Biomechanics of Resistance
Exercise
ES 342
S i 2010
Spring
Plans for the next two weeks:
 Pushing projects to 4/1
 No, I’m not kidding.
 We will be in the weight room in NIN Hall for every class
periodd untill then
h
 MAY not have class on 3/18. You still must show up to find
out I am not cancelling outright just yet.
out.
yet
Key Point
 Specificity
S ifi it is
i a major
j consideration
id ti when
h one is
i
designing an exercise program to improve
performance in a particular sport activity.
activity The
sport movement must be analyzed qualitatively or
qquantitativelyy to determine the specific
p
joint
j
movements that contribute to the whole-body
movement. Exercises that use similar joint
movements are then emphasized in the resistance
training program.
Musculoskeletal System
 Skeleton
 Skeletal Musculature
 Muscles function by
 Origin = proximal
(toward the center of the
ppullingg against
g
bones
body) attachment
 Bones rotate about joints
 Insertion = distal (away
 Forces are transmitted
from the center of the
through the skin to the
body) attachment
environment
 Can be divided into
 axial skeleton
 appendicular
pp
skeleton..
Figure 4.1
41
Major Body Movements
 Planes of movement are relative to the body in the anatomical position
unless
l otherwise
h i statedd
 Common exercises that provide resistance to the movements and related
sport activities are listed in the coming slides
 Simple
Si l rules:
l
 Flexion is any movement that takes us away from the anatomical position in




the sagittal plane
Extension is anyy movement that takes us towards the anatomical position
p
in
the sagittal plane
Abduction is any movement that takes us away from the anatomical position
in the frontal plane
Adduction is any movement that takes us toward the anatomical position in
the frontal plane
Rotation occurs in the transverse plane if we begin the movement in the
anatomical position—This movement, especially can occur outside the
transverse plane!
l !
Figure 4.16
4 16
Reprinted, by permission, from Harman, Johnson, and Frykman, 1992.
Figure 4.16
4 16 (continued)
Reprinted, by permission, from Harman, Johnson, and Frykman, 1992.
Key Terms
 agonist: The muscle most directly involved in bringing about
a movement; also called the prime mover.
 antagonist: A muscle
l that
h can slow
l ddown or stop the
h
movement.
 synergist: A muscle that can assist the agonist.
A Lever
 Body movements primarily act through the bony levers of the
skeleton
 The lever applies a force on the object equal in magnitude to
but opposite in direction from FR
Key Term—Mechanical
Term Mechanical Advantage
 Ratio of:
 The moment arm through which an applied force acts to
 The moment arm through
g which a resistive force acts
 A mechanical (dis)advantage:
 Greater than 1.0 allows the applied (muscle) force to be less
than the resistive force to produce an equal amount of torque
 Less than 1.0
1 0 is a disadvantage in the common sense of the term
E
Example
l off Mechanical
M h i l (Di
(Dis)advantage
) d
t g
 MM /MR = 5 cm/40 cm = 0.125
 < 1.0  mechanical disadvantage.
A First-Class
First Class Lever (the Forearm)
 Muscle force and resistive
force act on opposite sides of
the fulcrum
 Because MM is much
h smaller
ll
than MR, FM must be much
greater than FR
 Mechanical Disadvantage
A Second-Class
Second Class Lever (the Foot)
 Muscle force and resistive force act on
the same side of the fulcrum
 Muscle force acts through a moment
arm longer than that through of the
resistive force
 Due to mechanical advantage, the
required muscle force is smaller than
the resistive force.
 Because MM is greater than MR, FM is
l than
less
h FR.
A Third-Class
Third Class Lever (the Forearm)
 Muscle force and resistive force act
on the same side of the fulcrum
 Muscle force acts through a moment
arm shorter than that through of the
resistive force
 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
 Because MM is much smaller than MR,
FM mustt be
b muchh greater
t th
than FR
Th P
The
Patella
t ll and
d Mechanical
M h i l Advantage
Ad
t g
 (a) Having a patella increases the mechanical advantage of the quadriceps
muscle
l group
 maintains the quadriceps tendon’s distance from the knee’s axis of rotation
 (b) No patella means the tendon falls closer to the knee’s
knee s center of rotation
 shortens the moment arm through which the muscle force acts and decreases
the muscle’s mechanical advantage
Moment Arm of the Muscle &
M h i l Ad
Mechanical
Advantage
t g
 The distance from the joint axis of rotation to the tendon’s
line of action varies throughout the joint’s ROM
 When the moment arm (M) is shorter, there is less
mechanical advantage
Moment Arm of the Resistance
 The moment arm (M) through
which the weight acts changes
with the horizontal distance
from the weight to the elbow
Key Point
 Most off the
h skeletal
k l l muscles
l operate at a considerable
d bl mechanical
h
l
disadvantage. Thus, during sports and other physical activities,
forces in the muscles and tendons are much higher than those
exerted by the hands or feet on external objects or the ground.
What does this mean for people with
l g llevers?
longer
?
Variations in Tendon Insertion
 The points at which tendons are attached to bone
 Farther from the joint center results in the ability to lift heavier
weights
g
 Arrangement  loss of maximum speed
 Arrangement reduces the muscle’s force capability during faster
movements
Tendon Insertion and Joint Angle
g
 The slide shows changes in joint
angle with equal increments of
muscle shortening
 tendon is inserted (a) closer to joint
center
t
 (b) farther from the joint center
 Configuration (b) has a larger
moment arm
  greater torque for a given
muscle force
  less rotation per unit of muscle
contraction
  slower movement speed
Musculoskeletal System
 Sagittal plane slices the body into
left-right sections
 Frontal plane slices the body into
f t b k sections
front-back
ti
 Transverse plane slices the body
into upper-lower sections
Human Strength and Power
 Strength: The capacity to
exert force at any given
speed
 Power: The mathematical
product of force and
velocity at whatever
speed
Factors in Human Strength
g
 Neural Control
 more motor units are involved in a contraction
 the motor units are greater in size
 the rate of firing is faster.
faster
 Muscle Cross-Sectional Area
 Force a muscle can exert is related to its cross
cross-sectional
sectional area
 Volume is not the same as CSA
 Arrangement
g
of Muscle Fibers
 Pennation
 Greater angles  increased force production
Muscle Fiber Arrangements
 Muscle fiber arrangements and an example of each
Factors in Human Strength (cont)
 Muscle Length
 At
A resting
i llength:
h
 actin and myosin filaments lie next to each other
 maximal number of potential cross-bridge sites are available
 the muscle can generate the greatest force
 When stretched:
 a smaller proportion of the actin and myosin filaments lie next to each other
 fewer potential cross-bridge sites are available
 the muscle cannot generate as much force
 When shortened:
 the actin filaments overlap
 the number of cross-bridge sites is reduced
 there is decreased force g
generation capability
p
y
Muscle Length and Actin
and
d Myosin
M i Interaction
I t
ti
 Interaction between actin and
myosin filaments
 Muscle is at its resting length
 Muscle is contracted or
stretched
 Muscle force capability
p
y is
greatest when the muscle is at
its resting length because of
i
increased
d opportunity
t it ffor actinti
myosin cross-bridges
Factors in Human Strength
 Joint Angle
 Resistance arm changes
h
dduring joint rotation
 Force arm changes during joint rotation
 Actin-myosin
y
cross-bridge
g interactions change
g duringg joint
j
rotation
 Muscle Contraction Velocity
N
Nonlinear
li
 Generally, ability of a muscle to generate force declines as the
velocity of contraction increases
 Joint Angular Velocity
 There are three types of muscle action
Key Term—Joint Contraction Types
 concentric muscle action:
 Muscle shortens because the contractile force is greater than the resistive force
 Forces generated within the muscle and acting to shorten it are greater than the
external forces acting at its tendons to stretch it
 eccentric muscle action:
 Muscle lengthens because the contractile force is less than the resistive force
 Forces generated within the muscle and acting to shorten it are less than the
external forces acting at its tendons to stretch it
 isometric muscle action:
 Muscle length does not change because the contractile force is equal to the
resistive force
 Forces generated within the muscle and acting to shorten it are equal to the
external forces acting at its tendons to stretch it
Force–velocity curve for eccentric and
concentric
t i actions
ti
Factors in Human Strength
 Strength-to-Mass Ratio
 In sprinting and jumping,
jumping the ratio directly reflects an athlete’s
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.
 Body Size
 As body size increases, body mass increases more rapidly than
does muscle strength.
g
 Given constant body proportions, the smaller athlete has a
higher strength-to-mass ratio than does the larger athlete.
Joint Biomechanics:
C
Concerns
iin R
Resistance
i t
TTraining
i i g
 Back
 Back Injury
 The lower back is particularly vulnerable.
 Resistance trainingg exercises should ggenerallyy be performed
p
with the
lower back in a moderately arched position.
 Intra-Abdominal Pressure and Lifting Belts
 The “fluid
fluid ball
ball” aids in supporting the vertebral
ertebral column during resistance
training.
 Weightlifting belts are probably effective in improving safety. Follow
conservative recommendations.
recommendations
Intra abdominal pressure
Intra-abdominal
 Valsalva maneuver
 Glottis is closed keeping air from
escaping the lungs
 Muscles of the abdomen and rib cage
contract
 Creates rigid compartments of liquid in the
lower torso
 Keeps air in the upper torso
 The
Th “fl
“fluid
id bball”
ll” resulting
l i ffrom
contraction of the deep abdominal
muscles and the diaphragm
p g
Joint Biomechanics:
C
Concerns
iin R
Resistance
i t
TTraining
i i g
 Shoulders
 Prone to injury during weight training
 Poor structure
 Unique external forces
 Warm
W
up with
i h relatively
l i l lilight
h weights
i h
 Follow a program that exercises the shoulders in a balanced way
 Remember to address both directions in all three planes
 Example:
l Front Flies
l andd Backk Flies
l
 Maintain controlled speeds of movement
 Knees
 Prone to injury because of its location between two long levers
 Minimize the use of wraps
How Can Athletes Reduce the Risk of
R i t
Resistance
Training
T i i g IInjuries?
j i ?
 Perform
P f
one or more warm-up sets
 relatively light weights
 especially
p
y for exercises that involve extensive use of the shoulder
or knee
 Perform basic exercises through a full ROM
 Use
U relatively
l i l lilight
h weights
i h
 when introducing new exercises
 resumingg trainingg after a layoff
y of two or more weeks
 Do not ignore pain in or around the joints
(continued)
How Can Athletes Reduce the Risk of
R i t
Resistance
TTraining
i i IInjuries?
j i ? (continued)
(
i
d)
 Never attempt lifting
lf
maximall lloads
d without
h proper
preparation
 technique instruction in the exercise movement
 practice with lighter weights
 Performingg several variations of an exercise results in:
 more complete muscle development
 better joint stability
 Take care when incorporating plyometric drills into a training
program
Questions?
Sources of Resistance
t Muscle
to
M l Contraction
C t ti
 Gravity
 Applications to Resistance Training
 When the weight is horizontally closer to the joint, it exerts less resistive
torque
q
 When the weight is horizontally farther from a joint, it exerts more
resistive torque
 Weight-Stack
Weight Stack Machines
 Gravity is the source of resistance, but machines provide increased
control over the direction and pattern of resistance
Sources of Resistance
t Muscle
to
M l Contraction
C t ti (cont)
(
t)
 Inertia
 When a weight is held in a static position or when
it is moved at a constant velocity, it exerts constant resistance
onlyy in the downward direction
 However, upward or lateral acceleration of the weight requires
additional force
 Friction
 Friction is the resistive force encountered when one attempts to
move an object while it is pressed against another object
Sources of Resistance
t Muscle
to
M l Contraction
C t ti
 Fluid Resistance
 Fluid resistance is the resistive force encountered by an object moving
through a fluid (liquid or gas), or by a fluid moving past or around an object
or through an orifice.
 Elasticity
 The more an elastic component is stretched, the greater the resistance.
 Negative Work and Power
 Negative work refers to work performed on, rather than by, a muscle.
 The rate at which the repetitions are performed determines the power
output.
t t