Although osteoporosis is considered a geriatric disease, factors affecting bone strength are most influential during child growth and development. This article reviews what is known and still unclear in terms of bone growth, development and adaptation relative to physical activity before and during puberty. Bone is responsive to certain exercise protocols early in puberty and less so in postpubertal years, where bone strength, rather than bone mass, being the outcome of interest. Mechanical loading and high impact exercise promote bone strength. Intense training before and during puberty, however, may negatively affect bone development. Future research should focus on increasing our mechanistic understanding of the manner by which diverse physical stressors alter the integrity of bone. Longitudinal studies that examine the extent to which muscle and bone are comodulated by growth in children are also recommended.
Panagiota Klentrou, Mary Lou Nishio, Michael Plyley and Brock University
The purpose of this study was to compare the ventilatory breakpoints during exercise between young boys and adult men. Twenty-four active volunteers, 12 boys (10.8 ± 0.3 years of age) and 12 men (24.6 ± 1.1 years of age) with similar aerobic power (53.6 ± 4.2 to 55.7 ± 3.5 mlkg−1min−1), participated in the study. Each participant completed a standardized Physical Activity Questionnaire and anthropometric variables were measured. The exercise testing consisted of two graded tests to exhaustion on a treadmill. Heart rates were recorded every minute and gas exchange parameters were obtained every 30 s throughout the tests. Each ventilatory breakpoint was determined using a three-part model. Although the relative VO2peak values were not significantly different between the boys and the young adults, the boys reported significantly (p < .05) higher total activity than the adults. The boys had a significantly higher (p > .05) first ventilatory breakpoint expressed as a percentage of VO2peak than the adults (64.9% vs. 57.7%). Only 50% of the boys demonstrated a discernable VB2, and the VB2 of this group was not significantly different from that of the adults.
Panagiota Klentrou, Jill Slack, Brian Roy and Michel Ladouceur
The effects of 12 wk of exercise training using weighted vests on bone turnover and isokinetic strength were evaluated in postmenopausal women randomly assigned as exercisers (EX; n = 9) or controls (CON; n = 7). Training included 3 multimodal exercise sessions per wk wearing weighted vests. The vest load was progressively increased each wk to a maximum of 15% of body weight. Bone turnover was determined from resting levels of serum osteocalcin and NTx. Knee and ankle strength were measured at 60°/s and 180°/s using an isokinetic dynamometer. After 12 wk, NTx decreased by 14.5% (P ≤ .05) in EX, with no significant changes in osteocalcin. EX also showed a 40% (P ≤ .05) improvement in ankle plantar-flexion strength at 60°/s. Relative body fat significantly decreased and fat-free mass increased in EX. Exercise compliance was 80%. These findings support the use of progressive exercise training using weighted vests in postmenopausal women.
Cameron Mitchell, Rotem Cohen, Raffy Dotan, David Gabriel, Panagiota Klentrou and Bareket Falk
Previous studies in adults have demonstrated power athletes as having greater muscle force and muscle activation than nonathletes. Findings on endurance athletes are scarce and inconsistent. No comparable data on child athletes exist.
This study compared peak torque (Tq), peak rate of torque development (RTD), and rate of muscle activation (EMG rise, Q30), in isometric knee extension (KE) and fexion (KF), in pre- and early-pubertal power- and endurance-trained boys vs minimally active nonathletes.
Nine gymnasts, 12 swimmers, and 18 nonathletes (7–12 y), performed fast, maximal isometric KE and KF. Values for Tq, RTD, electromechanical delay (EMD), and Q30 were calculated from averaged torque and surface EMG traces.
No group differences were observed in Tq, normalized for muscle cross-sectional area. The Tq-normalized KE RTD was highest in power athletes (6.2 ± 1.9, 4.7 ± 1.2, 5.0 ± 1.5 N·m·s–1, for power, endurance, and nonathletes, respectively), whereas no group differences were observed for KF. The KE Q30 was significantly greater in power athletes, both in absolute terms and relative to peak EMG amplitude (9.8 ± 7.0, 5.9 ± 4.2, 4.4 ± 2.2 mV·ms and 1.7 ± 0.8, 1.1 ± 0.6, 0.9 ± 0.5 (mV·ms)/(mV) for power, endurance, and nonathletes, respectively), with no group differences in KF. The KE EMD tended to be shorter (P = .07) in power athletes during KE (71.0 ± 24.1, 87.8 ± 18.0, 88.4 ± 27.8 ms, for power, endurance, and nonathletes), with no group differences in KF.
Pre- and early-pubertal power athletes have enhanced rate of muscle activation in specifically trained muscles compared with controls or endurance athletes, suggesting that specific training can result in muscle activation-pattern changes before the onset of puberty.
Mathew Yao, Izabella Ludwa, Lauren Corbett, Panagiota Klentrou, Peter Bonsu, Kimberley Gammage and Bareket Falk
Bone properties, reflected by speed of sound (SOS), and physical activity levels were examined in overweight (OW) girls (n = 19) and adolescents (n = 22), in comparison with normal-weight (NW) girls (n = 21) and adolescents (n = 13). Moderate-to-vigorous physical activity (MVPA) was higher in NW than in OW in both age groups. Tibial SOS was lower in OW compared with NW in both age groups. MVPA correlated with tibial SOS, once age was partialed out. The results suggest that overweight girls and adolescents are characterized by low tibial SOS, which may be partially attributed to lower physical activity levels.
Bareket Falk, Laura Brunton, Raffy Dotan, Charlotte Usselman, Panagiota Klentrou and Davie Gabriel
Ten prepubertal girls and 15 young women were tested for maximal torque, peak rate of torque development, electro-mechanical delay (EMD), and time to peak rate of torque development during isometric elbow flexion. Absolute peak torque (17.0 ± 7.7 vs. 40.5 ± 8.3 Nm) and peak rate of torque development (105.9 ± 58.6 vs. 297.2 ± 113.0 Nm·s−1) were lower in the girls (p < .05). Normalized to muscle cross sectional area, torque was similar (8.27 ± 2.74 vs. 8.44 ± 1.65 Nm·cm−2), as was peak rate of torque development, normalized to peak torque (6.21 ± 1.94 vs. 7.30 ± 2.26 Nm·s−1/Nm). Both, time to peak rate of torque development (123.8 ± 36.0 vs. 110.5 ± 52.6 ms) and EMD (73.2 ± 28.6 vs. 51.9 ± 25.6 ms), were longer in the girls, although EMD’s difference only approached statistical significance (p = .06). Age-related isometric strength differences in females appear to be mainly muscle-size dependent. However, the time to peak torque and EMD findings suggest differential motor-unit activation which may functionally manifest itself in fast dynamic contractions.
Jennifer Dekker, Katlynne Nelson, Nigel Kurgan, Bareket Falk, Andrea Josse and Panagiota Klentrou
This study examined resting levels of catabolic and anabolic osteokines related to Wnt signaling and their responses to a single bout of plyometric exercise in child and adolescent females. Fourteen premenarcheal girls [10.5 (1.8) y old] and 12 postmenarcheal adolescent girls [15.0 (1.0) y old] performed a plyometric exercise trial. One resting and 3 postexercise blood samples (5 min, 1 h, and 24 h postexercise) were analyzed for sclerostin, dickkopf-1 (DKK-1), osteoprotegerin (OPG), receptor activator of nuclear factor kappa-β ligand (RANKL), and transforming growth factors (TGF-β1, TGF-β2, and TGF-β3). Premenarcheal girls had significantly higher resting sclerostin, TGF-β1, TGF-β2, and TGF-β3 than the postmenarcheal girls, with no significant time effect or group-by-time interaction. DKK-1 was higher in premenarcheal compared with postmenarcheal girls. There was an overall significant DKK-1 decrease from baseline to 1 h postexercise, which remained lower than baseline 24 h postexercise in both groups. There was neither a significant group effect nor group-by-time interaction in OPG, RANKL, and their ratio. RANKL decreased 5 min postexercise compared with baseline and remained significantly lower from baseline 24 h following the exercise. No changes were observed in OPG. OPG/RANKL ratio was significantly elevated compared with resting values 1 h postexercise. In young females, high-impact exercise induces an overall osteogenic effect through a transitory suppression of catabolic osteokines up to 24 h following exercise.
Panagiota Klentrou, Kirina Angrish, Nafisa Awadia, Nigel Kurgan, Rozalia Kouvelioti and Bareket Falk
Purpose: This study examined osteokines related to Wnt signaling at rest and in response to plyometric exercise in 12 boys [10.2 (0.4) y] and 12 girls [10.5 (0.4) y]. Methods: One resting (preexercise) and 3 postexercise (5 min, 1 h, and 24 h) blood samples were analyzed for sclerostin, dickkopf-related protein 1 (DKK-1), osteoprotegerin (OPG), and receptor activator of nuclear factor kappa-β ligand (RANKL). Results: Girls had higher resting sclerostin than boys [187.1 (40.1) vs 150.4 (36.4) pg·mL−1, respectively; P = .02]. However, boys had higher DKK-1 [427.7 (142.3) vs 292.8 (48.0) pg·mL−1, respectively; P = .02] and RANKL [3.9 (3.8) vs 1.0 (0.4) pg·mL−1, respectively; P < .01] than girls. In girls, sclerostin significantly decreased 5-minute and 1-hour postexercise (χ2 = 12.7, P = .01), and RANKL significantly decreased 5-minute postexercise (χ2 = 19.1, P < .01) and continued to decrease up to 24-hour postexercise, with large effect sizes. In boys, DKK-1 significantly decreased 1-hour postexercise and remained lower than preexercise 24-hour postexercise (χ2 = 13.0, P = .01). OPG increased in both boys (χ2 = 13.7, P < .01) and girls (χ2 = 11.4, P = .01), with boys having significantly higher OPG at 5-minute and 1-hour postexercise, whereas in girls, this increase was only seen 24-hour postexercise. Conclusion: Plyometric exercise induces an overall anabolic osteokine response favoring osteoblastogenesis over osteoclastogenesis in both boys and girls although the timeline and mechanism(s) may be different.
Aysha M. Thomas, Kayleigh M. Beaudry, Kimbereley L. Gammage, Panagiota Klentrou and Andrea R. Josse
Background: Physical activity (PA) declines during early adulthood with higher rates of inactivity in university students. The authors aimed to examine the frequency, intensity, time, type of PA, and the barriers to PA participation in Canadian students during the first year of university. Methods: Questionnaires assessing PA variables were administered to 301 first-year students at the beginning and end of the academic year. Results: PA decreased over a year. Males engaged in more vigorous activity minutes, more strength training, and more organized sports than females (P < .05). Females participated in more fitness activities than males (P < .05). Intramural (noncompetitive and school organized) sport participation remained constant throughout the year. Significant intrapersonal barriers to PA engagement included stress and perceived self-skill; significant interpersonal barriers included lack of friends and peer influence; and significant structural barriers included homework, class schedule, and overcrowded facilities. Conclusion: PA and sport participation declined, and some differences existed between the sexes. Focus should be placed on reducing the barriers that students’ experience that may impact their PA. Interventions/programming to promote different aspects of PA should focus on noncompetitive sport and recreation activities, as well as activities that students can do on their own time. Ensuring the availability of adequate campus facilities is also important.
Raffy Dotan, Cameron Mitchell, Rotem Cohen, Panagiota Klentrou, David Gabriel and Bareket Falk
Children differ from adults in many muscular performance attributes such as size-normalized strength and power, endurance, fatigability and the recovery from exhaustive exercise, to name just a few. Metabolic attributes, such as glycolytic capacity, substrate utilization, and VO2 kinetics also differ markedly between children and adults. Various factors, such as dimensionality, intramuscular synchronization, agonist-antagonist coactivation, level of volitional activation, or muscle composition, can explain some, but not all of the observed differences. It is hypothesized that, compared with adults, children are substantially less capable of recruiting or fully employing their higher-threshold, type-II motor units. The review presents and evaluates the wealth of information and possible alternative factors in explaining the observations. Although conclusive evidence is still lacking, only this hypothesis of differential motor-unit activation in children and adults, appears capable of accounting for all observed child—adult differences, whether on its own or in conjunction with other factors.