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1
Biomechanics of Human Movement
SCOTT K. LYNN GUILLERMO J. NOFFAL
OBJECTIVES
After completing this chapter, you will be able to:
Comprehend units of biomechanical measurements.
Apply velocity and joint angle speciﬁ city to training.
Understand the length–tension and force–velocity–power relationships.
Conceptualize Newton’s laws of motion and apply them to training.
Evaluate and compare different modes of resistance.
KEY TERMS
Acceleration
Angular Motion
Biomechanics
Balance
Displacement
Distance
Force
Friction
Gravity
Inertia
Length
Mass
Momentum
Power
Rotary Inertia
Stability
Stretch-Shortening Cycle
Time
Torque
Velocity
Weight
Work
CHAPTER 5
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2PART 1 Basic Science
Introduction
Biomechanics has been deﬁ ned as “the study of the structure and function of biological
systems using the means and methods of mechanics” (1). This deﬁ nition divides the
word biomechanics into two parts: bio (biological system) and mechanics. In the ﬁ eld
of strength and conditioning, the biological system that we are most concerned with is
the human body’s musculoskeletal system. This involves all tissues directly involved with
producing, preventing, or inﬂ uencing movement (muscles, bones, ligaments, tendons,
cartilage, etc.). Also, “mechanics” is deﬁ ned as the study of the inﬂ uence of force on
bodies. Therefore, this chapter examines how we can manipulate forces in a strength
and conditioning setting to produce the desired effect on the structures (tissues) and
functions (movements) of the human musculoskeletal system.
Biomechanics is the science of applying mechanical principles to biological systems such as the
human body.
For the strength and conditioning specialist, a basic knowledge of biomechanics is
essential in order to be able to evaluate human movement and then be able to design
and prescribe appropriate movements (exercises) aimed at increasing the overall
efﬁ ciency of movement. Forces are required to produce any type of human movement
and there are various different types of forces and aspects of those forces that must be
considered by the strength and conditioning specialist. A sound knowledge of basic
mechanical principles will allow for the prescription of appropriate movements at the
appropriate intensity to produce the desired movement outcomes without increasing the
chance for injury (acute or chronic) of any of the movement structures/tissues.
This chapter will be divided into the following sections:
1. Basic biomechanics: Biomechanical concepts essential to the strength and condi-
tioning specialist will be deﬁ ned.
2. Human musculoskeletal mechanics: Mechanical characteristics of the human mus-
culoskeletal system that affect movement will be discussed.
3. Biomechanics of resistance: An examination of the biomechanics of various forms of
resistance used in a strength and conditioning setting.
4. Progressing/regressing movement: An application of how a basic knowledge of
biomechanics can be used to make an exercise easier or harder to suit different
individuals.
It should be noted that many terms used in this chapter are not used according to
their strict mechanical deﬁ nition but have been simpliﬁ ed so that their applications in a
strength and conditioning setting can be more clearly understood.
BASIC MECHANICS
Biomechanics can be simply deﬁ ned as the effect
of forces on the structure and function of living
systems. In the ﬁ eld of strength and conditioning,
there are several mechanical concepts that must
be understood in order to fully comprehend how
to most effectively and safely achieve our training
goal. If we think of the simple example of some-
one lifting a barbell, some of the key mechanical
concepts include force, distance, speed, inertia,
mass, weight, velocity, acceleration, torque, power,
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CHAPTER 5 Biomechanics of Human Movement
and momentum. Many of these are derived from
three basic variables—length, time, and mass (2).
The basic unit of time is measured in either sec-
onds, minutes, or hours; however, since many
sporting or lifting movements are short in dura-
tion they are most often measured in seconds. The
basic dimension of length sometimes also known
as space is measured in inches, feet, and yards in
the United States, but the scientiﬁ c community has
adopted the metric system, which utilizes centi-
meters, meters, kilometers, and so on. Lastly, the
basic dimension of mass is commonly measured
in kilograms.
To illustrate many of these biomechanical prin-
ciples, we will examine one of the most basic of
exercises, the bench press. First, in the process of
lowering the bar to your chest or pushing it back
up, the bar moves through space in a fairly straight
line. Naturally, if you are a tall individual with long
arms, you will be moving it a greater distance than
someone with short extremities. Thus, distance
is deﬁ ned as the total path traveled by the bar.
Displacement is deﬁ ned as a straight line between
where the movement started and where the move-
ment ended. Although there is a difference between
the two terms (distance vs. displacement) they are
most often used interchangeably to describe how
far the object has traveled. So, in the bench press
example, if a beginner lifter struggles to push the
weight up and it does not take a direct line from
his or her chest to the ﬁ nish position, he or she
will have pushed the bar a greater distance than a
more experienced individual who is able to push
it straight up (Fig. 5.1). For the more experienced
lifter, the movement was much more efﬁ cient as
the distance the bar traveled was much smaller.
Similarly, if the beginner lifter took much lon-
ger to push the bar up, he or she would also have
a lower bar velocity (often referred to as speed),
which is deﬁ ned as distance divided by time.
Velocity can be measured in any unit that divides
a measure of distance by a measure of time. The
most common units used to measure velocity
include m · s−1, km · h−1, mi · h−1. Therefore, in
order to calculate velocity you must measure the
distance the object moved and the time it took to
cover that distance. Distance and time can be mea-
sured is several different ways, these include the use
of a tape measure and timing gates, video equip-
ment, or electronic transducers.
Another illustration of velocity is the example
of a sprinter ﬁ nishing the 100-m race in 10 seconds.
This sprinter has achieved an average velocity of 10
m · s−1 over the course of the race. This gives the
strength and conditioning specialist some infor-
mation but in order to tailor appropriate training
mation is needed. If we obtained 10 m split times
for this race, we could calculate many velocities
throughout the race and this would give us much
Fig 5.1
Beginner lifter Experienced lifter
FIGURE 5.1 The path that the barbell travels for (A) a beginner and (B) and experienced lifter. Notice that
the distance travelled is much greater for the beginner lifter; however, the displacements are the same.
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4PART 1 Basic Science
amount of force being applied to an object can be
calculated by multiplying mass of the object times
its acceleration. Mass is a measure of the quantity of
matter within the object, and, in the human body,
it would be the sum of all the tissues that make up
our bodies (bones, muscles, ﬂ uids, etc.)
Force = Mass · Acceleration
Mass can be thought of as a measure of the linear
inertia of a body. Inertia is deﬁ ned as the resistance
to changes in motion, and, therefore, an object
with a large mass will be more difﬁ cult to get mov-
ing or to stop once it has begun moving than an
object with a smaller mass. Mass is often equated
to weight; however, they differ in that weight is
mass multiplied by the acceleration due to gravity
(which we assume to be a constant value of 9.81 m
· s−2 on the earth). Therefore, a person with a mass
of 100 kg would weigh 981 N while on earth, while
in space in a zero gravity environment this person
would be weightless and while standing on the
moon this person would weigh 162 N (the acceler-
ation due to gravity on the moon is approximately
1.62 m · s−2). Thus, while the weight will change
race that need the most work. For example, if there
is a large drop in velocity toward the end of the
race, the strength and conditioning specialist may
need to design a training program to work on the
endurance of the athlete to ensure that he or she
is better able to maintain the velocity through the
nish line. Thus, it becomes increasingly impor-
tant to examine not just velocity, but how velocity
is changing over time.
Acceleration = Velocity ·Time−1
Acceleration is a measure of how velocity changes
over time (deﬁ ned as velocity divided by time). In
the bench press example, acceleration or decelera-
tion of the bar is achieved through the application of
force. Force is deﬁ ned as a push or pull that moves
or tends to move an object. The unit of force in the
metric system is the Newton, while in the United
States the pound (lb) is more commonly used. The
REAL-WORLD APPLICATION
Acceleration Forces in Lifting Weights
One component of acceleration that is constantly acting
on the human body and sports implements is accelera-
tion due to gravity. In the example of an arm curl with a
dumbbell:
F = m (a + g)
where F, force; m, mass of the dumbbell; a, instantaneous
acceleration; g, acceleration due to gravity (9.81 m · s−2);
with a concentric muscle action in the dumbbell curl, grav-
ity is a resistance force that results in negative acceleration.
With an eccentric muscle action to lower the dumbbell, the
force of gravity results in positive acceleration.
Q1
BOX 5.1
Calculating Average Velocity during Sprint
Running
The example is an athlete completing a 40-m sprint. We
have electronic timing lights at 10, 20, and 30 m from the
starting light gate. The average speed over the last 10
m might be used as an indication of maximum running
speed. Let us assume that the times and distances are as
follows:
Distance 10 20 30
Time 1.741 2.890 3.995
Velocity is the change in distance over change in time.
Therefore, velocity = (30 – 20)/(3.995 – 2.890) = 10/1.105
= 9.05 m · s−1.
BOX 5.2
Calculating Work and Power during Weight
Training
Let us use the example of a bench press to demonstrate
the calculation of work and power. If the mass of the bar
is 80 kg then the gravity force that this equates to is 784
N (80 × 9.81). During the repetition, the lifter moves the
bar through a distance of 0.70 m and then the work com-
pleted is approximately 549 J (80 × 9.81 × 0.7). Now, let
us assume that the lift is completed in 1.5 seconds. The
average power output during the lift can be calculated
as work divided by time, which equals 366 W (549 ÷ 1.5).
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CHAPTER 5 Biomechanics of Human Movement
is to increase the amount of weight being lifted.
Another simple modiﬁ cation is to increase the
number of repetitions to increase the workload.
This introduces the concept of Work, which is
deﬁ ned as force times displacement and is mea-
sured in Joules. As you increase the number of rep-
etitions you also increase the displacement over
which a force has been applied, therefore increas-
ing the amount of work done. If there are two indi-
viduals lifting the same amount of weight over the
same distance then these individuals are doing the
same amount of work; yet if one of these individ-
uals is capable of producing the lift in a shorter
period of time then it is said that this person is
more powerful.
Work = Force · Displacement
Power is calculated in two different ways—as work
divided by time or as force multiplied by veloc-
ity, and it is measured in Watts. Commonly used
“slow-moving” exercises such as the bench press,
squat, and dead lift only produce approximately
half the power of the faster Olympic lifts (3). As
can be seen from the formula, the optimization of
both force and velocity is necessary for the greatest
power output, and while large loads require large
amounts of force to get moving, the movement
speed is too low for optimal power. Conversely,
lighter loads can be accelerated to high speeds but
do not include the necessary force production to
achieve greatest power.
Power = Force · Velocity
Power = Work · Time−1
depending on where the individual is standing,
the mass will remain constant. It is ﬁ tting that the
unit of force bears Isaac Newton’s name as he has
been credited with the discovery of gravity. Gravity
is a mutually attractive force between two bodies
that possess mass. Since the mass of earth is much
greater than that of anything on its surface, it will
attract or pull all objects toward its core. Gravity is
an important concept for strength and condition-
ing practitioners as weight training includes lifting
and lowering objects against and with the force of
gravity. It should be noted that gravity always pulls
objects toward the center of the earth and thus only
acts in the vertical direction.
Momentum is the product of mass and veloc-
ity and it is an important concept for strength and
conditioning specialist since momentum alone can
continue the motion of an object. Unlike previous
Aristotelian views that a constant force application
was needed to maintain motion, Newton found
that an object’s inertia (mass) while on the move
had a tendency to maintain that motion and only
an external force acting on the object will slow it
down and eventually stop. Therefore, the greater
the momentum of an object the greater the exter-
nal forces needed to subsequently stop it.
Momentum = Mass · Velocity
Momentum and inertial patterns of the sport should
be mimicked during training.
There are many ways to increase the intensity of
a workout session. Naturally, the most obvious
REAL-WORLD APPLICATION
Understanding the Difference between Velocity and Acceleration
An example that most are familiar with involves discussing
how you would change velocities while driving your car
(i.e., either speed up or slow down). In order to accelerate
must be pressed. Conversely, in order to decelerate your
car, or decrease the speed, you must press on the brakes.
Thus, if while traveling at 40 mph you decide to increase
your speed to 50 mph, an acceleration is needed. And if you
are going at 40 mph and you wish to stop your car (velocity
of zero), a deceleration is needed. The rate at which these
accelerations/decelerations happen becomes extremely
important as well. Assume you are traveling at 40 mph in
your car and an animal jumps into the road directly in front
of you, you need to decelerate from 40 mph to zero mph in
a very short period of time to avoid hitting the animal. This
scenario requires an extremely large deceleration as a large
change in speed must happen over a short period of time.
However, if you are going at the same speed and you see
the trafﬁ c light change to red at 500 yd in front of you, you
can apply the brake more lightly and slow down gradu-
ally over a longer period of time. Thus, the same change in
speed over a longer period of time requires a much smaller
magnitude of deceleration.
Q2
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6PART 1 Basic Science
This is a very important biomechanical vari-
able for the strength and conditioning specialist
as modifying the placement of the weight or resis-
tance from the axis of rotation can be an effective
tool in either increasing or decreasing the muscu-
lar effort needed to successfully complete a move-
ment. Figure 5.3 demonstrates this concept as
Figure 5.3A would required much more muscular
torque than Fig. 5.3B to move the same mass. A
real-life example of this would be having some-
body do leg raises in the supine position with the
legs straight and then with the legs bent at the
knees. Bending the knees shortens the torque arm
distance and decreases the amount of muscular
torque needed to perform this exercise.
Whereas inertia relates to an object’s resistance
to being moved or stopped from moving (in a
linear sense), rotary inertia refers to an object’s
resistance to being spun (angular motion). Linear
inertia can be easily represented by the mass of the
object; however, in order to calculate the rotary iner-
tia you need to measure both the mass of the object
and how this mass is distributed relative to the axis
of rotation. A simple example demonstrating this
concept involves asking an individual to run with-
out bending their knees. They will obviously not be
able to run nearly as fast this way then when they
are able to ﬂ ex the knee during the swing phase of
the running gait. While the legs themselves are not
changing their mass as the hip ﬂ exes and extends,
the outstretched leg is maintaining the mass of the
lower leg and foot relatively far from the hip (axis
Q3
Q4
Up to this point we have only been considering
movements of objects or the body in a straight line,
or what is generally called linear motion; however,
many movements occur or take place about an axis
or fulcrum and are deﬁ ned as angular motion. In
the human body these angular motions occur as
our segments (foot, lower leg, thigh, etc.) rotate
about axes created at the joints (ankle, knee, hip,
etc.). Angular motion is measured in degrees
and in some instances can be described in radi-
ans (approximately 57 degrees) or revolutions
(1 rev = 360 degrees).
Force or power applied is determined by a com-
plex range of neural and mechanical interactions
within the muscle, between muscle and tendon, and
between muscle and the machines of the skeleton.
As discussed above, forces are needed to create
linear motion. The angular equivalent of force is
a torque (T), which is needed to create angular
motion, and is expressed in foot-pounds (ft lb) or
Newton-meters (N · m).
Torque = Force · Distance (length of the lever arm).
In order to lift a dumbbell the biceps brachii mus-
cle must produce a torque in the upward direc-
tion. How much torque is produced depends on
the amount of force being utilized multiplied by
the torque arm. The torque arm is deﬁ ned as the
perpendicular distance between where the force
is being applied (the attachment of the biceps
on the bone) and the axis of rotation (the elbow
joint). In the example shown in Figure 5.2, lift-
ing the dumbbell through concentric activity of
the biceps requires a counterclockwise torque of
greater magnitude than the clockwise torque being
produced by the dumbbell. That is, force of the
biceps multiplied by the torque arm of the biceps
has to be greater than the weight of the dumbbell
multiplied by the distance this dumbbell is from
the axis of rotation in order to produce concen-
tric elbow ﬂ exion. If the opposite is true and the
counterclockwise torque created by the dumbbell
is greater than the clockwise torque created by
the biceps brachii, eccentric elbow extension will
be the resulting motion as the muscle will have
allowed the dumbbell to “win.” Isometric activity
would occur when the magnitude of the torque
produced by the muscle is equal to the torque pro-
duced by the dumbbell.
M
u
s
c
l
e
t
o
r
q
u
e
D
u
m
b
b
e
l
l
t
o
r
q
u
e
Dumbbell moment arm
Biceps moment arm
FIGURE 5.2 The biceps muscle torque (counterclockwise) and
the torque produced by the dumbbell (clockwise), in an elbow
exion exercise.
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CHAPTER 5 Biomechanics of Human Movement
closer to the wire rendering them more stable and
less likely to tip over (angular motion) to one side or
the other due to the greater rotary inertia (Fig. 5.5).
The term balance implies control of equilib-
rium, whereas stability is resistance to loss of
equilibrium. One of the ways individuals increase
their stability is by increasing their base of support.
This base of support is deﬁ ned as the two-dimen-
sional area formed by the supporting segments of
the body (Fig. 5.6). Coaches often ask their play-
ers to spread their feet shoulder-width apart ren-
dering them more stable. Increasing the base of
support enhances stability because it increases the
distance your line of gravity has to move before it
is ends up outside this base, causing a loss of bal-
ance. Once the line of gravity is outside the base
of support, the body will experience a destabiliz-
ing torque from the pull of gravity that will tend
to topple the body over. Lowering your center of
gravity also increases stability by decreasing the
magnitude of this destabilizing torque by reducing
rotation (your feet on the ground). Olympic lifting
of rotation). If instead the runner ﬂ exes the knee
during the swing phase, this will bring the lower
leg and foot closer to the hip (axis of rotation) and
in so doing decreasing the rotary inertia and allow-
ing the entire leg to now ﬂ ex forward at a faster
rate. This allows for the person to get through the
recovery (swing) phase in a much shorter time and
therefore run much faster (Fig. 5.4).
Though this example demonstrates how bringing
the mass closer to the axis of rotation promotes faster
rotations, there are instances when rotation is not
desired and the goal is, then, to increase the rotary
inertia. Good examples of this are individuals walk-
ing along a tightrope. These daredevils often carry
a long pole in their hands that is bent down from
weights attached at its ends. The weights serve two
purposes: it puts mass far away from the performer
and brings the center of gravity of the individual
Muscle Torque = F * force arm = 20 lbs * 1 foot = 20 ft lbs
Muscle Torque = F * force arm = 20 lbs * 1.5 feet = 35 ft lbs
(A)
(B)
FIGURE 5.3 A. A longer force arm with the same mass = more torque required to move the object; (B) A
shorter force arm with the same mass = less torque required to move the object.
FIGURE 5.4 There is much more knee ﬂ exion during the swing
phase of running (B) than there is when walking (A). This allows
the runner to decrease the rotary inertia of the leg and move it
much faster.
FIGURE 5.5 The pole used by a tightrope walker increases their
rotary inertia.
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8PART 1 Basic Science
ball the base of support becomes the portion of
the ball that is in contact with ground. The stabil-
ity challenge can then be altered by how much air
you put in the ball. If the ball is pumped up with
a lot of air, it will be very rigid and you will be
balancing on a really small area of the ball. This
makes maintaining balance more difﬁ cult as the
base of support is extremely small. To make this
challenge easier, you can remove air from the ball
so that the ball become softer and more of it then
comes in contact with the ground, increasing the
base of support.
Friction can sometimes be used to increase
the difﬁ culty of a certain task or exercise. The two
factors related to friction are the nature of the two
surfaces attempting to slide past one another (the
coefﬁ cient of friction) and the amount of force
pressing the two surfaces together (the normal
force). Monarch cycle ergometers use increased
tension of the belt around the wheel to increase
the friction and increase the resistance. Football
coaches stand on top of blocking sleds to increase
the force pressing the sled to the ground and in
so doing making it harder for football players
to push the sled across the grass. Application of
talcum powder to the hands in order to remove
moisture and get a better grip is an example of
changing the nature of the surfaces in contact.
competitions require not only for the athlete to
lift the weight above their heads but also need to
demonstrate control of the weight by balancing
it for 3 seconds. This balance is difﬁ cult due to
the high center of gravity position since it is not
uncommon for these athletes to be lifting more
than two times their own body weight. Therefore,
a shorter lifter would have a stability advantage
over a much taller lifter as the same small move-
ment of the load would produce a greater destabi-
lizing torque in the taller lifter. The most unstable
foot position possible in a human being is stand-
ing on one foot. When standing on one foot our
base of support becomes the length or width of
the foot, and if the center of gravity falls outside of
the dimensions of the foot, there will be a loss of
balance. The strength and conditioning specialist
can use this foot position during many different
exercises to train stability. By training in this very
unstable position, one can further develop the
body’s sensory and muscular recruitment strate-
gies needed to maintain balance. Several pieces of
equipment frequently in use in strength and con-
ditioning facilities also have the goal of creating
an unstable surface to allow for the training of the
ability to maintain balance.
One such recent invention is the BOSU ball
(Fig. 5.7). If we stand on the platform side of this
Narrow/small base of support
Wide/large base of support
Weight vector
(center of gravity)
within the base of
support = balanced
Weight vector
(center of gravity)
not within the base of
support = loss of balanced
C D
A
B
FIGURE 5.6 The position of the center of gravity relative to the base of support determines whole body
stability and balance.
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CHAPTER 5 Biomechanics of Human Movement
the muscle increases. Conversely, as the muscle
is shortened from this optimal length, increasing
numbers of the actin/myosin crossbridges become
overlapped and are no longer able to produce the
power strokes that allow for the production of ten-
sion/force. Therefore, as the length of a muscle
decreases so too does the amount of tension/force
that the active component of muscle can produce.
The use of various methods of resistance training
can produce force-curve characteristics similar to
those of the sport.
The passive component of muscle only produces
force/tension when the muscle is lengthened. You
can think of your muscles as elastic bands. If you
shorten an elastic band beyond its resting length
it does not create any tension/force. The only way
to produce tension/force of an elastic band is to
stretch it beyond its resting length so that it then
tries to snap back to its original shape. Therefore,
when a muscle is shortened the total length–ten-
sion curve involves only the inverted U shape of
the active component. However, as the muscle is
increasingly lengthened, the tendon and connec-
tive tissues are stretched beyond their resting length
and produce an increasing amount of force with
increased lengthening. Therefore, the total tension/
force achieved when a muscle is stretched beyond
its resting length is the sum of both the active and
passive components (Fig. 5.8).
HUMAN MUSCULOSKELETAL
MECHANICS
LENGTH–TENSION RELATIONSHIP
There are two types of tissues that can create ten-
sion in a muscle: (a) the active component consist-
ing of the acting and myosin muscle proteins and
(b) the passive component consisting of the con-
nective tissue within the muscle belly which comes
together on either end to form the muscle tendon.
The tension/force that can be created by these two
different types of tissues changes as the length of
the muscle changes throughout a movement (4,5).
The length–tension curve for the active compo-
nent of muscle is an inverted “U” shape. The peak
of this curve (where the maximum active tension/
force can be produced) corresponds to the posi-
tion where the muscle is in an optimal position
to allow the most actin/myosin crossbridges. As
the muscle is increasingly stretched beyond this
length, these crossbridges are torn apart so the
amount of tension/force the active component
of muscle can produce decreases as the length of
FIGURE 5.7 Using a BOSU ball to alter the size of the base of
support during a squatting exercise.
Force or tension
Active force
Shortening Lengthening
Passive force
Resultant force
Resting length
FIGURE 5.8 The active, passive, and resultant forces for the
typical muscle length tension curve.
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10 PART 1 Basic Science
the position of his or her center of gravity or take a
step to widen the base of support to avoid falling.
It has also been shown that as one ages, explosive
strength or power decreases more than the maxi-
mum isometric strength (7), which makes training
these fast muscular contractions essential in older
adults. Therefore, an understanding of the relation-
ship between force, velocity, and power presented
in Figure 5.9 (5) is essential for the strength and
conditioning specialist.
Training at velocities and joint angles speciﬁ c to the
sport will result in the greatest carryover to sport
performance.
If a muscle is maximally activated in an attempt
to produce movements at different speeds, several
important points must be observed:
1. As the speed of concentric contraction
increases, the force that can be produced dur-
ing those contractions decreases. Therefore,
the minimum amount of force a muscle can
produce is during a fast concentric contraction.
2. Greater tension can be developed during
an isometric contraction (velocity = 0) than
during any speed of concentric contraction.
FORCE–VELOCITY–POWER
RELATIONSHIP
Although the amount of force a muscle can pro-
duce is important, perhaps more important in
many human movements is the velocity at which
a muscle can develop this force. Often the terms
“strength” and “power” are used erroneously
interchangeably. Strength refers to a muscles abil-
ity to produce force in isometric or slow velocity
contractions, whereas power refers to a combina-
tion of force production and velocity (6). Training
only force development at slow speeds (strength)
may have negative implications for a wide range of
individuals.
Most sports require the application of maximal
power output rather than force.
Obviously, most sporting activities involve high
velocity, high power movements and therefore it
may not be effective to train any athlete to only
be able to slowly develop extremely large forces.
Also, in order to train older adults to avoid falls, we
must be concerned with the velocity of muscular
contraction as well. If an individual loses his or her
balance, he or she must move quickly and adjust
Q5
Force and power
ConcentricEccentric
Isometric
FMax
Velocity VmaxVmax 0
FIGURE 5.9 The force–velocity (purple), power–velocity (yellow) relationship.
Chandler_Chap05.indd 10Chandler_Chap05.indd 10 10/19/2011 4:34:56 PM10/19/2011 4:34:56 PM
11
CHAPTER 5 Biomechanics of Human Movement
PCSA). This is logical as one of the main goals of
resistance training is to increase the size and hence
the force-producing capacity of our muscles.
One muscle with an extremely large PCSA in
the human body is the gluteus maximus muscle
(9). With the force-generating capacity of this
muscle being so large, many smaller muscles must
compensate for it if it is not working efﬁ ciently.
Therefore, it is important that we train it appro-
priately as extremely common pathologies such as
low back pain have been associated with a loss of
neural drive to this muscle—termed “gluteal amne-
sia” (10).
STRETCH-SHORTENING CYCLE
Most human movements begin with motions in
the opposite direction to the intended movement.
In a vertical jump, this involves the initial ﬂ exion
of the knee/hips and dorsiﬂ exion of the ankles
used to accelerate the center of gravity downward.
This causes an eccentric stretch of the knee/hip
extensors and ankle plantarﬂ exors that is quickly
turned into a concentric contraction of these same
muscles to produce the upward motion of the cen-
ter of gravity, resulting in the jump. This eccentric
stretch followed closely by a concentric shorten-
ing has been termed the stretch-shortening cycle
(SSC) of a muscle. If there is a minimal time delay
between the eccentric stretch and concentric con-
traction, it has been shown that there is an increase
in the force produced as compared to an isolated
concentric contraction (11). The magnitude of
increase in concentric force depends on the move-
ment performed and the resistance being moved,
but is generally thought to be in the magnitude of
10% to 20%. Therefore, the SSC is critical in pro-
ducing high force and high power concentric mus-
cular contractions.
The SSC is inherent in almost all sporting move-
ments and this mechanism is critical for producing
high force and power.
JOINT ANGLE AND MUSCULAR
TORQUE
Muscles pull on bones at a distance from the axis
of rotation (joint) and therefore, they produce a
torque that attempts to produce angular motion of
the bones to which they are attached. The amount
3. Muscles can generate their greatest forces
while resisting motion during eccentric
(lengthening) contractions. These eccentric
or lengthening velocities are shown as nega-
tive velocities (X axis) in Figure 5.9. It should
also be noted that forces generated during
eccentric contractions also rise slightly and
then remain relatively constant as velocity
increases.
4. As was mentioned in the Basic Mechanics
section, Power = force × velocity. Since force
and concentric velocity have an inverse rela-
tionship (i.e., as velocity goes up, force goes
down), the point of peak concentric power
will occur somewhere between an isometric
and maximum velocity concentric contrac-
tion. The in vitro concentric power curve
derived from the force–velocity relation-
ship of skeletal muscle is highly dependent
on the movement being tested. Izquierdo
et al. (7) demonstrated that the best resis-
tances (forces) for the development of peak
power in the upper body were in the range
of 30% to 45% of maximum isometric force.
However, for explosive lower body move-
ments peak power was observed at 60% to
70% of maximum isometric force.
This force–velocity–power relationship can be
readily observed in strength and conditioning set-
tings. If we attempt to lift an extremely heavy load,
the velocity of movement will be extremely small
as we will need to produce maximal forces to move
this load and will not be able to get it to move
very quickly. When training with lighter loads we
are much more able to get the resistance moving
quickly; however, training with too light a load will
necessitate extremely small forces from our mus-
cles. Therefore, in order to achieve peak power we
must choose an appropriate resistance to allow for
PHYSIOLOGICAL CROSS-
SECTIONAL AREA
The physiological cross-sectional area (PCSA) of a
muscle is a measure of how many muscle sarco-
meres are arranged in parallel in that particular
muscle. This has been shown to determine the
maximum force generating capacity of the muscle
(8). Therefore a bigger muscle (larger PCSA) can
produce more force than a smaller muscle (small
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12 PART 1 Basic Science
middle, and ﬁ nally, a third class lever has the force
in between the axis and resistance (Fig. 5.11).
The distances between the axis and the force
(force arm [FA]) and the axis and the resistance
(resistance arm [RA]) help determine the types of
movements that each lever system is best designed
to perform. Those levers with a short RA and a
long force arm are said to have a large mechani-
cal advantage (calculated by dividing the FA by
the RA). This is because large resistances can be
moved short distances with small forces if a lever
is used that creates this mechanical advantage. For
example, if a 180 lb person wants to move a 900 lb
rock, they could do this most effectively by getting
a board and wedging it under the rock and then
balancing the board on an object really close to
the rock (creating the axis of rotation). If the dis-
tance between the axis and the rock is 2 ft this cre-
ates 1,800 ft-lbs of torque that must be overcome
in order to move the rock. Therefore, the person
would need to jump on the board 10 ft from the
axis to produce the required torque needed to move
the large rock, but the resultant displacement and
hence the velocity of the rock would not be large.
Human muscle bone levers have the muscles
attached really close to the joints creating extremely
short FAs. By comparison, our limb segments are
relatively long creating much longer RAs. This cre-
ates a mechanical disadvantage or a velocity/speed
advantage (calculated by dividing the RA by the
FA) in human muscle bone levers. This is due to the
fact that it will take a lot of force to get the resistance
moving (mechanical disadvantage), but once we get
it moving it will have a much larger displacement
of torque can be calculated by multiplying the
force by the force-arm distance. As a muscle causes
movement of the bones, the length of the force arm
changes. This means that with the same amount of
muscular force, there are changes in the amount
of torque generated as the muscle moves the joint
through its range of motion. In a simple hinge joint
like the elbow/knee, the ﬂ exors (biceps brachii/
hamstrings) are at a mechanical advantage at a joint
angle of 90 degrees as the force arm is the longest
in this joint position. As the joint angle increases or
decreases, the force arm decreases in length, creating
less torque with the same muscular force (Fig. 5.10).
LEVERS
The arrangement of the bones, muscles and joints
in the human body create simple machines called
lever systems. The anatomical levers of the body
cannot be changed, but when the system is well
understood, they can be used more efﬁ ciently
to maximize the muscular efforts of the body
(12). The three components of every lever system
include the axis (joint), the resistance (weight of
the segment being moved and any attached exter-
nal weight) and the force (muscle force). The loca-
tion of these three components with respect to
one another will determine the type of lever and
most importantly, the movement characteristics for
which they are best suited. The lever type is deter-
mined primarily by which of the three components
is located in between the other two. That is, a ﬁ rst
class lever has the axis in between the other two,
while a second class lever has the resistance in the
Note: Red arrow = hamstring force
Blue line = force arm
FIGURE 5.10 The change in the torque generating capacity (force arm distance) of the hamstring muscles
as the knee joint angle changes.
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13
CHAPTER 5 Biomechanics of Human Movement
provided by this resistance can help accomplish
the goal of the training session, whether it is sim-
ply to make fundamental human movement pat-
terns more efﬁ cient or to increase strength, speed,
or both (i.e., power). The original and most com-
monly used form of resistance simply utilizes dif-
ferent forms of mass and the force of gravity. As
and hence velocity/speed. This concept is displayed
in research that examined the anatomical differ-
ences in the ankle/foot between a group of collegiate
sprinters and a group of height matched nonathletes
(13). It was discovered that the sprinters had longer
toes and also had 25% shorter Achilles tendon FAs
(Fig. 5.12). Therefore, the sprinters had shorter FA
and were also able, with their longer toes, to get the
force of the ground pushing back up on their foot
further from the axis of rotation at their ankle, creat-
ing a longer RA. This creates a greater velocity/speed
advantage that may be one mechanical reason why
sprinters can run faster than nonsprinters.
The musculoskeletal system is designed for speed and
range of motion rather than high force production.
BIOMECHANICS OF
RESISTANCE
In strength and conditioning settings, various forms
of resistance have been used to make movements/
exercises more challenging. The extra stimulus
A
FR
R
F
A
F
A
R
First Class
Second -Class
Third Class
FIGURE 5.11 First class, second class, and third class lever systems.
Red = Non-sprinters
Blue = Sprinters
FA = force arm
RA = resistance arm
Force Resistance
FIGURE 5.12 The differences in anatomy noted by Lee and
Piazza (2009) between sprinters and nonsprinters. Sprinters
had a shorter force arm and a longer resistance arm, creating a
Chandler_Chap05.indd 13Chandler_Chap05.indd 13 10/19/2011 4:34:57 PM10/19/2011 4:34:57 PM
14 PART 1 Basic Science
However, the ﬁ rst half of this equation (ma) is not
constant as the mass must be accelerated at the start
of the lift and decelerated at the end of the lift. If a
mass is moved extremely slowly through the range
of motion, the effect of the “ma” term becomes neg-
ligible; however, for rapid movements with large
changes in speed, this term becomes extremely
important and can cause great variation in the resis-
tance felt by the muscles throughout the range of
motion. It has been shown that doing high-speed
lifts with free-weight resistance requires in excess
of 190% of the weight of the load in order to pro-
duce these high accelerations (14,15). That means
that if doing a bench press with 130 lb, the resis-
tance at the beginning of the concentric phase can
actually exceed 250 lb as the momentum of the
eccentric phase is quickly absorbed and the bar is
accelerated concentrically to a high rate of speed.
Then, once the weight is moving at a high rate of
speed, the resistance provided to the muscles can
decrease to almost zero if the weight is accelerated
fast enough that the magnitude of the (m × a) term
equals that of the (m × g) term. This gives the lifter
the feeling that the bar is temporarily ﬂ oating and
almost thrown into the air, which can be dangerous
in a bench press movement. This is demonstrated
by studies showing that well-trained athletes can
spend up to 52% of the concentric phase of a high
speed lift attempting to decelerate and control the
the activity of the muscles producing the movement
during this portion of the lift (15).
Two common forms of mass used as resistance
in strength and conditioning settings include (i)
free weights and (ii) gravity-based machines.
Free Weights
Free weights are often thought to include only bar-
bells and dumbbells but can also come in many
other forms. Any object that has a mass and allows
for 6 degrees of freedom movement of that mass
can be considered a “free weight.” Other common
forms of free weights include: kettlebells, medicine
balls, weight vests, weighted ankle/wrist straps,
weighted sleds, training ropes, chains, and the sim-
plest form of resistance of all, the individual’s own
body weight. The biomechanics of these forms of
resistance follow the laws of inertia outlined above
and always have the resistance acting vertically
downward. Therefore, the force needed to move
these weights vertically can be determined using
the formula F = ma + mg. The force needed to
various technologies have advanced, other forms
of resistance have been developed that have cer-
tain biomechanical characteristics that are differ-
ent from mass and provide a different stimulus
to the human body during training. The follow-
ing section will examine several different forms of
resistance and discuss how the mechanics of these
then produce different training stimuli. This infor-
mation is important for the strength and condi-
tioning specialist so that the appropriate form of
resistance can be used to accomplish the speciﬁ c
functional goal of the training. It is important to
tailor the training to the particular goal of the pro-
gram to ensure success. An appropriate analogy
can be drawn to the engines in our automobiles.
A formula one car (designed for speed) will require
a much different engine than a truck designed to
haul and tow large loads. The strength and condi-
tioning specialist needs to choose the appropriate
form of resistance to ensure that we are building
the correct engines (muscles) to meet the goals of
the individual.
The following section will examine the bio-
mechanics of several different forms of resistance,
which will be divided into two main categories: (a)
those that use mass and the force of gravity as the
resistance and (b) those that do not use signiﬁ cant
mass and generate the mechanical resistance using
other means.
MASS
When using any form of mass as resistance, the
most important biomechanical concept that the
strength and conditioning specialist must keep in
mind is its inherent inertial properties. To be put
another way, mass resists changes to its state of
motion. So if it is not moving, large forces must
be applied to the mass to get it moving, and once
it begins moving, less force must be applied to
keep it moving. Most overlook the fact that, when
performing a lift with a 20-lb dumbbell, the resis-
tance provided to the human body throughout
that lift can vary from much greater than 20 lb to
overcome the inertia of that mass, to almost zero
if the mass in accelerated to a high enough speed
during the lift.
The force the person is applying to the mass can
best be calculated using the equation F = ma + mg.
The second part of this equation (mg) is constant
as the acceleration due to gravity (g = 9.81 m · s−2)
and the mass remain constant throughout a lift.
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15
CHAPTER 5 Biomechanics of Human Movement
that use cables and pulleys to allow us to direct the
resistance of a mass/gravity upward or horizon-
tally (Fig. 5.13). Older versions of these machines
ing/removing weighted plates but newer versions
of these machines use pin loaded weight stacks to
make the adjusting of the resistance much easier.
Engineers have also attempted to design gravity
based machines so that the resistance delivered to
the individual better matches our muscles ability
to produce force across the range of motion of the
joint. For example, in the free weight bench press
we are limited in resistance by the amount we are
able to move through our weakest point (sticking
point) near the bottom of the lift. Therefore, our
muscles are not getting challenged appropriately in
the upper part of the range of motion.
move these weights horizontally does not need to
overcome gravity and therefore can be determined
using F = ma. Therefore, adjusting how much a
mass is moved horizontally/vertically can be a
good method of progressing and regressing many
different movements as the amount of gravity that
must be overcome during the movement can be
altered.
Gravity-Based Machines
The resistance of any mass acts vertically down
which limits our ability to train certain muscle
groups. For example, using mass to train a verti-
cal shoulder press movement is appropriate but
in order to train the antagonist movement (i.e.,
lat pull down exercise), the gravity force needs to
be redirected. This is accomplished by machines
Q & A from the Field
If the elbow ﬂ exors are strongest at 90 degrees of ﬂ exion, why is the
“sticking point” in the midrange of the movement?
Q
A
The elbow is strongest in ﬂ exion at 90 degrees because
the length of the force arm, “the perpendicular dis-
tance from the muscle insertion to the axis of rotation,”
is maximal at 90 degrees. However, the RA, the perpen-
dicular distance from the point of force application to
the axis of rotation, is also greatest in the midrange
of the motion for an isotonic exercise. Although the
mechanical advantage of the elbow ﬂ exors is great-
est at 90 degrees, the increasing length of the RA in a
heavy isotonic exercise overcomes this advantage. The
sticking point will occur somewhere near 90 degrees
of ﬂ exion.
Q & A from the Field
Is the full-squat exercise dangerous from a biomechanical
perspective?
Q
A
Any resistance training exercise performed improperly
can cause injury. This increased risk of injury may be
related to excessive volume, excessive resistance, poor
form, inﬂ exibility, or fatigue. With adequate strength
and ﬂ exibility, any healthy athlete should be able to
squat safely to the point where the tops of the thighs
are parallel to the ﬂ oor. Depending on the demands of
the sport, some athletes may beneﬁ t from squatting
below this “parallel” position. Squats performed appro-
priately do not cause instability at the knee. Good form
is critical to protect the low back in heavy squatting
movements. The squat exercise is important to both
the general population and athletes because of its
functionality and similarity to athletic movements and
activities of daily living.
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16 PART 1 Basic Science
accelerations of the mass will negate the effects of
these variable resistance designs.
Understanding the mechanics underlying a piece of
resistance training or conditioning equipment will
assist in initial purchase decisions as well as exer-
cise selection.
This was originally overcome by creating
machines where the user would start the move-
ment in their weakest position (i.e., the bot-
tom position of the chest press movement), but
with the machine lever arm to the weight stack
extremely short. As the movement proceeded
from bottom to top the machine’s lever arm to
the weight stack would increase in length thus
also increasing the resistance felt by the user in
later stages of the lift. Various other gravity based
machines have also attempted to produce a vari-
able amount of resistance throughout a lift using
different designs.
One common design uses a cable or chain that
wraps over a variable-radius cam and alters the
moment arm distance to the resistance (weight
stack) as the user moves through the range of
motion (Fig. 5.14). Again, this allows for the user to
feel more resistance at portions of the lift where the
muscles are mechanically strongest and less resis-
tance where the muscles are less optimally posi-
tioned. However, all of these machines use mass
for the resistance and therefore the speed of move-
ment becomes really important as creating large
FIGURE 5.13 Two gravity based weight machines that allow the resistance of the mass to be redirected
using pulleys.
FIGURE 5.14 A variable radius cam.
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17
CHAPTER 5 Biomechanics of Human Movement
movement (see Fig. 5.17). Finally, there have been
power racks designed to be able to incorporate both
mass and air resistance in a wide range of total body
6 degrees of freedom movements (bench press,
squat, dead lift, power clean, etc.). These can com-
bine different amounts of mass and air resistance
(Fig. 5.18) or can be used with negligible mass and
only air resistance (Fig. 5.19), depending on the
goal of the training session.
Research has shown that pneumatic resistance
allows for greater movement velocities and also
produces greater muscles activity at the end range of
motion as compared to free weights in a bench press
movement (18). This is due to the fact that pneu-
matic resistance contains limited mass and there-
fore does not develop momentum at high speeds.
Athletic movements as well as many movements
in everyday life (i.e., regaining equilibrium after a
loss in balance) require high speed muscular con-
tractions and velocity speciﬁ c power production.
Therefore, training with pneumatic resistance could
provide some movement speciﬁ c advantages over
free weights. However, this work also suggests that
pneumatic resistance reduces the forces required
OTHER FORMS OF RESISTANCE
Pneumatic Resistance
In order to overcome the limitations associated with
training at high speed using mass as the resistance,
a technology was developed that creates the resis-
tance with air pressure (17). It has been contended
that this form of resistance does not have the inher-
ent limitations of mass and its inertial properties.
Therefore, high speed training involving large accel-
erations can be performed and the resistance can be
kept relatively constant throughout the movement.
The basic technology involves a compressor pump-
ing air into a cylinder (Fig. 5.15) (more air pressure
= more resistance, less air pressure = less resistance)
equipped with a piston that further compresses
the air during the concentric phase and is pushed
back out by the air during the eccentric phase of the
movement. As the air is further compressed during
the concentric phase of movement the resistance
increases and this is thought to match the force pro-
ducing capacity of our muscles during most move-
ments. The user also has control of the resistance
with hand buttons or foot pedals that can increase
(pump in more air) or decrease (let air out) the
resistance throughout the movement.
There has been a wide range of exercise equip-
ment designed using this pneumatic resistance tech-
nology. Some of these include machines designed for
high stability that guide the user through the range
of motion and train only speciﬁ c movements (chest
press, leg extension, leg press, etc.) (see Fig. 5.16).
There are also cable machines that can be adjusted
to provide the resistance in the desired direction for
the exercise and allow for a less controlled range of
cylinder
FIGURE 5.15 A Keiser pneumatic resistance machine showing
the air cylinder (with the piston inside).
FIGURE 5.16 A pneumatic resistance machine designed for
high stability by guiding the user through the range of motion.
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18 PART 1 Basic Science
trained both concentrically and eccentrically (19).
However, an examination of the differences between
groups subjects training with free weights (concen-
tric–eccentric) and hydraulics (concentric only)
revealed no differences in velocity, torque, power, or
force between groups (20). Therefore, there is some
controversy in the literature regarding the usefulness
of hydraulic resistance in a training program. It can
also be suggested that the elimination of the eccen-
tric phase created with this equipment may have
uses for special populations as it may also decrease
the muscle soreness experienced by the user.
Elastic Resistance
Various forms of elastic resistance have become
extremely common in strength and conditioning
settings recently. Elastics provide a variable amount
of resistance throughout a movement as the elastic
will produce more force the more it is stretched. It
also provides an eccentric resistance as all the force
that went into stretching the elastic will be returned
as the individual’s muscles control the speed at
which the elastic is returned to its original length.
to use the stretch shortening cycle at the end of
the eccentric and start of the concentric phase of
motion (18). Therefore, further investigation is
needed to identify the neuromuscular responses of
the human body to this alternate form of resistance.
Hydraulic Resistance
Another form of resistance similar to pneumatics
uses ﬂ
uid (generally oil) to create the resistance.
This form of resistance has the movement drive a
piston that forces the ﬂ uid through a small open-
ing creating the resistance. The difference between
pneumatic and hydraulic resistance comes in the
compressibility of the ﬂ uid being used for the
resistance. The air used in pneumatic resistance
is compressible and, therefore, the forces put in
to compressing it during the concentric phase are
returned during the eccentric phase. The oil used
in hydraulic resistance is essentially incompress-
ible and, therefore, hydraulic resistance does not
provide any eccentric resistance during movement.
It has been shown that greater gains in peak
torque can be achieved when movements are
FIGURE 5.17 Adjustable pneumatic resistance machines that allow for the resistance to be directed in any
direction and allow for much less control of the range of motion.
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19
CHAPTER 5 Biomechanics of Human Movement
training purposes. However, the strength and con-
ditioning specialist must have a basic knowledge of
all forms of resistance and how they can be com-
bined and altered so that an appropriate stimulus
can be selected to meet the goal of each individual
training program.
PROGRESSING/REGRESSING
MOVEMENT
As a strength and conditioning specialist, an
important skill is to be able to modify movements/
exercises to (i) increase the difﬁ culty of movement
to further challenge those who have mastered the
basic movement and (ii) decrease the difﬁ culty of
movement to allow those unable to perform the
basic movement a chance to develop the proper
strength and/or muscular recruitment strategies.
Progressing and regressing movements/exercises
requires a good basic knowledge of many basic bio-
mechanical principles. The simplest progressions
and regressions can be performed by simply manip-
ulating the variables of the equation presented in
the previous section (F = ma). If we assume that
progressions would generally involve creating
movements that require more force production.
This can be accomplished by either increasing the
mass or increasing the rate of velocity change dur-
ing the movement (acceleration or deceleration).
Conversely, we could easily regress a movement/
exercise by decreasing the mass being moved or by
moving more slowly (requiring less accelerations/
decelerations). Although this sounds logical, there
A
B
F Air F AirF Bar
F Air F Air
F Plate F Plate
F Bar
FIGURE 5.18 Combining mass (bar and plates) with pneumatic
resistance in a bench press exercise.
F Air F Air
FIGURE 5.19 Using a bar with negligible mass and only air
resistance in the bench press exercise.
Studies comparing the effects of training with
elastic resistance to training with mass as a resis-
tance also provide contradictory results. In a sample
of sedentary middle aged women, there was found
to be no differences in several functional and struc-
tural measures between training with elastic resis-
tance versus training with a weight machine (21).
Whereas, in a sample of recreationally trained col-
lege students, those who trained by simply doing
depth jumps (using body weight) increased their
vertical jump height, while those who trained with
elastic resistance (VertiMax) did not change their
jump height after training (22).
WHAT FORM OF RESISTANCE IS
BEST?
It should be clear from the sections outlined above
that no single form of resistance is ideal for all
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20 PART 1 Basic Science
The strength and conditioning specialist needs a
much more complete knowledge of biomechanics
to be able to tailor movement/exercise difﬁ culty to
the level of each individual.
The following section will use the example of
the single-leg Romanian dead lift (RDL) in order
to illustrate how biomechanical principles can be
used to progress or regress a movement/exercise.
We will begin with a biomechanical description of
the basic movement.
This basic exercise (shown in Fig. 5.20) has several
main goals when used in a training program. Some
are other factors that must be kept in mind. For
example, moving extremely slowly through a bench
press movement may seem like a regression (less
accelerations) when it may actually make the exercise
more difﬁ cult. Newton’s First Law (Law of Inertia)
tells us that an object in motion wants to remain in
motion; therefore, getting the bar moving quickly
in certain phases of the lift would require less mus-
cular effort to keep it moving in other phases. This
becomes important in overcoming points in the
range of motion where the muscle length– tension
relationship and angle of pull of muscle are at their
least optimal (sticking point). Therefore, simply try-
ing to manipulate the variables of that equation in
order to progress/regress movements is not enough.
Hip abd muscle force
A
B
Trunk
weight
BW
Hip extensor
muscle force
Leg
weight
FIGURE 5.20 The basic single-leg Romania dead-lift exercise.
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21
CHAPTER 5 Biomechanics of Human Movement
REGRESSION: ARMS OUT
The ﬁ rst basic regression involves doing the exer-
cise with your arms out in a “T” position as shown
in Figure 5.22. This simple modiﬁ cation of the
tance, which increases your rotary inertia in the
frontal plane. This increased resistance to angular
motion makes it easier to keep your center of grav-
ity within your base of support and maintain your
balance.
REGRESSION: IPSILATERAL
BALANCE AID
In this middle regression, the client would use
a balance aid in the same arm as the stance leg
(Fig. 5.23). This regression does not have an effect
in the frontal plane as the force produced by the
balance aid passes directly through the axis of rota-
tion of the hip and therefore does not increase or
of these goals include (i) training the hip abductors
to increase frontal plane control of the pelvis, (ii)
training the hip extensors in the sagittal plane, and
(iii) to train the balance and proprioceptive sys-
tems in single limb stance.
If we examine this movement in the fron-
tal plane, we can see that the axis of rotation is
created at the hip of the stance limb. Gravity then
pulls on the rest of the body (person’s left in
Fig. 5.20) and produces a torque that is attempt-
ing to spin the pelvis clockwise. The hip abduc-
tors then produce a force on the other side of
the axis of rotation to counter the body weight
(clockwise) torque with the muscular (counter-
clockwise) torque needed to maintain a steady
pelvis.
In the sagittal plane, tilting of the trunk ante-
riorly creates a clockwise torque attempting to
produce ﬂ exion at the stance hip. We then have
two counterclockwise torques acting on the other
side of the axis of rotation attempting to pro-
duce hip extension: (a) the torque created by the
weight of the contralateral leg (which will be of
smaller magnitude than the torque created by the
trunk as the mass of a single leg will be much less
than the mass of the trunk) and (b) the muscle
force created by hip extensors that is needed to
absorb the excess trunk torque in the eccentric
phase and overcome it to produce the concen-
position.
We will now examine how a good knowledge
of biomechanics can be used to create three regres-
sions and two progressions of the basic RDL exer-
cise. The following continuum (Fig. 5.21) shows
each of the ﬁ ve exercise modiﬁ cations that can be
used to make the exercise less challenging (regres-
sions) and more challenging (progressions). Note:
The further the exercise is to the right, the harder
the exercise is; the further the exercise is to the left,
the easier the exercise is.
Basic
RDL
Progression
(Harder)
Regression
(Easier)
Ipsilateral
weight Contralateral
weight
Arms
out
Ipsilateral
balance aid
Contralateral
balance aid
FIGURE 5.21 The single-leg RDL progression/regression continuum.
FIGURE 5.22 The single-leg RDL exercise—arms out
regression.
Chandler_Chap05.indd 21Chandler_Chap05.indd 21 10/19/2011 4:35:18 PM10/19/2011 4:35:18 PM
22 PART 1 Basic Science
REGRESSION: CONTRALATERAL
BALANCE AID
In the third regression, the client would use a
balance aid in the opposite arm as the stance leg
(Fig. 5.24). This regression makes this exercise
easier in both the sagittal and frontal planes. Since
the mass is still the same distance from the axis of
rotation of the hip in the sagittal plane, this regres-
sion would have the same effect on the hip exten-
sors as would the ipsilateral balance aid. However,
this regression decreases the challenge for the hip
abductors in the frontal plane as the balance aid
produces an extra counterclockwise torque to help
decrease the torque required from the hip abduc-
tors. The effect of this regression occurs in the sagit-
tal plane as there is another force to help ﬁ ght the
weight of the trunk that is pulling the stance phase
hip into ﬂ exion. This decreases the force required
from the hip extensors to both slow down the hip
exion in the eccentric phase and to create the hip
extension in the concentric phase.
The ipsilateral balance aid would also increase
the base of support in the anterior–posterior
direction, therefore, making it easier to maintain
balance in this direction. However, it does not
increase the base of support in the mediolateral
direction.
Hip abd muscle force
BW
Balance
aid force
B
Hip extensor
muscle force
Trunk weight
Balance aid force
Leg
weight
A
FIGURE 5.23 The single-leg RDL exercise—ipsilateral balance aid regression.
Chandler_Chap05.indd 22Chandler_Chap05.indd 22 10/19/2011 4:35:20 PM10/19/2011 4:35:20 PM
23
CHAPTER 5 Biomechanics of Human Movement
to the ipsilateral balance aid regression except
for now the weight is producing a force in the
opposite direction (downward). This progression
has no effect in the frontal plane as the line of
action of the weight force passes directly through
the axis of rotation of the hip and therefore does
not increase or decrease the torque required from
the hip abductors. The effect of this progression
occurs in the sagittal plane as the weight produces
an extra clockwise torque that must be absorbed
by increasing the eccentric force created by the hip
extensors.
The use of various methods of resistance training
can produce force-curve characteristics similar to
those of the sport.
balance the clockwise torque created by the body
weight. This requires less force to be produced by
the hip abductors in order to maintain pelvic sta-
bility in the front plane during the movement.
The contralateral balance aid also increases the
base of support in both the anterior–posterior and
mediolateral directions. This makes it much easier
to maintain the center of gravity within the base of
support and maintain balance.
PROGRESSION: IPSILATERAL
WEIGHT
The ﬁ rst progression involves adding a weight
(generally a dumbbell) to the same hand as the
stance leg (Fig. 5.25). This progression is similar
Hip abd muscle force
A
BW Balance
aid force
B
Trunk weight
Balance aid force
Leg
weight
Hip extensor
muscle force
FIGURE 5.24 The single-leg RDL exercise—contralateral balance aid regression.
Chandler_Chap05.indd 23Chandler_Chap05.indd 23 10/19/2011 4:35:24 PM10/19/2011 4:35:24 PM
24 PART 1 Basic Science
Weight
Hip abd muscle force
A
BW
Weight
B
Trunk
weight
Hip extensor
muscle force
Leg
weight
FIGURE 5.25 The single-leg RDL exercise—ipsilateral weight progression.
PROGRESSION: CONTRALATERAL
WEIGHT
The second progression involves adding a weight
to the opposite hand as the stance leg (Fig. 5.26).
This progression makes the exercise more challeng-
ing for both the hip extensors and hip abductors.
Now, the downward force produced by the weight
is also producing a clockwise torque in the frontal
plane and therefore the hip abductors must pro-
duce a much greater force to keep the pelvis stable.
The effect of this contralateral weight in the sagittal
plane is the same as with the ipsilateral weight as
the dumbbell is the same distance from the axis of
rotation of the hip; therefore, the extra clockwise
torque that must be absorbed by increasing the
force created by the hip extensors is the same in
both progression conditions.
Summary
A good fundamental knowledge of biomechanics
is essential for any strength and conditioning pro-
fessional. This knowledge is imperative in order to
ensure the prescribed exercises are tailored to the
correct level, using the correct form and amount of
resistance, and reinforcing the appropriate move-
ment pattern to achieve the functional goals of the
training session as quickly and safely as possible.
Chandler_Chap05.indd 24Chandler_Chap05.indd 24 10/19/2011 4:35:29 PM10/19/2011 4:35:29 PM
25
CHAPTER 5 Biomechanics of Human Movement
Weight
Hip abd muscle force
A
BW
Weight
B
Trunk
weight
Hip extensor
muscle force
Leg
weight
FIGURE 5.26 The single-leg RDL exercise—contralateral weight progression
Maxing Out
1.
You want to incorporate some Olympic lifting into the
strength and conditioning program for the volleyball
team. Problem is the athletes are having real trouble
learning to perform the lifts correctly. How could
biomechanics be used to assist you in teaching the
athletes?
2.
The football coach has told you that he only wants his
players completing single joint exercises on pin-loaded
resistance machines and at slow speed. His rationale
is that he does not want the athletes injured in the
weight room. From your biomechanics knowledge
you do not believe such a program is optimal but you
have to convince the coach. Write a discussion paper
outlining the basis for including ground supported,
multijoint movements including high speed exercises
such as jump squats and Olympic lifts.
Chandler_Chap05.indd 25Chandler_Chap05.indd 25 10/19/2011 4:35:37 PM10/19/2011 4:35:37 PM
26 PART 1 Basic Science
CASE EXAMPLE
Extending the Application of a Simple Contact Mat Timing System to Derive More
Pertinent Mechanical Measurements
BACKGROUND
You have just been employed as a strength
and conditioning coach with a small college
that has limited performance testing equip-
ment and no current budget to purchase more
sophisticated equipment. The program has a
simple electronic timing system that can record
contact time and ﬂ ight time during vertical
jumping. In the past, only the ﬂ ight time has
been recorded and provided to the athletes and
coaches but you would like to feedback more
extensive information that is more understand-
able and relevant.
RECOMMENDATIONS/CONSIDERATIONS
One of the problems with just providing ﬂ ight
time is that the athletes cannot really relate to
the measure. They want to know how high they
have jumped. Also, in terms of quantifying leg
power ﬂ ight time does not adequately quantify
the explosiveness of the athlete or account for
athletes of differing body weights. Based on your
biomechanics knowledge you recommend to
the coaches that the jumps be performed from
an approach run, that the athletes jump onto
the mat then jump vertically upward for maxi-
mum height, landing back on the mat. You also
recommend record body mass on the day. From
these additional measures, jump height and
power will be calculated.
IMPLEMENTATION
Each sporting squad is tested prior to a skills ses-
sion. They are instructed beforehand to avoid
strenuous activity for the previous 48 hours.
Measurement of body mass. Body mass is
measured in kilograms for each athlete using an
electronic weighing scale.
Measurement of Jump Performance. As a
group the athletes are instructed in the correct
technique for performing the test.
Stand approximately three strides back from the
contact mat. Step into the mat area, landing with
both feet on the mat, then jump vertically upwards
for maximum height landing back on the mat. The
hands are to be held on the hips throughout the test.
At the end of each trial, the contact time and
subsequent ﬂ ight time will be recorded.
CALCULATIONS
Flight to Contact Ratio. A useful and easily cal-
culated measure is simply ﬂ ight time divided by
contact time.
Jump Height. Jump height can be estimated
based on the ﬂ ight time and the assumption of
simple projectile motion. The formula is
Jump height = (g × ﬂ ight time × ﬂ ight time)/8
where g = 9.81m · s−2
Work Done. Once jump height has been
determined, the work done during the concen-
tric phase of the jump can be calculated as
Work = F · d = mass × g × jump height
Absolute Power Output. As we have a measure of
contact time prior to the jump, we have an esti-
mate of the time over which the work calculated
above was completed. In most movements, the
duration of the eccentric and concentric phases
are roughly equal, which is a reasonable assump-
tion in this case. So the concentric time is equal
to the contact time divided by 2. Absolute power
is then calculated as the Work done divided by
the concentric time.
Relative Power Output. Relative power output
is calculated as absolute power output divided
by body mass. This gives an indication of the
power to weight ratio for the athlete.
relative power = absolute power · body mass−1
RESULTS
The following results were obtained on six ath-
letes and the subsequent additional measures
calculated. As you can appreciate, the use of bio-
mechanics principles has provided for a much
more in depth and relevant analysis of vertical
jump performance.
Q6
Chandler_Chap05.indd 26Chandler_Chap05.indd 26 10/19/2011 4:35:45 PM10/19/2011 4:35:45 PM
27
CHAPTER 5 Biomechanics of Human Movement
CASE EXAMPLE (Continued )
Extending the Application of a Simple Contact Mat Timing System to Derive More
Pertinent Mechanical Measurements
Athlete A B C D E F
Mass (kg) 80 78 82 79 69 74
Contact time (s) 0.561 0.493 0.587 0.534 0.521 0.508
Flight time (s) 0.567 0.587 0.543 0.602 0.511 0.519
FLT:CON ratio 1.011 1.191 0.925 1.127 0.981 1.022
Jump height (m) 0.394 0.423 0.362 0.444 0.320 0.330
Work done (J) 309 323 291 344 217 240
Absolute power (W) 1,103 1,312 991 1,290 832 944
Relative power (W · kg−1) 13.8 16.8 12.1 16.3 12.1 12.8
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Chandler_Chap05.indd 27Chandler_Chap05.indd 27 10/19/2011 4:35:46 PM10/19/2011 4:35:46 PM
Queries
[Q1] Please check if edit made to the sentence beginning “In the example of…” is OK.
[Q2] Please check if edit mae to the sentence beginning “However, if you are…” is OK.
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[Q6] Please check “to feedback more extensive information” for correctness in the sentence beginning “In
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Chandler_Chap05.indd 28Chandler_Chap05.indd 28 10/19/2011 4:35:47 PM10/19/2011 4:35:47 PM
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