Gender differences in pressure pain thresholdin a repeated measures assessment

ESMERALDA GARCIA1, DEBORA GODOY-IZQUIERDO1,

JUAN F. GODOY1, MIGUEL PEREZ1, & ISABEL LOPEZ-CHICHERI1,2

1Depto. Personalidad, Evaluacion y Tratamiento Psicologico, Facultad de Psicologıa. Universidad de

Granada, Granada, Spain and 2Hospital Viergen de las Nieves, Unidad de Dolor, Granada, Spain

AbstractThe objective of this study was to evaluate gender differences in the pain threshold, considering thetype of pressure point, its location and the repetition of the assessment. The pressure pain thresholdwas evaluated in 30 healthy volunteers (12 men and 18 women) in three assessment sessions separatedby 15 min and 7 days, respectively. Each assessment session was in turn composed of two trials in eachof which 24 different pressure points (symmetrically located), representing the 18 tender points for thediagnosis of fibromyalgia and six control points, were assessed. Gender differences were found in thepain threshold for all of the points and the measures taken, women showing a lower pain threshold incomparison to men and being these differences more pronounced for control points than for tenderpoints, the former reaching statistical significance in all cases. The analysis of the influence of repeatedmeasures on gender differences in the pain threshold showed a distinct pattern of recuperation in menthan in women, although only one difference in pain threshold was significant. The utility of the tenderpoint concept to study gender differences in pain threshold and the mechanisms that may explaindifferent patterns of recuperation between genders are discussed.

Keywords: Algometry, gender differences, pain, pressure pain threshold, tender points

Introduction

The existence of differences between men and women in the experience of pain has been

shown in epidemiological investigation (Unruh, 1996) as well as in the laboratory, where

pain was induced experimentally (Fillingim & Maixner, 1995; Riley, Robinson, Wise, Myers

& Fillingim, 1998). That women show a lower pain threshold than men is a consistent

pattern in experimental laboratory research, which has been substantiated using various

types of pain induction: Electrical (e.g., Kosek & Hansson, 2003), thermal (e.g., Myers,

Robinson, Riley & Sheffield, 2001), ischemic (e.g., Manning & Fillingim, 2002), chemical

(e.g., Drewes, Babenko, Birket-Smith, Funch-Jensen & Arendt-Nielsen, 2003), and

mechanical stimulation (e.g., Chesterton, Barlas, Foster, Baxter & Wright, 2003).

Nonetheless, to detect gender differences in experimental pain the most useful stimuli

appear to be mechanical (Riley et al., 1998; Rollman & Lautenbacher, 2001) or electrical

(Lautenbacher & Rollman, 1993). Aside from differences in pain threshold, it has also been

Correspondence: Debora Godoy-Izquierdo. Depto. Personalidad, Evaluacion y Tratamiento Psicologico, Facultad de Psicologıa.

Campus Universitario de Cartuja. 18071. Universidad de Granada, Granada, Spain. E-mail: deborag@ugr.es

Psychology, Health & Medicine,

October 2007; 12(5): 567 – 579

ISSN 1354-8506 print/ISSN 1465-3966 online ª 2007 Taylor & Francis

DOI: 10.1080/13548500701203433

found that women have a greater ability to discriminate between different pain sensations

and a lower tolerance of pain (Vallerand & Polomano, 2000).

Despite these results, many questions remain in the field: in the first place, the magnitude

of the gender difference effect itself. According to Chesterton et al. (2003), it is not the

direction of gender differences but their magnitude which is questioned, since not all studies

find statistically significant differences between men and women (e.g., Isselee, De Laat,

Bogaerts & Lysens, 1998; Isselee, De Laat, Lesaffre & Lysens, 1997; Jensen, Andersen,

Olesen & Lindblom, 1986; Lee, Lee, Kim, Kim & Chung, 1994; Sandrini, Antonaci, Pucci,

Bono & Nappi, 1994; Vatine, Shapira, Magora, Adler & Magora, 1993). On the contrary,

Riley et al. (1998), in a meta-analysis on gender differences, found an effect size (amount of

variance explained by the variable) of .55 for the pain threshold variable, which, depending

on the method, ranged from moderate to large. Fillingim & Maixner (1995) also affirmed

that it is a consistent and robust effect.

The other problem comes due to the explanation that the authors give about the

differences found among the studies. Some authors explain these differences, by suggesting

that those studies that fail to demonstrate significant gender differences have presented

designs with scant statistical power (Riley et al., 1998) or experimental rigor (Fillingim &

Maixner, 1995), because there are confounding variables like age (Lee et al., 1994) and

repeated measures (Chesterton et al., 2003), or because the found differences are small,

inconsistent, and subject to variations based on the experimental protocol used (Berkley,

1997). Furthermore, the variability of the designs of the studies themselves makes their

comparison difficult.

There is no agreement as to which are the mechanisms that explain gender differences in

pain sensation either. France & Suchowiecki (1999) propose four mechanisms: (a) effects of

the gender of the experimenter, (b) sexual differences in perceptive discrimination,

(c) gender differences of a hormonal nature, and (d) gender differences in the central modu-

lation of pain. Nonetheless, these are not the only explanations that have been suggested. The

differences have also been related to differences in the anxiety response in men and women or

to different gender or social roles (Myers et al., 2001; Robinson, Riley, Brown & Gremillion,

1998). Berkley (1997) proposes hypotheses related to the anatomical and functional

differences between men and women as the cause of gender differences in pain.

The present study is framed within a broader investigation of the metric properties of the

Dolorımetro Electronico Portatil1 (DEP1; Portable Electronic Dolorimeter) (Perez, Lopez de

la Osa, Ballesteros, Jimenez & Godoy, 2000) and the utility of the pressure pain threshold in the

diagnosis of musculoskeletal diseases. This paper attempts to explore gender differences in

pressure pain threshold more precisely, keeping under control variables which have been

potential sources of confusion, such as the use of repeated measures (Chesterton et al., 2003)

and the existence of localization differences in pain threshold (Fillingim & Maixner, 1995).

The present study evaluated 24 points distributed over the entire anatomy of the

individuals, all of them bilaterally. Of all the evaluated points, 18 were tender points,

recognized by the American College of Rheumatology as criteria to make a diagnosis of

fibromyalgia (Wolfe et al., 1990), and six were control points, or high pressure pain

threshold points. They were included since all these points present different characteristics

regarding sensibility to pain, at least in certain musculoskeletal pathologies. The tender

point has been object of some studies, which intended to shed light on its utility in the

diagnosis of certain pathologies, as well as on its sensitivity and specificity. Some works have

manifested that patients with fibromyalgia and other musculoskeletal diseases showed

generalized sensitivity independently of the type of point (Carli, Suman, Biasi &

Marcolongo, 2002; Granges & Littlejohn, 1993; Tunks, Crook, Norman & Kalaher,

1988) and that individuals without pain also show greater sensibility in tender points

568 E. Garcia et al.

(Tunks et al., 1988). Nonetheless, despite that their sensitivity and specificity have been

shown to be questionable, some authors continue to defend their utility (Granges &

Littlejohn, 1993). On the other hand, control points are usually freely established by the

experimenter. We have chosen the control points located as will be presented in the

procedure section to assure they were at musculoskeletal zones not situated in areas with high

sensorial innervation or near vital or reproductive organs, as literature has pointed out these

locations may affect pain assessment (Fillingim & Maixner, 1995; Landrine & Brown, 1993;

Robinson & Short, 1977; Velle, 1987).

No studies have been found that evaluate gender differences in pressure pain threshold

incorporating tender as well as control points, although the number of tender points is

generally higher in women (Croft, Scholum & Silman, 1994). Our interest in including these

points is to evaluate whether the concept of the tender point, or even the control point, may

help to illuminate some aspects of the existing controversy over gender differences in

experimental pain.

The main objective of this study was to evaluate the existence of significant differences in

pressure pain threshold between men and women. At the same time, the effect of the

location of the pressure point, the type of point pressured (tender vs. control) and the

repetition of the assessment on gender differences are studied. It is expected to find

significant differences between genders as a function of the location and type of pressure

point and the repetition of the assessment.

Method

Participants

Thirty healthy people not suffering from chronic or acute pain just before or during the

study participated, of which 12 were men (age M¼ 45.5 years, SD¼ 23.54) and 18 were

women (age M¼ 40.94 years, SD¼ 14.42). The participants proceeded from different

collectives (housewives, students, workers), with the objective of including subjects from

various ages and social groups.

All the participants had signed a declaration of informed consent in which they were

explained the characteristics of the evaluation which they were going to be subjected to.

Measures

The DEP1 (Perez et al., 2000) was used to assess the pressure pain threshold. The DEP1 is

an algometer that uses a psychophysical methodology, concretely, an ascending method of

limits, to evaluate the pressure pain threshold. The stimuli presented were different levels of

pressure that the experimenter progressively applies over the subjects body surface,

increasing in intensity by 1 kg/s. The apparatus has a range of 0 – 5000 g of pressure and a

sensitivity of 0.5 g. In a study by Garcıa-Fernandez et al. (in press), the widely acceptable

reliability and validity of the DEP1 is presented.

An anamnesis was also specifically designed for this study, which included demographic

information and relevant medical history information from each participant, such as medical

incidences or pain having occurred in the last few days.

Procedure

Once informed consent was signed by all the possible participants interested in taking part in

this study, each person completed a brief but detailed anamnesis. This anamnesis collected

Gender differences in pain threshold 569

information about demographic variables and allowed the control of variables related to the

clinical history of the person (illness, medication, traumatic injury, having been subject to

surgical operations, frequent pain, psychological disorders) and to medical incidents within

the last 3 days (medication, complications, invasive techniques). In this way, individuals

with a chronic musculoskeletal disorder, or for whom pain is frequent, people who do take

analgesic medication in a non-sporadic manner, who had suffered traumatic injury, any

surgical operation or had suffered pain frequently during the last 6 months, or anyone with a

psychological disorder, were excluded from the study. People with medical incidents in the

last 3 days, such as taking analgesic medication, the apparition of medical complications or

having been subject of invasive techniques were also excluded. These criteria were also

maintained along the study, making sure no one had any medical or therapeutic new

condition. Hence, finally, 30 healthy people participated in the study.

Once the anamnesis was completed and groups were formed, the assessment of the

pressure pain threshold with the algometer DEP1 described above was begun. The pressure

pain threshold is defined as the amount of pressure (in kg/cm2) necessary to elicit a pain

sensation in the individual (Granges & Littlejohn, 1993) distinct from that of unpleasantness

or discomfort.

The assessment procedure was composed of three assessment sessions, each divided into

two consecutive trials. Between the first and second assessment session, there was a delay of

15 min, and between the second and the third, there was a delay of 7 days. The rationale

behind choosing these time intervals is based on previous findings (i.e., Chesterton et al.,

2003; Kosek, Ekholm & Nordemar, 1993) and derived in parallel from the objective of

assuring time enough not to provoke physiological sensitization in the first case, and to avoid

any change in pain threshold between assessments due to a temporal reason or clinical new

conditions in the second case.

In each of these trials, 24 bilateral pressure points (18 tender points and six control points)

were assessed in a random sequence identical for all subjects. All these points were

distributed in 12 pairs with a symmetrical location on each side of the body. The tender

points assessed were those proposed by the American College of Rheumatology for the

diagnosis of fibromyalgia (Wolfe et al., 1990), located as follows:

. Occiput: at the suboccipital muscle insertions.

. Low cervical: at the anterior aspects of the intertransverse spaces at C5 – C7.

. Trapezius: at the midpoint of the upper border.

. Supraspinatus: at origins, above the scapula spine near the medial border.

. Second rib: at the second costochondral junctions, just lateral to the junctions on upper

surfaces.

. Lateral epicondyle: 2 cm distal to the epicondyles.

. Gluteal: in upper outer quadrants of buttocks in anterior fold of muscle.

. Greater trochanter: posterior to the trochanteric prominence.

. Knee: at the medial fat pad proximal to the joint line.

The control points were located at the hypothenar eminence, the anteromedial surface of the

medial tibia and the medial ulna.

Data analysis

First, with the objective of illustrating the distribution of the data, an exploratory analysis of

them was carried out. Given that the data did not always meet requirements for normality or

570 E. Garcia et al.

for homogeneity of variance, it was decided to apply non-parametric analysis in later phases.

This is also recommended when sample sizes are small, as occurred in our study (although

using non-parametric statistics do not solve this problem). Specifically, the comparison of

unrelated samples was done using the Mann – Whitney test (U), and when studying the

effect of the moment of measure variable on the pain threshold, the Wilcoxon test (Z) and

the Friedman test for related samples were used. Additionally, the effect size was calculated

using the Cohen’s d coefficient (Cohen, 1998, 1992), useful even when sample sizes are

small, as in this study. Given that sample sizes are not the same for both groups, the

coefficient value was calculated with the following formula (Cohen, 1988):

d ¼ ðM1 �M2Þ=p½ðN1 � 1Þ sd1

2 þ ðN2 � 1Þ sd22�=ðN1 þN2 � 2Þ

� �

1 and 2 being the values corresponding to the men and women group measurements,

respectively.

For data storage and analysis, the statistical package SPSS 11.0 for Windows was used.

Results

Women and men did not differ in age (U¼ 104.500, p¼ .882) or educational level

(U¼ 97.500, p¼ .647).

For the total of pressure points and sessions of measurement, significant differences were

found between men and women (U¼ 61000, p5 .05), women showing lower pressure

values (M¼ 3595.63, SD¼ 713.92) than men (M¼ 4106.09, SD¼ 577.10). When an

analysis of effect size was carried out to detect gender differences, the results showed an

elevated effect size for the mean of the total of all the assessed points (Cohen’s d¼ .80).

To study possible gender differences for each type of point separately (tender vs. control),

the mean for both trials in the three assessment sessions both for the tender and control

points, respectively, were used. Table I shows the mean, standard deviation, statistics, and

significance levels of the differences in pain thresholds between men and women. Figure 1

shows these differences graphically.

Significant differences were observed in a large part of the conditions studied, that is to

say, considering the type of pressure point for each session of assessment, concretely for

control points in session 1 (CON1: U¼ 49.000, p5 .05), for control points in session 2

(CON2: U¼ 59.500, p5 .05), for tender points in session 3 (TP3: U¼ 58.000, p5 .05)

and control points in session 3 (CON3: U¼ 44.500, p5 .01) and for the total mean

considering all the three sessions for control points (CONTOT: U¼ 52.500, p5 .05),

which stands as evidence for gender differences in pain threshold, showing for women lower

values in the pressure performed to reach threshold, which occurred also for all the points

and sessions considered. At the same time, it may also be seen that the differences are

significant for control points more frequently than for tender points. These results are

supported by d effect sizes values, which range from .74 to .91 for control points, being for

the total of sessions considered for this kind of points of .85, whilst they range from .47 to

.84 in the case of tender points, being for the total mean of .66 (Table I).

Once gender differences for tender and control points were established, it was interesting

to study the differences in each of the points separately. Given that each point had a

symmetrical correspondent, we will not refer to the 24 individual pressure points but to

the 12 different symmetrical locations, since a previous analysis of the right and left sides

did not show asymmetries in pressure pain threshold in a sample of healthy subjects and

patients suffering from several chronic pain-related conditions (Garcıa-Fernandez et al.,

Gender differences in pain threshold 571

unpublished document) (left tenderþ control points MED¼ 2572.13, SD¼ 920.26; right

tenderþ control points MED¼ 2500.34, SD¼ 900.65; Z¼71.441, p¼ .150). The results

showed that, specifically, the points that captured significant differences between men and

women were points 9 and 10 (U¼ 56.000, p5 .05, this test performed with the means of

trials and sessions), for which discrepancies due to gender, considering both trials of the

Table I. Mean, standard deviation, statistics and significance of the difference in pain threshold between men and

women for each type of pressured point.

Measure Group M (g/cm2) SD U p d a

TP1 Men 3592.23 639.84 73.000 .138 .58

Women 3210.62 676.36

CON1 Men 4621.56 481.26 49.000 .012* .91

Women 4049.49 706.80

TP2 Men 3491.32 783.05 77.000 .189 .47

Women 3128.96 754.01

CON2 Men 4546.42 552.54 59.500 .040* .74

Women 3993.93 855.27

TP3 Men 3765.85 645.73 58.000 .034* .84

Women 3143.83 790.47

CON3 Men 4619.18 629.35 44.500 .007** .81

Women 4046.97 753.31

TPTOT Men 3616.47 652.17 70.000 .108 .66

Women 3161.14 718.12

CONTOT Men 4595.72 528.63 52.500 .019* .85

Women 4030.13 737.56

TP, mean of tender points; CON, mean of control points; the numbers 1, 2 and 3 refer to the session number;

TOT, total mean of the sessions at these points. aA positive value of d indicates that the mean for men is higher than

for women. *p5 .05. **p5 .01.

Figure 1. Differences in the pressure pain threshold between men and women. 1, 2 and 3 refer to the first, second

and third assessment session, respectively. TP: Tender points; CON: Control points; TOT: Total mean for points.

*p5 .05; **p5 .01.

572 E. Garcia et al.

three sessions, were of 923.86 g on average; points 13 and 14 (U¼ 49.500, p5 .01), for

which discrepancies were of 278.26 g on average; points 17 and 18 (U¼ 39.000, p5 .01),

for which discrepancies were of 990.75 g on average; points 21 and 22 (U¼ 45.500,

p5 .01), for which discrepancies were of 942.26 g on average; and points 23 and 24

(U¼ 58.000, p5 .01), for which discrepancies ranged were of 619.07 g on average. Points

15 and 16 were close to significance level for discriminating between genders (U¼ 64.500,

p¼ .07), being discrepancies of 459.98 g on average for these. For all these points, men

showed a higher pain threshold. For significant results, the first three pairs of points

correspond to the group of tender points, and the others to the control points.

Another interesting aspect of the analysis was to assess how the repetition of the measures

influences the appearance of gender differences and, if so, in which session was found the

greater differences between men and women. Table II shows the contrast statistics obtained

for each trial and the significance reached. Significant differences between men and women

(U¼ 59.000, p5 .05) were only found in the fifth trial (corresponding to the first trial of the

third session), although others were close to reaching significance, such as the first trial of

the first session. Effect sizes values range from .50 to .77 for all the trials, but d reached .84

only for the first trial of third session.

Visual inspection of the data (Figure 2) shows how these differences gradually increase

over time. When the differences between trials were analyzed in each group by a Friedman

test, neither men nor women showed differences between sessions (p4 .05).

It is interesting, upon inspection of the graph, that the pattern for women is slightly

different from that for men. The tendency in both cases shows a lowering profile of the pain

threshold in the first and second session, accentuated between the first and second trial in

the first session. Nonetheless, the pattern differs in the third session, separated from the

other two by 7 days, in that women showed a pain threshold similar to that obtained in the

second session, whilst men increased their pain threshold to the levels found in the first trial

of the first session. A more detailed analysis of the data showed significant differences for

men between the second and third sessions (Z¼72.118, p5 .05), although this was not

true in the case of women (p4 .05).

Table II. Mean, standard deviation, statistics and significance of the difference in pain threshold between men and

women for each trial.

Measure Group M (g/cm2) SD U p d a

1A Men 3914.53 523.07 63.000 .057 .77

Women 3482.80 586.22

1B Men 3793.38 652.25 72.000 .128 .59

Women 3365.05 766.09

2A Men 3744.42 718.33 76.000 .176 .50

Women 3363.54 794.64

2B Men 3768.50 723.43 71.000 .117 .60

Women 3324.55 752.45

3A Men 3941.51 596.39 59.000 .038* .84

Women 3378.20 719.11

3B Men 3904.84 681.61 69.000 .099 .70

Women 3363.86 820.22

1, 2 and 3 refer to the first, second and third assessment session, respectively. A and B refer to the trial number in

each session (A¼First trial; B¼Second trial). aA positive value of d indicates that the mean for men is higher than

for women. *p5 .05.

Gender differences in pain threshold 573

Discussion

It is important to restate that this is the first study on gender differences in pain threshold

which evaluated gender differences in 24 points, bilaterally located and grouped into tender

and control points. On the other hand, although other studies have also used repeated

measures (Chesterton et al., 2003), there were taken in the same session. The present study

obtained repeated measures immediately (at 0 min), in the short term (at 15 min) and at a

long delay (at 7 days), which also permits the investigation of the differences in the pattern of

evolution of the pressure pain threshold between men and women.

The results show, as was expected, differences in pressure pain threshold between men

and women, where women showed a lower mean threshold than men at the evaluated

points. The effect size found for the total of all the points (.80) can be classified as elevated,

in accordance with Cohen’s convention for effect size (Cohen, 1992). This value is even

higher than that found in their meta-analysis on gender differences by Riley et al. (1998).

Gender differences in pain threshold have abundant support, showing that women have a

lower tolerance for painful stimulation or signal (Riley et al., 1998; Vallerand & Polomano,

2000). Our study confirms these differences.

However, as it has been established in this paper, a differential pattern was observed for

men and women in pressure pain threshold for tender and control points at different

assessed locations. With respect to gender differences in control and tender points, the

results showed that these differences were more accentuated at the control points (Table I

and Figure 1), showing that these may be more sensitive than tender points for detecting

gender differences. Effect sizes tend to support this conclusion, being in all the cases notably

higher for control points than for tender points (with the unique exception of session 3, in

which they are very similar), indicating an elevated effect size for the former type of points

(.85), whilst it is mostly moderate for tender points (.66). This result appears to repeat when

points that better discriminate between men and women are studied, considering that we

found significant gender differences in two of the three assessed pairs of control points,

whereas differences were found in only three of the nine pairs of tender points evaluated. In

this way, it may be concluded that the assessment of the pressure pain threshold at control

points may be more useful for capturing gender differences in pain threshold in comparison

with tender points.

Figure 2. Pattern of evolution of the pressure pain threshold (means) over all trials in men and women. 1, 2 and 3

refer to the first, second and third assessment session, respectively. A and B refer to the trial number in each session

(A¼First trial; B¼Second trial). *p5 .05.

574 E. Garcia et al.

Yet, what is the reason that the greatest gender differences in the assessment of pain

threshold are detected at control points? One possible explanation requires the reappraisal

of the notion of the tender point itself. The tender point is, by definition, that which is

sensitive to a pressure lesser than 4 kg/cm2 (Granges & Littlejohn, 1993). However, despite

that they have sometimes been understood to be specific to certain pathologies such as

fibromyalgia, some studies have manifested that tender points are more sensitive in the

majority of people, as in patients with different musculoskeletal pathologies (Carli et al.,

2002; Forseth, Forre & Gran, 1999; Granges & Littlejohn, 1993), and in individuals

without pain, although in this case there exist fewer tender points and they show a lesser

degree of sensitivity to pain (Campbell, 1986; Tunks et al., 1988). Given that these tender

points are more sensitive in all cases, it is logical that there do not exist such differences in

tender points as in control points, although they are significant in both cases (Tunks et al.,

1988).

In the same way, our results show that in both men and women tender points present

lower pain thresholds, which do not always allow discrimination between genders, while the

control points permit the observation of gender differences, demonstrating a lower pain

threshold in women. Therefore, the reason that the tender points allows less discrimination

between the two groups may be found in the location of the points themselves, situated at

sensitive areas or near vital organs, and in that men and women demonstrate similar pain

pressure thresholds at those locations.

Together with the type of point assessed, it would seem that the location of the point is

important. Experimental research on gender differences in pain threshold meets great

difficulty on establishing valid conclusions, given the abundant and contradictory literature,

in which each study evaluates different points. As Chesterton et al. (2003) indicate, studies

that investigate gender differences in different points cannot be compared, as the different

location of the points would affect the significance of the results (i.e., Gerecz-Simon, Tunks,

Heale, Kean & Buchanan, 1989; Hogeweg, Langereis, Bernards, Faber & Helders, 1992;

Orbach & Gale, 1989b; Petersen, Brennum & Olesen, 1992).

There exists an important paucity of studies that delve into this topic and include different

points in their protocol. In general, the studies assess a pair of points or make a more or less

exhaustive assessment of a specific region (for a review, see Riley et al., 1998), with

exceptions such as the study by Fisher (1987), which evaluated nine trigger points, and Lee

et al. (1994), with 13 points. Nonetheless, and despite their scarcity, some authors have

already shown that the assessment of pain varies with the location of the point, for example,

in association with the proximity to the reproductive organs (Landrine & Brown, 1993;

Robinson & Short, 1977; Velle, 1987). Fillingim and Maixner (1995) suggested that gender

differences in pain threshold might be independent of the point assessed, although they may

have a tendency associated with greater differences in anatomical areas with greater sensorial

innervation.

Our results go in the opposite direction. Of the 12 pairs of points assessed, only five

showed significant differences. These points, which were, in proportion, principally control

points, were situated at the intercostal space, the supraexternal quadrant of the gluteus

muscles, the knee cap, the anteromedial surface of the medial tibia, and the medial ulna, all

of them located at areas with little sensory innervation or reproductive importance. This

demands an explanation different to that of Fillingim and Maixner (1995), Landrine and

Brown (1993), Robinson and Short (1977) and Velle (1987), respectively.

Perhaps the gender differences depending on the assessed point are one of the reasons

which contribute to the high variance found in studies of gender differences in pain

threshold (Berkley, 1997; Riley et al., 1998) and in the pressure pain threshold specifically

Gender differences in pain threshold 575

(Chesterton et al., 2003), such that some studies may find significant or non-significant

differences in the pressure pain threshold depending on the assessed point.

In reference to the taking of repeated measures of the pain threshold in gender differences,

the results have showed that the greatest differences tend to appear in the final session of

assessment, specifically in the first trial of the third session (Table II). Effect sizes establish

moderate gender differences for all the trials, but only for those in the first trial of third

session effect size can be considered as elevated. Concretely, patterns of evolution are very

similar for men and women until the last session (Figure 2), in which men returned to a pain

threshold very close to that showed in the first session, whilst women tended to show a pain

threshold similar to that showed in the second session. In fact, only for men was a significant

difference found between values for the second and third session.

Given that the third session is separated from the former two by 7 days, it is unlikely that

variables associated with sensitization of the receptors in the case of women, or

desensitization in the case of men, account for the differences. Other possible mechanisms

must be taken into account. These mechanisms may act exclusively in the case of women, as

the pain thresholds do not appear to recuperate after a lapse of 7 days, contrary to what

happens in men. In both cases, a similar pattern can be seen until the third session, in which

in the first and second sessions (separated just by 15 min) both groups show a descent in

pain threshold, possibly due to mechanisms associated with sensitization of the receptors.

This was also reported in Chesterton et al. (2003), in which repeated measures were also

used, albeit with a different temporal interval.

However, after a lapse of 7 days, the pain threshold values return to levels similar to the

first trial of the first session in the case of men, but in the case of women the values are

similar to the last trial of the second session, which may suggest other factors associated with

central mechanisms implicated in pain perception, specifically with the descending

modulation of the perception of the painful stimulus.

Repeated nociceptive stimulation, during a sufficient period, may produce changes in the

excitatory properties of the spinal neurons, producing what has been termed ‘‘neuronal

plasticity’’ (Cervero, 2000). The pain transmission system is plastic and, as long as the pain

persists, the nervous system itself may change and diminish the threshold of pain perception

(Coderre, Katz, Vaccarino & Melzack, 1993; Porreca, Ossipov & Gebhart, 2002). The

results should be taken together with those of Kosek et al. (1993), who found, in a sample of

women only, similar pain thresholds in a 1-week delay, but the pain threshold increased

significantly given a greater delay (10 – 13 weeks). These results were also found by Jensen

et al. (1986), but not by Orbach and Gale (1989a).

Time probably exercises a differential effect in the pressure pain threshold in men and

women. It would be interesting to see if this pattern persists when the menstrual cycle of

women is controlled for, which may have been one of the sources of the differences in this

session. In the case the pattern continues to appear, it would explain the greater prevalence

of chronic pain pathologies in women. The experience of the painful stimulus evokes, even

after a considerable delay, the same previously learned reaction, not returning to habitual

levels, as occurs in men.

This article presents an important advantage over other works on gender differences in

sensitivity to pain: the high number of locations assessed, which permits the construction of

a relatively complete map of the differential distribution of the pain sensation response in the

human body. However, variables such as blood pressure, the reactivity of the subject or the

phase of the menstrual cycle in women were not controlled for. Unfortunately, as

Giamberardino, Berkley, Iezzi, de Bipontina and Vecchiet (1997) point out, there exists a

large and expanding list of psychological factors that have principal and interaction effects

576 E. Garcia et al.

on the pain threshold, which influences the judgment each person exercises over whether the

stimulus is painful and would modulate gender differences in pain threshold. That limits the

possibility of including all of the relevant characteristics in a single study (Lautenbacher,

1997).

The size of the sample must also be mentioned, since some authors (Riley et al., 1998)

claim that it would not be large enough to guarantee sufficient statistical power. We

intended to solve this problem with our sample by using non-parametric statistics, which

have greater statistical power in these cases, and using an estimation of effect size.

Nonetheless, posterior research must take this deficit into account.

In conclusion, we can affirm that gender differences in pressure pain threshold were

found, showing women a lower value than men, and that these differences were discovered

to be found principally in control points, rather than in tender points. The taking of repeated

measures with a varying delay showed a different pattern of recuperation of pain threshold in

men and women, which must be studied in detail in future research.

Acknowledgements

This research was supported by the Beca de Formacion de Personal Docente e Investigador de la

Junta de Andalucıa and the finantiation of the ‘‘Medicina Conductual/Psicologıa de la

Salud’’ Research Group (CTS-0267) conceded by the Junta de Andalucıa (Spain).

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