Muscular Strength in Children

Question 1

Development in muscular strength

Answer 1

• Muscle fiber number fixed soon after birth (between ages 1 to adolescence fibre diameter increases almost 3 fold)
• Muscle hypertrophy occurs as children grow via increased protein content (not cellular multiplication) Influenced by growth hormone IGF-I
• Muscle strength increases as muscle mass increases.
• Following puberty, circulating testosterone accelerates increases in muscle size and strength in boys.

Question 2

Dimensionality Theory

Answer 2

• According to dimensionality theory, areas should relate to body mass by the exponent 0.67.
• Areas should also relate to height by the exponent 2.0.
• Area/ height (Area has a dimension of 2 and height has a dimension of 1)
  
  • 2/1 = 2
• If muscle force is expected to relate to cross-sectional area of the contracting muscle then it should be expressed as M0.67 (Mass) or H2.0 (Height) if cross-sectional areas is the only factor involved in the development of strength during childhood.

Question 3

Dimensionality Theory - too simplistic

Asmussen, E. & Heeboll-Nielsen.J

Answer 3

Study of 7-16 yr old Danish schoolboys to determine the exponents for height relative to strength

• Data from children indicate that scaling factors for muscular strength are actually higher (higher than 2)
• This means that muscular strength is increasing at a greater rate with age than would be expected
• Girls tended to have a plateau of strength around 15 years of age while boys continues to get stronger

Question 4

Why might the dimensionality theory not be applicable to children?
What about comparing children with other children?

Answer 4

Why might the dimensionality theory not be applicable to children

• It assumes that subjects of different sizes are geometrically similar i.e. similar proportions.
• Children, when compared to adults have relatively larger heads and shorter legs

What about comparing children with other children?
- Debate is there is dimensional similarity between children. Some research disagrees suggesting that tissue composition is not constant throughout childhood and thus the theory of geometric similarity is questionable

Question 5

Effects of sports played and maturation differences

Jaric et al

Answer 5

In this study the researches measure isometric leg strength in adults, pubescent and pre-pubescent athletes involved in different sports to determine the optimal mass scaling exponent for strength.

• Adult elite athletes & pubescent athletes produced results very close to the expected 0.67 according to dimensionality theory.
• Pre-pubescent athletes this average was 1.14 suggesting that maturational differences can have a significant affect and that the rate of strength development in children before puberty increases at a greater rate than body size
• Children’s strength develops at a greater rate than can be explained purely by increases in body size

Question 6

Nerve Conduction velocity

Answer 6

Nerve conduction velocity is the speed at which motor and sensory impulses traverse a given section of nerve (m/s) and it is influenced by:

• Axonal diameter
  
  • Larger axon -> faster conduction velocity
  • Proximal segments (upper) are faster than distal (lower)
• Local tissue temperature
  
  • A decrease in local muscle temperature will reduce nerve conduction velocity
• Age
  
  • With ageing, adults nerve conduction velocity slows.
  • Children: Nerve conduction velocity increases rapidly in the first year of life.
• Height
  
  • Height is inversely related to nerve conduction velocity in adults.
  • If you’re taller, nerve conduction is slower as it has further to travel

Question 7

Nerve conduction velocity in children & adolescents

Malstrom & Lindstrom, 1997

Answer 7

Increases in nerve conduction velocity between ages 0 and 20 associated with age and muscle diameter (Malstrom & Lindstrom, 1997).

NCV in lower limbs: decreases with age
NCV in upper limbs: small increase with age

Conclusion: Increases in nerve conduction velocity in early childhood likely contribute to the development of strength.

Question 8

Motor unit recruitment and firing rate?

Answer 8

• Number of neurons present by early adulthood already exist at birth/early infancy.
• Intramuscular needle electrodes used in adults to confirm maximal motor unit firing rate decreases with age. Methods generally considered inappropriate for children.

Question 9

Recruitment and firing rates of type II motor units

Answer 9

Do children recruit a smaller percentage of their total motor unit pool? (Asmussen et al., 1955).
- Asmussen’s proposal along with subsequent researchers suggest that children recruit a smaller percentage of their total motor unit pool

In review paper, Dotan et al. (2012) hypothesised that children do not recruit or fully use higher-threshold type II motor units to the extent typical of adults.
- Type II units -> compromised in children

Lexell et al., 1992 - some evidence to suggest lower type II muscle fibre composition in children

Question 10

Transformation of type I to Type II fibres

Answer 10

Gradual increase in motor neuron impulse frequency during maturation may be responsible for increase in utilisation of type II motor units and also transformation of type I to type II fibres during growth (Dotan et al., 2012).

The figures show that the following occur while children age

• Muscle area increases until around the age of 25
• The number of fibres does not change
• The proportion of slow twitch (Type I) decreases which means that the proportion of fast twitch increases. This means that an increase in cross-sectional area of muscle as children age is due to an increase in size of muscle fibres and not the increase in the number of fibres
  
  • Type II muscle fibres increases significantly from the age of five (35%) to the age of twenty (50%).

Question 11

Agonist-antagonist co-contraction

Grosset JF et al

Answer 11

• When assessing force associated with agonist muscles, simultaneous activation of antagonist muscles detracts from the power output of the agonist muscle
• Thus higher agonist-antagonist co-contraction could partly explain why children have lower strength and power
• Some studies have reported greater coactivation in children
• Could greater agonist-antagonist co-contraction in children explain lower strength and power compared with adults?

Question 12

Central Inhibition

Answer 12

• Electrical stimulation used to evoke muscle contraction and compared with maximal voluntary contraction. Any observed difference attributed to central command.
• Negative feedback from peripheral receptors and/or inhibitory signals at the level of the motor cortex may limit force production.

Blimkie (1989) reported that motor unit activation (determined by twitch interpolation technique) for knee extensors was significantly higher in 16 yr olds compared with 10 yr olds.
- In children is there more central inhibition to prevent true maximal contraction in order to prevent muscle damage

Question 13

Development of muscular strength in children

Answer 13

Muscle cross-sectional area
- Is responsible for some but not all strength development

Nerve conduction velocity?
- Does influence strength development at some level

Recruitment of type II motor units?
- Yes in theory but more evidence is needed

Transformation of type I to type II fibres?
- Some theories that this is the case and some evidence to suggest it does

Agonist—antagonist co-contraction?
- Yes may contribute. But most of the evidence is for dynamic contraction and there appears to be less difference in children and adults in isometric contractions

Central inhibition?
- Yes some evidence of central inhibition in young children that is attenuated as they grow older and therefore the removal of that central inhibition contributes to strength development

Question 14

Muscle Damage

Answer 14

Children appear to experience less muscle damage from intense exercise

• Disruption of fibres and inflitration of inflammatory cells
• Release of creatine kinase into bloodstream (children produce less)
• Delayed onset muscle soreness (DOMS)
• Diminished strength
• Reduced range of motion

Question 15

Power duration curve (Graph)

Answer 15

There is considerable overlap between aerobic and anaerobic metabolism during exercise with almost all intensities and duration.

• Anaerobic metabolism responds rapidly to the energy requirement of exercise and is capable to satisfying the needs of very high intensity work
• Aerobic metabolic processers act more slowly to exercise but provide greater energy to sustained work.
  • 15 sec short burst of maximal exercise -> 88% anaerobic and 12% aerobic
  • 60-75 sec all out test -> equal between anaerobic and aerobic
  • 4 minutes -> 80% aerobic metabolism

Question 16

Habitual activity in children (Bailey et al., 1995)

Answer 16

• Over a 12-hr period the average duration of intense exercise in 6-10 yr old children is 3 seconds (Bailey et al., 1995).
• Average duration of all activities, regardless of intensity, was 6 s.
• 6 to 16 -> energy expenditure decline by almost 50%. However, anaerobic glycolytic capacity appears to increase as children grow.

Question 17

Assessment of ‘Anaerobic Fitness’

Answer 17

• Measurement of anaerobic enzymatic function and substrate ultilisation during anaerobic exercise not feasible.
• Measuring levels of biochemical markers of anaerobic glycolysis, particularly blood lactate.
  
  • Accuracy is not certain since it can be influenced by elimination rate metabolism and cell membrane transfer
• Oxygen deficit/debt
• Power production during
  
  • Wingate or treadmill exercise.
    + Such test can be limited by the proportion of anaerobic glycolysis involved
    + 30 sec Wingate is somewhere between 15 sec activity (88% anaerobic) and 60 sec activity (50% anaerobic)
• Field tests, e.g. sprints
  
  • Short term exercise fitness is not necessarily with anaerobic metabolic capacity

Question 18

Lab test: Wingate

Answer 18

• 30-s all out cycle test
• 2 indicators: peak power (usually obtain within the first 5 sec) & mean power output
• Absolute values of anaerobic power increase during childhood.
  
  • 30 sec duration is not enough to measure total anaerobic capacity and it is long enough for significant contribution from aerobic metabolism. The duration of a Wingate is a compromise between the two disadvantages

Question 19

Increase in power during childhood

Answer 19

• Between the ages of 12 and 17 peak aerobic power increases by 120% in males and around 65% in females. Most data indicates that anaerobic power adjusted for body mass also rise during childhood indicating that anaerobic fitness increases at a greater rate than can be explain by body mass alone
• Techniques for assessing lean leg volume in adults are not appropriate for children
• 1) anaerobic power improves during the growing years and 2) something beyond body or muscle size accounts for these improvements

Question 20

Mechanisms for increase in power

Answer 20

Mass independent influence on aerobic power -> 30 to 40% of the risk in peak and mean values between the ages of 8 and 14 years