Participants were randomly assigned to either (a) TCWI or (b) IM group, and followed up for over four days. Ethical approval was obtained from the Lebanese German University Institutional Review Board, and upon the confirmation of eligibility, each participant received an information sheet describing the study, signed voluntarily a consent form for participation, and filled a data collection form. To ensure confidentiality, participants’ data were accessible only by the principal investigators upon the need.
Physically active Lebanese adults aged between 19 and 44 years were recruited to participate in this study. To ensure that the participants will tolerate the intensive muscle-damaging protocol, they should be engaging in mild to moderate sports activities over two to three times per week . Participants with a history of serious lower limb trauma (i.e., fractures), meniscal or ligamentous tears, sensitivity to cold temperature, cardiopulmonary or inflammatory diseases, or individuals participating in strength and plyometric training exercises were all excluded.
The participants were recruited via WhatsApp messages sent to all the investigators’ contact lists. This message included a brief explanation of the study purpose, eligibility criteria, phone numbers of the investigators, and a request to forward this message to their contact list.
Upon eligibility, participants were familiarized with the experimental procedure. Afterward, all participants were stratified into two blocks (30 males and 30 females), and a randomization website (http://www.randomization.com) randomly allocated them equally into one of two intervention groups (TCWI or IM).
The assessment included a battery of tests including serum CK, one-repetition maximum (1-RM), countermovement jump (CMJ), and visual analog scale (VAS) respectively for baseline measurements. Testing followed the muscle damage protocol.
Firts, participants were asked to perform a knee extension exercise with the dominant lower extremity for strength testing. They started with a general warm-up of five minutes of moderate intensity jogging. This was followed by five minutes of muscle endurance exercise with a light load of 15 kg over 10 repetitions [3, 12]. Coutermovement jump testing followed strength testing after one hour. Finally, participants were asked to perform a maximal contraction of their right dominant leg (quadriceps) over four seconds, where an assessor immediately rated the level of soreness on the VAS .
After baseline measurements, each intervention (TCWI or IM) was conducted among the designated group. Testing was performed post muscle damage protocol at 2, 24, 48, and 72 H.
Muscle damage protocol
The muscle damage protocol consisted of five sets of 20 drop jumps from a 60-cm high box with 2 min of rest between sets. After each drop from the box and landing on the floor, participatans were instructed to perform a maximally explosive vertical jump upward and then land on the floor. Participants were told to flex their knees to at least at 90° during all landings and to keep their hands on their hips during the jumps. Moreover, they were verbally motivated to exert maximal effort during every repetition .
Participants allocated to the TCWI group completed a session of 15-min in cold water with a temperature of 12 °C . Participants were seated while immersing their whole body in water except their head and neck. The temperature was continuously measured by a mercury-in-glass thermometer and was maintained at 12 °C by adding blocks of ice when needed.
Participants in the IM group were seated and subjected to a circular local massage through the use of an ice cube. Massage was applied for 15 min with no extra pressure added by the tester, for each thigh, over the region of the quadriceps muscle .
It is noteworthy to mention that the modalities were applied by a qualified physical therapist for all the participants.
Data collection method
Serum creatine kinase
Muscle damage was tested via the serum CK blood test. This marker was chosen due to its dramatic increase in blood after EIMD, and its low cost . Blood samples were taken from participants at pre-exercise, then at 2H, 24H, 48H, and 72H post-exercise for all participants in both intervention groups. The blood samples were clotted for 30 min, then centrifuged for 20 min at room temperature. The serum was then stored at -20 °C until analysis. Finally, serum CK activity was analyzed with an enzymatic method at 37 °C .
Muscle performance is defined as the overall capability of a muscle or muscle group , and is divided into (a) speed, (b) coordination, (c) dynamic balance, (d) agility, (e) flexibility, (f) muscle strength, and (g) muscle power.
Muscle strength is defined as the maximal amount of force produced by a muscle or by a group of muscles in a specific exercise for one repetition . 1-RM test, is a gold standard , and a reliable method for evaluating the maximal strength with test–retest measures recorded high ICC = 0.990 , and a confidence interval of (CI 95%) . 1-RM test procedures for the measurement of muscular strength followed the recommendations of the American College of Sports Medicine . The starting position of the knee extension exercise was 90-degrees of knee flexion. Weights were gradually adjusted until the subject failed to safely and correctly complete a full range of motion of extension. This load is recorded and referred to as the 1-RM . A rest time of two minutes was taken between sets .
Power is related to the strength and speed of movement and is defined as the work produced by a muscle per unit of time (force × distance/time) . According to Markovic et al. , explosive muscle power is the main determinant of performance. Increase in leg power showed to affect the vertical jump height which is captured by the CMJ. CMJ begins with the knees flexed  and ends when the pelvis, defined by the four superior iliac spines, achieves its lowest position . Therefore, participants were instructed to drop jump from a 60-cm box, then perform a maximal bipodal CMJ, then land on the floor with knees flexed to at least 90°. To ensure measurement accuracy and reliability, participants were instructed to keep their hands on their hips during the jump and to keep their legs and hips extended until contact is made (end of landing phase) . CMJ was repeated three times and the highest vertical measure was recorded. ICC values showed moderate to excellent inter-session reliability for the bipodal CMJ (ICC average: 0.80, range: 0.46–0.97) according to Schwartz et al.  CMJ test showed excellent reliability (ICC = 0.98) and showed the highest correlation with the explosive power factor (r = 0.87) . CMJ power values were recorded by the "My Jump" application. According to Fernández et al. , “My Jump” showed an excellent validity for CMJ height (r = 0.995, P < 0.001) as compared to the force platform, which is the gold standard for estimating the explosive power of the lower limbs . Fernández et al.  stated that there was almost perfect agreement between the force platform and “My Jump” for the CMJ height (ICC = 0.997, P < 0.001; Bland–Altman bias = 1.1 ± 0.5 cm, P < 0.001).
DOMS is defined as a delayed feeling of soreness (about 24H after the exercise) accompanied by muscle stiffness, aching soreness, and/or muscular tenderness . Muscle soreness peaks within 72H and slowly resolve within 5 to 7 days . DOMS was assessed by the VAS of 10 degrees, ranging from “no soreness” (0) to “severe soreness”  . VAS showed excellent reliability in the assessment of DOMS, the ICC range was 0.98–0.99 for VAS .
Data were analyzed using the Statistical Package for the Social Sciences (SPSS) version 21.0 for windows. Descriptive statistics (mean, standard deviation) were calculated for participants’ age and the main key variables of the study. The Shapiro–Wilk test was used to check the normality of the data distribution. A probability (p) value of > 0.05 indicates that there is no significant difference between groups.
Regarding data analysis, a two-way (group x time) mixed factor analysis of variance (ANOVA) test was used to compare the mean differences between groups, TCWI and IM, for each dependent variable (CK, 1-RM, CMJ, and DOMS) at different timelines (Baseline, 2 H, 24 H, 48 H, and 72 H). Moreover, the T-test was used to indicate the significance of the difference between variables.