Energy systems in swimming

Full text

In: World Book of Swimming: From Science to Performance ISBN: 978-1-61668-202-6
Editors: L. Seifert, D. Chollet and I.Mujika ©2010 Nova Science Publishers, Inc.
Chapter 11
ENERGY SYSTEMS IN SWIMMING
Ferran A. Rodríguez1 and Alois Mader2
1Institut Nacional d’Educació Física de Catalunya, Universitat de Barcelona, Spain
2Deutsche Sporthochschule Köln, Cologne, Germany
ABSTRACT
Swimming performance can be described as the result of the transformation of the
swimmer’s metabolic power into mechanical power with a given energetic efficiency.
Most of the energy produced by the swimmer is utilized to overcome water resistance or
drag, and the rate of energy expenditure theoretically increases with the cube of the
velocity. This energy is generated by the sum of the immediate (phosphagen), short-term
(anaerobic glycolysis) and long-term (oxidative phosphorilation or aerobic) energy
delivery systems. The relative contribution of each system has been frequently
determined on research developed in other types of exercise activities or from linear
calculations. The modeling of the energy metabolism behavior using computer
simulation, combined with physiological field measurements, offers new insights and
improved estimations on the relative contribution of the energy delivery systems.
Key words: Energy metabolism, bioenergetics, aerobic/anaerobic energy expenditure,
maximal oxygen uptake, computer simulation
1. INTRODUCTION
This chapter (i) briefly reviews the bases of the energy delivery systems during exercise,
(ii) presents a computer simulation model of the dynamic behavior of energy metabolism
during swimming, (iii) describes the relative contribution of the aerobic and anaerobic
systems to energy expenditure during swimming in different events, and (iv) summarizes the
practical applications of swimming bioenergetics for performance and training.

Ferran A. Rodríguez and Alois Mader
2
2. ENERGY METABOLISM DURING EXERCISE
In swimming, as in all forms of human locomotion, muscles generate the energy or power
to propel the body through the water. Thus, swimming performance can be described as the
result of the transformation of the swimmers metabolic power into mechanical power with a
given energetic efficiency [19]. As in any form of exercise, chemical processes transform the
chemical energy stored within the muscle or borne by the blood into mechanical energy.
Energy for any biological function is obtained from the breakdown (catabolism) of food
into three basic chemical compounds: carbohydrates, lipids, and proteins. These can be stored
and ultimately transformed into another chemical compound, adenosine triphosphate (ATP),
which is the only fuel that can be used directly for muscle force generation or for any cellular
function. In muscle, energy from the hydrolysis of ATP by the enzyme ATPase activates
specific sites on the force-developing elements, which allows the muscle filaments
(myofibrils) to generate force. ATP is also needed to re-uptake calcium ions for muscle fiber
relaxation. There are three mechanisms involved in the breakdown and resynthesis of ATP
[2,10]:
1. Phosphagen system. Phosphocreatine (PCr) is broken down enzymatically to creatine
(Cr) and Pi which is transferred to ADP to re-synthesize ATP. This is in turn broken
down enzymatically to ADP (adenosine diphosphate) and inorganic phosphate (Pi) to
yield energy for muscle activity. This mechanism is commonly called phosphagen or
anaerobic alactic (i.e. does not use oxygen nor produces lactate) system and is the
sole source of energy for muscles lasting only a few seconds (up to 3-5 s at maximal
intensity). Only two thirds of phosphagen can be depleted without the activation of
glycolysis.
2. Glycolytic system. Glucose 6-phosphate, derived from muscle glycogen or blood-
borne glucose, through anaerobic glycolysis is converted to lactate and re-synthesizes
ATP by substrate-level phosphorylation reactions. This mechanism is named the
glycolytic or anaerobic lactic (i.e. lactate producing) system and represents a
medium-term energy delivery system. This system needs a few seconds to reach
near maximal rate and delivery of ATP plateaus after ~1 min at maximal power.
3. Aerobic system. The products of carbohydrate, lipid, protein, and alcohol metabolism
can enter the tri-carboxylic acid (TCA or Krebs) cycle in the mitochondria and be
oxidized to carbon dioxide and water in the presence of oxygen. This mechanism is
known as oxidative or aerobic (i.e. oxygen using) system and can be seen as a long-
term source of energy. This system takes about 40s-1.5 min to reach maximal power
but energy delivery can be maintained for about 5-7 min at that rate and almost
indefinitely at submaximal rates of exercise.
Figure 1 presents an overview of the main metabolic pathways of energy supply from
carbohydrate, lipid, and protein substrates, as well as the three mechanisms of energy
delivery.

Energy Systems in Swimming
3
Figure 1. Overview of the main pathways of energy metabolism with indication of the three metabolic
energy delivery systems: (1) phosphagen or anaerobic alactic, (2) glycolytic or anaerobic lactic, and (3)
oxidative or aerobic (3). The purpose of the three metabolic systems is to continuously re-synthesize
ATP to avoid a significant fall of intramuscular ATP concentration.
3. METABOLIC POWER PRODUCTION DURING SWIMMING
In physiological conditions ATP concentration does not decrease much during muscular
contraction. We will consider the rates of ATP splitting and resynthesis equal for all practical
purposes. Therefore, the overall energy output per unit of time (
E

) can be expressed by the
following simple equation:[2]
2
OVcabLrCPPTAE 





++=∝
(1)
where variables (note the dot above meaning ‘per unit of time’) indicate the net rates of ATP
and PCr splitting, lactate production and O2 consumption (and thus the rate of energy
expenditure by the three mechanisms for energy supply), and the constants b and c, the
amount of ATP re-synthesized per unit of lactate formed or O2 consumed. This equation helps
us to understand the notion of a continuum of energy supply to the working muscles, and how
the rates of energy expenditure can be estimated via measurement of PCr stores (estimated as
a function of the swimmer’s body mass), muscle lactate formation and elimination (estimated

Ferran A. Rodríguez and Alois Mader
4
as blood lactate accumulation), and oxygen uptake. For the sake of simplicity we will express
equation 1 as follows:
aerlacalacmet PPPP ++=
(2)
where Pmet is total metabolic power, and Palac, Plac, and Paer, metabolic power from anaerobic
alactic (phosphagen), lactic (glycolytic), and aerobic (oxidative) sources. However, this
classification is somewhat simplistic because exercise requires the simultaneous delivery of
more than one type of energy source, and must be viewed as a continuum of energy supply.
Figure 2 shows a diagram of the total energy output (i.e. Pmet) and absolute contribution of the
three energy systems during maximal swimming, as a function of time in top male swimmers
obtained by computer simulation. The diagram illustrates the concept of simultaneous
activation of the metabolic mechanisms for the maintenance of the high energy output needed
to swim at maximal speed over the different competitive distances.
Figure 2. Total energy output (
tot
E

) and share of the three energy delivery systems during maximal
swimming as a function of time. Data (expressed as mLO2·kg-1·min-1) correspond to absolute values
for 50 to 1,500 m freestyle swimming at competitive speed in top male swimmers as obtained by
computer simulation.
Swimming can be interpreted as a simple thermodynamic process in which chemical
energy (E) is transformed into mechanical work (W in joules) with a given efficiency (e);
since W (J) per unit of time (s-1) is mechanical power (Pw in watts, W):
E/We;WE =→
(3)

Energy Systems in Swimming
5
That implies that the energy required to swim at a certain speed does not depend only on
the metabolic power developed by the swimmer but also on his or her swimming efficiency,
which is known to be the most relevant factor in swimming performance.[19]
All mechanical power output (P0) is delivered by the transformation of metabolic power
(Pm) into mechanical work. In this process, a large part of the chemical power contained in
the muscle is transformed into heat, with a given metabolic efficiency (em), which does not
vary much between subjects:
mmomom ePP;e/PP ⋅==
(4)
On the other hand, a considerable part of the mechanical power is used to overcome
water resistance or drag forces (Pd in watts),[3] which is theoretically dependent on the cube
of swimming velocity:[19]
3
vKP
d⋅=
(5)
where K is a constant incorporating the density of water, the drag coefficient, and the greatest
cross-sectional area - the last two factors largely depending on individual swimmers
characteristics. This relationship implies that small increments in swimming velocity will
dramatically increase the mechanical power needed, and thus the metabolic power that the
swimmer must generate. For example a 2% increase in velocity will require an 8% increase in
mechanical power.
But in reality, the total mechanical power output (Po) produced by the swimmer equals
not only power to overcome drag (Pd), but also power expended in giving pushed away
masses of water a kinetic energy change (Pk). The ratio of the useful power (to overcome
drag) to the total power output has been defined as the propelling efficiency (ep), and thus:
[19,20]
poop e/vKP;P/vKe 33 ⋅=⋅=
(6)
Hence, rearranging equations 4, 5, and 6, the metabolic power that a swimmer must
generate (Pm) is directly proportional to the velocity cubed times the constant K (basically
depending on the drag coefficient and cross sectional area), and inversely proportional to his
or her propelling and metabolic efficiency:
mpm ee/vKP ⋅⋅=3
(7)
Since metabolic efficiency is quite homogeneous among individuals, we will assume it is
a constant. This leaves propelling efficiency as the major denominator in the equation. This
factor must be taken into account when assessing the metabolic capacity of swimmers, since
the energy cost of swimming for submaximal intensities has often been expressed as a linear
function, yet it theoretically increases with v3.
pm e/vKP 3
⋅=
(8)

Ferran A. Rodríguez and Alois Mader
6
Another parameter expressing the relationship between energy and propulsion (i.e.
propulsive efficiency) is the energy cost of swimming (Cs), which can be defined as the
amount of metabolic energy spent in transporting the body mass (mb) of the swimmer over a
unit of distance.[3] Energy cost is usually reported in J per m per kilogram (J·m-1·kg-1) or
simply as kJ·m-1.
d/EC;dm/EC sbs =⋅=
(9)
Swimming performance (vmax) may then be quantified as:
msmax P·Cv =
(10)
At submaximal intensities (i.e. below 80% of
2max
oV

), almost the total mechanical power
output is generated by the aerobic metabolism (Paer), and since oxygen uptake can be
measured, we may establish the following relationship:
3
2vKoVP
aer ⋅=∝
(11)
The rate of anaerobic lactic energy expenditure (Plac, mLO2·min-1) can be calculated from
the oxygen energy equivalent of net blood lactate accumulation after exertion during
swimming using the 2.7 mLO2·mmol-1·kg-1 of body mass (mb) proportionality constant
calculated by di Prampero et al. (1978):[2,4]
( )
t/m]La[]La[.P bbrestbex erlac ⋅−⋅∝ 72
(12)
In high velocity swimming, the rate of alactic anaerobic energy contribution (Palac,
mLO2·min-1) can also be estimated according to the constant calculated by di Prampero et al.
(1978) for front crawl swimming (18 mLO2·kg-1):[4]
t/mP balac ⋅∝18
(13)
According to equation 4, the rate of total energy expenditure (Pm,
tot
E

, mLO2·min-1) in
high velocity swimming can be estimated from
2
oV

and blood lactate measurements after free
swimming and the two energy equivalent constants:[16,17]
)t/m()t/La.(oVEP bbtotm⋅+⋅+∝=1872
2
Δ

(14)
Another means of estimating the anaerobic contribution to swimming is based on the
calculation of the accumulated oxygen deficit as described by Medbø et al. (1988),[11] using
equation 10 and extrapolating to speeds higher than 80% of
max
oV 2

. However, this estimation
relies on the assumption that the high velocity swimming speed is proportional to v3 in its
total range of variation, which is not completely in agreement with experimental
results.[12,13,16,17]

Energy Systems in Swimming
7
4. THE ENERGY COST OF SWIMMING
Swimming performance depends on the swimmer’s metabolic power attainable at
competitive speed and the energy cost to swim at that speed (equations 9-10). Thus, the
energy cost of swimming (Cs) can be used to quantify propulsive efficiency, a major
determinant of swimming performance.
At competitive speeds, Cs is least in freestyle and greater in backstroke, butterfly and
breaststroke, respectively.[1] As can be seen in Figure 3 in a wide range of submaximal
speeds, breaststroke is the least economical stroke, and Cs increases linearly with velocity.
This relationship is probably due to wide intracycle variations in speed (i.e. the substantial
amount of energy spent accelerating the body during the pushing phase to compensate for the
loss of speed occurring in the non-propulsive phase). The other three strokes show
exponential increases in Cs with increasing velocity, with butterfly reaching a minimum at a
speed of about 1 m·s-1, probably due to the tendency of the body to sink at lower speeds.[1]
These results are highly correlated to maximal competitive speeds and time records in the
four strokes.
Figure 3. Mean values of the energy cost of swimming (Cs) in the four strokes as a function of speed.[1]

Ferran A. Rodríguez and Alois Mader
8
5. CONTRIBUTION OF THE ENERGY DELIVERY SYSTEMS DURING
MAXIMAL SWIMMING
Competitive pool events include races from 50 up to 1,500 m. The metabolic power
needed to swim at maximal speed and the relative contribution of the three energy systems
vary depending on the distance and, thus, on the swimming time at maximal intensity. On the
other hand, the power and capacity of the three energy systems are individual factors
determining performance capacity in swimming. A large part of training is devoted to the
improvement of the different energy production mechanisms.[19] However, there is a
remarkable disparity in the technical and scientific literature as to how each of these systems
is activated and their proportional contribution to the total energy output in the different
events. Table 1 summarizes studies estimating the relative contribution of the energy delivery
systems during freestyle swimming events from 50 to 1,500 m. A very large disparity
between estimates is evident, particularly in the shorter events, where the anaerobic
contributions are obviously under- or overestimated. In contrast the disparity in longer events
(800-1,500 m) is comparably smaller, presumably reflecting more accurate quantification of
the aerobic contribution due to the practicality of oxygen uptake measurements. In previous
reports, the relative contribution of each system has frequently been suggested from
estimations based on other types of exercise, on linear calculations, or assuming a linear
increase of metabolic power in supra-maximal speed (e.g. maximal accumulated oxygen
uptake estimations of anaerobic contribution). Differences on the mechanical efficiency of the
swimmers investigated may also account for the disparity.
Table 1. Suggested relative contribution of the energy systems during swimming. Data
(in percentage) are a summary of the literature and are expressed the range of values
from different authors.[1,5,9,12-14,18,21]
Distance*
Phosphagen (%)
Glycolytic (%)
Aerobic (%)
50 m
15-80
2-80
2-26
100 m
5-28
15-65
5-54
200 m
2-30
25-65
5-65
400 m
0-20
10-55
25-83
800 m
0-5
25-30
65-83
1,500 m
0-10
15-20
78-90
*For male freestyle swimmers. Relative energy contribution is assumed to be similar in females for a
given distance.
Based on experimental results during free swimming, we calculated the rates of energy
expenditure (
tot
E

) during 50, 100, 200, and 400-m crawl swimming events along a wide
range of speeds (i.e. circa 1.1-2.2 m·s-1) (figure 4). The metabolic requirements, calculated
using equation 14, increased exponentially with an exponent close to 3. This outcome is
consistent with the mechanical power requirements to overcome water resistance and drag
forces (see Equation 5), theoretically dependent on the cube of swimming velocity.[19]
Interestingly, when compared with computer simulation values based on measurements
obtained in sprinters by Ring, et al.,[14] this exponent seems to be somewhat lower for high-
velocity swimming (i.e. over 1.9 m·s-1), and can also be observed in Figure 4 as the two

Energy Systems in Swimming
9
values corresponding to male sprinters over 2 m·s-1. This outcome may be explained by the
greater body height of male sprinters, which is related with faster swimming at high speed,
and/or by other biomechanical factors such as the hydrodynamic lift.
Figure 3. Rate of total energy demands (
tot
E

) during front crawl swimming in the range of submaximal
and maximal speeds (ca. 1.1-2.2 m·s-1). Values (filled circles) have been calculated using equation 13
from free swimming experimental data. Empty triangles (and dashed line) correspond to computer
simulation results in 50-m sprinters.[14] See text for further details.
6. MODELING OF THE AEROBIC AND ANAEROBIC ENERGY
YIELD AS A FUNCTION OF SWIMMING SPEED: A NEW APPROACH
In recent years, a model of the dynamics of energy muscle metabolism has been
developed by Mader.[6,8] This model assumes that the dynamic changes of the
phosphorylation state of the high-energy phosphate system, and the adjustment of the rate of
glycolysis and oxidative phosphorylation as a result of contraction, as well as the lactate
distribution from muscle to a passive compartment, can be calculated by a system of
differential equations. To summarize, Mader’s model is based on the regulation of ATP
production in muscle cells by oxidative phosphorylation (OxP) and glycolysis as a function of
the phosphorylation state of the cytosolic high-energy phosphate system. In this model, the
activation of OxP can be established including free cytosolic [ADP] as substrate and a second
driving force which results from the difference of free energy between mitochondria
(ΔGOXap ~ (1+ n)*Δp) and cytosol (ΔGATPcyt). Glycolysis is regulated at the level of PFK

Ferran A. Rodríguez and Alois Mader
10
mainly by free [ADP] and [AMP]. PFK is inhibited by a decreasing pH resulting from lactate
accumulation. The steady state characteristics of the activation control exhibit sigmoid types
of kinetics. The ATP/PCr equilibrium is calculated by algebraic equations. The dynamic
behavior of the metabolic control of ATP production as function of ATP consumption is
calculated by a system of two first order non-linear differential equations, which include the
steady state characteristics of OxP and glycolysis as well as a time delay for oxygen transport.
Lactate distribution and elimination is calculated using a two compartment model with an
active lactate space (lactate production) and a passive space, which includes lactate oxidation
and glucose resynthesis. After translation into a computer simulation model, the simulation of
the dynamics of the metabolic behavior is performed by a stepwise solution of the differential
equation with a Runge-Kutta-Fehlberg-method. The model has been applied to the computer
simulation analysis of energy supply during 50-m swimming[13,14], and 100, 200, and 400-
m crawl.[12,13,18]
Based on this computer simulation analysis (a detailed description and assumptions are
beyond the scope of this chapter but can be found elsewhere[6]) a much more precise picture
of metabolic processes involved during a race can be reproduced. Figure 5 displays an output
diagram resulting from the computer simulation of metabolic parameters during rest, exercise,
and recovery for 400 m freestyle swum at 3 min 45 s. Input parameters were adjusted to meet
the expected metabolic pattern of an elite 400-m specialist (i.e. very high oxidative and
relatively low glycolytic power). Figure 6 shows the corresponding time-dependent relative
contribution of the three energy delivery systems.
Figure 5. Computer simulation of metabolic parameters during rest, exercise, and recovery for a 400-m
freestyle race (3 min 45 s). See further details in the text.

Energy Systems in Swimming
11
Figure 6. Share of the three energy delivery systems during a 400-m race (3 min 45 s) as calculated by
dynamic computer simulation. Values are percentage of energy output from aerobic, alactic
(phosphagen), and glycolytic (lactic) processes, respectively.
To quantify the energy share during swimming at maximal speed, the dynamic behavior
of the energy metabolism during 50 to 1,500-m events has been simulated and a summary of
computed results are presented in Table 3. Input parameters (i.e.
max
OV 2

, maximal glycolytic
power, maximal ATP-PCr availability in the active muscles, percentage of active muscle
mass, and space for lactate distribution) were specified to meet the expected swimmer’s
power output (according to equation in Figure 4) and specific metabolic profile (e.g.
relatively high glycolytic power and low aerobic power in sprinters, high aerobic power, and
low glycolytic power in long-distance swimmers).
Table 3. Share of energy systems during freestyle swimming competitive events in top-
level swimmers obtained by computer simulation. Data are in percentage of total energy
output (Etot). See text for further details.
Distance
Time* (min:s)
Phosphagen (%)
Glycolytic (%)
Aerobic (%)
50 m
0:22.0
38
58
4
100 m
0:48.0
20
39
41
200 m
1:45.0
13
29
58
400 m
3:45.0
6
21
73
800 m
7:50.0
4
14
82
1,500 m
14:50.0
3
11
86
* Reference times for top-level male freestyle swimmers (2008). The relative energy patterns in female
swimmers are assumed to be relatively similar for a given distance.

Ferran A. Rodríguez and Alois Mader
12
The prevalence of anaerobic processes during sprinting events (i.e. 22 to 48 s ca.), leads
to a progressive predominance of the aerobic processes in middle- (i.e. 200 and 400 m) and
long-distance events (i.e. 800 and 1,500 m).
In 50m events a large muscle mass with a high percentage of fast twitch fibers (i.e. with
high glycolytic power) is required to produce a remarkably high energy output (over 200
mLO2·kg-1·min-1). ATP and PCr stores are rapidly depleted and glycolysis is almost
immediately activated to maintain energy output and becoming the most important source of
energy for muscle contraction. However, the short duration of these events prevents a high
level of acidosis from occurring, and maximal blood lactate will typically be at the range of
12-14 mmol·L-1. The activation of the aerobic system can be neglected because the race can
be completed without breathing.
100-m events, contrary to traditional views, require the complete and rapid activation of
both the glycolytic and aerobic energy delivery processes, as well as the capacity to sustain
high lactate concentrations in the active muscles. These high rates of activation (with short
time constants of oxygen uptake and glycolysis) must also be maintained to sustain the high
energy output needed to swim at maximal speed. Average values for the time constant τ = 22
s (ranging from 12 to 32 s) have been measured using breath-by-breath gas analysis.[15] In
fact, the simulations show that τ decreases when a fast PCr depletion takes place. Muscle
lactate will rapidly accumulate to values in excess of 30 mmol·L-1 causing acidosis (pH
reduction) and decreasing glycolysis probably through the inhibition of phosphofructokinase,
a key enzyme in the glycolytic pathway. This sequence of events will cause in turn a decrease
of the muscular rate of contraction and the onset of fatigue if the aerobic processes are not
able to sustain energy output during the last two thirds of the race. Blood lactate during
competition would typically rise up or exceed 20 mmol·L-1. Interestingly, during 100-m
maximal swims, peak
2
OV

correlated significantly with swimming speed (r = .79) and to 400
m (r = .75), confirming that both a high aerobic power and a fast activation of the aerobic
mechanisms are needed.
Middle-distance events require very high
max
OV 2

values (i.e. maximal aerobic power), as
well as moderate (400 m) to high (200 m) glycolytic power. The rate of activation of the
aerobic system is lower in 400-m events as compared to 100-m (average time constant equals
about 28 s),[15] but top-level specialists show the highest
max
OV 2

among swimmers, with
values in the range of 70-75 mL·kg-1·min-1 (60-65 for females) and a large body mass.
Maximal blood lactate values are typically in the range of 16-18 mmol·L-1 (200 m) and 14-16
mmol·L-1 (400 m). Interestingly, the fastest swimmers will also be able to swim faster at low
blood-lactate concentrations.[7]
Long-distance events are characterized by a predominance of the aerobic energy delivery
processes. Long-distance specialists show very high
max
OV 2

values, many in excess of 75
mL·kg-1·min-1 (65 for females), and low glycolytic power. A metabolic balance is required,
with muscle glycogen oxidation occurring at a high rate and
2
OV

reaching values close to
maximal. Glycolysis is in turn activated, reaching a steady state at a higher level of muscle
lactate. Maximal blood lactate values are typically in the range of 8-12 mmol·L-1.
Figure 7 displays graphically the percentage relative contribution of the three energy
delivery systems and the simplified aerobic-anaerobic share during swimming at maximal
speed (50 to 1,500 m) as a function of time obtained by computer simulation.

Energy Systems in Swimming
13
Figure 7. Share of the three energy delivery systems (A) and simplified aerobic-anaerobic contribution
(B) during freestyle swimming at maximal speed as a function of time. Data (in percentage of total
energy demands,
tot
E

) were obtained by computer simulation. Symbols denote values corresponding to
50, 100, 200, 400, 800, and 1,500 m freestyle, respectively. See text for further details.
7. PRACTICAL APPLICATIONS
For performance:
1) Comprehension of the metabolic energy demands in various swimming events may
help the coach to determine which ones (i.e. sprint, middle-distance, or distance) best
match the individual metabolic capacities of a swimmer.

Ferran A. Rodríguez and Alois Mader
14
2) Computer simulations of the dynamic metabolic behavior, together with individual
metabolic data collection, may help the coach determining the optimal race (pacing)
strategy during competition.
For training:
1) A large amount of training is devoted to improve the metabolic capacities of the
swimmer. Knowledge of the metabolic requirements of the different swimming
events and strokes may help the coach to develop training plans to enhance the
underlying metabolic capacities.
2) Metabolic testing may identify which metabolic capacities are being improved,
maintained or decreased through training, and how the swimmer’s metabolic profile
corresponds to the optimal energy system development for given events and strokes.
8. CONCLUSION
Swimming performance is the result of the transformation of metabolic power into
mechanical power with a given energetic efficiency. Most of the energy produced will be
utilized to overcome drag, and the rate of energy expenditure will increase approximately
with the velocity cubed. Energy is generated by the sum of the immediate (ATP-PCr), the
short-term (anaerobic glycolysis), and the long-term (oxidative phosphorilation or aerobic)
energy delivery systems.
The energy cost of swimming is used to quantify propulsive efficiency, a major
determinant of swimming performance. At competitive speeds, propulsive efficiency is
greater in freestyle and lower in backstroke, butterfly, and breaststroke, respectively.
To quantify the energy share during swimming at maximal speed, the dynamic behavior
of the energy metabolism has been simulated. In 50-m events ATP and PCr stores are rapidly
depleted and glycolysis is almost immediately activated to maintain energy output. 100-m
events require the complete and rapid activation of both the glycolytic and aerobic energy
systems, since the decreasing glycolytic energy supply is compensated in part by increasing
oxidative ATP production during the last two thirds of the race. Middle-distance events
require very high
max
OV 2

values, as well as moderate to high glycolytic power. Long-
distance events are characterized by a predominance of the aerobic energy delivery processes.
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