Written by Kieron Vorster, Strength & Conditioning Coach at fit8.
How a child should train changes significantly depending on where they are in their physical development. Understanding the physiology behind pre- and post-pubescent growth allows parents and coaches to structure training that is safe, effective, and appropriate for each stage. This guide covers what the science tells us about how children develop physically, and what that means for training on and off the court.
How Children Grow: The Physical Foundations
Height
A child reaches 50% of their adult height by the age of two. The peak rate of growth in height occurs at approximately 11.4 years in girls and 13.4 years in boys.
Weight
Peak rate of growth in body weight occurs at approximately 12.5 years in girls and 14.5 years in boys.
Bone Development
During the first 14 to 22 years of life, membranes and cartilage are transformed into bone through ossification. Bones tend to fuse in the pre-teen years and are fully fused by the early twenties. On average, girls reach full bone maturity several years before boys.
Exercise increases bone width and bone density by depositing more minerals in the bone matrix, which increases bone strength. Calcium is an essential nutrient for bone growth throughout this period.
Muscle Development
In males, total muscle mass increases from approximately 25% of total body weight at birth to around 50% or more in adulthood. The most significant gains occur when muscle development peaks at puberty, corresponding to a tenfold increase in testosterone production. Muscle mass peaks in males between ages 18 and 25.
In females, muscle mass increases more slowly, reaching approximately 40% of total body weight in adulthood. This is largely attributable to hormonal differences at puberty. Muscle mass peaks in females between ages 16 and 20. The increase in oestrogen at puberty promotes greater fat deposition, which is the primary driver of body composition differences between the sexes after puberty.
Fat
Fat cells begin developing early in foetal life and each fat cell can increase in size at any age. Research suggests the total number of fat cells becomes fixed early in life. The amount of fat accumulated depends on diet, exercise habits, and heredity.
At birth, fat accounts for approximately 10 to 12% of total body weight. At physical maturity, fat content is approximately 15% for males and 25% for females, primarily due to hormonal differences.
The Nervous System
As children grow, they develop better balance, agility and coordination as their nervous system matures. Myelination of nerve fibres — the formation of a protective sheath that speeds the transmission of electrical impulses — must be complete before fast reactions and skilled movements are fully developed.
Myelination of the cerebral cortex occurs most rapidly during childhood but continues well beyond puberty. The full development of a skill or activity depends on the full maturation of the nervous system. The development of strength is also influenced by myelination.
How Children Respond to Exercise: Key Differences from Adults
Strength and Neural Maturity
High levels of strength, power and skill are not achievable if a child has not reached neural maturity. Myelination of many motor nerves is incomplete until sexual maturity, meaning neural control of muscle function is limited before that point.
Cardiovascular and Respiratory Function
Blood pressure at rest and during sub-maximal exercise is lower in children than in adults, increasing progressively to adult levels through the late teen years. Children have a lower stroke volume both at rest and during exercise. Blood flow to active muscles during exercise can be proportionally greater in children than in adults due to lower peripheral resistance.
Maximum heart rate is higher in children than in adults and decreases linearly as children age. Children under 10 frequently have maximum heart rates above 210 beats per minute. The average 20-year-old has a maximum heart rate of approximately 195 beats per minute.
Aerobic Capacity
VO2 max peaks between ages 17 and 21 in boys, then decreases linearly with age. Girls show the same trend, although the decrease begins earlier, generally between ages 12 and 15.
Running Economy
The following factors contribute to improvements in running economy as children mature: stride frequency, gait mechanics, musculotendinous elastic energy storage, surface area to body mass ratio, changes in body composition, thermal response to exercise, substrate utilisation, anaerobic capacity, and ventilatory capacity. Of these, only stride frequency has thus far been proven to be significant in studies with children aged 8 to 20 years.
Anaerobic Capacity
Children have a limited ability to perform anaerobic activities. Resting levels of ATP and PCr in children are similar to those of adults, so activities of less than 10 to 15 seconds should not be significantly compromised. Activities that tax the anaerobic glycolytic system — those lasting from 15 seconds up to 2 minutes — will be lower in children.
Children have lower lactate concentrations in both blood and muscle at maximal and supramaximal rates of work, and cannot attain high respiratory exchange ratios during maximal exercise. Anaerobic mean and peak power outputs are lower in children than in adults, even when adjusted for body mass.
Thermoregulation
Children rely more on convection and radiation for heat loss, enhanced through greater peripheral vasodilation. They have a greater ratio of body surface area to mass, meaning more skin surface area relative to body weight — an advantage in cool conditions for losing heat, but a disadvantage when environmental temperature exceeds skin temperature, as children then gain heat from the environment more rapidly.
A child's lower capacity for evaporative heat loss is largely the result of a lower sweat rate. Individual sweat glands in children produce sweat more slowly and are less sensitive to increases in core body temperature than those in adults.
How Children Adapt to Training
Body Composition
With both resistance and aerobic training, boys and girls will decrease body fat and fat mass while increasing fat-free mass, although the increase in fat-free mass is smaller in children compared with adolescents and adults. There is also evidence of significant additional bone growth as a result of exercise training, beyond that seen with normal growth.
Strength
Studies have shown that resistance and strength training carries a low risk of injury in children. Resistance training may offer protection against injury by strengthening the muscles that cross a joint. A conservative approach is recommended when prescribing resistance exercises for children, particularly pre-adolescents.
Pre-pubescent children can improve their strength through resistance training. These strength gains are attributable largely to neurological factors, with little or no change in muscle size.
Anaerobic Adaptations
Following anaerobic training, children show increased resting levels of PCr, ATP and glycogen, increased phosphofructokinase activity, and increased maximal blood lactate. Ventilatory threshold has been shown to increase with endurance training in boys aged 10 to 14.
Children and adolescents can be trained aerobically and anaerobically using the same general model as adults, but it is prudent to be conservative to reduce the risk of injury, overtraining and loss of interest in sport.
Growth and Maturation
Regular training has no apparent effect on growth in height. It does affect weight and body composition. The age at which peak height velocity occurs is generally not affected by regular training, nor is the rate of skeletal maturation.
Resistance Training Recommendations for Pre-Pubescent Children
Training Equipment
- Must be appropriate in design to accommodate pre-pubescent size and degree of maturity
- Should be cost effective
- Must be safe, free of defects and inspected frequently
- Should be located in an uncrowded area, free of obstructions, with adequate lighting and ventilation
Programme Considerations
- A pre-participation physical examination is mandatory
- The child must have the emotional maturity to accept coaching and instruction
- Adequate supervision by coaches knowledgeable in strength training and the specific requirements of pre-pubescent athletes is essential
- Strength training should form part of a comprehensive programme designed to increase motor skills and fitness levels
- Sessions should be preceded by a warm-up and followed by a cool-down
- Emphasis should be on dynamic concentric contractions through a full range of motion
- Competition between athletes is prohibited
- No maximal lift should ever be attempted
Prescribed Programme
- Training is recommended 2 to 3 times per week for 20 to 30 minute sessions
- No resistance should be applied until technique is properly demonstrated
- 6 to 15 repetitions equal one set; 1 to 3 sets per exercise
- Weight or resistance is increased in 0.5 to 1.5kg increments after the athlete completes 15 repetitions in good form
Age-by-Age Resistance Training Guidelines
Age 7 and Under
Introduce basic exercises with little or no weight. Teach exercise techniques and the concept of a structured training session. Progress from bodyweight calisthenics, partner exercises and lightly resisted exercises. Keep volume low.
Ages 8 to 10
Gradually increase the number of exercises and training volume. Practise exercise techniques in all lifts and keep exercises simple. Begin gradual, progressive loading, carefully monitoring tolerance to exercise stress.
Ages 11 to 13
Teach all basic exercise techniques with emphasis on form. Continue progressive loading. Introduce more advanced exercises with little or no resistance.
Ages 14 to 15
Progress to more advanced youth resistance programmes. Add sport-specific components. Continue to emphasise exercise technique. Increase volume.
Age 16 and Over
Move to entry-level adult programmes once all foundational knowledge has been mastered and a basic level of training experience has been gained.
Motor Ability and Sport Performance
Motor ability in boys and girls generally increases with age through the first 17 years, although girls tend to plateau at around the age of puberty for most measures. This is primarily due to increased oestrogen levels, which promote greater fat deposition, reduced muscle mass relative to boys, and a more sedentary lifestyle.
Improvements in motor ability result primarily from development of the neuromuscular and endocrine systems, and secondarily from increased activity levels.
Youth Fitness
Over recent decades there has been a marked increase in time children spend on screens and a corresponding decrease in physical activity. When activity levels fall below a certain threshold and fat levels rise above a certain level, children and adolescents face an increased risk of chronic disability and disease. An active lifestyle must be encouraged from early in life.
Key Takeaways for Parents
- Children develop physically at different rates, particularly during puberty.
- Exercise can support healthy bone development and body composition.
- Strength gains before puberty are primarily neurological rather than muscular.
- Resistance training can be safe and beneficial when properly supervised.
- Training programmes should be adapted to a child's age, maturity and stage of development.
- An active lifestyle during childhood is essential for long-term health and athletic development.
Conclusion
Children are not simply smaller versions of adults. Their bones, muscles, nervous system and cardiovascular systems are all developing throughout childhood and adolescence, and these changes influence how they respond to training.
Understanding the differences between pre- and post-pubescent athletes allows parents and coaches to provide training that is appropriate for a child's stage of development. While children can improve strength, fitness and athletic performance through training, programmes should be designed with their age, maturity and individual needs in mind.
Research shows that regular exercise and appropriately supervised training can contribute positively to bone health, body composition, fitness and overall athletic development. Equally important is encouraging an active lifestyle from an early age, helping children establish habits that support both long-term health and sporting participation.
By recognising how growth and maturation affect performance, parents and coaches can make more informed decisions and create an environment that supports the ongoing development of young athletes.
Junior Tennis Training: The Pre & Post Puberty Guide for Parents and Coaches
Written by Kieron Vorster, Strength & Conditioning Coach at fit8.
How a child should train changes significantly depending on where they are in their physical development. Understanding the physiology behind pre- and post-pubescent growth allows parents and coaches to structure training that is safe, effective, and appropriate for each stage. This guide covers what the science tells us about how children develop physically, and what that means for training on and off the court.
How Children Grow: The Physical Foundations
Height
A child reaches 50% of their adult height by the age of two. The peak rate of growth in height occurs at approximately 11.4 years in girls and 13.4 years in boys.
Weight
Peak rate of growth in body weight occurs at approximately 12.5 years in girls and 14.5 years in boys.
Bone Development
During the first 14 to 22 years of life, membranes and cartilage are transformed into bone through ossification. Bones tend to fuse in the pre-teen years and are fully fused by the early twenties. On average, girls reach full bone maturity several years before boys.
Exercise increases bone width and bone density by depositing more minerals in the bone matrix, which increases bone strength. Calcium is an essential nutrient for bone growth throughout this period.
Muscle Development
In males, total muscle mass increases from approximately 25% of total body weight at birth to around 50% or more in adulthood. The most significant gains occur when muscle development peaks at puberty, corresponding to a tenfold increase in testosterone production. Muscle mass peaks in males between ages 18 and 25.
In females, muscle mass increases more slowly, reaching approximately 40% of total body weight in adulthood. This is largely attributable to hormonal differences at puberty. Muscle mass peaks in females between ages 16 and 20. The increase in oestrogen at puberty promotes greater fat deposition, which is the primary driver of body composition differences between the sexes after puberty.
Fat
Fat cells begin developing early in foetal life and each fat cell can increase in size at any age. Research suggests the total number of fat cells becomes fixed early in life. The amount of fat accumulated depends on diet, exercise habits, and heredity.
At birth, fat accounts for approximately 10 to 12% of total body weight. At physical maturity, fat content is approximately 15% for males and 25% for females, primarily due to hormonal differences.
The Nervous System
As children grow, they develop better balance, agility and coordination as their nervous system matures. Myelination of nerve fibres — the formation of a protective sheath that speeds the transmission of electrical impulses — must be complete before fast reactions and skilled movements are fully developed.
Myelination of the cerebral cortex occurs most rapidly during childhood but continues well beyond puberty. The full development of a skill or activity depends on the full maturation of the nervous system. The development of strength is also influenced by myelination.
How Children Respond to Exercise: Key Differences from Adults
Strength and Neural Maturity
High levels of strength, power and skill are not achievable if a child has not reached neural maturity. Myelination of many motor nerves is incomplete until sexual maturity, meaning neural control of muscle function is limited before that point.
Cardiovascular and Respiratory Function
Blood pressure at rest and during sub-maximal exercise is lower in children than in adults, increasing progressively to adult levels through the late teen years. Children have a lower stroke volume both at rest and during exercise. Blood flow to active muscles during exercise can be proportionally greater in children than in adults due to lower peripheral resistance.
Maximum heart rate is higher in children than in adults and decreases linearly as children age. Children under 10 frequently have maximum heart rates above 210 beats per minute. The average 20-year-old has a maximum heart rate of approximately 195 beats per minute.
Aerobic Capacity
VO2 max peaks between ages 17 and 21 in boys, then decreases linearly with age. Girls show the same trend, although the decrease begins earlier, generally between ages 12 and 15.
Running Economy
The following factors contribute to improvements in running economy as children mature: stride frequency, gait mechanics, musculotendinous elastic energy storage, surface area to body mass ratio, changes in body composition, thermal response to exercise, substrate utilisation, anaerobic capacity, and ventilatory capacity. Of these, only stride frequency has thus far been proven to be significant in studies with children aged 8 to 20 years.
Anaerobic Capacity
Children have a limited ability to perform anaerobic activities. Resting levels of ATP and PCr in children are similar to those of adults, so activities of less than 10 to 15 seconds should not be significantly compromised. Activities that tax the anaerobic glycolytic system — those lasting from 15 seconds up to 2 minutes — will be lower in children.
Children have lower lactate concentrations in both blood and muscle at maximal and supramaximal rates of work, and cannot attain high respiratory exchange ratios during maximal exercise. Anaerobic mean and peak power outputs are lower in children than in adults, even when adjusted for body mass.
Thermoregulation
Children rely more on convection and radiation for heat loss, enhanced through greater peripheral vasodilation. They have a greater ratio of body surface area to mass, meaning more skin surface area relative to body weight — an advantage in cool conditions for losing heat, but a disadvantage when environmental temperature exceeds skin temperature, as children then gain heat from the environment more rapidly.
A child's lower capacity for evaporative heat loss is largely the result of a lower sweat rate. Individual sweat glands in children produce sweat more slowly and are less sensitive to increases in core body temperature than those in adults.
How Children Adapt to Training
Body Composition
With both resistance and aerobic training, boys and girls will decrease body fat and fat mass while increasing fat-free mass, although the increase in fat-free mass is smaller in children compared with adolescents and adults. There is also evidence of significant additional bone growth as a result of exercise training, beyond that seen with normal growth.
Strength
Studies have shown that resistance and strength training carries a low risk of injury in children. Resistance training may offer protection against injury by strengthening the muscles that cross a joint. A conservative approach is recommended when prescribing resistance exercises for children, particularly pre-adolescents.
Pre-pubescent children can improve their strength through resistance training. These strength gains are attributable largely to neurological factors, with little or no change in muscle size.
Anaerobic Adaptations
Following anaerobic training, children show increased resting levels of PCr, ATP and glycogen, increased phosphofructokinase activity, and increased maximal blood lactate. Ventilatory threshold has been shown to increase with endurance training in boys aged 10 to 14.
Children and adolescents can be trained aerobically and anaerobically using the same general model as adults, but it is prudent to be conservative to reduce the risk of injury, overtraining and loss of interest in sport.
Growth and Maturation
Regular training has no apparent effect on growth in height. It does affect weight and body composition. The age at which peak height velocity occurs is generally not affected by regular training, nor is the rate of skeletal maturation.
Resistance Training Recommendations for Pre-Pubescent Children
Training Equipment
Programme Considerations
Prescribed Programme
Age-by-Age Resistance Training Guidelines
Age 7 and Under
Introduce basic exercises with little or no weight. Teach exercise techniques and the concept of a structured training session. Progress from bodyweight calisthenics, partner exercises and lightly resisted exercises. Keep volume low.
Ages 8 to 10
Gradually increase the number of exercises and training volume. Practise exercise techniques in all lifts and keep exercises simple. Begin gradual, progressive loading, carefully monitoring tolerance to exercise stress.
Ages 11 to 13
Teach all basic exercise techniques with emphasis on form. Continue progressive loading. Introduce more advanced exercises with little or no resistance.
Ages 14 to 15
Progress to more advanced youth resistance programmes. Add sport-specific components. Continue to emphasise exercise technique. Increase volume.
Age 16 and Over
Move to entry-level adult programmes once all foundational knowledge has been mastered and a basic level of training experience has been gained.
Motor Ability and Sport Performance
Motor ability in boys and girls generally increases with age through the first 17 years, although girls tend to plateau at around the age of puberty for most measures. This is primarily due to increased oestrogen levels, which promote greater fat deposition, reduced muscle mass relative to boys, and a more sedentary lifestyle.
Improvements in motor ability result primarily from development of the neuromuscular and endocrine systems, and secondarily from increased activity levels.
Youth Fitness
Over recent decades there has been a marked increase in time children spend on screens and a corresponding decrease in physical activity. When activity levels fall below a certain threshold and fat levels rise above a certain level, children and adolescents face an increased risk of chronic disability and disease. An active lifestyle must be encouraged from early in life.
Key Takeaways for Parents
Conclusion
Children are not simply smaller versions of adults. Their bones, muscles, nervous system and cardiovascular systems are all developing throughout childhood and adolescence, and these changes influence how they respond to training.
Understanding the differences between pre- and post-pubescent athletes allows parents and coaches to provide training that is appropriate for a child's stage of development. While children can improve strength, fitness and athletic performance through training, programmes should be designed with their age, maturity and individual needs in mind.
Research shows that regular exercise and appropriately supervised training can contribute positively to bone health, body composition, fitness and overall athletic development. Equally important is encouraging an active lifestyle from an early age, helping children establish habits that support both long-term health and sporting participation.
By recognising how growth and maturation affect performance, parents and coaches can make more informed decisions and create an environment that supports the ongoing development of young athletes.