Download Muscle Structure

Structure & Contraction
• Muscle Belly
• Attached to bone via tendon
• Covered in fascia
• Composed of many fascicles
• Fascicle
• Composed of many bundles of muscle fibers
• Muscle Fiber
• AKA… muscle cell
• Composed of many myofibrils
• Myofibrils
• Organized into many sarcomeres
• Sarcomeres: the contractile unit of muscle
• Contain myofilaments
• Myofilament
• Two varieties:
• Actin (thin)
• Myosin (thick)
Fast Twitch (Type II)
•
•
•
•
•
•
White
More anaerobic
Larger
Fatigue quickly
Fast contraction speed
Type IIa & IIb:
• IIa have more aerobic
ability due to more
capillaries; therefore
fatigue more slowly than
Type IIb
• Utilized in sprinting, jumping,
etc
Slow Twitch (Type I)
• Red
• More aerobic
• Smaller
• Fatigue slowly
• Slow contraction
speed
• Utilized in longdistance, endurance
activities
• Muscles contain both Fast Twitch and Slow Twitch fibers
• Amount varies between muscles & individuals
• Muscle composition is genetic
• Composition cannot be changed by training
An Analogy for Sliding Filaments in a Sarcomere
Imagine that you are standing between two large
bookcases loaded with books. These large bookcases
are several meters apart and are positioned on rails so
that they can be easily moved. You are given the task
of bringing the two bookcases together, but you are
limited to using only your arms and two ropes. Standing
centered between the bookcases, you pull on the two
ropes — one per arm — which are tied securely to
each bookcase. In a repetitive fashion, you pull each
rope toward you, regrasp it, and then pull again.
Eventually, as you progress through the length of rope,
the bookcases move together and approach you. In this
example, your arms are similar to the myosin molecules,
the ropes are the actin filaments, and the bookcases
are the z discs to which the actin is secured, which make
up the lateral ends of a sarcomere. Similar to the way
you would remain centered between the bookcases, the
myosin filaments remain centered during normal muscle
contraction (Figure 2B).
• Sliding filament theory
• Myosin attaches to actin (called crossbridge formation)
• Actin slides over myosin, shortening the sarcomere
• Myosin let’s go, reaches further on actin and forms new
crossbridge
• This process requires ATP!
•
•
Muscles are controlled by nerves
The nerve and all of the muscle fibers it controls is
called a motor unit
•
Precise movements
• Recruit many motor units (1500-3000)
• Each motor unit has few fibers (8-50)
•
Unrefined movements
• Recruit fewer motor units
• Each motor unit has many fibers (600-2000)
• All or None Principle
• When a nerve impulse reaches a certain
magnitude, all fibers in the motor unit will
contract
• If the impulse magnitude is not met, none of the
fibers contract
• The length of the sarcomere can impact its ability to contract with
maximal strength.
• Maximal force developed when an optimal # of crossbridges are formed
• Because a muscle is made of sarcomeres, the same can be true
for the whole muscle.
• Optimal # of crossbridges achieved by:
• Optimal muscle length
• Optimal joint angle
• YOUR TASK: Determine the optimal muscle length for
maximal production of tension.
• When muscle is stretched (long):
• Sarcomeres further apart
• Fewer crossbridges can form
• Myosin can’t reach actin
• Less force is produced
• When muscle is fully contracted (short):
• Crossbridges interfere with one another
• Fewer crossbridges actually form
• Less force produced
Muscle tension during elbow flexion at constant speed
Sport Books Publisher
15
• In addition to sarcomere length, what other
factors affect muscle force production?
• Joint angle
• What length produces the most force?
• Muscle cross-sectional area
• Increased muscle mass leads to increased force production
• Number of motor units recruited
• Age
• Increasing age results in decreased force production… Why?
• Muscle fibers lost due to decreased activity levels, atrophy & apoptosis
• Gender
• Females ~70% as strong as males
• When strength:weight ratio is accounted for females are
just as strong as males
• Male testosterone production allows for more muscle
growth
• Speed of movement
• As speed increases, force developed decreases… Why?
• Crossbridges cannot form & re-form fast enough
• Muscle fiber type
• What type produces the most force?