Normal Growth and Motor Development

• Normal growth and maturation of the child's musculoskeletal system and development of motor skills are determined by numerous factors, including genetic (familial inheritance), nutritional, hormonal, illness, physical activity, social conditions, race, culture, climate, and geographic location.

• This chapter describes normal growth and maturation of the pediatric musculoskeletal system; normal patterns of motor skill development; and methods of evaluating growth, maturation, and development and identifying abnormalities.

Somatic Growth

• Somatic growth refers to the increase in weight, height, and organ size.

— Assess somatic growth by comparing a child's height and weight to a population of other children at the same chronologic age using Centers for Disease Control and Prevention growth charts.

— These charts include the range of height, weight, head circumference, and body mass index (BMI) measured in a sample of American children aged birth to 20 years.

— While frequently used as a marker of health and nutritional status, somatic growth is not a reliable indicator of biological maturity. There is significant individual variation in the timing (when the growth spurt occurs) and tempo (rate or speed at which growth spurt occurs) of growth.

• Growth rate varies with age, is greatest from birth to 2 years, declines during childhood, and briefly increases again during the adolescent growth spurt (Table 1-1, Figure 1-1).

• During the prepubertal stage between 6 and 12 years of age, growth averages 3 to 3.5 kg and 6 cm per year, with minimal difference between boys and girls (Figure 1-1).

ADOLESCENT GROWTH SPURT

• The adolescent growth spurt begins at about 9 to 10 years of age for girls and 11 to 12 years of age for boys.

— Males experience a growth spurt about 2 years after the onset of puberty, heralded by the onset of gonadal enlargement.

— Girls experience a growth spurt 6 months after the appearance of breast buds. The growth spurt starts peripherally with enlargement of the hands and feet, then progresses centrally to the arms and legs, and lastly the trunk.

— The peak growth rate occurs earlier for trunk length and later for leg length compared with stature; thus the late childhood growth spurt is characterized by rapid trunk growth and the early adolescent growth spurt is characterized by rapid growth of the legs.

HEIGHT

• The rate of height growth accelerates until it reaches a maximum, termed the peak height velocity (PHV), which occurs about 2 years after the start of the adolescent growth spurt.

• The growth spurt lasts anywhere from 24 to 36 months (Figure 1-2).

• Standard deviations of age at PHV range from 0.7 to 1.2 years, indicating significant individual variation in the timing of the growth spurt.

• Age at PHV is a reliable indicator of somatic maturity in boys and girls.

• In boys PHV occurs at an average age of 14 years at a rate of 10.3 cm (4.3") per year and then decelerates to a stop by age 18 years.

• Girls start their growth spurt at an average age of 12 years at a rate of 9 cm (3.8") per year, reach PHV, and stop growing approximately 2 years earlier than boys (usually by age 16 years).

• The onset of menses generally follows maximum growth in height (PHV) by 1 year and therefore indicates a rapid deceleration in growth and limited additional gains in stature (Figure 1-2).

• Peak height velocity and magnitude of height gained is 3 to 5 cm greater in boys than in girls.

• Girls achieve a final mean adult height of 163.8 cm compared with 176.8 cm for boys, for an average adult height difference of 13 cm between men and women. This difference in final adult stature is because of the smaller PHV and the earlier termination of growth in girls compared with boys.

• Pubertal growth accounts for almost 25% of final adult height.

• The genetic contribution to final adult height is approximately 60%.

• There is a trend for youth who attain PHV at an earlier age to be slightly taller at that age, but ultimately there seems to be no relationship between age at PHV and final adult stature.

• Early maturers generally have a higher PHV than late maturers, and late maturers on average are taller when the growth spurt begins; consequently the mean adult height of early and late maturers is usually the same.

• Height differences among boys with differences in age of pubertal onset will generally disappear by late adolescence.

• Similarly, children with constitutional growth delay will "catch up" with their peers by late adolescence.

ESTIMATING ADULT HEIGHT

• Mid-parental height is a frequently used method to estimate a child's genetic height potential based on the child's gender and the biological parents' height, with a standard deviation of approximately 2".

— Mid-parental height for boys = paternal height + maternal height + 5 (inches)/2

— Mid-parental height for girls = paternal height + maternal height — 5 (inches)/2

• Multiplier method

— Height at given chronological age (cm) X multiplier = adult height (Figure 1-3)

WEIGHT

• On average, peak weight velocity (PWV) is greater in boys than girls. Peak weight velocity coincides with PHV in boys, but occurs about 6 to 9 months after PHV in girls.

• During puberty, girls reach PWV at age 13 years at a rate of 8.5 kg per year followed by a decrease to less than 1 kg per year by 15 years.

— Pubertal weight gain in girls is caused primarily by continuous increase in fat mass rather than an increase in skeletal and muscle mass.

• Boys reach PWV at age 14 years at a rate of 9.5 kg a year followed by a decrease to less than 1 kg per year by 17 years.

— Pubertal weight gain in boys primarily is caused by increases in height (skeletal mass) and muscle mass with a stable fat mass.

• Weight gains during puberty account for approximately 40% to 50% of ideal adult weight in both sexes.

BONE GROWTH

• Primary ossification centers develop before birth in the long bones.

• Primary ossification centers develop during infancy in small bones such as the patella and tarsal bones.

• Secondary ossification centers, within the epiphysis of long bones, develop during infancy and early childhood and fuse with primary ossification centers during late childhood, adolescence, and early adult life.

• The cartilage between the primary and secondary ossification centers of long bones becomes the epiphyseal growth plate.

— The epiphyseal plate is responsible for longitudinal growth and is subject to pressure or axial forces.

— Long bones of the upper and lower extremities (femur, tibia, fibula, humerus, radius, ulna) grow in length through the process of endochondral ossification, the proliferation of cartilage cells in the epiphyseal growth plate which then ossify to bone.

• The growth in diameter of bones around the diaphysis occurs by appositional ossification, the deposition of bone beneath the periosteum.

— The pelvis, carpals, and tarsals grow by appositional ossification.

— The phalanges, metacarpals, and spine grow by appositional and endochondral ossification.

• The clavicle is formed through intramembranous ossification, which unlike endochondral ossification, does not require a preexisting cartilage model.

• At the ends of each long bone the epiphysis is covered by articular cartilage and forms the joint surface.

— The development of normal joints requires a functioning neuromuscular system to allow normal motion.

• Ring epiphyses surround the periphery of round bones, such as the tarsal bones and vertebrae, which grow circumferentially.

• Apophyses are growth plates at the surface of bones such as the iliac crest.

— Traction apophyses are growth plates at the sites of muscle-tendon attachments, such as the tibial tubercle or ischial tuberosity, and thus are subject to traction forces.

— Apophyses contribute to adult bone shape and may look like a bony outgrowth or bump.

• Ossification begins first in the scapula, humerus, radius, and ulna, and then additional ossification centers develop in a predictable order (Table 1-2).

• On average, ossification centers develop earlier in girls than boys.

• Long and short tubular bones are mature when the diaphyses and the epiphyses fuse.

• Round or irregularly shaped bones are mature when they achieve final adult shape.

• During the fetal period, the metaphyses are composed of woven bone that has a high collagen content, giving it the flexibility needed for birth. By 4 years of age, the majority of woven bone has been converted to lamellar bone.

• Cortical bone thickness increases throughout childhood, resulting in increased diaphyseal thickness with age. Conversion to lamellar bone gives mature bone greater tensile strength but much less flexibility.

• The peak velocity of bone mineral accumulation lags behind PHV by an average of 1 year.

• Greater than 90% of peak skeletal bone mass is present by age 18 years.

• The rate of bone growth is precisely regulated, and each growth center contributes a specific percentage of final bone length.

• The trunk grows most rapidly during childhood.

• Growth of the upper limbs occurs earlier than the lower limbs, which grow the fastest during adolescence.

• The foot grows earlier than the rest of the lower limb and achieves its adult length earlier than the rest of the body; half of the adult foot length is achieved between 12 and 18 months of age.

• From infancy through early childhood, a child's lower extremities progress from a varus to valgus position.

• Lateral bowing of the tibia is common during the first year, and bowleg is common during the toddler years.

• Knock-knee is most prominent between 3 and 4 years of age.

• The normal bowlegged position evolves into a maximum average valgus angle of 12 degrees between ages 2 and 3 years and then eventually self-corrects to the average adult valgus angle of 5 degrees.

• A child's level of habitual physical activity does not affect the rate of skeletal maturation and has no effect on final adult body stature.

GROWING PAINS

• Growing pains are defined as limb pains that cannot be traced to trauma or disorders of bone, muscle, or joints, and are common in the pediatric age group, with an incidence of 4% to 36%.

• They most commonly occur from 3 to 5 years of age and from 8 to 12 years of age and more often in females than males.

• Pain is usually in the lower extremities and occurs at rest or during the night and not with physical activity.

• The children have a normal physical examination and no evidence of other systemic disease.

• The etiology is unknown; growing pains are benign and self-limited, with no effect on growth velocity.

ASSESSING SKELETAL MATURITY

• Level of ossification, or skeletal maturation, is the best indicator of biological maturity.

• Progression from a cartilaginous skeleton to a fully ossified adult skeleton is radiographically visible.

• Assessment of skeletal age (SA), or bone age, is based on bone development.

• A single radiograph of the left hand is most commonly used to assess skeletal maturity.

— The bones are compared with those in a standard radiographic atlas, either Greulich-Pyle or Tanner-Whitehouse, using a defined set of criteria.

— The skeletal ages derived from the different atlases are not equivalent and cannot be used interchangeably.

• Skeletal age may be compared to chronological age (CA), expressed as a difference between SA and CA or as a ratio of SA to CA.

— For example, a child with an SA of 11.8 years and a CA of 10.1 years is said to have advanced skeletal maturity for CA. Skeletal age minus CA yields a difference of +1.7 years, and SA divided by CA yields 1.2. A ratio above 1.0 indicates advanced skeletal maturity; conversely, a ratio below 1.0 indicates delayed skeletal maturity.

• Children may be classified as having an SA that is advanced, average, or delayed.

— Children whose SA is within 1 year of CA are classified as average maturers.

— Children whose SA is 1 year or more behind CA are classified as delayed or late maturers.

— Children whose SA is 1 year or more ahead of CA are classified as advanced or early maturers. Early maturers tend to be heavier and taller compared with late maturers at all ages, but final adult height typically is similar.

• Skeletal age is better correlated with stage of pubertal development than with CA and can be useful in predicting adult height in early or late maturers.

• Height, weight, and stages of pubertal development have become the main clinical tools for monitoring adolescent development because of the cost, inconvenience, and radiation exposure associated with using radiographs to assess skeletal age.

CHANGES IN BODY COMPOSITION

• Body composition is most often described as a two-compartment model, a combination of lean or fat-free mass (FFM) and fat mass (FM).

• The primary components of FFM are bone, skeletal muscle, and nonskeletal muscle soft tissue; the primary component of FM is adipose or fat tissue.

• Subcutaneous "baby fat" develops during the first year of life and gradually is burned up by increased mobility in early childhood.

• Fat mass and FFM gradually increase as body size increases between 2 and 6 years of age, but on average FM decreases more in boys than in girls due to increased energy expenditure and decreased caloric intake.

• Body physique remains relatively stable from 6 to 12 years of age, and FFM on average is 80% in a prepubertal child.

• Body mass index is weight divided by height squared (kg/m2) and is related to FFM in children and adolescents.

— Body mass index increases in infancy, decreases through early childhood to a low point around 5 or 6 years of age, then rises again through rest of childhood and adolescence.

— Children with the same BMI can have significantly different percentages of fat and FFM, and therefore elevated BMI is not uniformly a good indicator of fatness in childhood and adolescence.

• Fat-free mass undergoes a well-defined spurt during adolescence (Figure 1-4).

• Throughout adolescence lean or fat-free body mass increases from 80% to 90% in boys but decreases from 80% to 75% in girls as they accumulate more subcutaneous fat.

• During the interval of PHV, boys gain approximately twice as much FFM as girls (14 kg vs 7 kg), while girls gain twice as much fat mass as boys (3 kg vs 1.5 kg).

• Women have almost twice the percentage of body fat as men.

• Girls continue to gain FM but not FFM into late adolescence (16-20 years of age).

— Girls enter puberty with 15.7% average body fat, and as adults average 26.7%.

— Boys enter puberty with 4.3% average body fat, and as adults average 11.2%.

• Adult males have 150% of the FFM of an average adult female.

• Young women have more subcutaneous fat deposits in the pelvis, breast, upper back, and arms, compared with young men.

• The effect of heredity on body mass and composition is about 40%.

• Children who have more fat after 6 years of age have a higher risk of retaining fat through childhood and into adulthood.

CHANGES IN MUSCLE MASS AND STRENGTH

• The percentage of muscle, or muscle mass, increases with age.

• During middle childhood, muscle strength, coordination, and endurance increase through maturation and training.

• Small differences in muscle strength begin to appear between boys and girls in middle to late childhood.

• The greatest gains in muscular mass and strength occur during adolescence in girls and boys, with boys showing greater gains in both.

• The muscular strength gains in early adulthood occur at a much slower rate than in puberty.

• In boys, muscle strength increases linearly with age from early childhood through age 13 or 14 years and then experiences a marked acceleration through late adolescence into early or mid-20s.

• In girls, muscle strength also increases linearly through age 15 years but without an adolescent spurt.

• For boys this increase in muscle is a dominant change during puberty. The increase in strength is more than predicted from growth in stature.

• On average, the peak gains in muscular strength and power occur after PHV but around PWV. The peak increase in muscle strength lags behind the increase in mass, occurring in the final stage of pubertal development.

• Muscle mass peaks around 3 months after height spurt in boys and girls, and the increase in mass is double that in boys compared with girls.