The Interplay of Muscle Pain and Stiffness
Creators: name: McCarney, Luke
Publisher: name: RMIT University
Year: 2025
Subjects: classificationCode: 420702
subject: Exercise physiology

description: Musculoskeletal pain disorders are associated with increased muscle pain and stiffness, leading to the widely held belief by allied health clinicians and patient groups alike that increased muscle stiffness is directly associated with increased muscle pain. These disorders are commonly assessed through muscle palpation techniques and patient-reported ratings of pain and stiffness that are subjective in nature since they involve clinician and patient interpretation. However, the validity and reliability of muscle palpation techniques are questionable. 
Myotonometry objectively measures muscle stiffness and has been used to investigate whether an interplay exists between muscle stiffness and pain. Some studies have reported that an interplay exists, supported by recognised muscle neurophysiological changes occurring with chronic pain. However, other studies have reported no interplay, likely due to the high variability in an individual’s perception of pain and muscle stiffness found in the population. Hence, there is no agreement as to the existence of this interplay. This thesis specifically investigated whether an interplay exists between objective stiffness measures and subjective pain and stiffness ratings, addressing shortcomings in the literature. 
For this thesis, three studies were conducted. The first investigated the validity and reliability of a purpose-built myotonometer (PBM; UNSW, Sydney, AU) against the MyotonPRO (Myoton AS, Tallinn, EE). Fifteen adult participants took part in this investigation (7 female, age = 29±5 years, height = 173±11 cm, weight = 84±20 kg). First, maximal voluntary isometric contraction (100% MVIC) was performed on a Biodex isokinetic dynamometer (Biodex, New York, US) to calculate 30% and 60% of MVIC. Biceps brachii (BB) muscle stiffness was then measured three times at each muscle contraction intensity; at rest or 0%, 30% and 60% MVIC. PBM stiffness data was correlated to MyotonPRO data. Repeated measures ANOVA investigated stiffness changes across the three contraction intensities. Intra-rater reliability was calculated for the myotonometers for each of the contraction intensities. Strong, positive correlations were found between the PBM and MyotonPRO stiffness data (p &lt; 0.0001, ρ = 0.90). Significant increases in stiffness were found by both myotonometers with increasing muscle contraction intensity (p &lt; 0.001). Intra-rater reliability was excellent for both myotonometers for each contraction intensity (ICC &gt; 0.90). This investigation verified the validity and reliability of the myotonometers and showed that muscle stiffness changes can be detected with increasing muscle contraction intensity.
The second study determined whether myotonometry can measure changes in muscle stiffness throughout a three-week recovery period from delayed-onset muscle soreness (DOMS). It also investigated the interplay between objective stiffness measures and subjective pain and stiffness ratings over the three-week recovery period. 
The myotonometers were used to measure plantar flexor (PF) stiffness at baseline and over a three-week recovery period in 23 adult participants (9 female, age = 25±4 years, height = 176±10 cm, weight = 82±23 kg). DOMS was induced by downhill (-13), backward walking on a treadmill (Quasar, h/p/cosmos, Traunstein, DE) at 2.2 kph. Stiffness of the medial gastrocnemius (MG), lateral gastrocnemius (LG) and Achilles tendon (AT) were measured at six timepoints. Participants also self-rated their PF pain and stiffness at these timepoints using a numerical rating scale (NRS). The timepoints were: i) pre-DOMS (baseline); ii) within 10 minutes following DOMS; iii) 48 hours post-DOMS; iv) one-week post-DOMS; v) two weeks post-DOMS; and vi) three weeks post-DOMS. Two-way repeated measures ANOVAs were conducted to investigate PF stiffness changes over the recovery period. Myotonometer stiffness data were correlated with NRS pain and stiffness ratings to examine interplay. Over the recovery period, significant stiffness changes were found by both myotonometers (p &lt; 0.0001). Weak, non-significant correlations were found between the objective stiffness measures and the subjective pain and stiffness ratings. Hence, this investigation showed that myotonometry can detect changes in muscle stiffness following DOMS, but no interplay was found. 
The third study investigated PF stiffness (MG, LG and AT) over a six-week eccentric exercise training program. It also investigated the interplay between objective stiffness measures and subjective pain and stiffness ratings over the training program. Ten adult participants took part in this study (5 female, age = 27±5 years, height = 173±10 cm, weight = 83 ±26kg). Eccentric training was performed using the same downhill and backward treadmill walking protocol used in the second study. This walking protocol was repeated weekly over six weeks (once per week) totalling six bouts of exposure. PF stiffness was measured at three timepoints per week over the six-week program (a total of 18 timepoints). Participants self-rated PF pain and stiffness at each timepoint using the NRS. The timepoints were: i) pre-exercise (baseline); ii) within 10 minutes following exercise; and iii) 48 hours post-exercise. Two-way repeated measures ANOVAs were conducted to investigate the effect of the training program on PF stiffness. Myotonometer stiffness data were correlated to NRS pain and stiffness ratings to investigate interplay between these factors. Over the training program, significant stiffness changes were found by the MyotonPRO (p &lt; 0.05), but not the PBM. Weak, non-significant correlations were found between the objective stiffness measures and the subjective pain and stiffness ratings. Although this investigation found significant changes in MyotonPRO stiffness measures, no clinically meaningful changes were found. Further, no interplay was found. 
In summary, myotonometry was found to be a valid and reliable method to detect stiffness changes. No significant interplay between objective stiffness measures and subjective ratings of pain and stiffness was found. Throughout the studies, the variability of the group stiffness data was found to be high (CV = 25%). In comparison the intra-individual variability was low (CV = 3%). The high variability of the group stiffness data found at each timepoint precluded any meaningful comparison of the group stiffness data over time. Hence, a key recommendation for allied health settings is that myotonometry should only be used to assess and monitor stiffness changes in individual patients and not groups. A final consideration is that future work investigates methods to normalise stiffness data to reduce the high group stiffness variability. This may allow for meaningful comparison of group stiffness data.