Jenkins, Nathaniel D. M. BA, CSCS*D, NSCA-CPT*D; Palmer, Ty MEd, CSCS
Muscle actions may be generally broken down into 3 types: concentric, eccentric, and isometric. However, concentric and eccentric muscle actions are the only 2 that are dynamic–that is, muscle length changes while force is being generated (24). Dynamic constant external resistance (DCER) training using machines or free weights employs movements that contain both an eccentric and a concentric muscular action (13). While both actions are extremely important to the completion of a lift, the characteristics of concentric and eccentric muscle actions are stark in comparison. One critical differentiation is that higher levels of muscle tension can be generated during eccentric muscle actions (3). Moreover, there is a significant amount of mechanical stress accrued during this part of the lift due to the lengthening of the muscle while cross bridge formation is occurring. Since mechanical stress is thought to be the chief factor stimulating muscular adaptation, researchers have concluded that the eccentric portion of a lift is that which induces many of the adaptations from resistance training (2,14,19,23). However, the damage produced by eccentric muscle action has the potential to cause a significant amount of soreness, fatigue, and inflammation (17). Recently, concentric-based training through the use of implements such as weighted push/pull sleds has become a popular method believed to evade the detriments associated with eccentric training.
While eccentric muscle actions may indeed be responsible for many of the adaptations seen in muscle, there have been numerous studies that have concluded that concentric-only training also results in favorable changes to skeletal muscle. For example, Evetovich et al. (12) showed that 12 weeks of concentric isokinetic training increased quadriceps femoris strength by 15.5%. In addition, Housh et al. (16) demonstrated that 8 weeks of unilateral concentric-only DCER exercise significantly increased quadriceps femoris cross-sectional size (3.3%) and strength (39.7%) in the trained leg. Based on these results, it is clear that concentric-only resistance training is capable of inducing morphological adaptations and increasing strength in human skeletal muscle.
Traditionally, sleds are used as a means to perform resisted towing in an attempt to improve sprint speed by increasing stride length (1,21). Sleds are usually connected to an athlete fitted with a harness or belt that is in series with a rope, strap, or cable. Much of the literature available today explores the benefits and drawbacks of these implements used for this means. However, sled training can also be a unique concentric contraction training method that consists of pushing, pulling, or performing standard resistance training movements with attached straps (i.e., press, row, fly) (see Video, Supplemental Digital Content 1, http://links.lww.com/SCJ/A19, which demonstrates the chest press while using a sled).
The use of a sled forces the active muscles to shorten for creating movement (i.e., concentric contraction), resulting in greater contractile forces compared with resistive forces. For movement to occur, however, the static inertia of the sled must first be overcome. The inertia of the sled is composed of the weight of the sled (includes any additional resistance added to the sled in the form of weighted plates) plus the amount of friction between the bottom of the sled and the surface material (turf, rubber, grass, etc.). According to Newton's first law of motion, the greatest amount of inertia will likely occur at the beginning of sled movement due to the sled's initial stationary position (11). Therefore, a large amount of muscular force must be generated early in the range of motion to overcome the resistive and frictional forces preventing the sled from moving. Once enough force has been created for movement to commence, less force is needed thereafter due to a decrease in static inertia.
Depending on the specific exercise being performed with the sled, every repetition may require significant muscular tension at the beginning of the range of motion to overcome these initial forces. For example, when using straps to perform movements like a press or row, the sled is static at the beginning of every repetition. This is due to a discontinuation in applied muscular force in between concentric contractions by the prime movers. If, however; the sled is being pushed or pulled across a given distance through a series of continuous and rapid muscular contractions so that the sled does not cease to stop, the amount of force necessary to maintain movement decreases. This phenomenon is also based on Newton's first law (11), which states “an object in motion tends to remain in motion.” These characteristics of sled training are unique and must be considered when employing their use.
In addition, when pulling or pushing a sled, the eccentric contractile forces are minimal in comparison with the concentric contractile forces due to a lack of eccentric loading. For example, when an athlete is pulling a sled, the muscles of the quadriceps are responsible for unilaterally extending the knee when “pushing off” from the ground producing a concentric contraction (8). This action is responsible for generating explosive amounts of force and power to propel the added resistance of the sled across the ground surface. However, it is essential for coaches to understand that while this phase of knee extension is occurring for one leg, the opposite leg may experience minimal eccentric loading. This loading seems to be insignificant because body weight is always unilaterally supported and there is no flight phase (see Video, Supplemental Digital Content 2, http://links.lww.com/SCJ/A20, which demonstrates the backward walking sled drag). Minor eccentric muscle actions also occur in muscles that control the descent of the lower limbs against gravity. For example, the tibialis anterior controls the dropping of the foot after heel strike (as when walking) (5). Further, if the sled is used in a way that the individual is sprinting, while pushing or pulling the sled, a greater eccentric component will be present due to the addition of a true flight phase (22).
Another distinguishing feature of sled training is that there is negligible opportunity to store elastic energy and/or preload the muscle (18). In other words, the force produced is completely reliant on isolated concentric muscle action. Often, coaches attempt to replicate this by performing exercises where a barbell rests on the pins of a power rack. For example, the bench press may be modified so that the bar rests on the pins at or near the athlete's sticking point (i.e., at the bottom portion of an athlete's range of motion) before the beginning of every repetition. In theory, this creates a greater demand on the musculature concentrically and, in turn, forces the muscle to adapt by generating greater amounts of force without relying on the stored elastic energy that is transferred from the eccentric portion of the lift. Furthermore, some lifts, such as the deadlift, do not begin with an eccentric muscle action. Additionally, there are few sports that require an athlete to begin with a concentric contraction without having the benefit of an eccentric muscle action beforehand. Examples include an American football lineman exploding off the line of scrimmage at the snap of the ball or a sprinter taking off out of his or her blocks. Further applications will be explored.
One of most highly touted benefits of these implements is their ability to improve conditioning while also contributing to increases in strength. Based on personal observation, heart rate and breathing rate responses, as measured by a heart rate monitor and visual observation, respectively, are both very high to this type of work. Interestingly, previous research has shown that eccentric exercise is not as metabolically demanding as concentric exercise. This has been shown to be a result of less motor unit recruitment (9), lower lactate response (6,15), lower heart rate (6,7,15), lower ventilatory drive (7), and lower oxygen consumption (7), among other variables. In a study by Durand et al. (10), when workload was matched during concentric- and eccentric-only bouts of leg extensions, the concentric-only workload resulted in a greater heart rate and lactate response and a greater change in plasma volume (10). These enhanced acute physiological responses to concentric training should in turn result in greater chronic physiological adaptations.
Due to the absence of an eccentric component, work with a sled does not result in as much delayed onset muscle soreness, fatigue, or as drastic of acute reductions in strength as are characteristically seen with eccentric exercise based on the observations of the authors. For this reason, this type of work is typically tolerated at both higher volumes and a greater frequency in athletes.
Sleds (such as the one pictured) are currently being built so that they may be pushed or pulled at various joint angles. When straps are attached to the front, they may be used for traditional movement patterns, such as the chest press, row, curl, front raise, rear deltoid fly, and pec fly, to name a few (Figures 1 and 2). The uses of these sleds to mimic traditional patterns used in the weight room are limited only by the coach's imagination and the strength of the individual athlete. By simply switching which set of handles you use in the case of pushing (Figures 3 and 4) or by manipulating torso angle in the case of traditional movement patterns, the coach has greater control over the joint angles used to complete a movement. Of particular interest for some, especially those interested in the rehabilitative purposes of such implements, is the backward walking sled drag (Figure 5). This movement is simply a terminal knee extension without eccentric stress or the shear forces typically associated with a seated leg extension machine. Moreover, the external load can be increased so that it far exceeds what is normally possible with typical rehabilitative implements such as bands. For this reason, it has a great application to the rehabilitation of those with knee pathologies. Additionally, pushing the sled requires an athlete to use each leg independently, which is prototypical of movements involving the lower body in sport (see Video, Supplemental Digital Content 3, http://links.lww.com/SCJ/A21, which demonstrates a vertical handle push and the required unilateral triple extension). However, in the weight room, the most commonly recommended lower-body exercises are performed using double-leg support. Not only does sled pushing require unilateral support and stability, it also reinforces leg drive and extension at the ankle, knee, and hip: qualities useful in many sports, including rugby and American football. As previously mentioned, the sled can also be used to develop starting strength in movements like the deadlift or pin press, which do not contain an eccentric component before the concentric portion of the lift.
Yet another benefit of sleds is that they allow for easy loading and unloading of weight plates so that resistance can be changed at will. In addition, some sleds on the market today are capable of handling significant loads. In this author's experience, external loads exceeding 900 pounds can often be added to these sleds. This makes them extremely adaptable to the needs of the weakest and strongest individuals.
The physiological responses generated by concentric-only exercise were discussed in depth earlier. Some of these responses are, in fact, what we see while observing athletic populations using sled-type implements. If observation alone is not enough to validate this, Berning et al. (4) demonstrated that pushing and pulling a motor vehicle is an exhausting task that requires high amounts of anaerobic energy output (4). However, the authors concluded that the substantial metabolic and neuromuscular stresses produced by this training need to be considered when being placed into a strength and conditioning program. Despite the obvious load differences, the pushing and pulling of a sled is very similar to the pushing and pulling of a car. With that in mind, while not quite at same the intensity level of pushing or pulling a motor vehicle, one could still expect a high metabolic demand with the pulling or pushing of a sled.
The use of a sled allows for the strength and conditioning professional or personal trainer to select the amount of weight used to create the desired training effect. For example, if the goal is pure muscular endurance and/or conditioning, the coach may choose to use a lighter load over an increased distance. This may be accomplished by performing a “shuttle” or a “suicide” while pushing the sled. The coach may also pick a distance and then set a time in which the distance must be completed; by lowering the time allowed to complete the distance in subsequent bouts, the coach may effectively increase intensity to elicit a training effect. On the contrary, if the goal is muscular strength, the coach may choose a very high load that can only be pushed or pulled over 1–20 yd. If the goal is hypertrophy, a moderate to moderate-high load may be used over an intermediate distance (i.e., 15–35 yd). It is important to note that eccentric muscle actions appear to maximize the hypertrophic response to resistance exercise (23). Consequently, the sole use of a sled for resistance training may not maximize hypertrophy. Although distance may be used to quantify volume, repetitions may be used too: this is especially true when programming movements like the chest press or row.
The load used is largely determined by the athlete and is ultimately at the discretion of the coach. Factors such as surface type play a role in the selection of a load because certain surfaces such as turf provide less friction than a surface such as grass. Therefore, the sled will be much harder to move on grass than it will be on turf if the load is equivalent. In the case of both strength and hypertrophy development with the sled, the load used may be great enough that if the athlete were to try to continue past the prescribed distance or repetitions, he or she would not be able move the sled. Keogh et al. (20) also described cues and some basic program design that a coach may use when using a heavy sprint-style sled pull to build sprint speed. They suggested observing stride length, step rate, and joint angles as well as the time it takes for the athlete to complete the prescribed distance as a way to monitor performance and individualize the exercise prescription. Their results also suggested that for sled pulls performed for durations less than 20 seconds, trained subjects should be able to complete 3 sets with 3 minutes of rest while experiencing little to no decline in performance (20). Finally, due to the nature of the exercise, independent of the load or distance used, a strong conditioning effect may be observed. While this has not been validated by research, it is the opinion of the author after careful inspection of this mode of training and the response of athletes.
Next, in the authors' experience, sleds generally allow individuals to withstand much greater volumes and frequencies of training. Therefore, coaches may choose to implement the use of sleds as a “finisher” for the muscle group worked at the end of a resistance training session in addition to the typical volume of such a session (Table 1). A sled workout may also be added in as a second session later in the day as a way of providing an additional stimulus (Table 2). If this is done, coaches must be sure to monitor the rate of recovery in their athletes and adjust volume and or frequency accordingly. Likewise, they can be used to comprise an entire workout for the day (Table 3). For example, a full-body sled-training workout may be used in season when a coach does not want to introduce high amounts of eccentric stress during frequent high levels of competition (Table 4).
Sports, such as baseball, softball, soccer, hockey, and basketball, that often play numerous times per week make it hard for the strength and conditioning coach to schedule regular strength training sessions, especially when there is concern of applying too much of a stimulus. While the coach does not want to completely eliminate a training stimulus and lose many of the gains made during the off-season and preseason training program, he or she must also be careful not to interfere with subsequent athletic performance. In this case, the addition of a full-body concentric-based sled workout may provide the stimulus needed to maintain adaptations while making it easy for the athletes to recover in time for the next competition.
It is essential to mention that sled-based exercises should not make up the bulk of training, especially when sports performance is the goal. Eccentric muscle actions are critical to athletic success, the prevention of injury, and explosive performance (i.e., importance of a countermovement in a vertical jump for the generation of power). Therefore, programming should include resistance training that includes both eccentric and concentric contractions. Concentric sled training should be used as a complement or when decreased training stress is needed to ensure optimal performance in ensuing competition, such as when a coach is forced to prescribe training within 24–48 hours of a contest.
Finally, and perhaps most importantly, the novelty of this method of training creates a unique stimulus among those using it. As a result, increases in compliance among individuals of all ages, sizes, and strength may be seen.
The applications of the sled are seemingly unending. At first glance, the use of sleds seems reserved for very narrow populations and a select assortment of movements. However, the variety of ways in which they may be employed is in fact limited by the imagination. Although this article mainly discussed how sled training may be applied to athletic use, there is opportunity for practitioners in a wide array of disciplines to benefit from their use. Such implements may be found with a quick search of the Internet. Even if specialty sleds like the one pictured in this article are not readily available, or are outside of one's budget, traditional dragging sleds or things such as tires with a fixed strap can be used in their place.
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