Increased range of motion (ROM) with stretching has been attributed to neural, mechanical, and psycho-physiological factors. Neural factors that can increase ROM include neural responses such as muscle spindle reflexive disfacilitation (decreased firing of nuclear bag and nuclear chain fibres), stretch-induced pre-synaptic inhibition, reciprocal inhibition, as well as small to trivial contributions from recurrent (Renshaw cells) and autogenic (Golgi tendon organs) inhibition (Behm, 2018b; Behm et al., 2016a; Behm and Chaouachi, 2011). Mechanical alterations contributing to improved flexibility can include thixotropic effects (shear stress and temperature-induced decreases in viscoelasticity), increased tissue compliance (decreased stiffness) and changes in fascicle angle, length and rotation (Behm, 2018b; Behm et al., 2016a; Behm and Chaouachi, 2011; Blazevich et al., 2012). Psycho-physiological factors refer to an increased stretch tolerance that allows the individual to accommodate greater stretch-induced discomfort or pain and thus force the joint through a greater ROM (Magnusson et al., 1996; 1997).
Menthol based topical analgesics are reported to work as counterirritants to decrease pain sensation (Galeotti et al., 2002; Macpherson et al., 2006). Menthol inhibits transient receptor potential ion channels such as TRPA1 that evoke thermal and pain sensations (Macpherson et al., 2006). In addition, menthol inhibits neuronal membrane [Ca.sup.++] channel currents enhancing analgesic properties (Galeotti et al., 2002). In contrast, Yosipovitch et al. (1996) indicated that thermal pain thresholds were unaltered after menthol and alcohol application. Menthol has also been reported to provide a significant reduction in vascular conductance within 1 and 10 minutes of application (Olive et al., 2010). In addition, topical menthol can have a rapid effect on reducing ipsilateral and contralateral arterial blood flow (Topp et al., 2011b). It is unknown whether menthol-based pain or discomfort inhibition and decreased vascular conductance or blood flow would contribute to a greater stretch tolerance and hence greater ROM.
There is extensive evidence that the unilateral application of interventions such as muscle fatigue, stretching, and rolling can induce increased fatigue (Halperin et al., 2014a;b; 2015; Kawamoto et al., 2014), ROM (Behm et al., 2016b; Chaouachi et al., 2017) and pain thresholds (decreased pain sensitivity) (Aboodarda et al., 2015; Cavanaugh et al., 2017) in contralateral limbs, respectively. Since fatigue, stretching, rolling, and menthol based topical menthols can influence contralateral actions (i.e. fatigue, ROM, blood flow) and sensations (i.e. pain threshold), would unilateral application of a menthol based topical analgesic have global pain relief and augment the stretch threshold increasing ROM of the treated and untreated limbs.
As there have been no studies to the authors' knowledge that have examined the local and global effects o f a me n th o l ba s ed topical an al g esi c o n RO M, the obj ec ti v e of the present study was to administer a menthol based topical analgesic unilaterally and observe the effects upon the passive static, active and ballistic ROM of the treated and contralateral untreated hip flexion ROM. Based on the aforementioned literature demonstrating non-local effects, it was hypothesized that the unilateral application of a menthol based topical analgesic would enhance stretch tolerance and increase ROM in the treated and untreated limbs.
A further objective was to detect possible sex ROM differences due to the application of a menthol based topical analgesic.
A priori statistical power analysis (G Power: Universitat Dusseldorf) was conducted to determine the minimum number of participants needed to achieve an alpha of 0.05, power of 0.8 and effect size of 0.5. The analysis of ROM with the Behm et al. (2016b) and Chaouachi et al. (2017) studies indicated that sample sizes of 8 and 12 respectively were needed. Hence, 14 participants consisting of seven males (21.7 [+ or -] 1.5 years, 1.80 [+ or -] 0.07 m, 84.2 [+ or -] 8.1 kg, BMI: 25.9 [+ or -] 2.3) and seven females (20.5 [+ or -] 1.3 years, 1.65 [+ or -] 0.06 m, 67.6 [+ or -] 16.9 kg, BMI: 25.2 [+ or -] 1.8) were recruited from the undergraduate student population, with an age range of 18-24 years. Participants were excluded if: (a) they had any pre-existing health conditions, such as cardiovascular problems or any musculoskeletal injuries and (b) they were unable to properly perform the stretches involved in the study. Prior to testing, all participants were provided with information regarding the nature of the study, the requirements upon participation in each data collecting session (i.e. attendance to all four sessions upon commitment to study, warming up for 5 minutes on the cycle ergometer prior to testing) and each participant completed an informed consent form. The study approved by the institution's Interdisciplinary Committee on Ethics in Human Research (20192235-HK) and was conducted according to the Declaration of Helsinki.
Fourteen participants were tested for passive static, active and ballistic hip flexion ROM of both legs, pre- and post-interventions. The double blind, repeated measures (crossover) experimental design included the random allocation of four conditions: 1. Placebo with static stretching (SS), 2. Placebo with dynamic stretching (DS), 3. Menthol based topical analgesic with SS, 4. Menthol based topical analgesic with DS. After the pre-test measures, the menthol based topical analgesic or placebo gel was applied unilaterally to the right hamstrings by the researcher with latex free gloves, followed by a 15-minute rest period to achieve the full effects of the topical analgesic (Johar et al., 2012; Topp et al., 2011a; 2011b). The gel was applied to the leg (no prior preparation such as shaving, abrading or alcohol cleansing) superficially without massage. After the 15 minute waiting period, a dynamic warm-up was employed, which consisted of a five-minute cycling period on a cycle ergometer at 70 rpm/Kp. Following this warm-up, either unilateral SS (three repetitions of 30-seconds with 30-second rest between repetitions) or DS (three repetitions of 30 kicks at 1Hz with a 30-second rest between repetitions) of the treated (right leg: placebo or topical analgesic) leg was performed. Post-intervention tests of both legs were performed immediately thereafter (post-test) and 10 minutes after the intervention (post-10-min). Both legs were tested to determine the local and non-local (global or crossover) effects of the unilateral interventions (topical analgesic vs. placebo and SS vs DS).
Menthol based topical analgesic versus placebo gel intervention
A volume of 5 ml of menthol based topical analgesic gel (Biofreeze: Performance Health Inc: Akron Ohio) or placebo gel was applied unilaterally to the entire length and breadth of the right hamstrings. The placebo gel supplied by Performance Health Inc was formulated to look, and smell similar, as well as have a similar gel composition. Whether the gel applied to the hamstrings was a placebo or topical analgesic was blinded to all participants and researchers. To...