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Sports Med 2004; 34 (14): 967-981
R
EVIEW
A
RTICLE
0112-1642/04/0014-0967/$31.00/0
2004 Adis Data Information BV. All rights reserved.
Does Overtraining Exist?
An Analysis of Overreaching and Overtraining Research
Shona L. Halson
1,2
and Asker E. Jeukendrup
1
1 Human Performance Laboratory, School of Sport and Exercise Sciences, University of
Birmingham, Edgbaston, UK
2 Department of Physiology, Australian Institute of Sport, Belconnen, ACT, Australia
Contents
Abstract ....................................................................................967
1. Incidence of Overtraining.................................................................971
2. Overreaching Research ..................................................................972
2.1 Performance ........................................................................972
2.2 Mood State .........................................................................973
2.3 Physiology ..........................................................................973
2.4 Biochemistry .........................................................................973
2.5 Glycogen Depletion .................................................................974
2.6 Immune System ......................................................................974
2.6.1 Glutamine .....................................................................976
2.7 Hormones ...........................................................................976
2.7.1 Cortisol and Testosterone .......................................................976
2.7.2 Other Hormones ...............................................................977
2.8 Autonomic Nervous System ...........................................................977
2.8.1 Catecholamines ...............................................................977
2.8.2 Heart Rate Variability ...........................................................978
3. Overtraining Research ....................................................................978
4. Conclusions and Directions for Future Research .............................................979
Athletes experience minor fatigue and acute reductions in performance as a
Abstract
consequence of the normal training process. When the balance between training
stress and recovery is disproportionate, it is thought that overreaching and
possibly overtraining may develop. However, the majority of research that has
been conducted in this area has investigated overreached and not overtrained
athletes. Overreaching occurs as a result of intensified training and is often
considered a normal outcome for elite athletes due to the relatively short time
needed for recovery (approximately 2 weeks) and the possibility of a supercom-
pensatory effect. As the time needed to recover from the overtraining syndrome is
considered to be much longer (months to years), it may not be appropriate to
compare the two states. It is presently not possible to discern acute fatigue and
decreased performance experienced from isolated training sessions, from the
states of overreaching and overtraining. This is partially the result of a lack of
diagnostic tools, variability of results of research studies, a lack of well controlled
studies and individual responses to training.
968 Halson & Jeukendrup
The general lack of research in the area in combination with very few well
controlled investigations means that it is very difficult to gain insight into the
incidence, markers and possible causes of overtraining. There is currently no
evidence aside from anecdotal information to suggest that overreaching precedes
overtraining and that symptoms of overtraining are more severe than overreach-
ing. It is indeed possible that the two states show different defining characteristics
and the overtraining continuum may be an oversimplification. Critical analysis of
relevant research suggests that overreaching and overtraining investigations
should be interpreted with caution before recommendations for markers of over-
reaching and overtraining can be proposed. Systematically controlled and moni-
tored studies are needed to determine if overtraining is distinguishable from
overreaching, what the best indicators of these states are and the underlying
mechanisms that cause fatigue and performance decrements. The available scien-
tific and anecdotal evidence supports the existence of the overtraining syndrome;
however, more research is required to state with certainty that the syndrome
exists.
The process of intensifying training is commonly As it is possible, by definition, to recover from a
employed by athletes in an attempt to enhance per-
state of overreaching within a 2-week period,
[2,3]
it
formance. As a consequence, the athlete may experi-
may be argued that this condition is a relatively
ence acute feelings of fatigue and decreases in per-
normal and harmless stage of the training process.
formance as a result of a single intense training
However, athletes who are in an overtrained state
session. Athletes may experience similar or height-
may take months or possibly years to completely
ened symptoms following completion of the intensi-
recover, in which time an athlete’s career may be
fied training period and may be identified as over-
seriously compromised. Therefore, the usefulness
reached. Continual training and/or non-training
and relevance of research purely investigating over-
stress is suggested to result in a state of overtrain-
reaching must be considered. In essence, it is gener-
ing.
[1]
To date, there are a number of investigations
ally thought that symptoms of overtraining, such as
that have examined the effects of an intensified
fatigue, performance decline and mood distur-
training period that resulted in overreaching, on
bances, are more severe than those of overreaching.
numerous physiological, biochemical, psychologi-
However, there is no scientific evidence to either
cal, immunological and hormonal variables (for a
confirm or refute this suggestion. Importantly, as
recent review, see Urhausen and Kindermann
[1]
).
there is no diagnostic tool to identify an athlete as
However, information regarding overtraining is
overtrained, diagnosis can only be made by exclud-
based almost entirely on anecdotal information. The
ing all other possible influences on changes in per-
majority of studies examine athletes in a state of
formance and mood state. Therefore, if no explana-
overreaching, as it is not ethical to deliberately
tion for the observed changes can be found, over-
induce a state of overtraining.
training is diagnosed. Hence, there is no objective
Overreaching is often utilised by athletes during evidence that the athlete is indeed overtrained. Ad-
a typical training cycle to enhance performance. ditionally, in the studies that induce a state of over-
Intensified training can result in a decline in per- reaching, many of the physiological and biochemi-
formance; however, when appropriate periods of cal responses to the increased training are highly
recovery are provided, a ‘supercompensation’ effect variable, with some measures in some studies dem-
may occur with the athlete exhibiting an enhanced onstrating changes and others remaining unaltered.
performance when compared with baseline levels. The aim of this review is to critically examine
2004 Adis Data Information BV. All rights reserved. Sports Med 2004; 34 (14)
Does Overtraining Exist? 969
literature on both overreaching and overtraining and tions are not entirely satisfactory; however, at pre-
to determine if there is sufficient and appropriately sent they provide the most accurate description of
accomplished scientific evidence to indicate that the the conditions and are commonly cited in the litera-
state of overtraining does indeed exist. ture.
The lack of common and consistent terminology
In addition, the term overtraining and overreach-
in the study of overtraining is one of the many
ing will be considered the product or outcome, while
problems associated with research in this area. For
intensified training will be considered the process.
the purpose of this review the following definitions
This is to avoid confusion surrounding the use of the
will be used:
[4]
term overtraining as both the outcome and the pro-
cess of this state.
•
Overtraining: an accumulation of training and/or
non-training stress resulting in long-term decre-
The process of intensified training leading to
ment in performance capacity with or without
overreaching and/or overtraining is often viewed as
related physiological and psychological signs
a continuum
[6]
(see figure 1). The continuum sug-
and symptoms of overtraining in which restora-
gests that increased stress, or overload, results in a
tion of performance capacity may take several
disruption of homeostasis and a temporary decrease
weeks or months.
in function.
[7]
The stress may arise from training,
psychological stress or illness. The resultant acute
•
Overreaching: an accumulation of training and/or
fatigue can cause a positive adaptation or improve-
non-training stress resulting in short-term decre-
ment in performance provided appropriate recovery
ment in performance capacity with or without
is allowed. This is considered a normal training
related physiological and psychological signs
response and this progressive increase in training
and symptoms of overtraining in which restora-
load followed by sufficient recovery results in en-
tion of performance capacity may take from sev-
hanced performance and is the basis of effective
eral days to several weeks.
training programmes. However, if the balance be-
These definitions suggest that the difference be-
tween appropriate training stress and adequate re-
tween overtraining and overreaching is the amount
covery is disrupted, an abnormal training response
of time needed for performance restoration and not
may occur and a state of overreaching may develop
the type or duration of training stress or the degree
(see figure 1).
of impairment.
[5]
While this may be correct, there is
According to this continuum, if athletes undergo
currently no evidence to suggest that the nature and
periods of intensified training in the absence of
extent of the training that will result in these condi-
appropriate recovery, they may not respond posi-
tions is identical. In fact, it has been anecdotally
tively to the training and progressive fatigue and
suggested that overreaching may occur more in team
decreased performance ensues. Once a state of over-
sports and explosive/power sports whereas over-
training would be more prevalent in endurance
sports. Similarly, the degree of impairment exper-
ienced in each of these states may not be similar and
may indeed be expected to be significantly worse in
the overtrained compared with the overreached
state. The above definitions also imply that there
may be an absence of psychological signs associated
with the conditions. Increased mood disturbance can
be used to discern the acute fatigue and decreased
performance experienced following a single bout of
intensive exercise and that of longer term overreach-
ing symptoms.
[2]
For these reasons, the above defini-
Increasing state of fatigue
Continual intensified training with inappropriate recovery
Increasing severity of symptoms
Single training
session
Overreaching Overtraining
Fi
g
. 1. The overtrainin
g
continuum.
2004 Adis Data Information BV. All rights reserved. Sports Med 2004; 34 (14)
970 Halson & Jeukendrup
reaching has occurred, one of two outcomes may tance (Alarm Reaction Stage). As adaptation is ac-
quired, the body’s resistance to the stressor rises
occur. Firstly, the athlete/coach/sport scientist may
above normal (Stage of Resistance). However,
recognise the symptoms associated with overreach-
under continual exposure to the stressor, resistance
ing and provide appropriate rest and recovery for the
drops to below normal levels and a Stage of Exhaus-
athlete. Following this, full recovery may occur and
tion ensues.
[13]
Selye observed this pattern of beha-
the process of overreaching may have stimulated a
viour using a variety of stimuli that included forced
supercompensation effect and performance may in-
muscular work.
crease to a level higher than that previously at-
tained.
[7]
The second possible suggested outcome
It is often suggested that overtraining is the result
following overreaching is the progressive develop-
of an accumulation of stressors that exceed an ath-
ment of a state of overtraining. The reduced per-
lete’s finite resistance capacity, similar to that which
formance ability that occurs as a consequence of
Selye observed.
[14]
Selye stated that “stress shows
overreaching may be the stimulus for an increase in
itself as a specific syndrome, yet it is non-specifical-
training in an attempt to improve the diminished
ly induced”, therefore, environmental, physical and/
performances. Alternatively, the reduced perform-
or emotional stressors may result in a variety of
ance may be unrecognised. If high levels of training
nonspecific responses.
[13]
This theory appears plau-
persist and/or rest and recovery is inadequate, the
sible and may correspond nicely with Israel’s theory
more serious state of overtraining is thought to de-
of sympathetic and parasympathetic forms of over-
velop. Other contributing stressors include frequent
training; however, further research to substantiate or
competition, monotonous training, psychosocial
counter this notion is required.
stressors, illness/ infection and heavy travel sched-
It is evident that research in the area of overtrain-
ules.
[5]
ing is lacking in a variety of aspects. Generally,
there is a lack of well controlled investigations that
While the fundamentals of the overtraining con-
include appropriate measures of performance as
tinuum appear logical, there is no evidence to con-
well as baseline and recovery periods of assessment.
firm that overreaching will develop into overtrain-
Changes in performance and recovery time are pres-
ing and that symptoms of overtraining are worse
ently two of the only methods to both diagnose
than those of overreaching. An example of this
overreaching and overtraining and to try to partition
ambiguity is evidenced by the theories of Israel,
[8]
the two conditions. However, a large proportion of
which suggest that two types of overtraining exist.
investigations do not include such measures. There-
One type is suggested to be the result of sympathetic
fore, it is impossible to determine if athletes in these
nervous system dominance and the other character-
studies were overreached, as it is possible to have a
ised by parasympathetic dominance.
[8]
Again, this
positive adaptation to intensified training. There are
theory is often presented in reviews and discussions
also issues inherent to the lack of diagnostic tools to
on overtraining.
[6,9-12]
However, while it is common-
indicate a state of overreaching or overtraining as
ly thought that the parasympathetic form may be a
well as a variety of definitions in use to describe the
more advanced stage of overtraining and that this
conditions.
type is subsequent to the sympathetic form, there are
no data available to suggest that this occurs. Indeed,
Some of the major concerns associated with in-
there is no scientific evidence to indicate that over-
vestigations in this area include the following:
reaching itself precedes overtraining.
•
Different terminology and definitions exist,
which hinder the ability to compare the results of
Selye
[13]
proposed the notion that the body’s
research studies.
stress response is triphasic in nature and that the
body’s ability to resist stress is finite. Continual
•
No single definitive diagnostic tool exists, which
exposure to a given stressor or stressors according to results in the diagnosis of overreaching and over-
Selye, initially results in a decline in general resis- training in differing ways.
2004 Adis Data Information BV. All rights reserved. Sports Med 2004; 34 (14)
Does Overtraining Exist? 971
•
Numerous studies do not report or measure per- ance changes were not described. In a study of 170
formance. As a reduction in performance is one
college swimmers over a 4-year period, Raglin and
of the only methods to definitively diagnose
Morgan
[18]
classified 6.8% of swimmers as ‘stale’
overreaching and overtraining, and hence to de-
each season. However, on average, 32.1% of the
termine if an athlete has positively or negatively
swimmers studied showed signs of training ‘dis-
adapted to the training, it is essential that investi-
tress’ each season and 45.9% were ‘distressed’ in
gations report and measure performance appro-
more than one training session. Again, performance
priately.
was not assessed.
•
A lack of performance measures during intensi-
Koutedakis and Sharp
[19]
examined 257 elite ath-
fied training periods means we know little about
letes who were members of British National Teams
the time course of changes in performance and
and/or Olympic squads in a variety of sports over a
related markers that may be used to identify
12-month training season. Thirty-eight cases (15%)
overreaching. Most studies that do measure per-
of athletes were classified as overtrained and in 50%
formance simply test before and after intensified
of these cases a state of overtraining was said to
training, not during.
have developed in the 3-month competition phase.
•
The quantity and quality of the intensified train-
The incidence rate was slightly higher in male
ing is often not reported or measured. Therefore,
(17%) as opposed to female athletes (11%). Interest-
the duration and intensity of training required to
ingly, when sports were divided into predominantly
induce a state of overreaching is unknown.
aerobic and anaerobic events, there was no signifi-
•
Responses to intensified training are likely to be
cant difference in incidence of overtraining over the
individual and differences may exist between
study period. Athletes were diagnosed as over-
different sporting activities.
trained if they were experiencing feelings of con-
•
As it is not possible to intentionally induce a state
stant fatigue and unexplained underperformance;
of overtraining in an athlete, it is necessary to
however, performance was not measured or report-
perform overreaching studies. Therefore, there is
ed.
a lack of investigations on overtrained as op-
As can be seen from the above studies relating to
posed to overreached athletes.
the incidence of overtraining, it is extremely diffi-
cult to interpret the reported findings for two major
1. Incidence of Overtraining
reasons. First, the terminology used is inconsistent
and the term ‘staleness’ gives no indication of
The incidence of overtraining among a variety of
whether the athlete is fatigued, overreached or over-
athletes appears to be relatively high. Over a 1-year
trained. Secondly, as there is no diagnostic test for
period, Morgan et al.
[15]
reported that of 400 swim-
overtraining and performance measures are often
mers who trained up to 14000 m/day, 5–10% were
not made, whether the athletes are actually over-
‘stale’. Both O’Connor et al.
[16]
and Hooper et al.
[17]
trained is questionable.
reported similar incidences of overtraining in swim-
Despite this relatively high suggested incidence
mers over a 6-month period. Hooper et al.
[17]
report-
of overtraining, it remains that there is very little
ed three of 14 swimmers as ‘stale’, based on a failure
scientific evidence describing this condition. Cross-
to improve performance over a designated time peri-
sectional studies on athletes suggested to be over-
od and consistent elevated fatigue ratings. However,
trained are extremely rare and therefore there is little
although high fatigue ratings may be indicative of
information detailing symptoms. In addition, these
overreaching, a failure to improve performance may
studies do not provide information on the develop-
alternatively suggest that the training volume and
ment of overtraining. A practical and ethical method
intensity was not sufficient to induce a positive
of trying to gain an understanding of the early signs
adaptation. Similarly, O’Connor et al.
[16]
classified
and symptoms of overtraining has commonly been
three of 11 swimmers as ‘stale’; however, perform-
2004 Adis Data Information BV. All rights reserved. Sports Med 2004; 34 (14)
972 Halson & Jeukendrup
to intensify training and induce a state of overreach- The importance of measuring performance is
ing. However, as mentioned in the introductory sec- highlighted when examining many of the commonly
tion, there are a number of problems associated with suggested markers of overreaching and overtraining.
previous research in this area. Many of these markers, described throughout this
article, are indicators of a positive adaptation to
training (e.g. reduced submaximal heart rate and
2. Overreaching Research
plasma lactate concentrations). Therefore, it is es-
In an attempt to develop an understanding of the
sential that possible indicators be considered in rela-
mechanisms of overtraining as well as markers that
tion to changes in performance.
may provide an early indication of impending over-
While a performance decline is necessary to indi-
training, a number of aspects thought to be associat-
cate a state of overreaching, the magnitude of per-
ed with overtraining have been investigated. These
formance decline may vary widely depending on the
include changes in performance, mood state, physi-
specific performance assessment. Hooper et al.
[22]
ology, biochemistry, glycogen depletion, the im-
reported a 2.4% increase in performance times in
mune system, hormones and autonomic balance.
swimmers who were overreached compared with a
The lack of definitive diagnostic criteria for over-
1.1% decrease in well trained swimmers. While only
training is reflected in much of the overreaching and
percentage change data were presented and not ab-
overtraining research by a lack of consistent find-
solute times, performance tests were completed over
ings. Additionally, almost all suggested markers are
100m for sprinters and 400m for distance swim-
based on findings of a relatively small number of
mers. Both Jeukendrup et al.
[3]
and Snyder et al.
[30]
investigations that have induced overreaching and
reported a decrease in maximal aerobic power
not overtraining. It is necessary to critically examine
achieved during a graded incremental cycle test to
the findings and conclusions of these studies so that
exhaustion of approximately 3–4% as a result of 2
they may be interpreted accurately.
weeks of intensified cycling training. Jeukendrup et
al.
[3]
reported a slightly larger decline in perform-
2.1 Performance
ance (5%) when the same subjects completed a
time-trial test with an approximate duration of 15
A number of research studies have intentionally
minutes. When researchers incorporate time to fa-
increased training volume and/or intensity to induce
tigue assessments, a larger decline in endurance
a state of overreaching. While this is an appropriate
capacity is evident.
[20,24]
Fry et al.
[20]
and Urhausen
means of inducing such a state, an increase in train-
et al.
[24]
reported a 29% and 27% decline in perform-
ing load does not guarantee that the athlete will
ance, respectively, when using a time to fatigue
become overreached. It is possible for athletes to
protocol.
respond positively to a period of intensified training
From this information, it appears that both the
and, therefore, it is critical that performance is mea-
type of performance test employed and the duration
sured. Additionally, if performance is measured and
of the performance test are important in determining
remains unchanged after the increased training load,
the changes in performance associated with over-
by definition the athlete cannot be diagnosed as
reaching or overtraining. As a variety of perform-
overreached.
ance assessments have been utilised in overreaching
Numerous studies have reported changes in a
research, it is difficult to compare changes in associ-
variety of physiological and biochemical responses
ated variables.
to intensified training; however, it is necessary to
discriminate between those that have induced a de- Debate exists as to which performance test is the
cline in performance,
[3,20-24]
those that did not
[25-27]
most appropriate when attempting to diagnose over-
and those that have either failed to measure or did reaching and overtraining. In general, time to fatigue
not report performance changes.
[16,28,29]
tests will most likely show greater changes in exer-
2004 Adis Data Information BV. All rights reserved. Sports Med 2004; 34 (14)
Does Overtraining Exist? 973
cise capacity as a result of overreaching and over- ilar decrease in
˙
VO
2peak
was also reported by Sny-
training. Additionally, they allow the assessment of der et al.
[30]
after 15 days of increased high-intensity
substrate kinetics and submaximal measures can be training (4.94 vs 4.65 L/min).
made at a fixed intensity and duration. However,
Jeukendrup et al.,
[3]
Lehmann et al.
[33]
and
these tests are not an accurate performance indica-
Urhausen at al.
[24]
all reported reduced maximal
tor, as they do not sufficiently reflect the true de-
heart rates (HR
max
) after increased training. This
mands of the athletic competition.
may be the result of a reduced power output ob-
served during maximal exercise due to an inability
2.2 Mood State
to attain a maximal effort. However, it is not clear
whether the decreased HR
max
and possibly a de-
An objective marker to indicate negative adapta-
creased cardiac output is the cause or the conse-
tion to training stress is clearly lacking in the re-
quence of premature fatigue.
search literature. However, there is general agree-
Lehmann et al.
[33]
reported a tendency towards
ment that the overtraining syndrome is characterised
increased resting stroke volume after an increase in
by psychological disturbances and negative affec-
training volume in middle- and long-distance run-
tive states.
[17]
ners. This was in conjunction with a decreased
In several studies in which subjects were identi-
HR
max
. A recent study by Hedelin et al.
[34]
reported
fied as overreached, clear signs of psychological
increased plasma volume and reduced HR
max
fol-
distress were observed.
[2,3,20,24]
However, increases
lowing a 50% increase in training volume in elite
in global Profile of Mood States (POMS) scores
canoeists. However, performance was not assessed
have also been reported in periods of increased
following recovery and, therefore, it could not be
training that have not resulted in a state of overtrain-
determined if the athletes were fatigued or over-
ing.
[31,32]
Increased global POMS scores were noted
reached.
in swimmers after 3 days of increased training
[27]
as
Reductions in maximal physiological measures
well as after 10 days of increased training.
[32]
In both
such as oxygen uptake and heart rate during incre-
of these studies, alterations in mood state occurred
mental tests to exhaustion after intensified training
in the absence of changes in performance. Over a
may be a consequence of a reduction in exercise
4-year period, the POMS questionnaire was able to
time and not related to abnormalities in physiologi-
correctly identify ‘stale’ athletes on an average of
cal function per se. Future studies should examine
81.45% of occasions in collegiate swimmers.
[16,32]
such measures during exercise at identical time-
However, in one study, performance was not mea-
points and identical intensities to determine if
sured and the team coach completed the classifica-
changes do indeed occur with overreaching inde-
tion of stale athletes based on an inability to train at
pendent of changes in intensity and duration.
a previous level.
[16]
These studies suggest that
changes in mood state may be a useful indicator of
2.4 Biochemistry
overreaching; however, the need to combine mood
disturbances with measures of performance is nec-
In the search for a reliable and valid indicator of a
essary.
state of overreaching and overtraining, a variety of
biochemical responses to an increased training load
2.3 Physiology
have been explored. Lowered submaximal and max-
Other studies have measured physiological vari- imal blood lactate concentrations have been ob-
ables that are related to performance in an attempt to served in a number of investigations.
[3,20,24,30,35]
examine markers and mechanisms of overreaching. Jeukendrup et al.
[3]
noted a shift to the right in lactate
An 8% decrease in peak oxygen uptake (
˙
VO
2peak
) curves in cyclists who underwent 2 weeks of intensi-
[4.8 vs 4.4 L/min] was reported after 14 days of fied training. Lehmann et al.
[35]
reported a decrease
intensified training in competitive cyclists.
[3]
A sim- in submaximal lactate values (2.9 vs 2.4 mmol/L) as
2004 Adis Data Information BV. All rights reserved. Sports Med 2004; 34 (14)
974 Halson & Jeukendrup
well as maximal values (11.3 vs 9.5 mmol/L). A consumed less carbohydrate than the responders
number of other studies have identified lower blood (5.3 vs 8.2 g/kg/day). However, importantly, muscu-
lactate concentrations after significant increases in lar power, sprint and endurance swimming ability
training load; however, changes were not signifi- were not affected in either the responders or the non-
cant.
[20,24,33,36]
responders. Costill et al.
[25]
concluded that the gly-
cogen levels of the non-responders were sufficient
When discussing changes in blood lactate con-
to maintain performance, but inadequate for the
centrations as a result of intensified training, it is
energy required during training and, therefore, fa-
important to consider glycogen status and possible
tigue resulted. As overreaching and overtraining are
decreases in muscle and liver stores due to increased
primarily defined by a reduction in performance, the
training. Future research should ensure that dietary
ability to ascertain whether the non-responders were
intake is reported and standardised on the day before
indeed overreached is limited.
all performance assessments.
These findings directed Snyder et al.
[30]
to ex-
Other biochemical markers such as concentra-
amine performance responses to intensified training
tions of creatine kinase, urea and iron levels have all
with the addition of sufficient dietary carbohydrate,
been considered as possible indicators of overtrain-
in an attempt to determine whether overreaching
ing. However, inconsistent findings and the inability
could still occur in the presence of normal muscle
to distinguish acute fatigue resulting from intensi-
glycogen levels. To ensure sufficient carbohydrate
fied training from overreaching or overtraining does
intake, subjects consumed 160g of a liquid carbohy-
not support the use of the majority of biochemical
drate in the 2 hours following exercise. Subjects
markers as diagnostic tools. Importantly, if de-
completed 7 days of normal training, 15 days of
creases in maximal and submaximal lactate are to be
intensified training and 6 days of minimal training.
used as indicators of overreaching, this must be in
Resting muscle glycogen was not significantly dif-
conjunction with a decline in performance. This
ferent when compared with normal training (530.9
ascertains whether or not the athlete demonstrates a
µmol/g dry weight) and intensified training (571.2
positive or negative training adaptation.
µmol/g dry weight) as determined by needle biopsy
of the vastus lateralis muscle. Subjects were report-
2.5 Glycogen Depletion
ed to be overreached; however, maximal power
As overreaching is thought to be brought about
output during an incremental cycle test was not
by high-intensity training with limited recovery, it is
statistically different after intensified training. Only
perceivable that the fatigue and underperformance
four of the eight subjects demonstrated both a de-
associated with overtraining is at least partly attribu-
cline in maximal power output and an increase in
table to a decrease in muscle glycogen levels. There-
responses to questionnaires. From the two studies
fore, two studies have been performed in an attempt
cited above, the role of carbohydrate intake and
to elucidate the role of carbohydrate and dietary
glycogen depletion in overreaching is unclear.
[25,30]
intake on performance after intensified train-
Again, this is partly due to inappropriate analysis of
ing.
[25,30]
performance.
Costill et al.
[25]
investigated this possibility by
examining the effects of 10 days of increased train-
2.6 Immune System
ing volume on performance and muscle glycogen
levels. Of the 12 swimmers participating in the Given the many anecdotal reports of increased
investigation, four were unable to tolerate the in- illness rates and upper respiratory tract infections
crease from 4000 to 9000 m/day and were conse- (URTI) in overreached and overtrained athletes,
[37]
quently classified as non-responders. The group of the role of exercise-induced immunosuppression has
non-responders consumed approximately 1000 kcal/ been explored. It seems plausible that the prolonged
day less than their estimated energy requirement and and/or intense exercise usually required to induce
2004 Adis Data Information BV. All rights reserved. Sports Med 2004; 34 (14)
Does Overtraining Exist? 975
overtraining, may increase both the duration of the Similarly, resting peripheral blood lymphocyte
numbers also appear not to be influenced by over-
‘open window’ and the degree of the resultant im-
training.
[39-42]
However, while cell numbers may
munosuppression. While this alteration in immune
remain constant, activation of lymphocytes may be
function is indeed possible and there are numerous
increased. Fry et al.
[39]
reported a significant in-
anecdotal reports of increased susceptibility to ill-
crease in the activation level of peripheral blood
ness in athletes who are overtrained, there is little
lymphocytes (CD25+, HLA-DR+, CD3+ : CD25+
scientific information to substantiate this inference.
ratio). Following intensified training that resulted in
Mackinnon and Hooper
[38]
increased the intensity
overreaching, Gabriel and Kindermann
[45]
also re-
of training of a group of 24 swimmers. Of those
ported increased HLA-DR+ T cells.
swimmers that were identified as overreached, one
Neutrophil numbers have been reported to be
in eight (12.5%) reported symptoms of URTI. Sur-
unchanged
[39,41,46]
and increased
[42]
in response to
prisingly, in the group of 16 athletes who responded
intensified training that resulted in a state of over-
positively to the intensified training, nine (56%)
reaching. Importantly, neutrophil function has not
exhibited self-reported symptoms of URTI. There-
been assessed in overreached athletes and, therefore,
fore, increased URTI incidence is likely to reflect
the relative contribution of neutrophil cells to possi-
the increase in training, regardless of the response of
ble immune dysfunction in overreached athletes is
the athlete to the increased physical stress.
unknown.
Whilst a plethora of literature exists on the effects
Natural killer cell numbers appear to be unaltered
of single exercise bouts and periods of increased
in athletes showing symptoms of overreaching.
[39,46]
training on URTI incidence, the above-mentioned
Currently, there are no reports in the literature that
study is the only investigation that has examined
document natural killer cell function in overreached
increased URTI incidence in conjunction with a
or overtrained athletes.
decline in performance indicative of a state of over-
The mucosal immune system response has been
reaching or overtraining. Similarly, a limited num-
examined in athletes who were reported to be over-
ber of investigations have been performed that ex-
trained using salivary IgA as a marker.
[47]
IgA is an
plore the relationship between immune suppression
important factor in host defence and has been ob-
and the overtrained athlete.
[23,39-42]
served in relation to increased URTI incidence in
As leucocytosis is typically the immediate re-
endurance-trained athletes.
[44]
To date, there are lim-
sponse to intensive exercise,
[43]
resting peripheral
ited data on changes in mucosal IgA as a result of
blood leucocyte numbers have been determined dur-
overreaching. Mackinnon and Hooper
[48]
reported
ing both periods of training that has resulted in
18–32% lower salivary IgA concentrations in three
overreaching and in athletes diagnosed as over-
athletes showing symptoms of overtraining com-
trained.
[23,40-42]
With the exception of Lehmann et
pared with those who were well trained. It is diffi-
al.
[23]
all previous studies have not demonstrated
cult to determine if these athletes were overreached
or overtrained based on the data presented. A recent
changes in leucocyte number in overreached sub-
investigation also reported lower IgA levels after
jects. Interestingly, Lehmann et al.
[23]
reported a
intensified training; however, this was not statisti-
significant decline in leucocyte number when the
cally significant.
[49]
training volume was increased. No changes were
observed following an increase in training intensity
In summary, the current information regarding
and during this condition a state of overreaching did
the immune system and overreaching seems to only
not develop. The clinical consequence of a reduction
confirm the role of intensified training in immune
in leucocyte number is presently unclear and
suppression. Whilst many cell numbers do not ap-
changes may simply reflect cell redistribution or
pear to change during overreaching, those cells that
increased cell turnover.
[44]
do alter, appear to simply reflect the nature of the
2004 Adis Data Information BV. All rights reserved. Sports Med 2004; 34 (14)
976 Halson & Jeukendrup
training performed. Therefore, immune parameters considered overtrained than in well trained swim-
may change in response to intensified training, inde- mers.
pendent of whether the training results in overreach-
At present, the role of glutamine in overreaching
ing or overtraining.
is not clear. While plasma glutamine concentration
may or may not decrease following periods of inten-
A further methodological concern in this area is
sified training, there is still no evidence to link low
the lack of functional cell measurements. It is gener-
glutamine levels with impaired immune function
ally accepted that exercise-induced leucocytosis is
and increased susceptibility to illness or infection.
transient and cell number changes have little clinical
significance.
[50]
Immunocompetance is best as-
2.7 Hormones
sessed by examining cell function as opposed to cell
number and future research investigating the im-
2.7.1 Cortisol and Testosterone
mune system and overreaching should address this
In a study examining the pituitary hormonal re-
concern.
sponse in overtrained endurance athletes, Urhausen
Overtrained athletes often anecdotally report an
et al.
[53]
reported no significant changes in resting
alteration in immune system function and the use of
cortisol (254 ± 19 vs 264 ± 28 nmol/L, normal vs
markers of immune function as a diagnostic test for
overtrained, respectively) when subjects were ex-
overtraining has been suggested. However, while
amined prior to and during a state of ‘short-term
some studies claim the athletes investigated are
overtraining’. Similarly, no changes in resting se-
overtrained, it is more likely that the athletes were
rum cortisol concentration were observed by Flynn
overreached. Whether immune function is seriously
et al.,
[21]
Mackinnon et al.
[41]
or Hooper et al.
[42]
impaired in overtrained athletes is unknown as the
Maximal cortisol responses, however, appear to
scientific data are not available.
be reduced during overreaching. Snyder et al.
[30]
2.6.1 Glutamine
reported a decrease in plasma cortisol concentration
As glutamine is an important substrate for cells of
from 514.8 ± 56.8 to 381.8 ± 52 nmol/L following a
the immune system, especially lymphocytes, macro-
period of intensified training that resulted in a state
phages and possibly natural killer cells,
[51]
during
of overreaching. Urhausen et al.
[53]
also reported
periods of immunological challenge glutamine pro-
similar reductions in maximal cortisol levels in
duction is increased. Therefore, it may be expected
overreached athletes.
that low plasma glutamine levels commonly ob-
The documented response of both total and free
served after prolonged exercise may result in reduc-
testosterone concentrations in overreached athletes
tions in immune function. In turn, this could lead to
is contradictory. Flynn et al.
[21]
observed decreased
an increased risk of infection and may partially
serum total and free testosterone levels coincident
explain the increased URTI incidence in endurance-
with a decrease in performance following intensive
trained athletes.
training. Vervoorn et al.
[54]
also reported lower tes-
tosterone levels in rowers following an increase in
Given the changes in plasma glutamine levels
intensive training; however, there were no signifi-
that are commonly observed after prolonged exer-
cant changes in performance and, therefore, the
cise and the function of glutamine in immune cells,
presence of a state of overreaching or overtraining
the role of this amino acid in overtraining has recent-
cannot be confirmed. There were no significant dif-
ly gained attention. Parry-Billings et al.
[52]
reported
ferences in resting testosterone levels during normal
lower plasma glutamine concentrations (503 µmol/
training and during a state of overreaching in endur-
L) in 40 athletes diagnosed as overtrained when
ance athletes identified as overreached by a signifi-
compared with controls (550 µmol/L). While Mack-
cant reduction in performance.
[53]
innon and Hooper
[38]
found no relationship between
incidence of URTI and glutamine, they observed The ratio of testosterone : cortisol is suggested to
23% lower glutamine concentrations in swimmers indicate the balance between androgenic-anabolic
2004 Adis Data Information BV. All rights reserved. Sports Med 2004; 34 (14)
Does Overtraining Exist? 977
activity (testosterone) and catabolic (cortisol) activi- overtraining; and (ii) a sympathetic form. The para-
ty.
[55]
Cortisol (a steroid and primary stress hor- sympathetic form is suggested to be characterised by
mone) and testosterone (a primary androgenic-ana- increased fatigue, apathy and altered mood state,
bolic hormone) are both released in response to high immune and reproductive function.
[8]
Lehmann et
intensity (>60% maximal oxygen uptake [
˙
VO
2max
]) al.
[9]
suggest that this form of overtraining is more
aerobic and anaerobic exercise.
[56]
The testoster- frequently observed and may be referred to as the
one : cortisol ratio is believed to be an indicator of modern form of overtraining. This form of over-
the positive and negative effects of training due to training is said to be the consequence of an imbal-
the opposing effects that the hormones have on ance between extended duration, high-intensity en-
growth, protein synthesis and muscle metabo- durance training and little regeneration, possibly in
lism.
[41]
combination with other non-training stress factors.
[9]
Catecholamine levels in urine and plasma can reflect
A decrease in the testosterone : cortisol ratio of
the activity of the sympathetic nervous system and
approximately 30% or a fall below 0.35 × 10
–
3
has
can, therefore, examine the possibility of a parasym-
been suggested as an indicator of a state of over-
pathetic-sympathetic imbalance or autonomic im-
training.
[55]
The usefulness of this ratio as a diagnos-
balance.
tic tool has not been supported in the literature. The
ratio has been shown to remain unchanged in over-
2.8.1 Catecholamines
reached athletes,
[53]
yet a decreased ratio has been
Basal urinary catecholamine excretion has been
reported in athletes who show no performance dec-
reported to be significantly reduced in overtrained
rements after intensive training.
[54]
Therefore, the
athletes.
[9]
Catecholamine excretion was negatively
ratio of testosterone : cortisol has not been proven to
correlated to fatigue ratings and following a period
have the ability to discriminate overreached athletes
of recovery, catecholamine excretion returned to
from well trained athletes.
baseline levels.
[9]
Increased resting plasma nora-
2.7.2 Other Hormones
drenaline levels were observed by Hooper et al.
[22]
in
Urhausen et al.
[53]
reported lower resting adreno-
athletes who were suggested to be overtrained. Leh-
corticotropic hormone (ACTH) levels and lower
mann et al.
[57]
also observed increased resting
exercise-induced ACTH release in overreached ath-
noradrenaline levels following a period of increased
letes. In addition, a reduced maximal plasma growth
training volume that resulted in performance incom-
hormone (GH) concentration was also reported.
[53]
petence. However, Urhausen et al.
[53]
could not rep-
Decreases in exercise-induced plasma hormone
licate the above findings and reported no significant
concentration, especially cortisol, appear to be evi-
differences in submaximal and maximal plasma cat-
dent in overreached athletes. Reports of resting hor-
echolamine concentrations in overtrained athletes.
mone concentrations are highly variable, with some
The differences in the above findings may be related
researchers showing increases, decreases and no
to methodological differences and high inter-indi-
change. It remains difficult to draw conclusions
vidual differences in catecholamine responses to
about possible changes in hormone concentrations
exercise. Some studies have used urinary 24-hour
due to different performance assessments and
catecholamine excretion as a measure of the auto-
changes in duration and intensity of exercise as a
nomic nervous system activity and others have used
consequence of intensified training. Additionally,
plasma or serum measures and, as such, comparison
plasma hormone concentrations may be different in
between measures are not directly possible. While
overtrained and overreached athletes.
24-hour catecholamine excretion is often considered
a more valid measure, catecholamine responses to
2.8 Autonomic Nervous System
an exercise challenge can give additional informa-
Israel
[8]
proposed that two types of overtraining tion on autonomic nervous system responses to ex-
might exist: (i) a parasympathetic or vagal form of ercise.
2004 Adis Data Information BV. All rights reserved. Sports Med 2004; 34 (14)
978 Halson & Jeukendrup
In the context of overreaching, the role of suggest that HRV in the standing position had a
changes in dopamine has not been thoroughly inves- tendency to decrease in response to intensified train-
tigated. At present, there is only one study that has ing in the subjects who were identified as over-
examined changes in dopamine as a result of over- reached.
[60]
However, intra-individual variability
reaching.
[57]
In this experiment, dopamine remained was high in this investigation and again performance
unchanged as a result of intensified training; howev- was not measured.
er, this was attributed to high inter- and intra-assay
From these studies, it is not possible to comment
variability (up to 30%), reflecting both high method-
confidently on possible changes in HRV. If per-
ological variation and high inter-individual varia-
formance is not assessed, we are unable to determine
tion.
[57]
whether the subjects had a positive or negative adap-
tation to training and, therefore, it cannot be deter-
2.8.2 Heart Rate Variability
mined if the athletes were overreached. This may
Heart rate variability (HRV) is the term used to
partly explain some of the lack of changes in mea-
describe the oscillation in the interval between con-
sures of HRV.
secutive heart beats.
[58]
HRV is commonly assessed
In summary, there are very few well controlled
by examining the intervals between successive R
studies that examine changes in possible indicators
waves, which is determined from the detection of
and mechanisms of overreaching. Most studies use
each QRS complex.
[58]
overreaching as a model for overtraining for practi-
To date, few studies have investigated HRV in
cal and ethical reasons; however, it cannot be certain
overreached athletes, with studies showing either no
that changes observed during overreaching are re-
change
[34,59]
or inconsistent changes.
[60]
The lack of
flected in overtrained athletes.
uniformity in findings is most likely related to dif-
ferent techniques and methods of presenting HRV
3. Overtraining Research
analysis, differing methods of identifying over-
reaching and individual variation in both HRV and
As mentioned in the introductory section, scien-
responses to training.
tific data on overtrained athletes are extremely
Hedelin et al.
[34]
increased the training load of scarce. One of the most commonly cited papers on
nine canoeists by 50% over a 6-day training camp. the mechanisms of overtraining is that of Barron et
Running time to fatigue,
˙
VO
2max
, submaximal and al.
[29]
In this experiment, an insulin-induced hypog-
maximal heart rates and maximal blood lactate pro- lycaemic challenge was administered to assess hy-
duction decreased (all previously reported responses pothalamic-pituitary function in overtrained ath-
to overreaching) in response to the intensified train- letes. This challenge stimulates the release of
ing; however, all indices of HRV remained un- ACTH, GH and prolactin. Athletes were also intra-
changed. On average, there were no significant venously administered luteinising hormone-releas-
changes in low- or high-frequency power, total pow- ing hormone (LHRH) and thyrotropin-releasing hor-
er or the ratio of low- to high-frequency power, both mone (TRH), which act at the level of the pituitary.
in the supine position and after head-up tilt. Howev- Overtrained athletes had significant decreases in
er, mood state was not measured and performance GH, ACTH and consequently cortisol responses in
was not assessed after a period of recovery. Similar- response to insulin administration, which returned to
ly, Uusitalo et al.
[59]
reported no change in intrinsic levels similar to that of asymptomatic runners fol-
heart rate and autonomic balance, assessed through lowing 4 weeks of rest. This suggests that there was
pharmacological vagal blockade, in female athletes impairment at the hypothalamic level. Responses of
following 6–9 weeks of intensified training. In addi- hormones released as a result of LHRH and TRH
tion, both the time domain and power spectral analy- (i.e. thyroid-stimulating hormone, prolactin, lutein-
sis in the frequency domain were calculated during ising hormone and follicle-stimulating hormone)
supine rest and in response to head-up tilt.
[60]
Results were unchanged. This demonstrated that there was
2004 Adis Data Information BV. All rights reserved. Sports Med 2004; 34 (14)
Does Overtraining Exist? 979
no evidence of pituitary dysfunction and hence the ed and the type of exercise test employed was un-
clear. Compared with normally responding subjects
impairment was at the level of the hypothalamus.
examined during the same period, the overtrained
This study
[29]
was one of the first and only studies
subject exhibited an increase in high-frequency and
investigating possible mechanisms of overtraining;
total power in the lying position during intensified
hormonal imbalance has since been cited by numer-
training, which decreased after recovery. The in-
ous authors as a mechanism of overtraining. While
crease in high-frequency power was suggested to be
this research provided new and interesting informa-
most likely the result of increased parasympathetic
tion, subject numbers were very small and individu-
activity.
[62]
al variation was high. Four overtrained athletes were
From the above-mentioned studies, it is not pos-
investigated in total, with only two subjects given
sible to conclusively comment on the similarities or
actrapid insulin alone. The prolactin responses of
differences between overreaching and overtraining.
the subjects to this challenge ranged from <1 to 98
This is primarily due to: (i) the lack of research in
ng/min/mL. Additionally, subjects were reported to
the overtraining area; (ii) a lack of baseline mea-
be recovered after a 4-week rest period. This sug-
sures as symptoms are often evident before a per-
gests that the athletes were, indeed, overtrained;
formance assessment can be made; and (iii) the
however, performance was not measured in this
variation in methodologies and outcomes in experi-
study at any timepoint. Although this study provides
ments investigating overreaching. From the study by
information on changes that may be associated with
Rowbottom et al.,
[40]
it appears there are few abnor-
overtraining, the results are not entirely conclusive.
malities in the variables that commonly indicate a
Rowbottom et al.
[40]
examined a combination of
state of fatigue due to increased exercise. This is
parameters in ten athletes from different sports who
similar to the research in the area of overreaching.
were diagnosed as overtrained. Athletes self-report-
Comparisons cannot be made between the maximal
ed difficulty maintaining their training programme
exercise responses in overreached athletes and those
and debilitating fatigue, which was not alleviated by
of overtrained athletes, as the information does not
rest or bed-rest; however, performance could not be
exist in overtrained athletes. There does appear to be
measured at baseline. Resting haematological, bio-
some similarities in hormonal abnormalities; how-
chemical and immunological measures were made
ever, once again this is based on very little scientific
and compared with established normal ranges. The
overtraining information.
only measured parameter that was significantly dif-
ferent to normal ranges was glutamine, indicating
4. Conclusions and Directions for
that in most haematological, biochemical and immu-
Future Research
nological aspects, these athletes were not different
from normal controls. A recent study by Smith and
Numerous scientific papers as well as popular
Norris
[61]
reported similar plasma glutamine levels
literature suggest a number of markers that can be
in five athletes who were diagnosed as overtrained
used to identify overreaching and overtraining. Ad-
and athletes who responded normally to training and
ditionally, some reports discuss possible mecha-
competition. While time taken to recover was not
nisms and causes of overtraining based on similar
reported, it was stated that athletes took longer than
information. It is essential that research in this area
4 weeks to return to baseline training volume and
is examined critically and it is evident that some of
intensity levels.
the previous research in this area should be inter-
preted with caution.
Hedelin et al.
[62]
reported increased HRV and
decreased resting heart rate in an athlete who was
The majority of knowledge on markers of over-
suggested to be overtrained. The athlete reported
training is based on the results of studies that have
accumulated fatigue and reduced performance;
deliberately induced a state of overreaching in ath-
however, the change in performance was not report-
letes. At present there is insufficient evidence to
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980 Halson & Jeukendrup
7. O’Toole ML. Overreaching and overtraining in endurance ath-
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tion of this review. The authors have no conflicts of interest
Sports Med 1994; 15 (1): 21-6
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176, Belconnen, ACT 2616, Australia.
2004 Adis Data Information BV. All rights reserved. Sports Med 2004; 34 (14)
