Growth

1
Growth Developmentof Skeletal Muscle
2
Skeletal, Striated, Voluntary Muscle
3
6 weeks from conception
4

• At the end of week 3, the intra-embryonic
  mesenchyme differentiates into three loose
  aggregate pairs of mesenchyme on each side of the
  neural tube (Paraxial, Intermediate, Lateral)
• Paraxial mesoderm differentiates into the future
  dermatome (dorsal surface), myotome (middle
  layer), and sclerotome (ventral layer), forming
  dermis, muscle, and connective tissue
  respectively.
• Intermediate mesoderm, will form the future
  urogenital system.
• Lateral mesoderm will develop into future body
  cavities and parts of the body wall.

5
Somite Development
Dorsal View of an Embryo at about 22 days (8
somite stage)

• The paraxial mesoderm will develop into paired
  cuboidal bodies, or somites. These will
  eventually develop into the bones (sclerotome),
  muscles (myotome), and dermis (dermatome) of and
  surrounding the axial skeleton.
• Somites appear as bumps on the dorsal surface of
  the embryo.
• At the end of week 3, 4-12 somites are present By
  the end of week 5, 42-44 can be counted. However,
  most appear between days 20-30, giving this
  period the title of the somite period of
  development.
• Somites appear cranially to caudally, beginning
  at the occipital end. They can be counted and are
  used to roughly estimate the age of the embryo.
• 5 - 7 coccygeal pairs disappear leaving 37 pairs
  of somites.

6
Law of Original Innervation
The myoblasts (future muscle cells) form
concurrently with the spinal nerves and they
migrate out from the notochord together. This
results in the formation of 31 spinal nerves with
associated skin, muscle, and connective tissue.
7
Dermatome an area of skin receiving mesenchyme
from a specific somite that is supplied by a
single spinal nerve and its ganglion
8
(No Transcript)
9
(No Transcript)
10
6 weeks
8 weeks
11
Mesenchyme
Myoblasts
Myotubules
Single Muscle Fibre
Peripheral Nuclei
12
Myogenesis

• Mesoderm pluripotent connective tissue cell
• Presumptive myoblast undergoing mitosis,
  mononucleated cell incapable of fusion or
  contractile protein synthesis
• Myoblast mononucleated cell not undergoing
  mitosis, cell capable of fusion and of
  synthesizing myofibrillar proteins
• Myotubule multinucleated cell from fusion of
  myoblasts, may contain sarcomeres depending on
  stage. Nuclei at centre in early stages migrating
  to periphery as matures to muscle fibre
• Muscle fibre mature multinucleated muscle cell
  with myofibrils

13
Growth of Muscle

• Muscle fibre number increases prenatally and for
  a short time postnatally
• Fiber number doubles between 32 weeks gestation
  and 4 months of age
• Girth and length increases continue into
  postnatal period
• Postnatal increase in muscle girth due almost
  entirely to hypertrophy not hyperplasia
• Fibres increase length by
• increase in of sarcomeres (major)
• increase in length of sarcomeres
• length increase primarily at musculotendinous
  juncton in respone to functional length

14
Muscle Composition

• FETUS - fibres small in number and widely
  separated by extracellular material
• TERM - still small, greater number, more closely
  packed
• ADULT - Larger diameter with little space between
  them
• Therefore
• decrease in sodium and chloride (extracellular)
• increase in potassium (intracellular)
• decrease in water content

15
Changes in Muscle Compositionin Water, Sodium
(Na), Chloride (Cl) and Potassium (K) as a
percentage of adult levels from 13 weeks of
gestation to Adulthood
16
Muscle Mass Estimated from Creatinine Clearance
Creatinine levels sensitive to diet and exercise
17
K40 estimate of Fat Free Mass

• Assumed constant proportion of potassium in Fat
  Free Mass
• Average values acceptable, whereas indivudual
  estimates have considerable error due to
  variability in proportion of potassium in Fat
  Free Mass

18
(No Transcript)
19
Estimated Muscle Massas a Percentage of Body
Weight (data collected from various sources)
20
Strength Differences

• No gender difference in strength if expressed per
  unit of cross-sectional area of muscle
• Disproportionate strength increase in male
  adolescence more in upper extremities than trunk
  or lower extremities
• No significant difference (7-17 years) in lower
  extremity strength after adjusting for height,
  between boys and girls

21
Peak Strength Velocityoccurs after Peak Height
Velocity
22
Muscle Width(distance and velocity curves)

• Peak velocities occur later than peak height
  velocity in boys and girls

23
Strength Maturity not reached until late
twenties
24
Resistance Training

• Children can weight train with individual
  monitoring
• Effort/Benefit ratio is very high prior to
  puberty
• Training prior to puberty has lasting effect

25
Fibre Typing

• Type I
• Red (Slow Twitch Fibres)
• Type II (IIa IIb)
• White (Fast Twitch Fibres)
• High proportion Type I and undifferentiated Type
  IIc fibres during early and mid-childhood in
  comparison to adulthood
• Little known about sex associated differences in
  fibre type distribution

26
Dahlström, M. (Karolinska Institute)The dancer
Physical effort, muscle fibre types, and energy
intake and expenditure

• Muscle fibre type composition in young dancers,
  even at the very beginning of their professional
  dance training, differs from that in the average
  individual and is characterised by a high of
  type I fibres, similar to that found in 20 years
  old dancers.
• This, together with the fact that detraining did
  not change the muscle fibre type composition,
  supports the idea that the high percentage of
  type I fibres in dancers is due to a selection of
  individuals with suitable muscle fibre
  composition to the dance profession rather than
  being an effect of training.