Lecture 4
Biomechanics of Resistance Exercise
MUSCULOSKELETAL SYSTEM
Skeleton
Muscles function by pulling against bones that
rotate about joints and transmit force through the
skin to the environment.
The skeleton can be divided into the axial skeleton
and the appendicular skeleton.
Skeletal Musculature
A system of muscles enables the skeleton to move.
Origin = proximal (toward the center of the body)
attachment
Insertion = distal (away from the center of the body)
attach-ment
KEY TERMS
 agonist: The muscle most directly
involved in bringing about a movement;
also called the prime mover.
 antagonist: A muscle that can slow down
or stop the movement.
 FA = force applied to the lever; MAF =
moment arm of the applied force; FR = force
resisting the lever’s rotation; MRF = moment
arm of the resistive force.
KEY TERM
 first-class lever: A lever for which the
muscle force and resistive force act on
opposite sides of the fulcrum.
O = fulcrum;
FM = muscle force;
FR = resistive force;
MM = moment arm of the muscle
force;
MR = moment arm of the resistive
force.
Mechanical advantage = MM /MR
= 5 cm/40 cm = 0.125, which,
being less than 1.0, is a
disadvantage.
● The depiction is of a first-class
lever because muscle force and
resistive force act on opposite
sides of the fulcrum. During
isometric exertion or constantspeed joint rotation,
FM · MM = FR · MR
●Because MM is much smaller
than MR, FM must be much greater
than FR; this illustrates the
disadvantageous nature of this
arrangement.
KEY TERM
second-class lever: A lever for which the
muscle force and resistive force act on the
same side of the fulcrum, with the muscle
force acting through a moment arm longer
than that through which the resistive force
acts. Due to its mechanical advantage, the
required muscle force is smaller than the
resistive force.
A SECOND-CLASS LEVER (THE FOOT)
Figure 4.4 (next slide)
The slide shows plantarflexion against resistance
(e.g., a standing heel raise exercise).
FM = muscle force; FR = resistive force; MM =
moment arm of the muscle force; MR = moment
arm of the resistive force.
When the body is raised, the ball of the foot, the
point about which the foot rotates, is the fulcrum
(O).
Because MM is greater than MR, FM is less than
FR.
The slide shows plantarflexion
against resistance (e.g., a
standing heel raise exercise).
–FM = muscle force; FR = resistive
force; MM = moment arm of the
muscle force; MR = moment arm
of the resistive force.
–When the body is raised, the ball
of the foot, the point about which
the foot rotates, is the fulcrum
(O).
–Because MM is greater than MR,
FM is less than FR.
KEY TERM
 third-class lever: A lever for which the
muscle force and resistive force act on the
same side of the fulcrum, with the muscle
force acting through a moment arm shorter
than that through which the resistive force
acts. The mechanical advantage is thus less
than 1.0, so the muscle force has to be
greater than the resistive force to produce
torque equal to that produced by the
resistive force.
A THIRD-CLASS LEVER (THE FOREARM)
 Figure 4.5 (next slide)
 The slide shows elbow flexion against
resistance (e.g., a biceps curl exercise).
 FM = muscle force; FR = resistive force;
MM = moment arm of the muscle force; MR
= moment arm of the resistive force.
 Because MM is much smaller than MR,
FM must be much greater than FR.
 –The slide shows elbow
flexion against resistance
(e.g., a biceps curl exercise).
 –FM = muscle force; FR =
resistive force; MM =
moment arm of the muscle
force; MR = moment arm of
the resistive force.
 –Because MM is much
smaller than MR, FM must
be much greater than FR.
–(a) The patella increases the mechanical advantage of the
quadriceps muscle group by maintaining the quadriceps tendon’s
distance from the knee’s axis of rotation.
–(b) Absence of the patella allows the tendon to fall closer to the
knee’s center of rotation, shortening the moment arm through which
the muscle force acts and thereby reducing the muscle’s mechanical
advantage.
 EXAMPL
 The patellofemoral joint reaction force C is
the equilibrant to the force of the
quadriceps muscles. For reasonable
assumption, we may consider the patella
as a moveable pulley, and the tendinous
attachments to the patella as the two
supporting strands (figure 1), thus Fm and
Ft are equal in magnitude.
 Q: In the position shown in the
figure, how much force develops the
quadriceps muscles when 158.9 lb
compression force acts on the
patellofemoral joint?
MOMENT ARM AND MECHANICAL
ADVANTAGE
 Figure 4.7 (next slide)
 During elbow flexion with the biceps
muscle, the perpendicular distance from the
joint axis of rotation to the tendon’s line of
action varies throughout the range of joint
motion.
 When the moment arm (M) is shorter,
there is less mechanical advantage.
As a weight is lifted, the moment arm
(M) through which the weight acts, and
thus the resistive torque, changes with
the horizontal distance from the weight
to the elbow.
MUSCULOSKELETAL SYSTEM
 Variations in Tendon Insertion
 tendon insertion: The points at which
tendons are attached to bone.
 Tendon insertion farther from the joint
center results in the ability to lift heavier
weights.
 This arrangement results in a loss of
maximum speed.
 This arrangement reduces the muscle’s
force capability during faster movements.
MUSCULOSKELETAL
SYSTEM
 Anatomical Planes of the Human Body
 The body is erect, the arms are down at
the sides, and the palms face forward.
 The sagittal plane slices the body into leftright sections.
 The frontal plane slices the body into
front-back sections.
 The transverse plane slices the body into
upper-lower sections.
HUMAN STRENGTH AND
POWER
Basic Definitions
 strength: The capacity to exert force at any
given speed.
 power: The mathematical product of force
and velocity at whatever speed.
HUMAN STRENGTH AND
POWER
 Biomechanical Factors in Human Strength
 Neural Control
 Muscle force is greater when: (a) more motor units are
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involved in a contraction, (b) the motor units are greater in
size, or (c) the rate of firing is faster.
Muscle Cross-Sectional Area
The force a muscle can exert is related to its cross-sectional
area rather than to its volume.
Arrangement of Muscle Fibers
Variation exists in the arrangement and alignment of
sarcomeres in relation to the long axis of the muscle.
KEY TERMS
 pennate muscle: A muscle with fibers that
align obliquely with the tendon, creating a
featherlike arrangement.
 angle of pennation: The angle between
the muscle fibers and an imaginary line
between the muscle’s origin and insertion;
0° corresponds to no pennation.
EXAMPLE
 The role of the
Gastrocnemius is to elevate
the foot (known as plantar
flexion). The medial and
lateral parts of the
Gastrocnemius generate the
same tension and both tend
to elevate the tendon.
 How much force in each
part of the Gastrocnemius if
we know that the angle of
pennation of the muscular
fibers is 60°and the
Achilles tendon tension is
400 N?
KEY TERM
 concentric muscle action: A muscle action in
which the muscle shortens because the contractile force is greater than the resistive
force. The forces generated within the
muscle and acting to shorten it are greater
than the external forces acting at its
tendons to stretch it.
KEY TERM
 eccentric muscle action: A muscle action in
which the muscle lengthens because the
contractile force is less than the resistive
force. The forces generated within the
muscle and acting to shorten it are less
than the external forces acting at its
tendons to stretch it.
KEY TERM
 isometric muscle action: A muscle action in
which the muscle length does not change
because the contractile force is equal to the
resistive force. The forces generated within
the muscle and acting to shorten it are
equal to the external forces acting at its
tendons to stretch it.
Center of Gravity
 What is the center of gravity?
 • the point around which a body’s weight is
equally balanced in all directions
 • also referred to as the center of mass or mass
centroid
 • (need not be physically located inside of a body)
Center of Gravity
 Why is the center of gravity of interest in the study
of human biomechanics?
 • it serves as an index of total body motion
Center of Gravity
 Why is the center of gravity of interest in the study
of human biomechanics?
 • the body responds to external forces as though
all mass were concentrated at the CG; this is
consequently the point at which the weight vector
is shown to act in a free body diagram
LOCATING THE HUMAN BODY
CENTER OF GRAVITY
 Locating the CG for a body
containing two or more
movable, interconnected
segments is more difficult than
doing so for a non-segmented
body, because every time the
body changes configuration, its
weight distribution and CG
location are changed. Every
time an arm, leg, or finger
moves, the CG location as a
whole is shifted at least slightly
in the direction in which the
weight is moved
 Question: The x,y-coordinates of the CGs of the
upper arm, forearm, and hand segments are
provided on the diagram below. Use the
segmental method to find the CG for the entire
arm, using the data provided for segment masses
from Appendix D.
Stability and Balance
 What is stability?
resistance to disruption of equilibrium: Stability is
defined mechanically as resistance to both linear
and angular acceleration, or resistance to
disruption of equilibrium.
 What is balance?
ability to control equilibrium: An individual’s ability
to control equilibrium is known as balance.
Stability and Balance
 What is the base of support? (area bound by the
outermost regions of contact between a body and
support surface(s))
Stability and Balance
 What can increase a body’s stability?
 • increasing body mass
 • increasing friction between the body and the
surfaces of contact
Stability and Balance
 What can increase a body’s stability?
 • increasing the size of the base of support in the
direction of an external force
Stability and Balance
 What can increase a body’s stability?
 • horizontally positioning the center of gravity
near the edge of the base of support on the side of
the external force
Stability and Balance
 What can increase a body’s stability?
 • vertically positioning the center of gravity as low
as possible
 The higher the CG, the greater the amount of
torque its motion creates about the support
surface.
HUMAN STRENGTH AND POWER
 Biomechanical Factors in Human Strength
 Strength-to-Mass Ratio
 In sprinting and jumping, the ratio directly reflects an
athlete’s ability to accelerate his or her body.
 In sports involving weight classification, the ratio helps
determine when strength is highest relative to that of other
athletes in the weight class.
 As body size increases, body mass increases more rapidly
than does muscle strength.
 Given constant body proportions, the smaller athlete has a
higher strength-to-mass ratio than does the larger athlete.
CAM-BASED WEIGHT-STACK MACHINES
 Figure 4.14 (next slide)
 In cam-based weight-stack machines, the
moment arm (M) of the weight stack
(horizontal distance from the chain to the
cam pivot point) varies during the exercise
movement.
 When the cam is rotated in the direction
shown from position 1 to position 2, the
moment arm of the weights, and thus the
resistive torque, increases.
 In cam-based weight-
stack machines, the
moment arm (M) of the
weight stack (horizontal
distance from the chain
to the cam pivot point)
varies during the
exercise movement.
 –When the cam is
rotated in the direction
shown from position 1 to
position 2, the moment
arm of the weights, and
thus the resistive
torque, increases.
 The force of muscular tension is resolved into two force components
one perpendicular to the attached bone and one parallel to the bone .
1- The rotary component: The component of muscle force acting
perpendicular to the bone that causes the bone to rotate about the
joint center. 2- Dislocating component: The component of muscle
force directed parallel to the bone that pulls the bone away from
the joint center. 3- Stabilizing component: The component of
muscle force directed parallel to the bone that pulls the bone
toward the joint center. Nota: a- Depending on whether the angle
between the bone and the attached muscle is less than or greater
than 90°. b- The angle of maximum mechanical advantage for
any muscle is the angle at which the most rotary force can be
produced.