Mechanics of Contraction

Question 1

The fundamental property of muscle is contractility; that is, the ability to
develop force, to shorten and perform work. Muscle mechanics is concerned
____ +___________

Answer 1

force

length.

Question 2

§ Isometric contraction

Answer 2

• A contraction that occurs when the muscle length is fixed.
• Force is developed but there is no external shortening and hence
no external work is done
• Force of weight = force developed by muscle

Question 3

§ Isotonic contraction

•

Answer 3

A contraction that occurs at a fixed level of force development
is isotonic
• Mismatch between tension generated by the contracting muscle
and the (constant) load on the muscle
• Further divided into: concentric and eccentric

Question 4

§ Concentric contraction

Answer 4

• A contraction that causes muscle to shorten
• Force of load < force developed by the muscle

• Positive work done on external load by muscle

Question 5

§ Eccentric contraction

Answer 5

• A contraction that occurs when the muscle is lengthening
• Muscle injury and delayed onset muscle soreness (DOMS) are
associated with eccentric contractions
• Strengthening exercises are often designed to involve eccentric
contractions. Muscle growth may be aided as a result of the
eccentric muscle damage caused by the release of cytokines
• External load does work on the muscle

Question 6

Tetanus

• Repetitive stimulation enables

Answer 6

myoplasmic calcium to rise to

saturating level and maximum force is developed

Question 7

§ Passive force

Answer 7

• The force required to stretch a muscle to a length where it can
contract

Question 8

§ Active force

Answer 8

• The force generated (above passive force) during a contraction

Question 9

Preload

•

Answer 9

The length of the muscle prior to a contraction (it determined the
isometric force as per the FL relationship)

Question 10

§ Afterload

Answer 10

• The force which the muscle must ‘overcome’ in order to shorten

Question 11

Isometric quick release

Answer 11

• When a contracting muscle is suddenly shortened and clamped
at a new, shorter length
o An experimental approach to measure the velocity of
cross-bridge reattachment

Question 12

§ Isotonic quick release

Answer 12

• When a contracting muscle is suddenly given a lighter load
(rapid drop in afterload) which is then held stable as the muscle
shortens against the lighter but constant new afterload
o An experimental approach to measure shortening
velocity

Question 13

Increasing tension in skeletal muscle:

o Skeletal muscle summation

Answer 13

In skeletal muscle, a single AP (red) leads to release of sufficient
calcium (black) to cause full activation of the contractile apparatus and
the muscle responds with a “twitch” (blue).
§ Because the twitch happens after the refractory period, a series of APs
in skeletal muscle can lead to summation or sustained tetanic
contraction.
§ Mostly all muscle contraction is due to tetani rather than single muscle
twitch

Question 14

Motor unit recruitment
§ Motor units differ in recruitment threshold
size principle:

Answer 14

• Low threshold need low force (weak neural drive) to be activated
• High threshold need high force (strong neural drive) to be
activated
§ Size principle:
• Smallest motor units are recruited first
• Largest motor units are recruited last
§ High force is produced when a large number of motor units are activated

Question 15

Dont see descending limb/ extrmemes of Force-Sarcomere-Length Relationship of Skeletal Muscle bc…

Answer 15

of connective tissue

Question 16

Hill’s Elastic Muscle Model

o

Answer 16

The model is constituted by a contractile element (CE) and two non-linear
spring elements, one in series (SE) and another in parallel (PE).

o The net force-length characteristics of a muscle is a combination of
the active and passive force-length elements
o Parallel elastic component:
§ Represents the passive force of connective tissues (fascia,
epi-,peri-,endo-mysium and SL)
o Series elastic component (SE):
§ no biological equivalent
elasticity

Question 17

Increased tensions in eccentric due to:

Answer 17

• Cross bridge breaking force > holding force at isometric length
• High tendon force to overcome internal damping friction

Question 18

Limitations of Hill model

Answer 18

§ Contractile element was essentially reduced to a ‘black box’
§ Contractile structures identified in the 1950s and 60s did not fit the
model
§ No anatomical structures were identified which fit the series element
§ The model could not accurately predict the timecourse of the rise in
force during an isometric contraction
§ Could not accurately predict the response to quick-release experiments