Menthol-based Topical Analgesic Induces Similar Upper and Lower Body Increases in Pain Pressure Threshold: A Randomized Trial

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.