Cameron J. R. Blimkie

Bone mineral density in adolescent female athletes: relationship to exercise type and muscle strength

PURPOSE This study investigated the influence of different exercise types and differences in anatomical distribution of mechanical loading patterns on bone mineral density (BMD) in elite female cyclists, runners, swimmers, triathletes, and controls (N = 15 per group). Associations between leg strength and BMD were also examined. METHODS Areal BMD (g x cm(-2)) was assessed by duel-energy x-ray absorptiometry (DXA) (total body (TB), lumbar spine (LS), femoral neck (FN), legs, and arms). Right knee flexion and extension strength was measured using a Cybex Norm isokinetic dynamometer at 60 degrees x s(-1). RESULTS Runners had significantly higher unadjusted TB, LS, FN, and leg BMD than controls (P < 0.05); higher TB, FN, and leg BMD than swimmers (P < 0.05); and greater leg BMD than cyclists (P < 0.05). Absolute knee extension strength was significantly (P < 0.01) correlated (0.33 < or = r < or = 0.44) with TB, FN, LS, and leg BMD for all groups combined. Weaker but still significant correlations (0.28 < or = r < or = 0.33) existed for normalized (per leg lean tissue mass) knee extension strength and all BMD sites, except FN BMD. There were no significant correlations between absolute or normalized knee flexion strength and any of the BMD variables. Absolute knee extension strength was entered as the second independent predictor for LS and leg BMD in stepwise multiple linear regression analysis (MLRA), accounting for increments of 4% and 12%, respectively, in total explained variation. CONCLUSION We conclude that running, a weight bearing exercise, is associated with larger site-specific BMD than swimming or cycling, that the generalized anatomical distribution of loads in triathlon appears not to significantly enhance total body BMD status, and that knee extension strength is only a weak correlate and independent predictor of BMD in adolescent females.

Gymnastic training and bone density in pre-adolescent females

Bone mineral density (BMD) was compared between 7-11-yr-old female gymnasts (GYM: N = 16) with a history of high volume impact loading (minimum of 15 h.wk-1 for past 2 yr) and healthy nonathletic controls (CON: N = 16). Whole body (WB) and regional areal BMD measures were determined by dual energy x-ray absorptiometry (DXA) normalized for height and body mass and also converted to bone mineral apparent density (BMAD). Volumetric BMD and bone cross-sectional areas were also measured by peripheral QCT (pQCT) at the left distal radius. GYM were significantly (P < 0.01) shorter (129.3 +/- 5.7 vs 136.7 +/- 4.4 cm; means +/- SD) and leaner (15.1 +/- 1.9 vs 19.6 +/- 4.3% body fat from DXA), and had significantly (P < 0.05) greater femoral neck (0.698 +/- 0.058 vs 0.648 +/- 0.064 g.cm-2) and trochanter (0.616 +/- 0.060 vs 0.530 +/- 0.084 g.cm-2) areal BMD than CON. GYM also had significantly higher whole body (0.101 +/- 0.009 vs 0.094 +/- 0.007 g.cm-3), femoral neck (0.245 +/- 0.060 vs 0.205 +/- 0.049 g.cm-3) and lumbar spine (0.227 +/- 0.014 vs 0.210 +/- 0.026 g.cm-3) BMAD compared with CON. Height normalized areal BMD measures were also significantly higher at all sites in GYM. Radial total (367.7 +/- 51.6 vs 307.4 +/- 27.6 mg.cm-3), trabecular (207.9 +/- 45.3 vs 163.8 +/- 31.4 mg.cm-3), and cortical (496.9 +/- 67.5 vs 429.8 +/- 33.8 mg.cm-3) BMD were also significantly greater in the GYM compared with the CON. In conclusion, high volume impact loading was associated with greater (compared with controls) whole body and regional bone mineral density in pre-adolescent female gymnasts.

Resistance Training During Preadolescence

SummaryHigh intensity resistance training appears to be effective in increasing strength in preadolescents. Children make similar relative (percentage improvement), but smaller absolute, strength gains compared with adolescents and young adults in response to similar resistance training programmes. Resistance training appears to have little if any effect on muscle size, and strength gains during training have been associated with increases in levels of neuromuscular activation and changes in intrinsic contractile characteristics of muscle. Although unsubstantiated, improved motor coordination probably also contributes to the increase in strength, especially for more complex strength manoeuvres. On the basis of limited information, training-induced strength gains are lost during detraining, and the decay in strength has been associated with a reduction in neuromuscular activation.Short term resistance training appears to have no effect on somatic, growth (height or weight) and body composition, and no proven positive influence on sports performance, injury rate or recovery from injury during preadolescence. Weighthfting has proved injurious to some children, especially when unsupervised and without instruction in proper weightlifting technique and load selection. In contrast, the risk of injury from prudently prescribed and closely supervised resistance training appears to be low during preadolescence. Lastly, short term resistance training appears to have no detrimental effect during preadolescence on either cardiorespiratory fitness or resting blood pressure.

Strength training: importance of genetic factors.

PURPOSE This study focuses on the quantification of genetic and environmental factors in arm strength after high-resistance strength training. METHODS Male monozygotic (MZ, N = 25) and dizygotic (DZ, N = 16) twins (22.4 +/- 3.7 yr) participated in a 10-wk resistance training program for the elbow flexors. The evidence for genotype*training interaction, or association of interindividual differences in training effects with the genotype, was tested by a two-way ANOVA in the MZ twins and using a bivariate model-fitting approach on pre- and post-training phenotypes in MZ and DZ twins. One repetition maximum (1RM), isometric strength, and concentric and eccentric moments in 110 degree arm flexion at velocities of 30 degrees x s(-1), 60 degrees x s(-1), and 12 degrees x s(-1) were evaluated as well as arm muscle cross-sectional area (MCSA). RESULTS Results indicated significant positive training effects for all measures except for maximal eccentric moments. Evidence for genotype*training interaction was found for 1RM and isometric strength, with MZ intra-pair correlations of 0.46 and 0.30, respectively. Bivariate model-fitting indicated that about 20% of the variation in post-training 1RM, isometric strength, and concentric moment at 120 degrees x s(-1) was explained by training-specific genetic factors that were independent from genetic factors that explained variation in the pretraining phenotype (30-77%). CONCLUSIONS Genetic correlations between measures of pre- and post-training strength were indicative for high pleiotropic gene action and minor activation of training-specific genes during training.

Skeletal adaptations to alterations in weight-bearing activity: A comparison of models of disuse osteoporosis

AbstractThe removal of regular weight-bearing activity generates a skeletal adaptive response in both humans and animals, resulting in a loss of bone mineral. Human models of disuse osteoporosis, namely bed rest, spinal cord injury and exposure to micro-gravity demonstrate the negative calcium balance, alterations in biochemical markers of bone turnover and resultant loss of bone mineral in the lower limbs that occurs with reduced weight-bearing loading. The site-specific nature of the bone response is consistent in all models of disuse; however, the magnitude of the skeletal adaptive response may differ across models. It is important to understand the various manifestations of disuse osteoporosis, particularly when extrapolating knowledge gained from research using one model and applying it to another. In rats, hindlimb unloading and exposure to micro-gravity also result in a significant bone response. Bone mineral is lost, and changes in calcium metabolism and biochemical markers of bone turnover similar to humans are noted. Restoration of bone mineral that has been lost because of a period of reduced weight bearing may be restored upon return to normal activity; however, the recovery may not be complete and/or may take longer than the time course of the original bone loss. Fluid shear stress and altered cytokine activity may be mechanistic features of disuse osteoporosis. Current literature for the most common human and animal models of disuse osteoporosis has been reviewed, and the bone responses across models compared.

Voluntary strength, evoked twitch contractile properties and motor unit activation of knee extensors in obese and non-obese adolescent males.

SummaryThe purpose of this study was to determine whether neural and/or muscular factors contributed to the inferior strength-related motor performances of obese adolescents. Subjects were 10 non-obese (14.6% fat) and 11 obese (32.3% fat) males matched for age (15–18 years), level of maturity (Tanner stages IV and V), lean body mass, and height. Peak torque (PT) was measured during maximal voluntary isometric (IS) and isokinetic (IK) knee extension (KE). Peak twitch torque (TT), time to peak torque (TPT), and half-relaxation time (HRT) of the knee extensors were elicited by percutaneous electrical stimulation. The interpolated twitch technique was used to determine the extent of motor unit activation (% MUA) during maximal voluntary IS KE. Knee extensor cross-sectional area (CSA) was determined by computed axial tomography taken at the mid-thigh. All strength and area measurements were made on the right side of the body. Obese subjects had significantly (P < 0.05) lower maximal voluntary IS and IK KE strength normalized for body weight, and significantly lower % MUA during IS KE. There were no significant differences (P > 0.05) between groups for absolute or normalized (for the product of muscle CSA and height) ISPT, IKPT, and TT, knee extensor CSA, or TPT and HRT. These results suggest that reduced MUA and a lower strength per mass ratio (due to excess fat) are probably important contributing factors to the poorer motor performances of the obese, especially for complex motor tasks involving large muscle groups and the support or moving of body weight.

Mid-femur geometry and biomechanical properties in 15- to 18-yr-old female athletes

PURPOSE Right-leg mid-femur geometry and biomechanical indices of bone strength were compared among elite cyclists (CYC), runners (RUN), swimmers (SWIM), triathletes (TRI), and controls (C)-10 subjects per group. METHODS Bone cross-sectional areas (CSA), volumes (Vol), and cross-sectional moments of inertia (CSMI) were assessed by magnetic resonance imaging (MRI), and cortical volumetric bone density (volBMD) was determined as the quotient of DXA-derived bone mineral content (BMC) and MRI-derived cortical bone volume. Bone strength index (BSI) was calculated as the product of cortical volBMD and CSMI. RESULTS RUN had higher (P < 0.05) size- (femur length and body mass) adjusted (ANCOVA) cortical CSA than C, SWIM, and CYC; and higher size, age, and years of sport-specific training- (YST) adjusted cortical CSA than SWIM and CYC. TRI had higher (P < 0.05) size-adjusted CSA than SWIM. SWIM and CYC had significantly larger (P < 0.05) size-adjusted medullary cavity CSA than RUN and TRI, and the difference between CYC and RUN persisted after additional adjustment for age and YST. RUN had significantly (P < 0.05) greater size-adjusted CSMI and BSI than C, SWIM, and CYC; and higher size, age, and YST-adjusted CSMI and BSI than SWIM and CYC. Mid-femur areal bone mineral density (BMD) was significantly (P < 0.05) higher for RUN compared with CYC only, but there were no other differences among groups for BMC, bone volumes, or volumetric total or cortical BMD. CONCLUSIONS Running, a weight-bearing exercise, is associated with more favorable geometric and biomechanical characteristics in relation to bone strength, compared with the weight supported activities of swimming and cycling. Differences may reflect skeletal adaptations to the specific mechanical-loading patterns inherent in these sports.

Anaerobic power of arms in teenage boys and girls: relationship to lean tissue

SummaryThe purpose of this study was to describe anaerobic peak and mean power characteristics of arms as a function of age, gender and body composition among 50 girls and 50 boys 14 to 19 years of age. Peak (pP) and mean power (MP) output were determined during arm cranking (Wingate anaerobic test). Fat-free weight (FFW) was estimated from skinfolds, and lean arm volume (LAV) was determined by water displacement, corrected for fat volume. PP and MP increased progressively and significantly (P<0.05) with age for boys but not for the girls. Boys had significantly larger absolute PP and MP outputs than girls at comparable ages. PP and MP corrected for lean tissue volume were greater in older than younger boys (P<0.05), but did not vary significantly with age for girls. Absolute (W) PP and MP were highly correlated with LAV (r = 0.82;r = 0.75) and FFW (r = 0.84;r = 0.78) among boys. The respectiver values for girls were 0.60 and 0.49 (LAV); 0.78 and 0.60 (FFW). Absolute peak and mean power, respectively, were more highly correlated with LAV (r = 0.90;r = 0.84) and FFW (r = 0.90;r = 0.85) for the combined data for boys and girls than for similar gender specific comparisons. In conclusion, both anaerobic PP and MP of arms during adolescence are highly correlated with lean tissue volume and fat-free weight, particularly among boys.

Fractures, physical activity, and growth velocity in adolescent Belgian boys

The relationship of fractures to physical activity and growth velocity in stature and metacarpal II bone dimensions was investigated in adolescent Belgian boys. Peak fracture incidence occurred between 12 and 14 yr of age and preceded the age at peak height velocity. The peak fracture rate occurred during mid adolescence (+/- 2 SD of the age at peak height velocity) and was twice as high as the rates before and after this period. The majority of fractures occurred during active participation in sports and general physical activities. The age at peak growth velocity for metacarpal cortical thickness, an indirect measure of bone mineral content, was about 6 months later than the ages at peak height velocity and peak growth velocity for metacarpal length. Peak fracture incidence occurred during a period when the amount of time spent in sports physical activity was low compared with later years. A lag in cortical bone thickness and mineralization, relative to linear skeletal growth, and unknown factors associated with active participation in sports, rather than an increase in the amount of physical activity, appear to be the predominant factors associated with the increased fracture incidence in Belgian boys during the growth spurt.

A comparison of bone geometry and cortical density at the mid-femur between prepuberty and young adulthood using magnetic resonance imaging

In upper extremity bones, a sexual dimorphism exists in the development of periosteal and endocortical bone surfaces during growth. Little is known about developmental patterns of bone geometry at weight-bearing bones like the femur. Using MRI and dual energy X-ray absorptiometry (DXA), this study assessed the differences in mid-femoral total (TA), cortical (CA) and medullary areas (MA), cortical thickness, and cortical density (BMD(compartment)) between prepuberty and young adulthood in 145 healthy subjects (94 females) 6 to 25 years old. Additionally, agreement between mid-femoral total bone volume (TV) measurements by DXA and MRI were investigated. In both sexes, TA, CA, MA, and cortical thickness were significantly larger in adults compared to prepubertal subjects (P < 0.001), and males had greater values than females. This sex difference persisted for TA, CA, and cortical thickness (P < 0.05), but not MA, after adjusting for femur length and weight. Mean (SD) cortical BMD increased from 1.05 (0.07) and 1.09 (0.10) g/cm(3) in prepubertal children to 1.46 (0.14) and 1.42 (0.1) g/cm(3) in young adults, females and males, respectively (P < 0.001). TV measurements by DXA were significantly greater than by MRI (P < 0.001) in young adults. In conclusion, periosteal and endocortical expansion and increasing cortical BMD are the growth processes found at the mid-femur in both sexes. Our findings contrast to that in upper extremity bones, where MA is constant in females during growth. The difference in femoral bone development may be due to higher strains caused by weight bearing and genetic factors. DXA, in contrast to MRI, is inaccurate in the determination of mid-femoral TV measures.