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ORIGINAL ARTICLE
Serum electrolyte concentrations and hydration status are
not associated with exercise associated muscle cramping
(EAMC) in distance runners
M P Schwellnus, J Nicol, R Laubscher, T D Noakes
...............................................................................................................................
See end of article for
authors’ affiliations
.......................
Correspondence to:
Professor Martin
Schwellnus, University of
Cape Town, UCT/MRC
Research Unit for Exercise
Science and Sports
Medicine, Department of
Human Biology, Faculty of
Health Sciences, University
of Cape Town, Boundary
Road, Newlands 7700,
South Africa; mschwell@
sports.uct.ac.za
Accepted 8 July 2003
.......................
Br J Sports Med 2004;38:488–492. doi: 10.1136/bjsm.2003.007021
Objectives: To determine whether acute exercise associated muscle cramping (EAMC) in distance runners
is related to changes in serum electrolyte concentrations and hydration status.
Methods: A cohort of 72 runners participating in an ultra-distance road race was followed up for the
development of EAMC. All subjects were weighed before and immediately after the race. Blood samples
were taken before the race, immediately after the race, and 60 minutes after the race. Blood samples were
analysed for glucose, protein, sodium, potassium, calcium, and magnesium concentrations, as well as
serum osmolality, haemoglobin, and packed cell volume. Runners who suffered from acute EAMC during
the race formed the cramp group (cramp, n = 21), while runners with no history of EAMC during the race
formed the control group (control, n = 22).
Results: There were no significant differences between the two groups for pre-race or post-race body
weight, per cent change in body weight, blood volume, plasma volume, or red cell volume. The immediate
post-race serum sodium concentration was significantly lower (p = 0.004) in the cramp group (mean (SD),
139.8 (3.1) mmol/l) than in the control group (142.3 (2.1) mmol/l). The immediate post-race serum
magnesium concentration was significantly higher (p = 0.03) in the cramp group (0.73 (0.06) mmol/l)
than in the control group (0.67 (0.08) mmol/l).
Conclusions: There are no clinically significant alterations in serum electrolyte concentrations and there is
no alteration in hydration status in runners with EAMC participating in an ultra-distance race.
E
xercise associated muscle cramping (EAMC) can be
defined as ‘‘a painful, involuntary contraction of skeletal
muscle that occurs during or immediately after exer-
cise.’’
1
Numerous studies have documented the serum
electrolyte changes that occur with endurance exercise.
2–6
Serum electrolyte and fluid disturbances have been asso-
ciated with the development of muscle cramps in certain
clinical conditions.
7–13
It is therefore often assumed that
EAMC is also caused by fluid imbalances, in particular
dehydration, and serum electrolyte abnormalities.
14–17
This
assumption is common,
15 16 18–20
despite the fact that very few
studies have examined the relation between changes in
serum electrolyte concentrations and the development of
EAMC. There are no well conducted studies that have
documented a relation between dehydration and muscle
cramping in athletes.
A prospective study of marathon runners showed no
association between EAMC and plasma volume changes or
changes in serum sodium, potassium, calcium, phosphate,
bicarbonate, urea, or creatinine concentrations.
21
Two limita-
tions of this study were that serum magnesium concentra-
tions were not determined and that serum electrolyte
concentrations were not documented in the recovery period
after the race. If abnormal serum electrolyte concentrations
return to normal during recovery, and this correlates with
clinical recovery from EAMC, it would support the hypothesis
that abnormal serum electrolyte concentrations are related to
EAMC.
Magnesium has been shown to play an important role in
muscle and nerve function.
22 23
Magnesium is also often
promoted, mostly by the industry, as the most important
electrolyte supplement for preventing skeletal muscle
cramping in athletes. Thus any study examining the
relation between EAMC and changes in serum electrolyte
concentrations should include measurement of serum mag-
nesium.
The relation between serum electrolytes, dehydration, and
EAMC has been reported in two other case series. There was
no association between EAMC and serum potassium con-
centrations in cyclists who cycled for between two and a half
and five hours, and no association between dehydration (per
cent body weight loss) and uncontrolled muscle contraction
in 44 triathletes.
24 25
Small subject numbers and the lack of
any control groups were limitations of those case series.
There is clearly a lack of research documenting the
association between EAMC, dehydration, and serum electro-
lyte status. The relation between EAMC and fluid and
electrolytes during the recovery phase from acute cramping
after exercise has also not been documented. This relation is
particularly important to document because a disassociation
between recovery from EAMC and changes in serum
electrolyte concentrations would strongly support the
hypothesis that changes in serum electrolytes are not related
to the aetiology of EAMC.
Our aim in this study was therefore to document the
relation between the development of EAMC in ultra-distance
runners and concomitant changes in serum electrolyte
concentrations and hydration status.
METHODS
Subjects
This prospective cohort study was conducted at the Two
Oceans Ultra-marathon, a 56 km road race held annually in
Cape Town, South Africa. The ethics and research committee
of the University of Cape Town Medical School approved the
study.
All runners who registered for the race were considered
potential subjects. In a pre-race advertisement campaign in
488
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the press and during registration for the race, a cohort of 72
runners was recruited. Forty five of these had a history of
regularly suffering from EAMC and 27 had no previous
experience of muscle cramping. A regular history of EAMC
was defined as having a history of EAMC during at least two
of every six consecutive races. Inclusion criteria were that all
subjects should be male between the ages of 20 and 60, have
a history of at least two years of active running as a registered
runner, have no history of medical illness, and have no
history of chronic or recent medicinal drug use.
A personal interview was conducted with each runner
during pre-race registration by one of the investigators (JN).
A questionnaire was first administered to the runners to elicit
personal details, medical history, and any history of EAMC. A
personal interview followed during which the information in
the questionnaire was confirmed and all testing procedures
were carefully explained. Written informed consent was
obtained from each subject.
On race day, 21 runners from the initial subgroup of 45
runners with a history of cramping, who were all part of the
cohort of 72 runners, suffered acute EAMC during or within
60 minutes of completing the race and formed the ‘‘cramp’’
group (n = 21). Data were collected from 22 of the 27 runners
who had no past history of cramping but who were all part of
the initial cohort of 72 runners. These runners formed the
control group (n = 22) and they had no past history of EAMC
and did not suffer any form of muscle cramping during or
after the race. Twenty nine runners of the original cohort of
72 were excluded for various reasons: 16 failed to comply
with the protocol during the race, nine had incomplete blood
samples, and four drank significant amounts of fluid after
completing the race but before arriving for immediate post-
race blood sample collection.
Pre-race assessment
Pre-race weighing was conducted on the morning of the race.
All runners were weighed at least 75 minutes before the start
of the race (0600 hours). Body mass was measured in full
race attire (running shorts, vest, and shoes). An electronic
scale (Soehnle, Germany) was used for all body mass
measurements. The scale was calibrated before weighing
sessions. Subjects were instructed not to drink any fluids or
consume any food between the weighing procedure and the
start of the race.
Pre-race blood samples were collected from all subjects in
the 75 minutes before the start of the race. Blood samples
were taken from the antecubital veins, with subjects in the
seated position. All the blood samples were clearly coded and
stored for later analysis. Blood samples were analysed for
haemoglobin concentration, packed cell volume, plasma
proteins, serum sodium, serum potassium, serum total
calcium, serum total magnesium, serum osmolality, and
plasma glucose.
The temperature on race day ranged from 14.3
˚
C to 23.8
˚
C,
with a relative humidity of 47%.
Post-race assessment
All subjects reported to the medical tent within five minutes
of completing the race and before drinking any fluid or
emptying their bladder. On arrival at the medical tent, blood
samples were immediately collected from all the subjects.
These samples were analysed for haemoglobin concentration,
packed cell volume, plasma proteins, serum sodium, serum
potassium, serum total calcium, serum total magnesium,
serum osmolality, and plasma glucose. Thereafter subjects
were weighed in full race attire (running shorts, vest, and
shoes) as before the race. Subjects were requested to return
60 minutes later for a repeat blood sample and an interview
to obtain race cramp history. Runners were allowed to
consume liquids ad libitum in this 60 minute post-race
period.
A second post-race blood sample was collected from
subjects at 60 minutes after the race. This blood sample
was analysed for serum sodium, serum potassium, serum
total calcium, serum total magnesium, serum osmolality, and
plasma glucose.
Race cramp history
All subjects were requested to monitor muscle cramp status
during the race. These data were documented at the 60
minute post-race interview. The following data were collected
from the EAMC group: muscle groups that cramped, the race
distance when cramps started, the duration of cramping
bouts, the recurrence of cramping bouts, cramping severity,
and relieving factors for an acute cramp.
Muscle groups were listed as quadriceps, hamstrings,
gastrocnemius, and other. Race distance was listed as less
than 28 km, 28 to 42 km, or more than 42 km. Landmarks
familiar to all Two Oceans marathon runners were used to
identify these distances that relate to the first half, the third
quarter, and the last quarter of the race, respectively.
Duration of EAMC was recorded as lasting for less than 30
seconds, 30 to 120 seconds, and more than 120 seconds.
Recurrence of EAMC referred to the number of repeat
episodes of the muscle cramp after the first episode. This
was quantified as one to two recurrences, three to four
recurrences, or more than four recurrences until completion
of the race.
Cramp severity was quantified according to the time the
runner was forced to stop or run slowly and the ability to
continue running afterwards. Any cramping where the
runner could continue running or walking within five
minutes was classified as mild. Cramping that required the
runner to stop for 10 to 15 minutes but with resumption of
walking or running was classified as moderate, and cramping
that required stopping for more than 15 minutes with
inability to resume a comfortable walking or running pace
was classified as severe. Relieving factors included stretching
the cramping muscle, massaging the cramping muscle, and
slowing of running pace.
Criteria for discharge
Discharge criteria were that subjects had to be pain-free, able
to walk freely, and have suffered no cramping activity for 30
minutes.
Analysis of blood samples
A centrifuge and two large cooler boxes filled with ice were
set up in a section of the medical tent for immediate
preparation and storage of blood samples. Serum was
obtained for sodium, potassium, calcium, magnesium, and
osmolality analysis. Blood for glucose analysis was obtained
in tubes containing potassium oxalate/sodium fluoride. Blood
for haemoglobin and packed cell volume analysis was
obtained in tubes containing K3 EDTA 15%. Aliquots of
whole blood were removed from the purple top vacutainers
and kept for later analysis of packed cell volume and
haemoglobin. The remainder of the vacutainer was kept on
ice until centrifugation at 2500 rpm for 10 minutes. This
separation of serum was within 45 minutes of obtaining the
blood sample.
Storage of serum samples was done at 220
˚
C in well sealed
tubes until analysis. This storage was achieved within 90
minutes of blood sampling. The blood samples were stored at
the Physiology Laboratories at the University of Cape Town
(UCT) Medical School. All analyses were completed within 10
days after collection. All samples were analysed in duplicate
Serum electrolytes, dehydration, and cramp 489
www.bjsportmed.com
with suitable standards and random intervals between
samples.
Serum sodium and serum potassium determinations were
done using ion selective electrodes on an ionised sodium/
potassium analyser (KNA1, Radiometer, Copenhagen,
Denmark). Serum total calcium and total magnesium
concentrations were determined using atomic absorption
spectrophotometry (Varian AA 1275 series atomic absorption
spectrophotometer, Varian Techtron, Musgrave, Australia).
The serum samples were diluted 1:40 with 0.1% lanthanum
chloride solution. The wavelengths of determination were
422.7 nm for calcium and 285.2 nm for magnesium. Serum
osmolality determinations were done using an automatic
osmometer (Osmette A, Precision Systems, Newton,
Massachusetts, USA). Particular care was taken to ensure
with storage of samples before measurement of serum
osmolality. Storage may result in water evaporation, causing
an error of measurement. All samples were stored in paraffin
wax sealed Eppendorf tubes until analysis could be under-
taken. Plasma glucose determinations were done on fluoride/
oxalated plasma using an automated glucose analyser
(Beckman Glucose Analyzer 2, Beckman Instruments, Brea,
California, USA). This glucose analyser was based upon an
enzymatic glucose oxidase technique. Serum protein deter-
minations were done on the serum samples using the
standard Biuret method.
Haemoglobin determinations were done on whole blood
samples using the standard cyanmethaemoglobin technique
(Drabkins solution) for haemoglobin estimation. Packed cell
volume determinations were carried out in duplicate using a
microhaematocrit centrifuge.
Two methods were used to determine changes in hydration
status of runners in this study. The first calculated changes in
pre-race and post-race body mass measurements. The
following formula was used to calculate the percentage
change in body weight after the race: [pre-race weight2post-
race weight]/pre-race weight6100. The extent of body weight
loss was used as an index of dehydration.
23 26
The second method to assess hydration status used the
formula that is applied to haematological data to calculate
the changes in blood volume, plasma volume, and red cell
volume that occurred during the race.
27
The changes were
calculated by applying the Dill and Costill formula to the
haematological data.
27
Statistical analysis
Statistical analysis of the data was conducted at the Medical
Research Council in Cape Town using SAS system software
on a Sunsperc 10 Server computer (SAS Institute Inc, SAS/
STAT User’s Guide, version 6, 4th ed; Cary, North Carolina: SAS
Institute Inc, 1989). Data were analysed using both para-
metric and non-parametric statistics. Where data were
skewed, non-parametric statistics were used. The Wilcoxon
two sample test was used for comparisons between groups
and within groups. The Tukey-type multiple comparison
post-hoc test was used to determine where the differences
between the groups lay. The Wilcoxon sign rank test was
used to test for within group differences. Significance was
accepted at the 0.05 level.
RESULTS
Descriptive data
The average age (years), weight (kg), and finishing time
(minutes) from the cramp and control groups are presented
in table 1. There were no significant differences between the
two groups for any of these variables.
Race cramping history
The clinical characteristics and race history of muscle
cramping in the cramp group are presented in table 2. The
most common muscle groups reported by the cramp group
were hamstring (48%) and quadriceps (38%). All the runners
(100%) reported cramping in the latter half of the race, with
most (67%) reporting cramping immediately after the race.
Most cramps (62%) reportedly lasted longer than 30 seconds
and most runners (71%) reported three or more cramping
episodes. Most cramps (57%) were moderate to severe in
intensity and were best relieved by slowing the running pace
(76%) or passive stretching (52%).
Serum electrolyte concentrations and haematological
data
The pre-race, immediate post-race, and 60 minute post-race
results for serum sodium, potassium, total calcium, and total
magnesium concentrations (mmol/l), serum osmolality
(mmol/kg), plasma glucose concentration (mmol/l), plasma
proteins (g/l), packed cell volume (%), and haemoglobin (g/dl)
from the cramp and control groups are given in table 3.
There were no significant differences between cramp and
control groups for pre-race, immediate post-race, and 60
minute post-race serum potassium and calcium concentra-
tions, osmolality, plasma glucose, plasma proteins, packed
cell volume, or haemoglobin results. The immediate post-race
serum sodium concentration was lower (p = 0.004) in the
cramp group (139.8 (3.1) mmol/l) than in the control group
(142.3 (2.1) mmol/l). The immediate post-race serum
magnesium concentration was higher (p = 0.03) in the cramp
group (0.73 (0.1) mmol/l) than in the control group (0.67
(0.1) mmol/l).
Table 1 Age, weight, and finishing time for the cramp
and control groups
Variable Cramp group (n = 21) Control group (n = 22)
Age (years) 36.6 (7.7) 42.4 (7.5)
Weight (kg) 77.7 (11.3) 72.8 (8.4)
Finishing time (min) 309.0 (41.3) 304.3 (35.5)
Values are mean (SD).
Table 2 Clinical characteristics and race history of
muscle cramping in the cramping group (n = 21)
Category Subcategory Percentage
Muscle group Quadriceps 38%
Hamstring 48%
Gastrocnemius 14%
Other 5%
Race distance when cramp
started
,28 km 0%
28 to 42 km 33%
.42 km 67%
Duration of cramping episode ,30 s 38%
30 to 120 s 38%
.120 s 24%
Number of recurring episodes 0 10%
1–2 19%
3–4 38%
.4 33%
Cramping severity Mild 43%
Moderate/severe 57%
Relieving factors Stretching 52%
Massage 24%
Slowing race pace 76%
Values are percentage of total group.
490 Schwellnus,Nicol,Laubscher,etal
www.bjsportmed.com
Hydration status
The average per cent change in body weight, blood volume,
plasma volume, and red cell volume between immediate
post-race and pre-race from both the cramp and control
groups are shown in table 4. There was no significant
difference between the two groups for any of these variables.
DISCUSSION
The two main findings of this study were first, that there is
no relation between any clinically significant changes in
serum concentrations of sodium, potassium, total calcium,
and total magnesium and the development of EAMC in ultra-
distance runners before or immediately after a race, or during
the period of clinical recovery from EAMC; and second, that
there is also no relation between the changes in hydration
status (measured by changes in body weight, plasma volume,
blood volume, or red cell volume) and the development of
EAMC in ultra-distance runners during or immediately after
a race. The results of our study also show that there is no
relation between the development of EAMC in ultra-distance
runners and changes in serum osmolality, blood glucose
concentration, and the concentration of plasma proteins
before and after a race.
The association between skeletal muscle cramping and
disturbances of serum electrolyte concentrations has been
well documented in a variety of medical conditions. These
include plasma volume contraction and extracellular hypo-
osmolality in patients undergoing haemodialysis, severe
dehydration after sweating, vomiting or diarrhoea, hypona-
traemia associated with whole body salt depletion, hypoka-
laemia and hyperkalaemia, hypocalcaemia, and more
recently hypomagnesaemia.
9–12 28
In most instances, the
postulated mechanism for the development of skeletal
muscle cramping is related to alterations in neuromuscular
excitability which can be induced by disturbances in serum
electrolyte concentrations. Clinically, patients with these
abnormalities present with increased generalised neuromus-
cular excitability, which can lead to generalised skeletal
muscle cramping.
Numerous studies have shown that disturbances in serum
electrolyte concentrations and hydration status can occur
following ultra-distance running.
2–4 6
It is therefore com-
monly assumed that there is a relation between changes in
serum electrolyte concentrations, hydration status, and the
development of EAMC in distance runners.
15 16 18–20
In the present study, serum electrolyte concentrations and
hydration status were not associated with the clinical
symptoms of EAMC. Small but statistically significant
differences in serum sodium and magnesium concentrations
between the cramping and control groups in the immediate
post-race period in the present study are too small to be of
clinical significance. Furthermore, the decrease in serum
sodium concentration following the race in the cramp group
is probably related to an increased fluid intake during the
race in this group.
29
Although drinking patterns were not
measured directly, increased drinking in the cramp group is
likely because of the well publicised belief that cramping is
caused by dehydration. The serum electrolyte concentrations
observed in this study are also similar to those reported by
others, and most importantly are too small to be of any
clinical significance.
21 24 25
This study was not designed to measure fluid balance in
the runners; therefore precise data on the type and volumes
of fluid ingested during the race, as well a specific losses
(sweat, urine, faeces) during the race, are not presented.
However, accurate and consistent data on the hydration
status immediately after the race (body weight changes,
changes in plasma volume, changes in blood volume)
indicate that runners with EAMC were less dehydrated than
Table 3 Values for pre-race, immediate post-race, and 60 minute post-race serum sodium, potassium, total calcium, and total
magnesium concentrations, serum osmolality, plasma glucose concentration, plasma proteins, packed cell volume, and
haemoglobin results for the cramp and control groups, expressed as mean (SD) and median (1st and 3rd quartiles)
Variable
Pre-race Immediate post-race 60 min post-race
Cramp (n = 21) Control (n = 22) Cramp (n = 21) Control (n = 21) Cramp (n = 13) Control (n = 16)
Mean (SD)
Sodium (mmol/l) 139.2 (2.1) 139.3 (2.0) 139.8 (3.1)* 142.3 (2.1)* 140.3 (1.9) 141.7 (1.7)
Potassium (mmol/l) 4.5 (0.4) 4.4 (0.4) 4.9 (0.6) 4.7 (0.5) 4.7 (0.5) 4.6 (0.5)
Calcium (mmol/l) 2.2 (0.1) 2.2 (0.1) 2.3 (0.2) 2.3 (0.1) 2.2 (0.3) 2.2 (0.2)
Magnesium (mmol/l) 0.81 (0.1) 0.83 (0.1) 0.73 (0.1)* 0.67 (0.1)* 0.75 (0.1) 0.73 (0.1)
Osmolality (mmol/kg) 284 (5) 282 (4) 280 (6) 284 (10) 284 (7) 283 (8)
Glucose (mmol/l) 6.3 (3.1) 6.1 (1.1) 6.8 (1.9) 6.5 (2.0) 6.3 (1.0) 6.5 (1.1)
Plasma proteins (g/l) 73.3 (5.3) 72.7 (3.9) 76.4 (5.2) 73.7 (15.5)
PCV (%) 40.0 (4.0) 42.0 (4.0) 40.0 (3.0) 40.0 (3.0)
Haemoglobin (g/dl) 15.5 (1.1) 15.5 (0.8) 15.7 (1.0) 15.5 (3.2)
Median (1st and 3rd quartiles)
Sodium (mmol/l) 139 (138, 140) 139 (138, 141) 140 (139, 142)* 143 (141, 144)* 141 (138, 141) 142 (141, 143)
Potassium (mmol/l) 4.5 (4.2, 4.6) 4.3 (4.2, 4.5) 4.9 (4.5, 5.4) 4.7 (4.5, 5.0) 4.8 (4.2, 5.0) 4.6 (4.3, 4.9)
Calcium (mmol/l) 2.2 (2.1, 2.2) 2.2 (2.1, 2.2) 2.3 (2.2, 2.4) 2.3 (2.2, 2.4) 2.3 (2.0, 2.4) 2.2 (2.1, 2.3)
Magnesium (mmol/l) 0.8 (0.8, 0.9) 0.8 (0.8, 0.9) 0.8 (0.7, 0.8)* 0.7 (0.6, 0.7)* 0.8 (0.7, 0.8) 0.7 (0.7, 0.8)
Osmolality (mmol/kg) 284 (280, 285) 282 (280, 286) 280 (277, 286) 283 (279, 291) 284 (279, 286) 285 (282, 287)
Glucose (mmol/l) 5.3 (4.7, 6.7) 5.8 (5.6, 7.1) 6.5 (5.6, 7.6) 6.7 (5.8, 8.1) 6.3 (5.9, 7.0) 6.6 (5.5, 7.4)
Plasma proteins (g/l) 71.9 (69.6, 74.3) 72.7 (71.1, 75.4) 75.0 (73.3, 78.8) 76.2 (75.0, 80.6)
PCV (%) 40 (40, 40) 40 (40, 40) 40 (40, 40) 40 (40, 40)
Haemoglobin (g/dl) 15.4 (14.6, 16.1) 15.7 (14.9, 16.1) 15.6 (15.2, 16.2) 16.2 (15.6, 16.9)
*Significant difference in the immediate post-race values between the cramp and control groups (p,0.05).
PCV, packed cell volume.
Table 4 Per cent change (pre-race to post-race) in body
weight, blood volume, plasma volume, and red cell
volume during the race for the cramp and control groups
Variable
Cramp group
(n = 21)
Control group
(n = 22)
Change in body weight (%) 22.9 (1.2) 23.6 (1.2)
Change in blood volume (%) 21.3 (5.5) 23.7 (4.0)
Change in plasma volume (%) 0.2 (6.3) 20.7 (8.6)
Change in red cell volume (%) 22.1 (16.2) 28.3 (12.6)
Values are mean (SD).
Serum electrolytes, dehydration, and cramp 491
www.bjsportmed.com
non-cramping runners at the time of presentation. The per
cent decrease in body weight (pre- to post-race) was less in
the cramp group (2.9%) than in the control group (3.6%).
The absence of any relations between clinical recovery from
EAMC and changes in serum electrolyte concentrations in
the 60 minute period after the race also do not support the
hypothesis that EAMC is associated with alterations in serum
electrolyte concentrations. Finally, the clinical picture of
EAMC is that of localised skeletal muscle cramping, and this
differs substantially from the generalised skeletal muscle
cramping observed in patients with serum electrolyte
changes secondary to systemic disease.
9–12 28
Conclusions
The results of our study do not support the common
hypotheses that EAMC is associated with either changes in
serum electrolyte concentrations or changes in hydration
status following ultra-distance running. An alternative
hypothesis to explain the aetiology of EAMC must therefore
be sought.
Authors’ affiliations
.....................
M P Schwellnus, J Nicol, UCT/MRC Research Unit for Exercise Science
and Sports Medicine, Newlands, South Africa
R Laubscher, Centre for Epidemiological Research, Medical Research
Council, Parow, South Africa
T D Noakes, University of Cape Town, Newlands, South Africa
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492 Schwellnus,Nicol,Laubscher,etal
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