Muscles Flashcards
Hennemans’s size principle
small motor units within a pool are recruited first
Specific tension
the maximum amount of force exerted by the fibers per unit pf PCSA (capacity of muscle fiber to produce force). Influenced by muscle fiber type (type 2 have higher specific tension.
Effects of velocity on contractile force
Decreased contraction speed can increase the force produced by the muscle as the filaments move past each other at a slower speed meaning there is an increased chance for a cross bridge to be formed.
Factors that contribute to muscle force
PCSA (greatest predictor of muscle force generating capacity) + pennation angle
Specific tension
Neural drive
Type and speed of contraction
Active and passive force
Neuromuscular fatigue
Function of titin
Titin attaches myosin to the z line. Titin contributes to muscle force by storing passive energy and also protects the sarcomere form over stretch.
Passive elastic force
How does PCSA and pennation angle contribute to muscle force
- PCSA = muscle volume/fibre length
- Fibre pennation angle is important as it relates to how much force is being transferred to the tendon. Increases pennation angle = decreased force onto tendon
- Both PSCA and pennation angle will change with contraction
- Fibre length is normally very different to muscle length
- optimal fibre length and sarcomere length occur at same point (greatest force generating capacity)
How does specific tension contribute to muscle force
Specific tension is the maximum number of force exerted by the fibers per unit of PCSA (capacity of muscle fibres to produce force)
- ST depends on muscle fiber makeup, higher specific tension in type 2B muscle fibers
- Postural muscle have more type 1 muscle fibres therefore lower specific tension
Effects that neural drive has on muscle force
Each muscle innervated by motor neuron, they receive input from descending pathways, when enough excitatory input is received they send down an AP. Every AP generated in Mu will generate and AP in muscle fibre resulting in active force production.
Force can be alter by number and discharge rate of single motor units
Smaller motor units are recruited first (type 1), this is because they reach AP threshold first (greater change in membrane potential in smaller units). With greater excitatory input the number of motor units increases and the size of the MU recruited increases. Hennemans size principle.
Effects that type and speed of contraction have on muscle force
Eccentric → isometric → isotonic
Each motor unit discharge produces more torque in eccentric. Therefore eccentric achieves same torque with least amount of MU achieve.
Actin and myosin hold on longer for each AP in eccentric + more stretch of elastic component (titin) when lengthening
With increasing rate of change of muscle length, the less force produced.
Force producing capacity depends on total number of attached cross bridges, as filaments slide past each other at faster rates fewer cross bridges are able to attach.
Hill mechanical models and how active and passive structures contribute to force
Hills mechanical model of the muscle tendon unit
Contractile component CC - muscle fibers, actin and myosin cross bridges
Series elastic component SEC - intracellular titin, tendon
Parallel elastic component PEC - connective tissue (epi, peri and endo + titin)
The stretch shortening cycle occurs due to both active and passive reasons
When a muscle tendon unit is stretched just prior to contraction the resulting contraction is more forceful than in absence of pre stretch
- Elastic recoil of SEC
- Stretch reflex of lengthened muscle
Concept of neuromuscular fatigue
Has components in the peripheral (muscle) and central (brain, spinal cord). Most common with eccentric exercises. Some sarcomeres resist the stretch more than others causing some overstretch and disrupted → membrane damage.
Peripheral neuromuscular fatigue
Peripheral = failure to produce force appropriate to drive:
- Neuromuscular junction, reduce relase of Ach (AP not initiated along muscle)
- Muscle cell membrane potential changes
- Reduce Ca release for excitation contraction coupling
- Accumulation of metabolites (lactate and H will change pH= alter cell membrane)
- Depletion of fuels such as ATP which allows for CB cycle
Central neuromuscular fatigue
Central = failure to drive motoneurons adequately (cant get them to AP threshold):
- Lower excitatory drive from cortex (motivation, pain ect)
- Increased inhibitory input
- Decreased muscle spindle input
2 main factors that can calculate total muscle force
Total muscle force = PCSA x specific tension
Maturation in embryo
0-8 weeks = embryo
week 3 = trilaminar (3 layers germ disc) ectoderm (neural), mesoderm (muscles + connective tissues), endoderm (epithelia)
9-38 weeks = fetus, period of functional maturation
Somites, adjacent to the neural tube differentiate to form dermatome (skin), myotome (muscles) and sclerotome (connective tissue)
