HKIN Physiologic changes at Puberty Exercise Physiology through the teens, and ramifications for training.

Unless otherwise noted, the information contained in this section of the course is from: Rowlands, T.W., Children’s Exercise Physiology, 2nd Ed.,Human Kinetics, Windsor ON, 2005

Outline Effects of Growth factors on Exercise Effects of Exercise on Growth Pubertal effects on fitness Training effects Aerobic fitness Anaerobic fitness Muscle strength Responses to Physical Training

Effects of Growth factors on Exercise GH/IGF-1 axis –GHRH (hypothalamus)- GH (Pituitary)- IGF-1 (liver) - peripheral tissue growth. Insulin. Effects of increased body size Biological age vs chronological age Gene expression.

Effects of Exercise on Growth 1.Is it a positive or negative relationship? 2.Caloric stealing! 3.No increase in serum IGF-1 levels from X’s. 4.+ve Correlation between VO 2 max and IGF-1 levels. 5.Rate and timing of peak height velocity 6.Conclusion

Effects of Puberty on Fitness. Increase in VO 2 max throughout childhood. (L*min -1 ) Big difference in genders during & after puberty There is a gender difference if we look at relative VO 2 max. Females decline steadily from age 8. Males stable. Increases mediated by increases in heart and muscle size. Increases in Aerobic endurance due to training are only significant during & after puberty.

Training effects on puberty Intensive exercise causes increase in sex hormone levels - principle of diminishing returns! Intensive exercise appears to cause hypoestrogenemia through inhibition of H-P-G axis

Baer J.T., Endocrine parameters in amenorrheic and eumenorrheic adolescent female runners. Int. J. Sports Med. 14: , 1993 GroupEstradiol level 1.Amenorrheic runners113 mmoles * L -1 2.Eumenorrheic runners247 mmoles*L -1 3.Eumenorrheic sedentary251 mmole*L -1

Training and Puberty Intensive training can cause delayed menarche of two years. Usually in conjunction with hyponutritional status. Generally, there is no evidence of pubertal delay from physical activity Again, the tall slender hips, thin athletic phenotype is typically a late maturer - Natural selection rather than causative by activity.

Aerobic fitness Glycogen stores are lower in children and increase with age. Little data to show increases in enzymatic levels from training, but Ericksson …. Prepubertal athletes show large increases in lactate tolerance (56%incr)

Aerobic fitness In general, aerobic metabolism declines as children age, with an increase in anaerobic glycolysis. Much of the improvements in “fitness” are tolerance to increasing lactate levels and running economy. At puberty, there is a sudden increase in endurance training response to aerobic metabolism. RHR response is similar to adults (69/81 bpm)

Anaerobic fitness Increases of % on wingate test Increases of 20% in treadmill run to exhaustion yrs, 12 weeks, but no improvement in 40 m. sprint time. (Mosher et al, 1985) Generally, anaerobic training yields small improvements, if any.

Strength changes in Lean Body Mass (LBM) agemalesfemales 11yrs11 kg 11 kg 17 yrs35 kg 22 kg

Strength agemalesfemales 11 yrs42%41% 17 yrs53%42% Lean muscle mass as a percentage of total body weight

Strength Does strength mirror increases in body height? Does strength mirror increases in muscle mass? Does strength mirror an increase in number of muscle fibers? Do nerves work better? (faster conduction velocities) Does female strength parallel males in children?

Strength May be related to angle of pennation Changes in central inhibition (golgi tendon organs? Neural recruitment and conduction velocity are likely reasons.

Strength Unpennated Unipennate Bipennated

Response to Strength training Increases of % over 8-12 weeks (subjects were U18) Increases of % were accompanied by 16.8% increase in EMG Males 9-11 yrs, increases of 35%(bench press) & 22% (leg press)

Thermoregulation in prepubertals Same as adults when ambient temp does not exceed 7 degrees above skin temp. Prepubertals do not tolerate heat as well as adults: greater dizziness, headaches, nausea and inability to persist.

fluid balance Water repletion is necessary during exercise bouts in excess of 60 minutes. Repletion is generally voluntarily stopped at about 70% of fluid losses. Smaller circulatory volume of children can be impacted to a greater degree than the adult. Must drink after satiation.