Loading modalities and bone structures at nonweight-bearing upper extremity and weight-bearing lower extremity: A pQCT study of adult female athletes
Introduction
High peak bone mass is associated with reduced risk of osteoporotic fractures later in life [1], [2], [3]. Different sports activities, in turn, are known to result in high peak bone mass particularly at the loaded bone sites [4], and some loading types, such as high-impact, odd-impact and high-magnitude loadings, have been shown to be clearly more effective than others [5], [6], [7], [8]. However, relatively little is known about the relationships between different loading types and structural characteristics of bone, which principally determine the whole bone strength [9]. At the lower extremities, the strongest bone structures appear in the athletes representing high-impact and odd-impact loading sports [10], [11], [12], [13], while the athletes in nonweight-bearing sports, such as swimmers, despite their considerable muscle work in training, do not seem to have improved bone structure at the femoral neck [13], [14]. At the upper extremities, strongest bone structures have been observed among athletes whose sports involve high-magnitude loading, such as weightlifting, or impact loading, such as tennis [15], [16], [17].
Crucial to bone adaptation is the mechanism by which the load is transmitted to and absorbed within the given musculoskeletal structure. Body weight and loading induced direct ground reaction forces and impacts acting through the closed kinetic chain apparently play a key role for lower extremities [18]. This is obvious for bones with a direct ground contact, such as the calcaneus and the contiguous straight-shafted tibia. The direct influence of concomitant muscle contraction forces during jumping and leaping cannot be ignored in these bones either. However, for the more proximal lower extremity sites, such as the femoral neck, the influence of muscle performance becomes probably more accentuated since continuous muscle work is required for maintaining the natural erect body position in normal human locomotion and other movements [19]. Even during normal daily exercises, the related peak hip joint moments have been shown to be positively associated with proximal femur bone mass [20], [21]. It is, however, evident that more vigorous loading, such as maximal jumping, can override the influence of light-intensity physical activities such as walking and jogging [11].
In contrast to the lower extremities, the upper extremities lack the regular weight-bearing component, a fact that may underline the role of muscle performance and modulate the skeletal response to incident loading. Accordingly, while structure of the weight-bearing tibia has clearly deteriorated in weightless conditions in space, the nonweight-bearing radius has not shown any general trend for bone loss [22].
The effect of exercise in strengthening the bones is commonly evaluated with respect to bone mineral density (BMD)—mostly the DXA-derived BMD. However, the interpretation of BMD is deemed inherently ambiguous and one is thus not able to accurately determine structural particulars, e.g., the cortical bone [23]. The contribution of cortical bone to the bone strength is evident [36]. Since the cortical bone also seems more responsive to loading (probably through corticalization of trabecular bone adjacent to the endosteal cortical surface) [24] than bone density, the predominance of BMD as the main outcome in exercise studies can be challenged. The main focus should be laid on assessing such bone structural particulars which are apparently most relevant in terms of mechanical rigidity [9].
The purpose of this cross-sectional study was thus to broaden and deepen the present, mostly DXA-based information [10], [11], [12], [13], [14], [15], [16], [17] on the apparent influence of different sports and associated loadings on bone structure. Specifically, we assessed the relationship between the bone structure and the loading modality, and estimated the contribution of muscle performance and related joint moments to the structure of weight-bearing and nonweight-bearing bones. The main research questions were whether the higher bone mass in athletes is used for building mechanically reasonable bone structure and whether the lack of constant weight-bearing at the upper extremity is reflected to the structure of its bones.
Section snippets
Subjects
A total of 113 premenopausal competitive national-level female athletes and 30 nonathletic referents participated in the study. The athletes were 21 volleyball players, 24 hurdlers, 23 racket players (13 tennis, 8 badminton and 2 squash players), 18 soccer players and 27 swimmers, and they were recruited from national sport associations and local clubs. The referents were volunteers who were recruited from a local medical school. The study protocol was approved by the Ethics Committee of The
Results
Group characteristics are shown in Table 1. The volleyball players were taller and heavier, and the referents and racket players were somewhat older than the other groups. As regards the athletes, the mean of the weekly training hours varied from 4.6 to 13.5 h. The referents reported 2.9 training hours a week on average. Physical activity of the reference group included various types of exercise from walking to more intense sports, such as aerobics and floorball. Compared with the referents,
Discussion
This pQCT study of female athletes showed that women, whose sports exert impact loading, had apparently stronger bones at both the weight-bearing and nonweight-bearing skeleton compared with the nonathletic referents. These findings corroborate the relevance of impact loading as an efficient mean to improve bone rigidity [10], [11], [12], [13], [14], [15], [16], [17]. In sport-specific and natural loading conditions, the ground impacts can differ a lot in terms of number, magnitude of force,
Acknowledgments
The authors thank all study participants for their great effort. We also thank Matti Pasanen, MSc for his statistical help, Virpi Koskue for the pQCT measurements, Taru Helenius for scheduling the measurements, Salla Peltonen for assisting the physical performance measurements and Seppo Niemi for his assistance in preparing the figures and tables. The financial support from the Juho Vainio Foundation, Helsinki, Finland, the Research Fund of the Tampere University Hospital, Tampere, Finland, and
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