The participants for this study consisted of 122 current CKD practitioners (82 men, 40 women) aged 18 and older from 9 countries, which included Argentina, Australia, Colombia, Germany, India, Malaysia, South Korea, UK, and US. The demographic details are shown in Tables 1, 2.

Demographics, training volume, and injury details were obtained using the online survey consisting of 22 questions. Demographic information included age, sex, height, weight, country of residence, years of CKD training experience, current belt rank, and purpose for learning CKD. Questions regarding training volume included frequency, intensity, and duration. Lastly, injury details consisted of questions pertaining to the cause, diagnosis, number of injuries, severity, and area of injury.

E-mails were sent to CKD school owners in the countries previously mentioned which outlined the purpose of the study as well as the directions. The directions included asking their students to take an online survey with the inclusion criteria being 18 years of age or older and currently involved in CKD training. The school owners informed their students that participation in the study was voluntary and that their responses would be kept confidential, which were also stated in the survey. The participants were also informed of the purpose of the study and asked to sign an informed consent form approved by the Hanseo University College of Health Science Human Studies Committee. The investigatory online survey was conducted between February 4, 2016 to March 21, 2016. The link to the online survey was given only to those who expressed an intention to participate in the study.

Safety is one of the most important aspects of CKD training. The techniques are designed to generate maximum power through the use of sequential motion while minimizing injuries to the joints. Competitions such as full contact sparring and tournaments are not a part of CKD training. A typical class is between 45 to 60 min and includes a combination of 10 components: warm up/cool down, stretching/flexibility, basic techniques, patterns, speed drills, defense drills, target training, close range defense, rehabilitation/strength training, and character development. The first component is warm-up/cool down exercises, which may include jumping jacks, running in place, lightly performing punches and kicks in the air, etc. This is followed by stretching and flexibility exercises which consist of 2 types: static and dynamic. Static stretching involves sustaining muscle-lengthening activities using body weight. Each position is held at the point of tension for 10–20 sec while the muscles are relaxed with deep abdominal breathing. Dynamic stretching consists of performing CKD movements slowly and using momentum to move joints throughout the entire range of motion to improve dynamic flexibility. The next component is basic techniques, which consist of basic punches, strikes, kicks, and blocks. The fourth component, patterns, involve preset combinations of basic techniques, which are different for each belt rank. The level of intensity in which the patterns are performed can be adjusted for various training purposes. Speed drills consist of a combination of offensive techniques that are intended to be performed at high speeds. Defense drills involve working with a partner to practice and develop skills needed for handling an attack or confrontation while wearing safety equipment. Target training uses equipment such as hand held targets and punching bags to apply CKD techniques against such objects of resistance. Close range defense is practiced in the event a student is grabbed by an attacker. Rehabilitation and strength training involves returning the body’s function to its preinjury state and increasing muscle strength and endurance. Lastly, character development trains the mind to adopt a positive attitude and belief system.

For cardiovascular development, basic techniques, patterns, speed drills, and defense drills are often performed for a certain period of time at a level of intensity to bring the heart rate up to 60%–80% of the maximum heart rate for 20 min. For anaerobic development, patterns, speed drills, and target training are performed at maximum speed for 1 or 2 min and then given a period of rest that is twice the length of time spent in activity. Conditioning, or strength training, involves additional target training exercises, calisthenics, and plyometric activities.

Statistical analysis was conducted with the IBM SPSS ver. 18.0 (IBM Co., Armonk, NY, USA). All data are reported as mean± standard deviation and as mean change over baseline. Prior to the main analysis, the Kolmogorov–Smirnov test was used to determine the normality of distribution of the examined variables. Since the data was not normally distributed, nonparametric Mann–Whitney U-test was used for comparing the variables. The examined variables included participant training experience, training volume, number of injuries, severity of injuries, injured body parts, causes of injuries, and diagnoses of injuries. Frequency analysis was used as an analysis method and verification was performed at a significance level of P<0.05.

As shown in Table 1, the mean age, height, and weight were 40.08±11.82 years, 171.82±9.11 cm, and 78.50±15.54 kg, respectively. Of the 122 participants, there were 82 men and 40 women.

Table 2 describes the characteristics of the participants. Among the 9 countries, the highest number of participants was from the UK (45.9%). Color belts accounted for more than any other rank (34.4%), followed by 1st degree black belts (19.7%). There was a wide range of training experience, but most participants had 1–4 years (38.5%). The average training experience of all the participants was 7.96 years and the most common reason for training was for health and fitness (20.2%), followed by self-defense (17.6%).

Although the belt curriculum is uniform across all CKD schools, each school is led by different instructors who may have different ways of teaching class. To gather data on the training volume for each participant, questions regarding the frequency, intensity, and duration were asked in the survey as shown in Table 3. The most common CKD training volume was a frequency of twice a week (37.7%) with moderate intensity (47.5%), and a duration of 45–60 min (68.0%). To obtain specific figures, values were given for 7 levels of intensity: extremely low (1), very low (2), low (3), moderate (4), high (5), very high (6), extremely high (7). For duration, 3 values were used to obtain a specific number of minutes: 45 min for ‘45 min or less,’ 52.5 min for ‘45 to 60 min,’ and 90 min for ‘more than 60 min.’ Using these values, the participants in this study trained with a frequency of 2.7 times per week, with moderate to high intensity, for a duration of 63.4 min per training session. Per week, this amounts to 2.85 hr of CKD training.

There was no significant correlation between training frequency and duration on injury. However, there was a significant correlation (P=0.009) between training intensity and injury as shown in Table 4.

To provide a more standardized way of expressing the injury rate of CKD practitioners, the annual number of injuries per 100 people was calculated with a formula used in a study by Birrer and Halbrook (1988) which investigated the injury rate of martial arts practitioners from a 5-year national survey. The annual rate of injury per 100 CKD practitioners was 11.73 (114 (total # of injuries)122 (total # of participants)×1007.96 (average years of experience)). Of the 122 participants, 53 (43.4%) sustained a total of 114 injuries during their entire training experience in CKD. The participants in this study had an average training frequency of 2.7 times per week and a duration of 63.4 min per session (171.18 min per week). 43.4% of those who participated in the study experienced a physical injury once (39.69%). Participants who experienced injury chose from among 6 degrees of severity ranging from ‘very minor’ (11.32%), ‘minor’ (35.88%), ‘somewhat minor’ (26.49%), ‘somewhat severe’ (16.96%), ‘severe’ (3.70%), and ‘very severe’ (5.65%), all of which were self-reported. The body parts that were most involved in injuries were ‘multi parts’ (47.19%) followed by ‘lower limb’ (18.86%), ‘ankle, foot, or toe’, ‘hand, wrist, or finger’, and ‘no reply’ which were equally 7.55% each, and ‘back’ (1.6%), and ‘head,’ ‘upper limb,’ ‘hip or groin,’ and ‘others’ which equally accounted for 1.89% each. Further details of injuries are shown in Table 5.

Improper technique was cited as the most common cause of injury (27.7%). The most common diagnoses of injuries were muscle sprains (21.9%) followed by bruises (14.3%) and ligament sprains (12.4%). A complete list of the causes and diagnosis of injuries is shown in Table 6.