Jackson Pui Man Wai, Ph.D.
533573313
Mechanics affecting muscle tension
Muscle length-tension relationship
The peak tension occurs at slightly greater than resting length
120% resting length, or sarcomere length is about 2.2 microns
Optimal length
All myosin cross-bridges connect with the actins
Shorter length
Myosin cross-bridges interference due to overlapping of actins
Longer length
Some or all actins are beyond the range of the myosin cross-bridges
Muscle velocity-tension relationship
The faster the contraction velocity, the smaller the tension developed
The amount of tension is proportional to the number of cross-bridges actively operating at a given time
(actomyosin formation), a fast movement decreases the probability of such an interplay
Angle of pull
The mechanical leverage (arrangement of the bone and muscle) during a motion affects the muscle tension
and the total work output developed by the muscle
Normally, the muscle tension required in the mid-range of joint angle is the largest comparing to smaller or
larger angles for any given load.
Type of Muscular Contraction
Static/isometric
Muscle produces tension without changing of muscle length
No movement of body segments
Counterbalanced either by an equal or opposite contraction of the antagonists, or by
supramaximal resistance to the muscular motion
Primary for stabilization of one segment in order to allow movement in another
segment, usually seen in maintain posture and balance of body movement.
E.g. fix shoulder joint in order to raise lower arm,
an upright torso: contraction of upper and lower back and abdominal
muscles.
Isometric strength can be assessed by performing a maximum voluntary contraction
(MVC).
Dynamic contraction Muscle produces tension with changing of muscle length
Observed movement of body segments
When a muscle contracts against a constant load/weight, the muscle tension is not
constant.
‘Isotonic contraction’ literally means same or constant (iso) tension (tonic), which
supposedly is one that produces the same amount of tension while shortening as it
overcomes a constant resistance. However, this is not true in a skeletal muscle
system, because the tension exerted by a muscle as it shortens is varying due to the
change of (1) the angle of pull, (2) the speed of shortening, and (3) the length of the
muscle fibers.
Old term ‘Isotonic Contraction’ which is defined as ‘muscle contracts with constant
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Jackson Pui Man Wai, Ph.D.
Concentric
Eccentric
533573313
tension’ is only valid in experimental setting and is more appropriate to be called
“isoweight or isoload contraction” (Wai).
Dynamic strength can be assessed by one repetition maximum (1 RM): the force
produced by a muscle group exerting against a resistance in one maximum effort.
Dynamic contractions can be classified into concentric, eccentric, isokinetic, and
plyometric contractions.
Muscle contracts and shortens.
e.g. elbow flexion, knee flexion, hip flexion, trunk flexion.
Muscle contracts and lengthens during active development of tension.
Movement
biceps is prime mover
biceps
Arm wrestling/pulling (draw)
isometric
Arm wrestling/pulling (winner)
concentric
Arm wrestling/pulling (loser)
eccentric
Lifting an object
concentric
Lowering an object
eccentric
quadriceps is prime mover
walking upstairs
concentric
walking downstairs /downhill
eccentric
Isokinetic
Muscle contracts at constant angular velocity controlled by an isokinetic machine
Maximum muscle tension can be developed throughout the joint’s range of motion
(ROM).
The higher the muscle force generated, the higher the resistance encountered.
The resistance produced by the machine is a measure of the muscle torque (torque
= force x displacement of an angular movement).
E.g. Cybex dynamometer, machine that simulate isokinetic movement: hydraulic
resistance machine, variable resistance machine, naturally movement: arm stroke
during swimming can be sensed as isokinetic movement.
Plyometric
Stretching a muscle through an eccentric (lengthening) phase followed by forceful
concentric (shortening) contractions.
Utilize the immediately release of elastic (potential) energy stored during stretched
phase.
E.g. jumping up after jumping from a height, continuing leaping over objects, rope
skipping
A concentric type (isoweight, free weight) weight training can only allow muscle to exert higher force
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Jackson Pui Man Wai, Ph.D.
533573313
(tension) in the mid-range of angle of pull throughout the range of motion therefore training effects will not
be as good as if muscle is required to produce a higher tension throughout the ROM, such as during
isokinetic contraction.
Accommodated/variable resistance weight training
Higher muscle tension can be developed throughout the range of motion
Variable lever arm system
The ratio of weight arm to force arm varies (increases) during exercise
Cam design - an oval shape pulley
Sliding load arm design
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