Existing literature provides a basic understanding of the biomechanics of a range of strongman exercises, with few studies extending to identify the biomechanical determinants of performance of strongman exercises [32,33,34]. Currently, there exists very little research evidence regarding how performance in one strongman event may be related to other strongman events. The only study that has empirically set out to answer this question has examined relationships between performance in strongman exercises, strength in TWTE and anthropometrics, with a limited number of strong correlations found between performance in individual strongman exercises (competition events) [8]. A limitation in the ability to answer this question is the wide variety of strongman event requirements, where various implements may be used for a given general movement pattern (e.g., for overhead pressing logs of varying diameters, axels, kegs, dumbbells, Viking machines can all be used), all of which can be performed to different competition requirements (as many repetitions as possible (AMRAP), 1RM, increasing load). As such, the following sections of the discussion will be organized under the general movement patterns of carrying/walking, pulling and static lifts. Qualitative analysis of strongman exercises in their most general form, along with quantitative results from studies of similar TWTE and CEA, may provide a greater understanding of strongman exercise performance determinants, injury risk and wider applications to other populations.
Carrying/Walking Exercises
Bilateral Load Carriage
The farmer’s walk and yoke walk are the most common bilateral carrying strongman exercises used in strongman training [10, 18]. Little biomechanical analysis has been performed on the yoke walk, with spinal motion, muscle activation and loading being measured [35]. Although differing in the absolute load and positioning of the load being carried, quantitative analysis of the farmer’s walk and other forms of load carriage may provide a greater understanding of the biomechanics of the yoke walk strongman exercise.
A systematic review comparing the biomechanics of backpack load carriage and unloaded walking showed backpack load carriage to be associated with an increase in stride rate (ES = 0.37) and a decrease in stride length (ES = − 0.32) when compared with unloaded walking [40]. The effect of backpack load carriage on spatiotemporal measures across the studies was small; however, effect sizes progressively increased as load increased [40]. Such findings are consistent with the farmer’s walk exercise, where substantially greater loads were used and greater differences existed compared with unloaded walking (stride rate: ES = 4.20, stride length: ES = − 3.40) [7]. Although a physical limit will be approached, whereby the athlete is no longer able to increase their stride rate with a decrease in stride length, it may be expected that the greater loads that can be used in the yoke walk when compared with the farmer’s walk would result in further increases in stride rate and decreases in stride length.
Statistically greater anterior/posterior, medial/lateral and vertical ground reaction forces were reported during the farmer’s walk when compared with unloaded walking [7]. Similar results have been reported when comparing backpack load carriage with unloaded walking, where greater propulsive and braking (anterior/posterior), and vertical ground reaction forces were reported during backpack load carriage [40]. The difference in anterior/posterior ground reaction forces observed between unloaded walking and backpack load carriage may partially be the result of the center of mass of the carrier being pulled backward when the load is positioned posterior to the centerline of the body.
Similar to the farmer’s walk, the athlete’s ability to maintain/minimize the reduction in their stride length while maintaining or increasing their stride rate during the yoke walk will result in a higher velocity and thus a greater performance outcome by the athlete [34]. Where greater braking and propulsive forces have been reported in the farmer’s walk than unloaded walking conditions [7], it would be suggested that greater performance in the farmer’s walk and yoke would be achieved by minimizing any potential increases in braking force while maximizing increases in propulsive force. A limiting factor contributing to the athlete’s ability to demonstrate these biomechanical determinants of performance in the farmer’s walk may be the grip strength of the athlete, as in most competitions the only artificial aid athletes can use to assist their grip is lifting chalk. Similarly, a limiting factor contributing to the athlete’s ability to demonstrate these biomechanical characteristics in the yoke walk may be the athlete’s ability to brace their trunk and hip musculature and tolerate high compressive loads [35]. The use of load carriage exercises in strength and conditioning programs of non-strongman athletes may support the development of these limiting factors, whereby it has been reported that exercises such as the farmer’s walk are typically included in programs to develop grip strength and total body strength [10].
Unilateral Load Carriage
The keg walk technique adopted in McGill et al. [35], whereby the keg is carried on a single shoulder, is just one technique which may be used by an athlete in a keg walk competition event or as a strength training exercise for non-strongman athletes. Other techniques to perform the keg walk may include; wrapping one’s arms around the keg in a hugged position on the anterior surface of their abdomen, lifting and carrying the keg using the handles positioned around the rim of the keg, or a combination of the aforementioned techniques. Individualized biomechanical analysis of each technique would therefore be required and as such is beyond the scope of this review.
Performance in the suitcase carry has been characterized by an athlete’s ability to maintain a vertical spinal posture (with respect to the frontal and sagittal anatomical plane) and a constant step cadence [31]. This may be deduced by the tendency for an increase in lateral bend and inability to maintain a set cadence as load is progressively increased [31]. As the load used in previous suitcase (McGill et al. [35]: ~ 31% bodyweight, Holmstrup et al. [31]: ~ 63% bodyweight) and unilateral dumbbell carriage studies may be less than what is expected to be used in strongman training, trunk bend, lumbar spinal loading, ground reaction force asymmetry, and changes in gait characteristics may be further magnified in a true strongman setting where greater loads are carried.
Future research on the biomechanics of strongman bilateral and unilateral carrying/walking type exercises may assist in determining the biomechanical demands of military physical fitness assessment exercises. Such assessments include the jerry can carry which is used to assess grip strength and load carriage speed, whereby military personnel carry jerry cans (usually of mass > 20 kg) a short distance (~ 20 m) in the fastest possible time [41]. Research into the biomechanical demands of the yoke walk may provide a foundation for future research into the demands placed on firefighters carrying breathing apparatus and firefighting equipment, and trail porters who have been known to carry loads of one-and-a-half times their body mass over vast distances [42]. As the practical guidelines on how to best condition these occupational groups for load carriage are limited, findings from strongman research may play a pivotal role in this process [43].
Pulling Exercises
Previous strongman biomechanical studies have only analyzed the biomechanics of athletes performing the heavy sled pull (> 100% body mass), which is typically used as a training tool to simulate the vehicle pull for strongman athletes, or as a strength and conditioning tool for other athletic groups [10, 32, 38]. Assessing the biomechanical similarities between persons performing the heavy and sub-body mass sled pull may assist in establishing the likely biomechanics of performing a vehicle pull.
Greater decreases in velocity, stride length and second-stride swing time, and greater increases in ground contact time have been found to occur when performing a sub-body mass sled pull at a sled load of 32.2% body mass compared with 12.6% body mass [44]. While no statistical difference in stride rate was reported between the two sub-body mass loading conditions, stride rate was statistically lower under both loading conditions than the unloaded condition [44]. No comparisons between loading conditions were made in the heavy sled pull study of Keogh et al. [32]; however similar changes in spatiotemporal parameters may be deduced from the lower velocity trials, whereby a reduced stride length and swing time, and increased ground contact time were reported [32].
When comparing joint kinematics between unloaded sprinting, sled pulls at 15%, 20%, 30%, and 40% body mass, statistically significant increases in knee and hip flexion at foot strike and toe off have been reported with an increase in sled load [45]. The greater knee and hip flexion at foot strike and toe off would likely result in the athlete attaining a more horizontal trunk position throughout the pull and increase the time and range of motion over which force was applied. Where the increases in sled mass in the study by Monte et al. [45] were associated with a decreased pull velocity, lower velocity during the heavy sled pull in Keogh et al. [32] and Winwood et al. [38] was similarly characterized by a more horizontal trunk position and greater knee flexion at foot strike. Qualitatively, it may appear that the more horizontal trunk orientation is a mechanism employed by the athlete to position the body so to optimize horizontal propulsive force production; however, more quantitative research is required to confirm this hypothesis.
The direction of the resultant ground reaction force of the athlete when performing the sled pull and strongman vehicle pull may also be dependent on the location at which the load is applied to the athlete’s body. A waist attachment site as opposed to a chest height attachment site on the athlete has been observed to result in the athlete attaining a more horizontal body position [46]. This is achieved through a greater trunk ROM and greater peak knee flexion during the stance phase of the sled pull [46]. A limiting factor when using a chest harness may be the increase in resistive trunk and hip extensor moment acting on the athlete. The vehicle pull strongman event is typically performed using a chest harness where the attachment site is located somewhere between the shoulder and the waist. To overcome the greater trunk and hip extensor moments and impart a greater horizontally directed propulsive force and impulse, the strength of the athlete’s trunk and hip flexors and their ability to maintain a predominantly horizontal position may both be determining factors of performance in the vehicle pull exercise.
From existing literature comparing the biomechanics of athletes performing sub-body mass sled pulls at varying loads, and the limited literature available on the heavy sled pull strongman exercise, it may be deduced that decreases in stride length and stride rate and increased trunk lean may be further magnified in the strongman vehicle pull where an increased resistive load is expected. Based on this knowledge and the relationship between increased sled load and decreased pulling velocity, it is suggested that greater performance in the strongman vehicle pull competition event may be characterized by the athlete’s ability to optimize the relationship between cadence and stride length, while attaining a total body position that enables greatest horizontal force production.
In addition to the heavy sled pull, strength and conditioning coaches often use a variety of similar resistive sprint training tools for the development of greater horizontal force production and sprinting ability in athletes [47, 48]. Such tools may include weighted vests, tires, and parachutes. Where such traditional forms of resistive towing training typically rely on ground reaction forces initially generated through the lower body, a strongman vehicle pull typically also includes the use of a thick rope in which the upper body musculature assists in the pull. The training benefits of simultaneous lower and upper body force application, as seen in the vehicle pull, may be particularly relevant in such sports as rowing and kayaking where simultaneous lower body pushing and upper body pulling forces are required. Another benefit may be seen during the competitive phases of an annual periodized plan, whereby athletes have reduced time to devote to strength and conditioning. At such times, total body exercises such as the truck pull and push press may allow high kinetic outputs to be generated through the primary upper and lower body musculature within the one exercise [49].
The current heavy sled pull research may be used as a basis for further research into the biomechanical demands of performing other variations of pulling type exercises such as the backward drag. The backward drag technique is used in firefighting and military physical fitness assessments and service, where service people may be required to drag victims out of danger [50, 51]. Further investigation into the biomechanical demands of a vehicle pull may be of benefit to military operations, as soldiers may be faced with instances where they are required to pull/push heavy equipment over short distances [51].
Static Lifting Exercises
A relative lack of quantitative biomechanical analysis exists on static lifts such as the atlas stone lift, log lift, and tire flip. To qualitatively analyze these three exercises, they may be broken down into phases and biomechanically analyzed alongside a variety of different TWTE and CEA.
Atlas Stone Lift
Of the three static strongman lifts analyzed in the current literature, the atlas stone may be seen as one of the most mechanically demanding and potentially injurious strongman exercises [4]. Quantitatively, little biomechanical analysis has been performed on the atlas stone lift, with just joint/muscle loading and muscle activation being measured in one study of three athletes [35].
In phase one of the atlas stone lift, the athlete attempts to lift the stone off the ground using a “hugged” grip and a lifting technique similar to a Romanian deadlift. Once the stone is off the ground, the athlete assumes a paused position with the stone resting in the lap. The most similar and comprehensive field of research related to the biomechanics of this movement is in the area of injury risk assessment/prevention for the manual handling stoop lifting technique, which is characterized by a bent back and straight knee posture until lift completion (fully erect standing position) [52]. Net moments and compressive forces acting on the spine have been reported to be similar between the stoop lifting technique and the often preferred squat lifting technique (characterized by a straight back and bent knee posture until lift completion) [52]. The insignificant differences in joint loading between these lifting techniques are supported by the findings of McGill et al. [35] where vertebral joint moments, muscular/joint compression and shear forces during the atlas stone lift were reported to be of a similar or lower magnitude to other strongman exercises analyzed including the farmer’s walk, yoke walk, keg walk and log lift.
The explosive movement initiated from the paused position at the end of phase one to the quarter-squat position at the end of phase two of the atlas stone lift may be the most similar to that of the beginning of the concentric phase of the box squat [53]. The box squat has been reported to result in statistically lower peak force production than the powerlifting or traditional style squat (box squat: 2528 ± 302, powerlifting: 2685 ± 301, traditional squat: 2680 ± 309 N) [53]. This was suggested to be due to the pause and transfer of load from the system to the box at the bottom of the squat, meaning the box squat may lose some of the benefits of the stretch-shorten cycle in terms of force production and loads lifted. There was however a statistically greater rate of force development in the box squat compared with the traditional and powerlifting style squats [53].
In phase three of the atlas stone lift, the athlete moves with the stone from a quarter-squat position to a full extension standing position, with the stone being transferred to a chest height ledge or over a bar. This final stage of the atlas stone lift may show biomechanical similarity to the concentric phase of the front squat, whereby the load is lifted in a squat like position on the anterior surface of the body to a full extension standing position. While still being in a flexed torso position throughout the entirety of the front squat, during the final stage of the atlas stone lift, the athlete often moves into a position of torso extension when the stone is passed onto the ledge/over the bar. The degree of movement of the torso into an extended state may be expected to differ between athletes of varying anthropometrics and performance standard, and may result in unique muscular and joint loading. Further quantitative analysis of the atlas stone lift is required to confirm this hypothesis.
It may be suggested that increasing a strongman athlete’s ability to utilize the stretch-shorten cycle in the transition from the end of phase one (bottom of squat) to the initial stages of phase two, while also promoting a high rate of force development throughout phase two, may be key in achieving greater performance in the atlas stone lift. Although a tacky substance is often used by athletes to assist in gripping the stone, a likely limiting factor of performance in the atlas stone lift may be the hugging grip strength of the athlete to initialize the lift of the stone off the ground during phase one.
The inclusion of the atlas stone lift into a strength and conditioning program may be of interest to military personnel or civilians required to perform physical lifting fitness assessments. Such an example of this form of test is the box lift and place assessment conducted in the Australian Army, whereby a box (up to 40 kg) must be lifted from the ground and placed on a ledge of height 1.5 m [54]. Where atlas stones may not be available at many strength and conditioning facilities, sandbags and heavy medicine (slam) balls may be lifted using a similar technique to the atlas stone lift.
Log Lift
The log lift is commonly used by strength and conditioning coaches as an alternative to traditional overhead lift variations, with the biomechanics of persons performing the log lift being relatively well analyzed compared with other strongman events [10, 35, 37, 39]. Existing literature has demonstrated not only the similarities of the log lift to the clean and jerk and push press but also the profound mechanical differences, where greater joint ROM was reported in the log variations, and greater force and impulses were reported in the barbell variation [37, 39]. These differences may be attributed to the log size and shape, as well as the training background of the athletes in these studies. There however remains a gap in the current strongman literature identifying the biomechanical determinants of greater performance in the log lift exercise. To the authors knowledge, literature on the biomechanical determinants of performance in all forms of strength-based overhead pressing exercises is lacking. As such, the advancement of researchers’ understandings of the biomechanical determinants of performance in the log lift is limited to phase one (movement of the log from the ground to a knee height or lap position) and phase two (movement of the log from a knee height/lap position to a racked position of the chest), which may exhibit some similarities to the power clean [39].
A greater 1RM in the power clean has been reported to be achieved by athletes that could minimize hip ROM during the first pull phase of the lift and produce a greater rapid extension of the hip during the second pull phase (r = 0.87) [55]. An athlete’s ability to execute the clean with minimal hip flexion during phase one and perform a powerful hip extension during phase two of the log lift is particularly evident in log lift competition events that require athletes to perform AMRAP in an allocated time at sub-maximal loads. These first two phases of the log clean appear kinematically similar to that of the atlas stone lift, suggesting there might be some commonalities in the determinants of performance of these two strongman lifts.
Greater performance (achievement of a successful lift at a greater barbell load) in the power clean has also been characterized by an athlete’s ability to keep the barbell close to their body throughout the second pull phase of the lift (likely through a greater net force application toward the body) [56]. This is likely to have great applicability to the log lift as the increased diameter of the log is expected to make achieving greater net force application toward the body more difficult due to the center of mass of the log being positioned further in front of the center of mass of the athlete.
Strength and conditioning coaches may consider lighter variations of the log lift exercise where the training of overhead vertical strength is required in an injury rehabilitation program or where an athlete has a history of shoulder instability. It has been suggested that using a neutral grip with the hands positioned shoulder width apart, as used in the log lift, promotes an anatomically optimal position for overhead pressing [57]. It must however be acknowledged that the greater diameter of the log compared with the bar may increase the lumbar loads during the log lift compared with barbell overhead lifts. As a result, the strength and conditioning coach may need to take into account an individual athlete’s injury history, movement competency, physical capacities, and sporting demands before determining the risks and rewards of these exercises.
Tire Flip
Although the tire flip is commonly used as a strength and conditioning training tool at a recreational and elite sporting level for power, strength, endurance, and metabolic conditioning training [10], biomechanical analysis of the exercise has only considered temporal determinants of greater performance [33]. Biomechanical analysis of the tire flip exercise may be particularly difficult as the lift is one of the few strongman lifts where the implement remains in contact with the ground throughout the entirety of the lift, thus quantifying the load lifted by the athlete may be difficult. Additionally, the dimension, mass and frictional characteristics of the tire and ground are likely to have an impact on the technique/biomechanics of the athlete performing the lift.
In phase one of the tire flip, the athlete begins the movement by lifting one side of the tire off the ground to just above the knee. This would appear to be biomechanically similar to aspects of the initial lifting phase of the conventional and sumo deadlift. The greater horizontally oriented trunk position at lift off during the tire flip may be more biomechanically similar to the trunk position at lift off during the conventional deadlift, when compared with the sumo deadlift [58]. Conversely, the wide stance used at lift off during the tire flip may be more biomechanically similar to the stance width at lift off during the sumo deadlift, when compared with the conventional deadlift [58].
In the second pull phase of the tire flip, the athlete moves the tire from the knee height position to the point of first hand release where the tire may be close to a 75° angle from horizontal. This phase of the tire flip may be biomechanically similar to the power clean, whereby the phase is characterized by a rapid acceleration of the implement from just above the knee, that primarily results from forceful triple extension of the ankle, knee and hip joints [33, 56]. Comparing successful versus unsuccessful attempts of the power clean revealed that greater performance may be characterized by an athlete’s ability to minimize forward barbell displacement during the second pull phase of the lift [56]. It is expected that the reduced forward barbell displacement relative to the centerline of the body minimizes the resistive joint torques experienced by the athlete and thus ensures maximal vertical force production.
In the third phase of the tire flip, the athlete catches the tire at approximately chest height by attaining a position of full extension of the wrist and hand, pronation of the forearm, flexion of the elbow and extension of the shoulder, before powerfully pushing the tire past its tipping point. The closest biomechanical assessment of such pushing movement may be that of a loaded cart (181 kg), where a decrease in horizontal force production as the height of force application increased (from a standing knuckle, elbow or shoulder height) was reported [59]. The tipping motion of the tire flip is however expected to result in profound biomechanical differences when compared with the translational/rolling motion of a cart. While the first two phases of the tire flip are expected to require a substantially vertical force component, it is expected that as the flip progresses to the third phase, the requirement of a greater horizontal force component becomes apparent.
The second pull phase time of the tire flip has already been identified as a key determinant of performance [33]. The athlete’s ability to move their body toward the tire and maintain or further advance (minimize) their body position relative to the tire during phase two is expected to be key factor in achieving a shorter second pull phase time. The ability to maintain or further advance one’s body position relative to a resistive load may be of particular interest to strength and conditioning coaches of rugby and American football athletes, where athletes are often required to block an opposing player’s movement or advance on an opponent’s ground. Strength and conditioning coaches may consider the use of the tire flip as an effective tool for the training of greater horizontal force and power production for rugby and American football athletes.