Peterson D.R., Bronzino J.D. (Eds.) Biomechanics: Principles and Applications
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20
Factors Affecting
Mechanical Work
in Humans
Ben F. Hurley
Arthur T. Johnson
University of Maryland
20.1 Exercise Biomechanics ................................
20-1
Equilibrium
•
Muscular Movement
•
Muscular
Efficiency
•
Locomotion
•
Effects of Age
•
Effects of
Exercise Training
•
Effects of Gender
•
Effects of
Genetics
References ...................................................20-9
High technology has entered our diversions and leisure activities. Sports, exercise, and training are no
longer just physical activities but include machines and techniques attuned to individual capabilities and
needs. This chapter considers several factors related to exercise and training that help in understanding
human performance.
Physiological work performance is determined by energy transformation that begins with the process
of photosynthesis and ends with the production of biological work (Figure 20.1). Energy in the form of
nuclear transformations is converted to radiant energy, which then transforms the energy from carbon
dioxide and water into oxygen and glucose through photosynthesis. In plants, the glucose can also be
converted to fats and proteins. Upon ingesting plants or other animals that eat plants, humans convert
this energy through cellular respiration (the reverse of photosynthesis) to chemical energy in the form of
adenosine triphosphate (ATP). The endergonic reactions (energy absorbed from the surroundings) that
produce ATP are followed by exergonic reactions (energy released to surroundings) that release energy
through the breakdown of ATP to produce chemical and mechanical work in the human body. The steps
involved in the synthesis and breakdown of carbohydrates, fats, and proteins produce chemical work and
provide energy for the mechanical workproducedfrom muscular contractions. The purpose of this chapter
is to provide a brief summary of some factors that can affect mechanical work in humans.
20.1 Exercise Biomechanics
20.1.1 Equilibrium
Anybody, including the human body, remains in stable equilibrium if the vectorial sum of all forces and
torques acting on the body is zero. An unbalanced force results in linear acceleration, and an unbalanced
20-1
20-2 Biomechanics
Nuclear
energy
Heat
ATP
ADP
Glucose
O
2
CO
2
Mechanical
work
FIGURE 20.1 Schematic of energy transformations leading to muscular mechanical work.
torque results in rotational acceleration. Static equilibrium requires that:
F = 0 (20.1)
T = 0 (20.2)
where F is vectorial forces, N, and T is vectorial torques, N m.
Some sport activities, such as wrestling, weight lifting, and fencing, require stability, whereas other
activities, including running, jumping,and diving, cannot beperformed unlessthereis managed instability.
Shifting body position allows for proper control. The mass of the body is distributed as in Table 20.1,
and the center of mass is located at approximately 56% of a person’s height and midway from side-to-
side and front-to-back. The center of mass can be made to shift by extending the limbs or by bending
the torso.
TABLE 20.1 Fraction of Body
Weights for Various Parts of the Body
Body Part Fraction
Head and neck 0.07
Trunk 0.43
Upper arms 0.07
Forearms and hands 0.06
Thighs 0.23
Lower legs and feet 0.14
1.00
Factors Affecting Mechanical Work in Humans 20-3
d
F
d
W
Fulcrum
Force
Load
FIGURE 20.2 A class 3 lever is arranged with the applied force interposed between the fulcrum and the load. Most
skeletal muscles are arranged in this fashion.
20.1.2 Muscular Movement
Mechanical movement results from contraction of muscles that are attached at each end to bones that
can move relative to each other. The arrangement of this combination is commonly known as a class 3
lever (Figure 20.2), where one joint acts as the fulcrum (Figure 20.3), the other bone acts as the load, and
the muscle provides the force interposed between fulcrum and load. This arrangement requires that the
muscle force be greater than the load, sometimes by a very large amount, but the distance through which
the muscle moves is made very small. These characteristics match muscle capabilities well (muscles can
produce 7 × 10
5
N/m
2
, but cannot move far). Since the distance is made smaller, the speed of shortening
Biceps
Triceps
FIGURE 20.3 The biceps muscle of the arm is arranged as a class 3 lever. The load is located at the hand and the
fulcrum at the elbow.
20-4 Biomechanics
of the contracting muscle is also slower than it would be if the arrangement between the force and the load
were different:
S
L
S
M
=
d
L
d
M
(20.3)
where S is speed, m/sec; d is distance from fulcrum, m; and L, M denotes load and muscle.
20.1.3 Muscular Efficiency
Efficiency relates external, or physical, work produced to the total chemical energy consumed:
η =
External work produced
Chemical energy consumed
(20.4)
Muscular efficiencies range fromclose tozeroto about 20 to 25%. The larger numbers wouldbe obtained
for leg exercises that involve lifting body weight. In carpentry and foundry work, where both arms and
legs are used, average mechanical efficiency is approximately 10% [Johnson, 1991]. For finer movements
that require exquisite control, small muscles are often arranged in antagonistic fashion, that is, the final
movement is produced as a result of the difference between two or more muscles working against each
other. In this case, efficiencies approach zero. Isometric muscular contraction, where a force is produced
but no movement results, has an efficiency of zero [Johnson et al., 2002].
Muscles generally are able to exert the greatest force when the velocity of muscle contraction is zero.
Power produced by this muscle would be zero. When the velocity of muscle contraction is about 8 m/sec
the force produced by the muscle becomes zero, and the power produced by this muscle again becomes
zero. Somewhere in between the above conditions stated, the power produced and the efficiency become
the maximum (Figure 20.4).
The isometric length–tension relationship of a muscle shows that the maximum force developed by a
muscle is exerted at its resting length (the length of a slightly stretched muscle attached by its tendons to
20
Muscle force (N)
Muscle power (N m/sec)
Velocity of shortening (m/sec)
10
0
0246 8
60
50
40
30
20
10
0
Force
Power
FIGURE 20.4 Force and power output of a muscle as a function of velocity. (Adapted and used with permission from
Milsum, 1966.)
Factors Affecting Mechanical Work in Humans 20-5
Hand cranking
Bicycling
Work load (Hp)
Gross efficiency (%)
20
16
12
8
4
0
0 50 100
Work rate (N m/sec)
150 200
0 0.1 0.2 0.3
FIGURE 20.5 The gross efficiency for hand cranking or bicycling is a function of the rate of work. (From Goldman,
R.F., 1978. Computer models in manual materials handling. In Drury, C.G. (Ed.), Safety in Manual Materials Handling,
National Institute for Occupational Safety and Health (NIOSH), Cincinnati, Ohio, pp. 110–116.)
the skeleton) and decreases to zero at twice its resting length. Maximum force also decreases to zero at the
shortest possible muscular length. Since muscular contractile force depends on the length of the muscle
and length changes during contraction, muscular efficiency is always changing (Figure 20.5).
Negative (eccentric) work is produced by a muscle when it maintains a force against an external force
tending to stretch the muscle. An example of negative work is found as the action of the leg muscle during
a descent of a flight of stairs. Since the body is being lowered, external work is less than zero. The muscles
are using physiological energy to control the descent and prevent the body from accumulating kinetic
energy as it descends.
Muscular efficiencies for walking downhill approach 120% [McMahon, 1984]. Since heat produced by
the muscle is the difference between 100% and the percent efficiency, heat produced by muscles walking
downhill is about 220% of their energy expenditure. Energy expenditure of muscles undergoing negative
work is about one-sixth that of a muscle doing positive work [Johnson, 1991], so a leg muscle going uphill
produces about twice as much heat as a leg muscle going downhill.
20.1.4 Locomotion
The act of locomotion involves both positive and negative work. There are four successive stages of a
walking stride. In the first stage, both feet are on the ground, with one foot ahead of the other. The trailing
foot pushes forward, and the front foot is pushing backward. In the second stage, the trailing foot leaves
the ground and the front foot applies a braking force. The center of mass of the body begins to lift over
the front foot. In the third stage, the trailing foot is brought forward and the supporting foot applies a
vertical force. The body center of mass is at its highest point above the supporting foot. In the last stage,
the body’s center of mass is lowered and the trailing foot provides an acceleration force.
This alteration of the raising and loweringof the body centerof mass, along with the pushing and braking
provided by the feet, makes walking a low-efficiency maneuver. Walking has been likened to alternatively
applying the brakesand accelerator while driving a car. Just as the fuel efficiency of the car would suffer from
this mode of propulsion, so the energy efficiency of walking suffers from the way walking is performed.
20-6 Biomechanics
2000
1500
Running
Cycling
Walking
Power (N
m/sec)
1000
500
0
0123456
Speed (m/sec)
78 9
FIGURE 20.6 Power required for walking, running, and cycling by an adult male. Curves for walking and running
interact at about 2.3 m/sec and show that walking is more efficient below the intersection and running is more efficient
above. (Redrawn with permission from Alexander, 1984.)
There is an optimum speed of walking. Faster than this speed, additional muscular energy is required
to propel the body forward. Moving slower than the optimal speed requires additional muscular energy
to retard leg movement. Thus, the optimal speed is related to the rate at which the leg can swing forward.
Simple analysis of the leg as a physical pendulum shows that the optimal walking speed is related to leg
length:
S ∝
√
L (20.5)
Unlike walking, there is a stage of running during which both feet leave the ground. The center of mass
of the body does not rise and fall as much during running as during walking, so the efficiency for running
can be greater than for walking.
At a speed of about 2.5 m/sec, running appears to be more energy efficient than walking, and the
transition is usually made between these forms of locomotion (Figure 20.6). Unlike walking, there does
not appear to be a functional relationship between speed and leg length, so running power expenditure is
linearly related to speed alone.
Whywould anyonewant to propelthe extra weight of a bicyclein addition to body weight? Onthe surface
it would appear that cycling would cost more energy than running or walking. However, the center of mass
of the body does not move vertically as long as the cyclist sits on the seat. Without the positive and negative
work associated with walking or running, cycling is a much more efficient form of locomotion than the
other two (Figure 20.6), and the cost of moving the additional weight of the bicycle can easily be supplied.
Many sports or leisure activities have a biomechanical basis. Understanding of the underlying biome-
chanical processes can lead to improved performance. Yet, there are limits to performance that cause
frustrations for competitive athletes. Hence, additional factors are sometimes employed to expand these
limits. A brief discussion of some of these factors is given here.
20.1.5 Effects of Age
It is well established that the maximal capacity to exercise declines with age. However, many factors that
change with age also affect maximal capacity to exercise. Examples of these factors include a gain in body
fat, a decrease in lean body mass, onset of disease, and a decline in the level of physical activity. For this
Factors Affecting Mechanical Work in Humans 20-7
reason, it is difficult to determine exactly how much of the loss in capacity to exercise with age can really be
attributed solely to the effects of aging. Nevertheless, the decline in cardiovascular or aerobic capacity with
advanced aging is substantial. Aerobic capacity is assessed by measuring the maximal amount of oxygen
that can be utilized during exercise and is known as V
O
2
max
. Pollock et al. [1997] reported more than a 30%
decline in V
O
2
max
per decade of aging in a 20-year longitudinal study in those who did not maintain high
levels of physical activity as they aged. This age-related decline was reduced substantially (∼5to8%per
decade) when exercise training was maintained throughout aging. Thus, there is no doubt that maximal
work capacity declines with advancing age. Although regular exercise (training) appears to reduce this
decline substantially, it cannot completely prevent the detrimental effects of age on aerobic capacity.
Muscular strength (maximal force production) is maintained for a much longer time period throughout
aging. Knee extensor strength, for example, does not become significantly reduced in men until they reach
their 60s [Lynchet al., 1999]. These losses then occur at a rate of ∼12 to 14% per decade. Loss of muscle mass
occurs at a rate of ∼6% per decade [Lynch et al., 1999]. There is some uncertainty about the magnitude
of muscle power (product of force and velocity) loss with age [Martin et al., 2000], but it is believed to
occur at a faster rate than strength [Skeleton et al., 1994]. There is also a loss of ∼8%perdecadeinforce
per unit of muscle mass (muscle quality) [Lynch et al., 1999], indicating that the loss of strength is more
than can be accounted for by strength alone.
20.1.6 Effects of Exercise Training
Forthe purpose of this review, exercise training will be divided intotwo training modalities, aerobicexercise
training and strength training. Aerobic exercise training consists of regular muscular activities that use
large amounts of oxygen to produce energy and include exercises such as walking, jogging, swimming,
and cycling. Strength training (ST) refers to the type of regular muscular activity that cause muscles to
contract (e.g., isotonic or so-called concentric exercise) or attempt to contract (e.g., isometric or eccentric
exercise) against resistance. Hence, this type of training is often called resistance or resistive training and
it can be performed with weight plates, machines, or by just using one’s body weight for resistance, such
as is done with certain kinds of calisthenics.
20.1.6.1 Aerobic Exercise Training
Many compensatory reactions allow the body to adapt to minor stresses, such as mild aerobic exercise, so
that homeostasis (equilibrium) can be maintained. For example, the increased energy demands of aerobic
exercise stimulate an increase in heart rate, respiration, blood flow, and many other cardiovascular and
metabolic reactions that allow the body to maintain homeostasis. As the intensity of exercise increases it
becomes more difficult for compensatory mechanisms to maintain homeostasis. After exceeding about
80% of an untrained person’s maximal exercise capacity homeostasis can no longer be maintained for
more than a few minutes before exhaustion results.
Regular aerobic exercise (training) elevates the threshold level so that a single exercise session can be
performed before disturbing and eventually losing homeostasis. It does this by elevating the maximal
physiological capacity for homeostasis so that the same intensity of aerobic exercise may no longer disrupt
homeostasis because it is a lower percentage of maximal capacity. In addition, training produces specific
adaptations during submaximal exercise that permit greater and longer amounts of work before losing
homeostasis [Hurley and Hagberg, 1998]. A good example of this is when blood lactate rises with increased
intensity of exercise. Prior to training, blood lactate concentration rises substantially when the intensity
of exercise exceeds about 75% of maximal oxygen consumption (V
O
2
max
).
Following training, the same intensity of exercise results in a lower concentration of blood lactate, so
exercise can now be performed at a higher fraction of the untrained V
O
2
max
before blood lactate reaches
the same level as before training. This adaptation is so profound that blood lactate levels are often only
slightly above resting values after training when performing submaximal exercise (e.g., <65% of V
O
2
max
).
Thus, exercise training allows an individual to perform a much greater amount of work during exercise
before homeostasis is disturbed to the point at which muscular exhaustion results.
20-8 Biomechanics
20.1.6.2 Strength Training
Muscular strength declines with age [Lindle et al., 1997; Lynch et al., 1999; Roth et al., 2001], disease
[Singh, 2002], and as a consequence of the administration of some medications [Singh, 2002] and can
be increased substantially in a relatively short time period with ST [Hurley and Roth, 2000; Hurley and
Kostek, 2001]. Muscle power can also be increased with ST, but the magnitude of increase depends on the
movement velocity of training during the muscle-shortening phase of exercise. Typical training modalities
of ∼2 sec during the shortening phase and ∼4 sec during the muscle-lengthening phase of movement
result in ∼30% increase in strength [Lemmer et al., 2000] and ∼25% increase in power [Jozsi et al., 1999].
However, increases of almost 100% have been reported when fast-velocity training has been incorporated
[Fielding et al., 2002]. Our group has reported increases in muscle mass of ∼12% with ST, within the first
couple of months of training, based on measurements made by magnetic resonance imaging of the entire
volume of the muscle being trained [Ivey et al., 2000]. The amount of force per unit of muscle volume
also increases substantially with ST [Tracey et al., 1999]. We have shown that ∼3 decades of age-related
strength loss and two decades of age-related muscle mass loss can be recovered/reversed within the first
couple of months of ST [Ivey et al., 2000; Lemmer et al., 2000].
20.1.7 Effects of Gender
Cardiovascular fitness and muscular strength are substantially higher in men compared to women. The
aerobic capacity in men is about 40 to 50% higher than women. In addition, upper body strength is ∼100%
higher and lower body strength is ∼50% higher in men [Lynch et al., 1999]. However, these differences are
diminished substantially when body composition is taken into consideration. For example, when V
O
2
max
is expressed with reference to body mass (ml/kg of body weight/min) this difference narrows to ∼20% and
to ∼10% when differences in muscle mass are taken into consideration. Differences in muscular strength
between men and women are also narrowed when normalized for fat free mass [Lynch et al., 1999]. There
does not appear to be any significant difference in responses to training between men and women when
expressed on a relative basis (% change), but men appear to have greater muscle mass gains than women
in response to ST when expressed in absolute terms [Ivey et al., 2000].
20.1.8 Effects of Genetics
We have observed great inter-individual variability in both the loss of muscle strength and mass with age,
known as sarcopenia [Lindle et al., 1997; Lynch et al., 1999], as well as the gain in muscle strength and
muscle mass with exercise training [Ivey et al., 2000; Lemmer et al., 2000]. For example, in a large number of
ST studies performed in our laboratory, the increases in strength for individual subjects range from <5to
>100% and muscle mass changes range from no appreciable change to >25% increase. These ST-induced
changes in strength and muscle mass are also highly variable in both young and older men and women.
In this context, after only nine weeks of a highly standardized quadriceps ST program in a healthy and
homogeneous group of young (20–30 yrs) and older (65–75 yrs) men and women, knee extension strength
gains ranged from 9 to 45 kg in the young group and from 2 to 39 kg in the older group [Ivey et al., 2000].
These strength changes amounted to a 10–51% increase in the young group and a 5–59% increase in the
older group. In this same study, the increases in MRI measured quadriceps muscle volume ranged from 19
to 344 cm
3
in the older subjects and 60 to 481 cm
3
in the young subjects. On a relative basis these increases
ranged from<1 to20% of the initial musclevolume of these subjects, further demonstrating the wide range
of inter-individual changes in the trained muscle mass resulting from this highly standardized, short-term
ST intervention. Others have observed similar large variations in strength and muscle mass changes with
training [Buchner et al., 1993]. Collectively, these data strongly support the notion that genetic factors are
involved in determining muscle strength and muscle mass responses to strength training.
The hypothesis that genetic factors may influence muscular strength is also supported by data from
both animals [Biesiadecki et al., 1998] and humans [Reed et al., 1991]. Biesiadecki et al. [1998] observed
a 1.5–5.2 fold divergence between the muscular strength of male rat strains with the lowest- and
Factors Affecting Mechanical Work in Humans 20-9
highest-strength levels, and Reed et al. [1991] observed significant genetic effects for absolute grip strength
normalized for body weight in 127 identical and 130 fraternal twin pairs. Heredity accounted for 65% of
the variance in grip strength, even after adjusting for the effects of weight, height, and age. Furthermore,
Seeman et al. [1996] reported that genetic factors accounted for 60–80% of the inter-individual differences
in lean body mass. More recently, Huygens et al. [2004] reported heritability of skeletal muscle mass of
up to 90%. In addition, Thomis et al. [1998] found evidence for a significant role of heredity in strength
responses to strength training. These responses were independent of the influence of genetic factors on
baseline strength values. These data provide strong support that genetic factors may influence sarcopenia
(i.e., alterations in strength and muscle mass with age) and strength and muscle mass response to strength
training. However, none of these studies assessed the effects of specific candidate genes or candidate gene
variations (polymorphisms) on strength or muscle mass response to training.
The heritability of musclemass and strengthindicate that specific genes contribute to differences in mus-
cle phenotype. More importantly, specific polymorphisms could at least partly explain the inter-individual
variability in sarcopenia and muscle response to ST. We recently explored the relationship of gene poly-
morphisms to muscle phenotypes in genes thought to have a plausible physiological connection to changes
in strength and muscle mass. We identified several candidate gene polymorphisms that were associated
with changes in strength or muscle mass with either age or ST. These included genes that code for ciliary
neurotrophic factor, insulin-like growth factors I and II, growth and differentiation factor 8 (myostatin),
Vitamin D, and androgen receptors. At the time of this writing, studies are ongoing to assess the role of
these genes in determining the extent to which strength and muscle mass respond to aging and ST. Based
on the substantial heritability of muscle phenotype, future research will likely find other gene variants
that help explain the inter-individual variability in both sarcopenia and muscle adaptation in response
to both aerobic and strength training. This information is likely to provide a basis for individualizing
exercise prescriptions for the prevention and treatment of sarcopenia and help explain why some people
make substantial adaptations to training, whereas others do not. Moreover, it will provide specific expla-
nations from a mechanistic perspective as to how genetics can influence mechanical and biological work
in humans.
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