Background: Both health professionals and consumers use menthol-based topical analgesics extensively for the temporary relief of pain from musculoskeletal ailments or injury. However, there are no reports of differences in the pain pressure threshold (PPT) or the relative effectiveness of topical analgesics to reduce pain in the upper and lower body muscles and tendons. Objectives: The objective of this study was to investigate whether differences existed in PPT and relative pain attenuation associated with a menthol based topical analgesic over a variety of upper and lower body muscles and tendons. Design: Randomized allocation, controlled, intervention study. Method: Sixteen participants (10 females and 6 males), who were tested on their dominant or non-dominant side. The order of specific muscle / tendon testing was also randomized, which included upper body (middle deltoid, biceps brachii, and lateral epicondylar tendon) and lower body locations (quadriceps, hamstrings, gastrocnemius, lumbo-sacral erector spinae muscles, and patellar and Achilles tendons). PPT was monitored before and 15-minutes following the application of a menthol based topical analgesic.

Experimental Design Using random allocation (slip of paper chosen from a box by one of the researchers), participants were either tested on their dominant or non-dominant side. Limb dominance was determined by the participants' self-report of the preferred hand used for writing/throwing (upper extremity) and for kicking a ball (lower extremity). Further, randomization was used to determine which muscle groups would be tested first: upper body versus lower body, and the order of muscle testing. The tested upper body limb muscles consisted of the middle deltoid, and biceps brachii. The lower body muscle groups consisted of the quadriceps, hamstrings, and gastrocnemius. In order to compare sensitivity differences between muscles and tendons, the patellar tendon, and Achilles tendon were also tested. In addition, as tennis elbow (lateral epicondylitis/epicondylosis) is a common injury [40], the lateral epicondylar tendon was also evaluated. The lower back was tested using the lumbo-sacral erector spinae muscles.

The pre-testing consisted of obtaining PPT (without topical analgesic application) using a handheld algometer (Lafayette Manual Muscle Test System™, Model 01163, Lafayette Instrument Company, Indiana, USA), from each muscle or tendon. The algometer was a hand-held muscle tester with a range of 0-300 pounds (136.1 kilograms) that consisted of a padded disc with a surface area of 1.7 cm2 attached to a microprocessor-control unit that measures peak force (pounds or kilograms). The unit has a digital readout for peak-applied pressure and provides a built-in calibration routine that verifies a valid calibration. The pain threshold was defined as the minimum pressure that induced pain [15]. Participants were instructed to provide a verbal report as soon as the quality of sensation changed from pressure to pain [15,16] at which point the algometer was removed. This study chose to measure the PPT via a pressure algometer as it has been shown to be a clinically and experimentally reliable method to assess pain [16,23,35]. The middle of the muscle belly for the biceps brachii, middle deltoid, hamstrings (biceps femoris), quadriceps (rectus femoris), and gastrocnemius was used. The measurement sites for the patella tendon and lateral epicondylar tendon were 1 cm distal from the bony origin. For the Achilles tendon, the measurement was taken at one third of the distance of the tendon length distal to its insertion. There were three readings from each testing position with a 5-second rest period between each reading.

Intervention The intervention involved the application of 4% menthol-based Biofreeze® (Performance Health, Akron, Ohio) to the muscles and tendons immediately following the pre-testing. Based on prior recommendations [22,45,32] varying volumes of the topical analgesic were applied in reference to the surface area of the muscle. The previously cited investigators recommended 1ml per 200 cm2 of muscle surface area. No significant force (light stroking actions to apply the analgesic) was used for the application of the menthol-based topical analgesic.

Inclusion Criteria:

1. Healthy individuals with no musculoskeletal or neural pathologies.

Exclusion Criteria:

1. Musculoskeletal or neural pathologies / injuries in the year before the experiment.

Excel sheet data can be obtained upon request.

December 14, 2020 onwards

Placebo and analgesic applications were identified by numbers which were only revealed after the statistical analysis.