Can Injuries Have a Lasting Effect on the Perception of Pain in Young, Healthy Women and Men?

Abstract

Background:

Pain is a characteristic, unpleasant sensory and emotional experience associated with actual or potential tissue damage. Pain is a subjective sensation, modulated by many factors such as age, sex, emotional state, national origin, or physical activity. Moreover, it is closely associated with intense physical activity, injuries, and traumas, which can significantly modulate pain tolerance.

Hypothesis:

We postulate that there are correlations between past injuries, physical activity, and intensity of pain perception (pain threshold and pain tolerance) in a population of healthy men and women.

Study Design:

Retrospective cohort study.

Level of Evidence:

Level 4.

Methods:

A total of 302 participants aged 18 to 32 years were included. The participants were divided into 2 groups (active and inactive individuals), in accordance with the scope of physical activity they had indicated. The test of pressure pain threshold and pressure pain tolerance was performed using an algometer.

Results:

Active women achieved significantly higher pain threshold and pain tolerance values in all measurements on the upper limb (except for the pain threshold on the left hand) compared with inactive women. In mediation analysis, the effect of injury remained significant only for the pressure pain tolerance in the dominant arm and the left hand in the female group. In the case of men, there were no significant differences in all measurements in view of the threshold and tolerance for pain between the groups of active and inactive and between men with injuries and without injuries.

Conclusion:

Intense, regular physical activity is a factor modulating the perception of pain. This was demonstrated as lowered sensitivity to pain stimuli in a population of healthy women.

Clinical Relevance:

Injuries should be treated as an important factor modulating the perception of pain. We recommend detailed monitoring of injuries during treatment and control of pain sensation.

Pain is a subjective sensation generated in a situation of real or potential tissue damage and is considered one of the strongest stressors for the body. It is also a perceptive experience, whose quality and intensity are modulated by neurophysiological, immunological, cognitive, psychological, social, and cultural factors.⁶ The literature emphasizes the key role of physical activity, defined as any bodily movement produced by skeletal muscles that requires energy expenditure,³⁶ in the perception, processing, and alteration of pain sensitivity.^11,24 Such activity, irrespective of one’s age, is a desirable health behavior, significantly contributing to the improvement of the quality of life, decrease in the risk of cardiovascular events,¹⁴ prevention of cancer,^3,15 decrease in the risk of neurodegenerative diseases,¹ and decline in mortality.³⁵

In clinical theory and practice, physical activity is considered one of the most important elements of prevention and treatment of pain.^9,25 This has been confirmed by both positive study results in individuals engaged in regular physical exercise^16,37 and increased risk of chronic pain observed in individuals preferring a sedentary lifestyle.^5,26

In the context of competitive sports, physical activity usually consists in sustained, many-hour effort often surpassing the adaptive capabilities of the young organism. It is inseparably connected with the feeling of pain, with diverse, usually multifaceted etiology (sterile inflammation of muscles, overstrain of joint structures, injuries, dehydration effects), which athletes experience on a daily basis. Naturally, repeated, and essentially unavoidable experiences of pain significantly affect the way the latter is perceived. On the other hand, pain modulation and control constitute an important skill that athletes learn and refine. Also, amateur athletes or physically active individuals demonstrate better pain control in a distressing situation, and the experience gained while managing pain significantly decreases their pain sensitivity as compared with physically inactive individuals.⁵ Consequently, increased tolerance to pain in physically active individuals may improve their physical endurance and thus allow for more intensive training and better performance in sports. Repeated injuries are subjectively perceived as less severe or burdensome and thus do not convey all the elements of nociceptive information. As such, they do not reflect the level of risk and downplay the actual consequences stemming from the extent and type of the trauma. In consequence, ignoring the signals generated by the nociceptive system translates not only into an increased number of sports injuries but also their greater extent and severity. In sports, pain is closely associated with intense physical activity, injuries, and traumas and, most of all, their accumulated effects, which lead to degenerative changes caused by excessive strain on the locomotor system. Therefore, the “realization” of pain experienced by an athlete points to the ultimate strain being exerted on the body, especially on those structures that are consistently subjected to maximum damage-causing forces and stress.²⁷ On the other hand, the number of injuries significantly determines pain tolerance³⁰ and seems to vary depending on the sports discipline practiced.³³

Recent years have witnessed a considerable increase in the number of research articles on physiological and psychological aspects of pain.^20,34 These studies mostly included physically active individuals (athletes) who experienced pain as a consequence of active participation in sports and for whom pain, as an indicator of acceptable limits of bodily strain, became an element of sports experienced.¹⁹ Few studies, on the other hand, pertain to the impact of injuries on pain sensitivity in nontraining healthy individuals who experience solely normal daily physical activity. In this context, it would be interesting to investigate whether pain sensitivity in such individuals is determined in the same way as in athletes, or whether the awareness of functioning in 2 parallel worlds—the world of sports, chosen voluntarily and treated in a special way, and the world of family and work—modulates their approach to pain and sports injuries.

Thus, the aim of this article is to determine whether there are any correlations between past injuries, physical activity, and intensity of pain perception (pain threshold and pain tolerance) in a population of healthy men and women.

Methods

Participants

The study was conducted from late May to early June 2016 and included 302 students (139 women, 163 men) aged 18 to 32 years. The participants were divided into 2 groups, in accordance with the scope of physical activity they had indicated:

• active individuals, who devoted at least 300 minutes a week, to physical activity performed regularly 2 to 3 times a week

• inactive individuals, who devoted not more than 100 minutes a week to physical activity and performed it usually during weekends.

The classification of the participants into those with and without injuries was based on their declaration in regards to locomotor system injuries experienced in the preceding 5 years.

All measurements were performed by the same investigator, always in the morning, and under the same conditions. The participants were informed about the purpose of the experiment, and all of them gave written informed consent to participate in the research.

Anthropometric data were obtained based on a single measurement performed directly before the study. Body height was measured with an anthropometer, and body weight was measured with an electronic scale (Radwag), with an accuracy of 1 cm and 0.1 g, respectively. The Bioethics Committee of the Regional Medical Chamber in Szczecin approved the study (No. 09/KB/V/2013).

Pain Measurement

The tests were carried out to assess the pressure pain threshold (PPT) and pressure pain tolerance (PTOL), both by means of algometer (Somedic SenseLab AB, Sweden), with probe area of 2 cm². The stimuli were applicated progressively at a rate of 50 kPa/s. The participants were instructed with regard to application of the algometer and then were given a chance to use the device. Each participant tested was informed about the meaning of the test and received tips on how to behave during the test. Before the relevant measurement, 2 attempts were made so that the person being tested could distinguish the sensation of pressure from pain and was able to stop the measurement of pressure at the right moment. Each participant received the same verbal instruction: “This procedure tests your ability to feel pressure pain above muscles. During the examination, some points will be pressed on your body, at the moment when you feel the first impression of pain in the examined area and not only pressure, please say ‘stop’ and press the button held in your hand.”

The PTOL was measured in the same way as the PPT, except that the patients pushed a button to stop the pressure stimulation when they really could not tolerate any further stimulation, and the value on the digital display was recorded as the PTOL. A mean value of the 2 measurements was used for further statistical analysis.

Pressure sensitivity measurements (PPT, PTOL) were made on both upper and lower limbs in the following places:

• the dorsal surface of the hand between the thumb and the index finger

• fat body around the knee joint

• humerus lateral epicondylus

Statistical Analysis

The distribution of data was checked using the Kolmogorov-Smirnov test. In further analyses, the parametric t test was applied, and continuous variables were expressed as the mean and standard deviation. A generalized linear model (GLM) with an identity or logit transformations was used for the mediation test. Three GLM models were constructed for each variable: the first basic GLM model (an identity transformation) for the association of the outcome (PPT/PTOL) with casual variable (injury); the second GLM model (a logit transformation) for the relationship between casual (injury) and mediator (activity); and the third GLM model (an identity transformation) for the effect of casual variable (injury) on the outcome (PPT/PTOL) when controlled for a mediator (activity). If a casual variable predicted the outcome in both the first and the third GLM model, the relationship was considered not fully mediated by a mediator variable (activity). All analyses were conducted using (Dell Statistica, Version 13).

Results

Anthropometric measurement results fell within the standard values for people aged 18 to 32 years (Table 1).

Table 1.

Characteristics of the groups studied

	Female (n = 139), Mean ± SD	Male (n = 163), Mean ± SD
Age, y	21.9 ± 4.9	21.6 ± 4.5
Weight, kg	59.8 ± 8.7	78.3 ± 10.5
Height, cm	167.4 ± 6.4	180.7 ± 6.8
Body mass index, kg/m2	21.3 ± 2.3	23.9 ± 2.4

Appendix Table A1 (available in the online version of this article) shows the difference in each variable between the sexes. As expected, PPT and PTOL were higher in men compared with women in all places tested. Active women achieved significantly higher pain threshold and pain tolerance values in all measurements on the upper limb (except for the PPT on the left hand) compared with inactive women. Interestingly, the women of both groups did not differ in pain threshold and pain tolerance measured on the lower limb.

In the case of men, no significant differences were found between the pain threshold, pain tolerance, and the level of physical activity.

Appendix Table A2 (available online) shows PPT and PTOL values in the groups of men and women who reported a sports injury experienced in the past. Women with previous injuries displayed significantly higher values of PPT and PTOL measured at the right, left, and dominant arms (PPT: right, P = 0.01; left,P = 0.005; dominant, P = 0.012; PTOL: right, P < 0.001; left,P = 0.011; dominant, P < 0.001) and higher values of PTOL measured at the left hand (P < 0.001) and the left knee (P = 0.042) as compared with women without injuries. The same measurements did not reveal any differences in the PPT and PTOL values in the male participants of the study. The results of mediation analyses are presented in Tables 2 to 4 for women and in Tables 5 to 7 for men. In mediation analysis, the effect of injury on the PPT/PTOL remained significant only for the right and dominant arm (PTOL, P = 0.006 and P = 0.023, respectively) (Table 2), and the left hand (PTOL, P = 0.002) (Table 3). The previous injury did not predict PPT or PTOL in men (Tables 5-7).

Table 2.

Generalized linear model P values^a: mediation analysis for the right, left, and dominant arm in female patients (n = 139)

	Arm PPT			Arm PTOL
GLM Model Formula	R	L	D	R	L	D
PPT/PTOL: Injury	0.035	0.011	0.034	0.001	0.021	0.002
Activity: Injury	5.58e-05
PPT/PTOL: Injury + Activity	0.096 0.370	0.138 0.006	0.096 0.360	0.006 0.042	0.300 0.0007	0.023 0.062

D, dominant; L, left; PPT, pressure pain threshold; PTOL, pressure pain tolerance; R, right.

^aP values marked in boldface correspond to PPT/PTOL versus injury relationship.

Table 4.

Generalized linear model P values^a: mediation test for the right and left knee in female patients (n = 139)

	Knee PPT		Knee PTOL
Model Specification	R	L	R	L
PPT/PTOL: Injury	0.701	0.517	0.191	0.047
Activity: Injury	5.58e-05
PPT/PTOL: Injury + Activity	0.857 0.614	0.744 0.429	0.264 0.767	0.077 0.803

L, left; PPT, pressure pain threshold; PTOL, pressure pain tolerance; R, right.

^aP values marked in boldface correspond to PPT/PTOL versus injury relationship.

Table 5.

Generalized linear model P values^a: mediation analysis for the right, left, and dominant arm in male patients (n = 163)

	Arm PPT			Arm PTOL
Model Specification	R	L	D	R	L	D
PPT/PTOL: Injury	0.395	0.691	0.713	0.714	0.968	0.946
Activity: Injury	0.924
PPT/PTOL: Injury + Activity	0.408 0.554	0.675 0.409	0.702 0.515	0.518 0.480	0.974 0.591	0.943 0.374

D, dominant; L, left; PPT, pressure pain threshold; PTOL, pressure pain tolerance; R, right.

^aP values marked in boldface correspond to PPT/PTOL versus injury relationship.

Table 7.

Generalized linear model P values^a: mediation analysis for the right and left knee in male patients (n = 163)

	Knee PPT		Knee PTOL
Model Specification	R	L	R	L
PPT/PTOL: Injury	0.757	0.960	0.904	0.390
Activity: Injury	0.924
PPT/PTOL: Injury + Activity	0.713 0.290	0.982 0.273	0.877 0.364	0.400 0.689

L, left; PPT, pressure pain threshold; PTOL, pressure pain tolerance; R, right.

^aP values marked in boldface correspond to PPT/PTOL versus injury relationship.

Table 3.

Generalized linear model P values^a: mediation test for the right, left, and dominant hand in female patients (n = 139)

	Hand PPT			Hand PTOL
Model Specification	R	L	D	R	L	D
PPT/PTOL: Injury	0.254	0.116	0.258	0.134	0.0003	0.094
Activity: Injury	5.58e-05
PPT/PTOL: Injury + Activity	0.747 0.032	0.241 0.394	0.761 0.030	0.439 0.068	0.002 0.279	0.319 0.096

D, dominant; L, left; PPT, pressure pain threshold; PTOL, pressure pain tolerance; R, right.

^aP values marked in boldface correspond to PPT/PTOL versus injury relationship.

Table 6.

Generalized linear model P values^a: mediation analysis for the right, left, and dominant hand in male patients (n = 163)

	Hand PPT			Hand PTOL
Model Specification	R	L	D	R	L	D
PPT/PTOL: Injury	0.524	0.513	0.593	0.451	0.302	0.331
Activity: Injury	0.924
PPT/PTOL: Injury + Activity	0.530 0.987	0.529 0.764	0.613 0.968	0.450 0.965	0.299 0.817	0.339 0.693

D, dominant; L, left; PPT, pressure pain threshold; PTOL, pressure pain tolerance; R, right.

^aP values marked in boldface correspond to PPT/PTOL versus injury relationship.

Discussion

This study is a natural consequence of the research on the perception of pain in athletes, which has been conducted for several years now.^21-23

Despite the growing body of evidence confirming the relationship between sensitivity to pain and physical activity in healthy individuals, the results of studies published to date have not been homogeneous. This pertains both to the stimuli used, most often mechanical and thermal, and to the specificity of the discipline studied. Ellingson et al⁵ observed a decreased intensity of pain in response to heat stimuli in physically active individuals. Higher pain threshold following application of heat stimuli was also found in triathletes as compared with nonathletes. However, there were no significant differences between the groups in a test using cold stimuli.⁷ These results would thus point to a lack of relationship between pain threshold and the density of receptors, the number of cold receptors being much higher. Also, Heneweer et al⁸ demonstrated that individuals involved in moderate physical activity displayed lower prevalence of low back pain than participants leading a sedentary lifestyle.

Another aspect of the subject analyzed was determination of the optimum intensity of physical activity that would allow a higher pain threshold without ignoring the pain information generated by the nociceptive system. The study by Heneweer et al⁸ showed that both inactivity and excessive physical activity were associated with an increased risk of chronic low back pain. The results of our research pertain to a relatively large sample of physically active participants, who nonetheless are nonathletes, indicating higher PPT values in physically active individuals. Research that corresponds to our findings are Norwegian population studies conducted by Landmark et al.^16,17 The prevalence of chronic musculoskeletal pain was 10% to 38% lower in participants who were involved in regular exercise compared with individuals who reported lack of physical activity.

Our findings support the thesis that physical activity is a key link in the “process” of shaping sensitivity to pain and that the latter is age specific, as physically active women displayed lower PPT and PTOL values than inactive men. This claim has been corroborated in a number of studies: Women have lower sensitivity to pain than men.^10,12,28,29 Considering the level of physical activity of men and women and its relationship with the intensity of pain perception, the PPT and PTOL values differed in the groups studied. In women, increased physical activity had a significant impact on the pain threshold and pain tolerance as compared with inactive women, except for left hand PPT, which may be the result of a less intense use of the nondominant upper extremity and both lower extremities.

In men, active participants did not differ from inactive participants with regard to the intensity of pain perception in the upper and lower extremities. This lack of difference may stem from the degree of physical activity declared (that is, the “active” participants may not actually be that active), as well as from the psychosocial context. Men are less willing to report pain.⁴ Furthermore, studies have shown that men report fewer pain incidents when interviewed by a female researcher, as was the case in our study.¹⁸

An interesting study conducted by Raudenbush et al³⁰ among male athletes confirmed that physical contact and frequent experience with pain play the key role in desensitizing athletes to pain. Despite being in pain, injured athletes did not give up competing and thus intensified and aggravated their injuries. The in-depth statistical analysis conducted by these researchers demonstrated that the severity of pain caused by a past injury correlated positively with greater willingness to continue playing despite the pain experienced.³⁰

In men, participants who had experienced an injury did not differ significantly from men without injuries in terms of PPT and PTOL values. This suggests that injuries are not a significant predictor of pain sensitivity. In women, we conducted a mediation analysis based on GLMs to assess the impact of individual variables: physical activity and injuries on PPT and PTOL. The analysis revealed that physical activity was a significant predictor of the left arm PPT, left arm PTOL, right hand PTOL, and dominant hand PPT. On the other hand, a past injury can predict PTOL in the dominant arm as well as the left hand. As for right arm PTOL, neither physical activity nor past injuries correlated with pain tolerance. Consequently, physical activity plays a more important role in the process of determining pain sensitivity than past injuries.

The results of our research also indicated that in men, the degree of physical activity and past injuries did not cause any changes in PPT and PTOL measured with an algometer. This might be related to the extent and severity of strain and injuries, which in the case of amateur sports activity are not “sufficient” to cause a change in pain perception like that observed in athletes.^20,30 In women, the pain variability may be related to the effect of sex hormones. Experimental sensitivity to pain changes in the course of the menstrual cycle, being higher in the luteal phase as compared with the follicular phase.³¹ Women in the high-estradiol and low-progesterone phases of the menstrual cycle demonstrated lower sensitivity to pain, with simultaneous increased binding of ligands to μ-opioid receptors observed in the brain.³² Our research did not include phases of the menstrual cycle, as numerous studies on this subject display methodological limitations. Moreover, they show that these effects are absent or minor at best.^2,13 Despite these remarks, the differences presented cannot be ignored, as they do not exclude a stronger effect of physical activity and past sports injuries on the perception of pain under conditions of changing female hormone levels.

Limitations

The main limitation of this study is that we did not assess the sample size before the study. However, it should be noted that the significant effect was observed in a smaller group of women. Thus, assuming a similar effect magnitude in both sexes, a lack of effect in men is very unlikely to be the group size issue. The lack of information on sex hormone levels, and the day and regularity of the menstrual cycle constitute limitations to the present study.

Conclusion

Physical exercise is an effective way of diminishing or eliminating pain. Intense and regular physical activity is a factor modulating the perception of pain. Direct and indirect experience of an injury may thus be treated as a significant factor leading to an increase of pain tolerance in women, at least in the body parts tested.

The study also showed that in a population of healthy men, the degree of physical activity and past injuries did not correlate with pain threshold and tolerance values.