers. At the total hip, swimmers conserved their baseline low Z-score values while soccers remained above the normal. At the femoral shaft, the cross sectional area had increased for the two groups with a higher magnitude in soccers (3.17% vs 2.31%; P < 0.05). The sub-periosteal width had increased in soccer players (P < 0.01) but not in swimmers. The Z-score showed an improvement of cross sectional moment of inertia and section modulus at femoral shaft section for soccers (P < 0.001), without any change in swimmers. Remarkably, swimmers had improved the Z-score of buckling ratio (P < 0.005), while soccers did not.

Discussion/conclusion

A season of training for sport characterized by impact had induced significant bone geometry improvement in late adolescent females despite expectations that this period of growth represents a plateau in bone maturation. This study adds to the existing literature describing intensive swimming practice as lacking an osteogenic effect when assessed by dual energy X-ray absorptiometry.

Introduction

Resistance to fracture is foremost among the factors contributing to bone health. It is currently well known that physical activity may induce bone tissue responses that influence the mechanical properties of bone. Among other things, those bone responses depend on the modalities of activity and the age of practice.

High impact activities are more efficient than activities characterized by weightlessness or experienced in an unloaded environment [1], [2], [3]. Such positive effects are currently observed on bone mineral density (BMD) [4], resistance to fracture [5], [6], bone remodeling [7], and bone ultrasound properties [8]. Some clinical studies have investigated the effects of exercise on the bone morphology pointing out the possibilities for bone to adapt also in geometry, inducing changes in mechanical properties. These studies were conducted using magnetic resonance imaging [9], [10], peripheral quantitative computed tomography [11], [12] and dual energy X-ray absorptiometry (DXA) [13]. DXA allows an indirect and less invasive method for assessing bone geometry using the Hip Structural Analysis (HSA) software [14], [15]. Using this technique, Ferry et al. [16] showed positive effects of soccer practice compared with the negative effect of swimming. In this cross-sectional study concerning the same subjects, the HSA parameters reflecting strength (cross sectional moment of inertia, section modulus, buckling ratio) favored soccer players. In contrast, the values of swimmers were in the lower range or below the normal, thus denoting weaker bone. Specifically, in this study, the upper limb BMD was lower for swimmers despite having a higher muscle mass. Theories describing the functional muscle-bone unit [17] suggest the bone response is partly under the mediation of muscle activity. However, it is possible that bone could develop in a paradoxical response when the training is performed in hypogravitational conditions [16], [18].

Age is another important factor for efficiency of physical activity. Pre- and early puberty may be the most opportune time to strengthen the female skeleton [19], [20]. Therefore, exercising during growth could represent a primary prevention against the effects of ageing on bone. Although physical activity has been shown to improve the bone density throughout the growing period, there are few studies dealing with its effects on bone geometry. The studies of Alvis et al. [21], [22] showed that a training program for 1 year in prepubertal subjects does not influence the hip structure assessed by HSA.

Whether such structural response can be linked to the period of growth or the duration of training stressors imposed on the skeleton is not clearly identified.

Moreover, bone maturation in females is reported to be complete at the end of adolescence [23]. Although the general understanding is that peak bone mass occurs by the end of the second or early in the third decade of life [24], accumulation is drastically reduced by 16 years of age in both the lumbar spine and femoral neck [23].

Beyond growth, the question of whether adolescent bone remains responsive to exercise warrants attention. Despite cross-sectional studies giving new information on the long-term adaptation of bone to physical activity [3], there are no relevant data on how bone geometry may respond to short-term constraints during adolescence.

Therefore, the purpose of the present study was to assess the changes in hip structural parameters and BMD after a season of training and competition in high-level late adolescent female soccer players and swimmers.