Over the past decade, training with kettlebells has become increasingly popular in the health/fitness industry globally. Kettlebells are large iron dome-shaped weights with a single handle cast in multiples and fractions of a Russian pood, which can be rounded as 16 kg in metric units. Historically, kettlebells were first used in early 18th-century Russia as a counterweight to measure farming stock. However, they soon became a tool for the males to show and compare their abilities in strength, endurance, and coordination as part of the social gatherings, which led to the early development of kettlebell exercises (2). In the present day, kettlebell training has been adapted in the health/fitness industry comprising ballistic moves performed against gravity for development of general physical fitness (2). Typical exercises performed in a kettlebell workout include swings, cleans, deadlifts, kettlebell presses, and snatches (Figures 1A–E). Kettlebell training often is performed in a circuit with specific work-to-rest ratios. For example, a typical circuit training routine comprises a format of 20 seconds of work followed by 10 seconds of rest, repeated for up to 4.5 minutes (11).
Over the past decade, research into kettlebell training has shown significant improvements in strength, power, and cardiovascular fitness among healthy young males (3,8,10). Moreover, several studies showed that kettlebell training is capable of improving cardiorespiratory endurance among healthy younger males and females (3,4) by producing a cardiac response of at least 76% of an individual’s maximum heart rate (HRmax) (ACSM’s Guidelines for Exercise Testing and Prescription, 9th edition). However, there has been limited research into the associated risks. This article aims to (a) demonstrate the physiological demands of kettlebell exercises and (b) identify risks pertaining to the use of kettlebells to provide fitness professionals with risk-management strategies that can help minimize the risk of injuries.
PHYSIOLOGICAL DEMANDS OF KETTLEBELL TRAINING
Withstanding the increasing interest in kettlebell training in exercise and sport settings, a number of scientific studies have been conducted investigating the effects of kettlebell training on cardiovascular fitness (3,4,11). For example, Hulsey et al. (4) compared the metabolic demands of kettlebell training with treadmill (TM) running in a group of moderately trained young men (n = 11, mean age = 21.4 ± 2.1 years) and women (n = 2, mean age = 21.4 ± 2.1 years) by analyzing average HR, oxygen consumption (V·O2), and rate of perceived exertion (RPE) during exercise. The kettlebell training routine involved completing as many kettlebell swings as possible in 12 minutes using either a 16-kg kettlebell (men) or an 8-kg kettlebell (women). After 48 hours, the participants completed a 10-minute TM run, which involved running at equivalent RPE as measured during the kettlebell routine. Results showed that the average HR in the kettlebell training routine (180 ± 12 beats per minute [bpm], 90% HRmax, all participants) was slightly higher than in the TM running routine (177 ± 11 bpm, 89% HRmax) with similar hard levels of RPE (15.3 ± 1.2 and 15.5 ± 1.2, respectively). However, the average V·O2 (34.1 ± 4.7 mL·kg·minutes) was 27% lower in the kettlebell routine than in the TM running routine (46.7 ± 7.3 mL·kg·minutes). Nevertheless, the results showed that kettlebell training can be an effective means in developing cardiovascular fitness by providing sufficient levels of cardiorespiratory stimulus, as recommended by ACSM (ACSM’s Guidelines for Exercise Testing and Prescription, 9th edition).
Similarly, Williams and Kraemer (11) compared cardiorespiratory and metabolic responses with a kettlebell high-intensity interval training (HIIT) routine (3 sets of 4 exercises, using a 20-second work and 10-second rest ratio for 12 minutes) versus a standard sprint interval cycling (SIC) protocol (3 sets of 30-second maximal cycling sprints and 4-minute rest) by analyzing HR, V·O2, and caloric expenditure in a group of well-trained younger men (n = 8, mean age = 21.5 ± 0.8 years). The results demonstrated that the kettlebell HIIT routine produced a significantly higher average HR (149.1 ± 7.4 vs. 139.6 ± 7.8 bpm), average V·O2 (22.6 ± 1.4 vs. 19.9 ± 1.0 mL·kg·minutes), and total caloric expenditure (144.8 ± 6.5 vs. 122.0 ± 7.3 Kcal) compared with the SIC routine. In another recent study, Howard et al. (3) compared the cardiorespiratory and metabolic demands by analyzing average HR, V·O2, and blood lactate of a 4-minute bout of tabata (20-second exercise and 10-second rest) intervals performing the two-handed kettlebell swing exercise with 8-kg (men) and 4-kg (women) kettlebells versus a volume-matched traditional kettlebell training regime (4 sets of kettlebell swings and 90-second rest) in a group of healthy, young men and women (n = 14; age range, 18–25 years). Results showed that both the average percentages of HRmax (81% vs. 73%) and V·O2max (71.0% ± 0.3% vs. 58.4% ± 0.3%) were significantly higher after the tabata interval training. Although these findings suggested that interval or traditional kettlebell training can produce sufficient cardiorespiratory responses to improve cardiorespiratory endurance (ACSM’s Guidelines for Exercise Testing and Prescription, 9th edition), Jay et al. (5) reported no significant effects of an 8-week, 20-minute kettlebell training program performed three times per week on the cardiovascular fitness of a group of middle-aged men and women (n = 40; combined mean age, 44.0 ± 8.0 years).
Notwithstanding its popular use in strength and conditioning programs, research examining the effects of kettlebell training on muscular strength and power has been limited (8,10). For instance, Lake and Lauder (8) examined the effects of a 6-week kettlebell training program (12 sets of 30-second maximal effort kettlebell swings, alternated with 30-second rest) performed twice per week on maximum (half squat, one-repetition maximum [1RM]) and explosive strengths (vertical jump height) in a group of active younger men (n = 12, age range = 18.0 and 27.0 years). The results showed significant improvements in both maximum and explosive strengths of the participants. More recently, Otto et al. (10) similarly reported that a 6-week periodized kettlebell training program showed significant improvements in maximum strength (1RM squat) and explosive power (vertical jump power) in a group of recreationally active and strength-trained younger men (n = 17, mean age = 22.9 ± 0.86 years). Kettlebell exercises also have been studied in injury prevention and rehabilitation programs. For example, Jay et al. (5) found that 8 weeks of training with the kettlebell swing exercise was beneficial in significantly reducing neck and lower back pain in a group of middle-aged men and women (n = 40, combined mean age = 44.0 ± 8.0 years). In addition, Zebis et al. (12) highlighted that kettlebell swings can effectively recruit and target the medial hamstring (semitendinosus), which plays an important role in the prevention of knee injuries by stabilizing against valgus collapse during dynamic movement. Finally, Anderson et al. (1) compared the trunk muscle recruitment in both single- and double-handed kettlebell swings to measure differences in single-handed versus double-handed kettlebell swings. The results showed higher trunk muscle activation levels in the single-handed kettlebell swing that can be as a result of the need for more stabilization of the core muscles for balance during the movement (1).
BIOMECHANICAL DEMANDS AND RISK OF INJURIES
The kettlebell swing forms the technical foundation for most kettlebell exercises (2). The kettlebell swing is a biomechanically complex exercise that incorporates ballistic eccentric and concentric contractions of the hip musculature, which recruit posterior chain muscles. A kettlebell swing is initiated by actively flexing the hip joint while restricting knee flexion, allowing the kettlebell to be displaced in a downward arc between the knees until a bottom position is reached (9). The placement of the kettlebell in the bottom position is dictated by individual hamstring flexibility and ability to maintain a neutral spine during the coordination of the hip and lower back. From the bottom position, the kettlebell is accelerated rapidly upwards by substantial knee and hip extension with full extensor muscle chain activation (9). This explosive contraction is known as hip drive that involves the hinging action of the hips during a kettlebell swing.
The rapid muscle activation patterns and joint loads of a kettlebell swing inevitably raise questions as to its safety and whether it can impose potentially damaging forces to the spine. In this regard, McGill and Marshall (9) reported that the spinal loads occurring during a 16-kg kettlebell swing do not exceed the National Institute for Occupational Safety and Health guidelines for maximal compressive loading of the lumbar spine (approximately 340 kg). This suggests the compression loads during a 16-kg kettlebell swing may not be problematic for individuals with no existing or history of injuries to the spine. However, the authors also reported that the swing imposed relatively high shear loads to the spine (Figure 2), which may aggravate preexisting lower back conditions vulnerable to shear stresses. Moreover, the ballistic nature of the kettlebell swing and the high weight increments of the kettlebells can encourage the use of heavier loads than what might typically be used in conventional barbell or dumbbell exercise. This can, as a result, further increase the risk of an injury to the lower back. Therefore, to reduce the risk of lower back injuries, trainers are first advised to conduct a thorough preexercise health screening before engaging their clients in kettlebell exercises. Secondly, trainers are advised to ensure appropriate weight selection according to the fitness level and needs of their clients. It is recommended that already physically active females start with 4-kg to 8-kg kettlebells, whereas males may begin with 12-kg to 16-kg kettlebells to develop basic strength (3,4,11). If the aim of the client is to develop more advanced strength and power, a progression to heavier kettlebells (>16 kg for males and >8 kg for females) is advised. To develop cardiorespiratory fitness, smallest weights should be preferred to begin with to prevent premature fatigue caused by an excessive buildup of metabolic by-products, such as lactate in the forearm muscles. Finally, trainers should constantly emphasize maintenance of a neutral spine while performing kettlebell exercises, which can reduce shear stress on the spine by producing a countermuscular force through the activation of the erector spinae muscles (9). If at any stage of a kettlebell exercise, lifting posture and alignment deteriorate, the exercise must be stopped to give the client sufficient time to rest and readjust the weight to minimize the risk of an injury.
In addition to the risk of injuries to the lower back, injuries to the wrist have commonly been reported among beginners as a result of the direct impact of the kettlebell over the wrist or due to off-center handling of the kettlebell (7). In a clinical case reported by Karthik et al. (7), a 39-year-old man presented with wrist pain in his right hand that had been going on for the last 3 months. The patient reported that the pain had started a few weeks after he started using kettlebell exercises for weight training. A thorough physical examination revealed that the patient had damaged his hand tendons (extensor pollicus brevis and abductor pollicis longus) as a result of the repetitive impact of kettlebell exercises that involved pressing techniques. In this regard, the authors cautioned against performing any kettlebell exercise with the wrists hyperextended to prevent the risk of injuries to the wrist. They also have stressed the importance of teaching the kettlebell users, especially beginners, about correct grip techniques and problems of off-center handle holding.
Poor ergonomic design of a kettlebell also can increase the risk of injuries. Traditionally manufactured kettlebells are one-size-fits-all with relatively thick vertically oriented handles, which may not be suitable for all individuals. This is unlike some traditional forms of weight training such as Olympic weightlifting, which caters for sex differences. For example, a men’s Olympic barbell is 2.2 m long and weighs 20 kg and has a diameter of 28 mm at the grip section. Whereas, a women’s Olympic barbell is similar to the men’s but is shorter at 2.10 m and lighter at 15 kg. It also has a smaller grip section diameter of 25 mm that allows women, who generally have smaller hands, to grip the bar more easily while reducing the risk of injury (http://www.iwf.net). Moreover, the body of a kettlebell is often too curved, which can cause impact injuries resulting from the excessive contact of the kettlebell with the wrist and forearm, when the kettlebell is moved incorrectly upwards during exercises such as the clean and snatch.
The kettlebell has become one of the most common forms of exercise equipment used in exercise settings. Despite its popularity and proven health and fitness benefits (4,5,8), the complex physiological and biomechanical demands of the kettlebell exercises raise concerns as to the adequacy of knowledge/awareness of the personal trainers instructing their clients in proper technique to minimize the associated risk of injuries. The following risk-management strategies are recommended for health/fitness professionals to use kettlebell exercises safely.
- Preexercise health screening: Before developing any kettlebell training program, a client’s needs assessment should be conducted through a comprehensive preexercise health screening system. Particular attention should be paid to any previous or existing musculoskeletal conditions such as lower back injuries, shoulder injuries, wrist or hand repetitive strain injuries, and degenerative joint conditions.
- Basic assessment of coordination: Before commencing any kettlebell exercise, a basic assessment of coordination should be conducted to measure the ability to hip hinge (the process of flexing and extending the hip) while keeping a neutral spine (Figure 3). For lower back safety, the clients should be reminded continuously to maintain a neutral spine when performing kettlebell exercises (6).
- Functional movement screening: Kettlebell exercises demand high muscular coordination and balance. Therefore, identifying and correcting obvious signs of faulty movement patterns before embarking on kettlebell exercises are crucial. Health/fitness professionals are advised to use established functional movement screen tests such as the overhead squat test.
- Core stability: Kettlebell swings require the repeated application of explosive force. Applying explosive force requires excellent core stabilization to brace the torso effectively to maintain a neutral spine. Therefore, core stability assessments (e.g., timed side-plank hold) are recommended before engaging a client in kettlebell training.
- Kettlebell positioning: Kettlebell cleans and snatches require correct rotational movement techniques to avoid excessive impact and bruising on the posterior forearm. To minimize excessive forearm impact, health/fitness professionals should instruct their clients to spear their hand through the kettlebell when performing kettlebell snatches, rather than flipping the kettlebell over, because they transition to the top position of the exercise. During a kettlebell clean, to minimize excessive posterior forearm contact, health/fitness professionals should instruct their clients to keep their elbow and forearm perpendicular to the floor when receiving the kettlebell in the racked position. Finally, during both lifts, a loose but controlled grip will allow the kettlebell to rotate rapidly enough to reduce forearm impacts.
- Kettlebell weight selection: Although there is no specific guideline for kettlebell weight selection, the goal, relative strength, and level of physical conditioning of a client should be considered when deciding the weight of kettlebells to be used in a training routine.
- Kettlebell design selection: Kettlebell designs should be selected according to the individual differences and needs of the clients. Kettlebells with a contoured design wrap around the wrist and forearm when performing kettlebell exercises such as the clean and snatch, assisting correct technique. Contoured kettlebells also have smaller, smoother handles, which may be more comfortable for clients with a smaller build.
- Personal protective equipment: The trainers should encourage their clients to use personal protective equipment such as weightlifting gloves and wrist guards and appropriate footwear with a secure fit and a flat and stable sole such as weightlifting shoes, which can prevent injury during certain kettlebell exercises.
BRIDGING THE GAP
Kettlebell training offers the chance to vary existing exercise regimes and can benefit cardiovascular fitness, core stability, muscular strength, and power. Moreover, kettlebell exercises can be used to improve hamstring function and potentially reduce neck and lower back pain. However, the ballistic and unorthodox nature of kettlebell training techniques can increase the risk of injuries, especially in unconditioned participants and beginners. Providing health/fitness professionals implement risk-management strategies including a comprehensive preexercise health screening system, functional movement screening, and appropriate weight and ergonomic selection, the risk of injuries can be minimized for the safe and effective use of kettlebells in exercise programs.
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