Sidorkewicz, Natalie MSc; McGill, Stuart M. PhD
Several qualitative studies investigating the sexual activity of people with low back pain (LBP) and/or injury have consistently reported that this population experiences a marked reduction in coital frequency. Approximately 34%1 to 84%2 of males with LBP have reported a decrease in the frequency of coitus. Sexual activity is a known indicator of quality of life3 and is recognized as an integral measure of health and disability; the Oswestry Disability Index Version 1.04 includes “sex life” in its measure of disability, and the World Health Organization's International Classification of Functioning, Disability and Health5 regards sexual relationships as an integral factor in the international standard to describe and measure health and disability.
The reported factors that are attributed to this reduced frequency are not only psychological (e.g., fear avoidance) but also mechanical. One study found that “sex life” was reported as causing additional pain in 84% of patients with chronic LBP and that sex life had improved 2 years postoperatively—this improvement was correlated strongest with a decrease in back pain (measured by visual analogue scale).6 In another sample of patients with chronic LBP, 64% (of a sample that was 89% male) reported worsening of pain due to sexual intercourse.7
In a questionnaire-based study,1 22% of males with LBP reported marked discomfort during intercourse—among these males, two of the most commonly reported sources of this discomfort were difficulty finding a position and with pelvic movements. Structured interviews of males and females with chronic LBP yielded similar results: after the onset of disabling LBP, the type of positions used were changed.8 These patients also disclosed that one of the important factors restricting sexual enjoyment was the LBP itself.8 It seems that a large percentage of males with LBP attempt to maintain their typical coital frequency but are limited by an exacerbation of pain because of the movements and postures of coitus.
Despite compelling qualitative evidence that pain during coitus results in coital frequency reduction and the recognized effect that sexual activity has on quality of life, health, and disability, a biomechanical analysis of movements and postures during basic coital positions has never been conducted. The objectives of this study were to first describe male spine movement and posture characteristics during coitus and compare these characteristics across 5 common coital positions. It was expected that lumbar spine movement would primarily occur in the sagittal plane of motion and that each coital position would have a distinct spine kinematic profile.
MATERIALS AND METHODS
All subject recruitment and data collection procedures were performed in accordance with the university's office of research ethics guidelines.
Ten healthy males (29.3 ± 6.9 yr, 176.5 ± 8.6 cm, 84.9 ± 14.5 kg) and 10 healthy females—with 4.7 ± 3.9 years of sexual experience with each other—were recruited for analysis in this study. Participants were excluded from this study if they had a history of spinal, abdominal, or hip surgery; a preexisting disabling back or hip condition; current and relevant musculoskeletal concerns; any sexual dysfunction that would prevent them from engaging in coitus for the duration of the data collection; and registered student status at the university.
Given the descriptive nature of this study, participants were given very little coaching; they were cued to “move as naturally as possible,” which was intentionally vague. Five coital positions were randomized by assigning a number to each position, using a random number generating function to determine the order of presentation, and placing an illustration of each position, in its randomized order, on the laboratory wall within sight of the couple. The couple was able to refer to the illustrations on the laboratory wall during the data collection to ensure that they were performing each position in the assigned order.
The electromagnetic motion capture system used to track the motion of the female participant's spine was limited to 32 seconds of data collection; because this system was synchronized with the motion capture system being used to track male spine motion, a trial duration of 20 seconds was used. Given this limited data collection time, and to avoid as much variation in speed and intensity as possible during each trial, participants signaled the researcher when they had reached what they considered to be their “natural coital speed and/or rhythm”—data collection for each trial began at this point.
Previous literature has identified common coital positions for those with LBP1,7,8—these positions, as well as commonly recommended positions for patients with LBP,9 and a biomechanical rationale influenced the inclusion of the following 5 coital positions in this study:
Rear-entry, in which the female is in the quadruped position and the male is kneeling behind her. Two variations of QUADRUPED were included in this study: one in which the female is supporting her upper body with her elbows (mQUAD1) and the other with her hands (mQUAD2).
Front-entry, in which the female is lying supine and the male is in the prone position on top of her. Two variations of MISSIONARY were included in this study: one which the male is supporting his upper body with his hands and the female is minimally flexed at the hips and knees (mMISS1) and the other in which the male is supporting his upper body with his elbows and the female is flexed at the hips and knees (mMISS2).
Rear-entry, in which the female is lying on her left side and the male is lying on his left side behind her with their hips and knees flexed (mSIDE).
Maximum Active Range of Motion
After completion of all coitus trials, each participant was asked to assume an upright standing posture (i.e., neutral) and bend forward, extend back, side-bend (to the left and right), and twist (to the left and right) at the waist as far as he or she could. This trial was considered the active range of motion (aROM) trial, in which it was assumed that maximum range of lumbar spine flexion, extension, lateral flexion, and axial rotation of the lumbar spine were achieved through active movement without any assistance.
To quantitatively measure the 3-dimensional (3D) lumbar spine kinematics, the torso and the pelvis were monitored using 8 optoelectronic motion capture cameras (Vicon MX20+, Vicon Motion Systems, Oxford, United Kingdom). These cameras monitored the location of 6 individual spherical reflective markers (Vicon MX, 12.5 mm in diameter, Vicon Motion Systems, Oxford, United Kingdom), adhered to the skin overlying the right and left acromion, iliac crests, and greater trochanters (these were removed after calibration), and 2 rigid marker clusters, 1 over the spinous process of the 12th thoracic vertebra and 1 over the sacrum, each instrumented with 5 noncollinear individual spherical reflective markers. All individual markers and rigid marker clusters were affixed to the participant's skin with adhesive tape. Three-dimensional lumbar spine kinematic signals were continuously collected for the duration of each trial and were sampled at a rate of 60 Hz. These data were collected using Vicon Nexus 1.7 software (Vicon Motion Systems, Oxford, United Kingdom) and securely stored on a password-protected personal computer.
Data processing was performed using Visual3D software (Version 4.96.11; C-Motion Inc., Rockville, MD). Raw kinematic data were filtered using a second-order, low-pass digital Butterworth filter with a cutoff frequency of 6 Hz.
The 3D coordinates of the calibration markers and rigid marker clusters were then used to construct a link-segment model of the torso and the pelvis. The relative orientation of the pelvis with respect to the torso was then computed to determine the 3D lumbar spine angles.
Using a custom computer program in MATLAB software (Version r2009B; The MathWorks Inc., Natick, MA), these spine angles were normalized to the maximum amplitude achieved during the maximum aROM trial and expressed as a percentage of these maximums (e.g., 50% of maximum flexion aROM). Speed of penetration was also calculated; a kinematic threshold—maximum hip flexion of the male participant—was used to determine the start of a penetration cycle.
An amplitude probability distribution function was then calculated for each position. This provided insight into the distribution of the varying spine angles (expressed as a percentage of full aROM) achieved during each trial (see Figure 1 for an example of variable spine motion during a given position). Specifically, the amplitude probability at a certain spine angle value is the probability that the spine angle is equal to that value or less than that.10 Thus, the amplitude probability distribution function determines the range of spine angles achieved (i.e., maximum, minimum, and median spine angles are values found at amplitude probabilities of 1.0, 0.0, and 0.5, respectively).
IBM SPSS Statistics software (Version 19, IBM Corporation, Somers, NY) was used for all statistical analysis. In this study, the independent variable was coital position and the dependent variables were the 3D lumbar spine angular displacements (expressed as a percentage of lumbar spine aROM) at amplitude probabilities of 0.0, 0.5, and 1.0, and penetration speed.
Separate univariate general linear models (factor: coital position = 5 levels, α= 0.05) were used at all 3 of these amplitude probabilities to assess whether each coital position had distinct spine kinematic profiles. This was followed by Tukey's honestly significant difference post hoc analysis to assess any main effects of coital position on spine kinematics.
Upon visual inspection of the kinematic data across all coital positions, it was found that the majority of the kinematic signal was in the sagittal plane (i.e., flexion/extension). For this reason, only findings pertaining to the sagittal plane of motion are discussed later. The sign convention for flexion and extension is negative and positive, respectively.
With respect to average rate of penetration cycles (see Table 1 for mean values), both variations of QUADRUPED, mQUAD1 and mQUAD2, were found to be significantly higher (F4,35= 9.271, P < 0.001) than mSIDE (P = 0.043 and P = 0.034, respectively) and both variations of MISSIONARY, mMISS1 (P = 0.003 and P = 0.002, respectively) and mMISS2 (P = 0.001 and P < 0.001, respectively).
Lumbar spine movement range varied depending on the coital position; however, across all positions, the majority of the range was in flexion. The mean values for each coital position at several amplitude probabilities, including 0.0, 0.5, and 1.0, are shown in Table 2 and Figure 2. The raw scores are also provided in Table 3 for the interested reader; the absolute maximum and minimum of each trial as well as an average of all local maxima and minima values are included because of the variability of spine motion during a trial (Figure 1).
Amplitude probability distribution function values were compared at probabilities of 0.0, 0.5, and 1.0. Significant differences were found at probabilities of 0.0 (F4,35= 17.717, P < 0.001), 0.5 (F4,35= 12.892, P < 0.001), and 1.0 (F4,35= 4.110, P = 0.008) (see Figure 3). At an amplitude probability of 0.0, which can be considered the lowest spine angle value achieved in each coital position, mSIDE was significantly lower than mQUAD1 (P < 0.001), mQUAD2 (P < 0.001), and mMISS1 (P < 0.001). mMISS2 was also significantly lower than mQUAD1 (P = 0.001), mQUAD2 (P = 0.001), and mMISS1 (P = 0.008) at an amplitude probability of 0.0. At an amplitude probability of 0.5, which can be considered the median spine angle value achieved in each coital position, mSIDE was significantly lower than mQUAD1 (P < 0.001), mQUAD2 (P < 0.001), mMISS1 (P < 0.001), and mMISS2 (P = 0.030). mMISS2 was also significantly lower than mQUAD1 (P = 0.001) at an amplitude probability of 0.5. At an amplitude probability of 1.0, which can be considered the highest spine angle value achieved in each coital position, mSIDE was significantly lower than mQUAD1 (P = 0.008) and mMISS1 (P = 0.020).
To our knowledge, a biomechanical analysis of coitus has never been conducted; the successful collection of kinematic data during this study demonstrates that such an analysis is feasible. The main objectives of this study were to describe male spine kinematics during coitus and compare movement and posture characteristics across 5 common coital positions.
This study documented spine mechanics in a sample that did not have a preexisting, disabling back or hip condition. As a result, the possible demands and risk factors of several coital positions for exacerbating specific subgroups of LBP were revealed. Despite a well-established biomechanical logic that if the loading of a structure exceeds its tolerance, damage to the structure will occur11—as well as the implicit contribution of motion to the load imposed on spinal structures11—the relevance of this study's findings to these subgroups may be a point in question. Marked improvement in patients' outcomes as a result of matching patients' signs and symptoms to their individualized treatment using an LBP subgrouping method has been reported by several groups.12,13 A critical component of the success of this subgrouping and treatment method is the clinician's determination of motions and postures that elicit LBP during the physical examination and provocative testing and the avoidance of these motions and postures during the patient-specific treatment program.12,13 In keeping with these well-established demonstrations of best clinical practice for the treatment of LBP,14–16 once the clinician has appropriately subcategorized the patient's LBP, initial guidelines presented here (Figure 4) may guide the clinician as to which coital positions contain motions and postures that may exacerbate their patient's subgroup of LBP. This will help enable more comfortable coitus for the patients with respect to their LBP and ensure consistency with their treatment program that aims to avoid motions and postures that trigger their LBP.
In general, male coital movement mainly occurred in the sagittal plane of motion; however, no participants achieved 100% of their spine aROM at any point during a coitus trial. This initial observation supports impressions made from the magnetic resonance images of the anatomy of sexual intercourse in the work of Schultz et al17,18; although the lumbar spine was not specifically examined, the inherent repetitive flexion-extension movement was clearly seen in the midsagittal plane images. Therefore, the following discussion is most applicable to patients whose LBP is exacerbated by spine flexion (i.e., flexion-intolerant), extension (i.e., extension-intolerant), and motion (i.e., motion-intolerant); a coital position is considered to be “spine-sparing” if the pain-provoking biomechanical variable (i.e., a motion and/or a posture) is avoided.
For those patients who are classified as flexion-intolerant via provocative testing, both mSIDE and mMISS2 would be considered the least spine-conserving because they reached the highest percentages of and fluctuated over the widest range of spine flexion, respectively. Conversely, both variations of QUADRUPED, mQUAD1 and mQUAD2, would be considered the most spine-sparing of the positions studied, followed by mMISS1. Although spine angular displacement during mQUAD2 remained within the flexion range, motion occurred through the least range. Hence, this would be contraindicated for the extension-intolerant patient. When advising the motion-intolerant patient, not one position included in this study would be recommended; however, coaching a hip-hinging technique19 to these patients may alter the spine kinematic profile in the positions so that coital movement is more hip-dominant, thus spine-sparing. This technique may also be beneficial for the flexion- and extension-intolerant patient, but the effectiveness of this movement pattern intervention will require further investigation. These findings provide a biomechanical explanation for patient reports on mechanical factors exacerbating LBP during coitus and contributing to reduced coital frequency.1,8
It is interesting to note that a seemingly subtle change in posture—for example, the male supporting his upper body with his elbows (mMISS2) or his hands (mMISS1) during different variations of MISSIONARY—altered the spine kinematic profile significantly; mMISS1 was among the more spine-conserving coital positions and mMISS2 was among the least for flexion-intolerant patients. Even a slight adjustment in the female partner's posture—for example, the female supporting her upper body with her elbows (mQUAD1) or her hands (mQUAD2) during different variations of QUADRUPED—affected the male spine kinematic profile. Our data (not presented here) show that the female partner achieved greater extension in mQUAD2 than in mQUAD1, so it is possible that the female spine posture affects the penetration angle and preferential contact of the penis.20 This suggests that the partner may be an integral factor in the intervention; however, this relationship requires further investigation.
Our findings and initial recommendations contradict the most frequently advised coital position for both male and female patients with LBP: the side-lying position.9,21 This may be due to conflicting biomechanical rationales influencing the development of recommendations. For example, White and Panjabi9 propose side-lying as the “best basic position for either partner with LBP,” because flexing at the hips and knees would relax the psoas and sciatic nerve, straightening the spine, and reducing a disc bulge; however, current research on intervertebral disc mechanics,22 commonly used tests for sciatic nerve tension in the clinical setting,19 and our results for mSIDE have shown the contrary. All other recommendations currently available for patients with LBP are based on conjecture, clinical experience,23 or popular media resources.24,25 Many health care practitioners feel uncomfortable discussing their client's sexual needs or do not address these needs at all26—perhaps the provision of recommendations qualified with empirical data will not only substantiate their clinical advice but also facilitate dialogue between health care practitioners and their patients regarding this important issue.
This analysis was limited to males, specific motion intolerances, and male-centric coital positions (due to instrumentation constraints). Expanding this biomechanical analysis of coitus to include female-centric positions, other motion and posture intolerances, and load intolerances will further the development of recommendations.
The role of the clinician is to assess the motions and postures that trigger LBP in the patient. Subsequently, the data provided here may guide the clinician's specific recommendations to the male LBP patient, including specific coital positions and movement strategies, to avoid the LBP triggers during coitus.
Future directions in the area of coitus biomechanics may address female-centric positions, populations experiencing pain (e.g., LBP), and effectiveness of movement pattern (e.g., hip-hinging) and posture adjustment (e.g., lumbar support)–based interventions.
* The success of this data collection demonstrates that a biomechanical analysis of coitus is feasible.
* Male spine movement, in the positions analyzed, is predominantly in the sagittal plane of motion (i.e., flexion/extension).
* Based on range of motion, the least to most recommended for a male flexion-intolerant patient are mSIDE, mMISS2, mQUAD2, mMISS1, and mQUAD1.
* Motion-intolerant patients are recommended to alter their coital movement from spine-dominant to hip-dominant.
* Subtle changes in either partner's posture may affect the spine kinematic profile of the male—the partner may aide in the intervention.
The authors acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) for its ongoing laboratory funding, and Dr. Edward Cambridge from the Department of Kinesiology, Faculty of Applied Health Sciences, University of Waterloo, for his invaluable assistance.
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lumbar spine; biomechanics; coitus; low back pain; sexual intercourse; quality of life; coital position