Social Laughter's Effect on Pain Tolerance
Phase: NA
Conditions: Pain
Sponsor: University of Texas at Austin

Mirthful laughter and muscle soreness/pain 
 
NCT02896075 
 
April 7, 2017 
 
 
 
 

Study Protocol 
Participants 
Forty adults (15 men and 25 women) 20-40 years of age were recruited. Participants were 
included if they were apparently healthy, normotensive, and free of musculoskeletal disease. 
Participants were excluded if they had known cardiovascular or musculoskeletal disease or if 
their pain tolerance was not reachable in a blunt force test on the quadriceps (maximal pressing 
force allowed by the IRB was 245 N). Participants were instructed to keep their normal daily 
routine including caffeine consumption throughout the study period to prevent the effects of 
lifestyle factors on pain tolerance measurements. The Institutional Review Board approved the 
study, and a written informed consent was obtained from all participants. The study was 
registered at the National Institutes of Health (ClinicalTrials.gov).   

Preliminary study to evaluate various pain tolerance tests 
 Hemodynamic factors are known to influence pain perceptions and pain responses 
(Bruehl & Chung, 2004) as the brain regions regulating the cardiovascular system are known to 
overlap substantially with those that contribute to anti-nociception (Randich & Maixner, 1984). 
In order to select the optimum pain tolerance protocol that would not evoke substantial 
hemodynamic responses, changes in blood pressure and heart rate in response to 3 commonly-
used pain tolerance tests (blood pressure cuff occlusion, blunt force application on muscles, and 
cold pressor test) were compared.  Participants were instructed to lie supine with a beat-by-beat 
blood pressure monitor (Portapres, Finapres Medical Systems, Enschede, Netherlands) that was 
placed on the middle phalanx of the middle finger on the left hand.  The Portapres was connected 
to the WinDaq data acquisition software (DATAQ Instruments, Akron, OH) to record heart rate 
and blood pressure for later analysis.  
A load cell (Transducer Techniques, Temecula, CA) attached to a pan head screw and 
metal handle was used to determine the load placed on the quadriceps of each participant.  The 
force transducer applied blunt force and measured the force by running through an amplifier 
(TMO-2, Transducer Techniques, Temecula, CA) connected to our data acquisition software 
(WinDaq). Blunt force was steadily increased until the participant indicated that the pain was not 
tolerable, at which point the test was terminated. The blunt force was applied half way between 
the anterior superior iliac spine and the top of the patella. The specific spot was marked in order 
to avoid testing directly on the same exact spot that could inflict residual tenderness. The rate of 
increases in applied force was controlled through feedback on the WinDaq data acquisition 
system. 

For the blood pressure cuff pain test, a standard blood pressure cuff was placed on the 
right upper arm. The cuff was gradually inflated at 15 mmHg per second until the participant 
indicated that they were at the limit of their pain tolerance.  Per the IRB guideline, the maximum 
blood pressure was set at 300 mmHg.  The cold pressor test was performed with the participants 
submerging their hand up to their wrist in ice water of about 4 oC. Participants were told to 
remove hand when they reached their pain tolerance.  
Main experimental protocol 
The main study was conducted using a randomized controlled cross-over approach. 
Participants reported to the laboratory a total of seven times. Participants underwent a 
preliminary testing session followed by three visits each for the two video watching 
interventions. The interventions were separated by a minimum of one week as a washout. Each 
intervention included one day of inducing muscle soreness in one leg through eccentric muscle 
contractions (see below), a second day of testing muscle soreness and pain tolerance and 
watching a 30-minute video (either a comedy or documentary), and a third day of testing muscle 
soreness and pain tolerance again to see if the effects of the video viewing persisted the next day 
(i.e., twenty-four hours after the video viewing).  
The second intervention was identical to the first intervention except that muscle soreness 
was induced on the contralateral leg and the video was switched to either comedy or 
documentary. The orders of the videos and legs were randomized by a research assistant flipping 
a coin, and participants watched a randomly-assigned video. If the video was a comedy, the 
participants picked stand-up comedians of their choice in order to meet with their individual 
preference of comedy/humor.  The video was watched for 30 minutes with two other people in 

the room to make it a social setting to induce greater degree of laughter. When the video was a 
documentary, an uninteresting serious documentary was chosen by the investigators.  
Eccentric exercise 
Participants began with a five-minute warm-up at 50 W on a cycle ergometer (Monarch 
Exercise, Vansbro, Sweden). Participants were then secured to the isokinetic dynamometer chair 
in a seated position with chest and waist straps (Biodex System 2, Shirley, NY).  The movement 
began at a speed of 45 degrees/second at a knee angle of 90 degrees.  The participants were 
instructed to resist maximally until full flexion was reached and completed 12 sets of 10 
repetitions of eccentric maximal voluntary actions. Participants aimed for 70% or better of the 
previously-tested one repetition maximum for the eccentric phase. Exercise sets were separated 
by one minute of rest. This protocol has been utilized successfully in previous studies to elicit 
delayed onset muscle soreness (Armstrong, Warren, & Warren, 1991; Prou, Guevel, Benezet, & 
Marini, 1999; Vassilis et al., 2008). 
Body composition 
The seven-site skinfold assessment was utilized to estimate body fat percentage using 
Lange skinfold calipers (Cambridge Scientific, Cambridge, MA). Body fat level was collected 
primarily as a descriptive variable but could potentially contribute to pain tolerance due to the 
association between body fat and inflammatory mediators (Gustafson, 2010) that could sensitize 
nociceptors to pain (Marchand, 2008).  
Muscle strength 
A reduction in muscle strength is an objective indication of muscle soreness (Byrnes & 
Clarkson, 1986).  Knee extensors of both legs were tested for maximum strength using the 
isokinetic dynamometer (Biodex System 2, Shirley, NY). Both the concentric and eccentric 

phases of the strength test were set at a speed of 60 degrees/second. Subsequently, isometric 
(static) muscle strength tests were performed. The lever was locked at 60 degrees and then at 90 
degrees, and the participant was instructed to try to extend the leg and hold it for 5 seconds. 
Pain 
The visual analog scale pain score (VAS) is an established instrument for the assessment 
of subjective pain (Jensen, Karoly, & Braver, 1986) that has been found to be reliable (Price, 
McGrath, Rafii, & Buckingham, 1983). It is a scale from 0 to 10 showing faces and descriptions 
of what participants may feel (Jensen et al., 1986). Participants picked the number (i.e., pain 
score) that corresponded best to how their sore leg felt. Participants also had their pain tolerance 
measured using the force test as an objective measure.  
Mood and psychological state 
The Coping Humor Scale (CHS) and the Situational Humor Response Questionnaire 
(SHRQ) were administered to all the subjects (Martin, 1996). The Positive and negative affect 
schedule (PANAS) was administered before and after watching the video to evaluate changes in 
mood state. High negative affect is characterized by subjective distress and unpleasurable 
engagement, and low negative affect by the absence of these feelings. Positive affect represents 
the extent to which an individual experiences pleasurable engagement with the environment 
(Watson, Clark, & Tellegen, 1988).  
 
Laughter 
In order to quantify the amount of laughter, a Pneumotrace II respiration transducer (UFI, 
Morro Bay, CA) was worn above the umbilical region while watching the videos. The respiration 
transducer was attached to the WinDaq data acquisition system (DATAQ Instruments, Akron, 

OH) to record the data. The irregular respiratory pattern was taken as indications of laughter as 
previously described (Bloch, Lemeignan, & Aguilera-T, 1991; Svebak, 1975). 
 
Statistical analyses 
 Descriptive characteristics were expressed as means±SEM. Paired t-tests were conducted 
between the various outcomes of the comedy and documentary treatments with the probability 
level set at p<0.05. To compare physiological changes caused by each pain tolerance test, linear 
mixed models were used. A random intercept was included for each participant since the 
experiment was repeated measures. ANOVA tables for each fitted model were constructed. In 
the case of significance, multiple comparisons were performed using the Bonferroni method. To 
control for important covariates and to investigate significant predictors of pain tolerance, linear 
mixed model analyses that include fixed and random effects were used.  The linear mixed model 
included fixed and random effects, to predict the pain tolerance. The fixed component described 
the sample’s relationship between the covariates, predictors, and pain tolerance while the random 
component described the variation within each participant. The inclusion of a random intercept 
for each participant in the mixed model was justified because of the large variance in pain 
tolerance.  Predictor variables included leg, time, and treatment, while covariates included sex, 
SHRQ, CHS, PANAS scores, and body fat percentage. A simple ordinary least squares 
regression was used to investigate if a relationship exists between the time spent laughing and 
the change in pain tolerance. To determine the total variability in pain tolerance, the intraclass 
correlation coefficient (ICC) was calculated.