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Implement Training for
Concentric-Based Muscle
Actions
Nathaniel D. M. Jenkins, BA, CSCS*D, NSCA-CPT*D and Ty Palmer, MEd, CSCS
Department of Health and Human Performance, Oklahoma State University, Stillwater, Oklahoma
Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided
in the HTML and PDF versions of this article on the journal’s Web site (http://journals.lww.com/nsca-scj).
SUMMARY
RECENTLY, THERE HAS BEEN
GREATER USE OF IMPLEMENTS IN
STRENGTH AND CONDITIONING
THAT ALLOW ATHLETES TO COMPLETE MOVEMENTS WHILE MINIMIZING OR ELIMINATING
ECCENTRIC STRESS. THE USE OF
SLEDS DURING TRAINING IS AN
EXAMPLE OF ONE OF THESE TOOLS
THAT IS A NOVEL IMPLEMENT FOR
ATHLETES STRIVING TO GAIN
CONCENTRIC STRENGTH WHILE
SIMULTANEOUSLY DECREASING
THE CHANCE OF ACQUIRING
MUSCLE SORENESS. THE PURPOSE OF THIS ARTICLE IS TO HELP
COACHES AND TRAINERS GAIN
INSIGHT ON THE PROPOSED BENEFITS AND PURPOSES OF USING
SLEDS AND TO EXPLAIN PRACTICAL
APPLICATIONS BASED ON RECENT
LITERATURE AND FROM THE
AUTHORS’ OWN PERSONAL EXPERIENCES AND OBSERVATIONS.
INTRODUCTION
uscle 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
M
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
Copyright Ó National Strength and Conditioning Association
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 concentriconly 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).
KEY WORDS:
concentric; resistance training; sled
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Instruments Requiring Concentric-Based Muscle Actions
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,
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VOLUME 34 | NUMBER 2 | APRIL 2012
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
Figure 1. Demonstration of the sled chest press.
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 concentriconly 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.
PRACTICAL APPLICATIONS
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
Figure 2. Demonstration of the sled row.
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 doubleleg 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
Figure 3. Demonstration of the low handle push.
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Instruments Requiring Concentric-Based Muscle Actions
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
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VOLUME 34 | NUMBER 2 | APRIL 2012
Figure 4. Demonstration of the vertical handle push.
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.
Figure 5. The starting position for the backward drag.
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
Table 1
An example of a workout with the use of the sled as a finisher
Upper-body hypertrophy
Bench press*
Chin-ups†
Seated dumbbell shoulder press†
Seated row†
Alternating dumbbell curls†
Finisher: A1. Sled chest press 4 3 30 yd‡
A2. Sled row 4 3 30 yd‡
RM = repetition maximum.
*67–85% 1RM; 3–6 sets.
†10RM; 2–3 sets of 10 repetitions.
‡Done in superset fashion with little to no rest between sets.
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 sledbased 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,
Table 2
Second session of the day—sled based
Lower-body sled session*
Sled push with high handles: 5 sets of 20–30 yd with a moderate to high weight
Backward walking sled drag: 3 sets of 20 yd with a moderate weight
Sled push with low handles: 3 sets of 20 yd with a moderate weight
*Assuming that the focus of the first session was lower-body.
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Instruments Requiring Concentric-Based Muscle Actions
Table 3
An example of a full-body sled session
Full-body sled day
A1. Sled push with high handles: 3 sets of 20 yd with moderate weight*
A2. Backward sled drag: 3 sets of 20 yd with moderate weight*
B1: Sled chest press: 3 sets of 10–12 with a moderate to high weight*
B2: Sled row: 3 sets of 10–12 with a moderate to high weight*
C1: Sled front raise: 2 sets of 12–15 with a low to moderate weight*
C2: Sled rear deltoid fly: 2 sets of 12–15 with a low to moderate weight*
D1: Sled push with low handles: 4 sets of 20 yd with moderate weight
*Done in superset fashion with no rest between exercises and 45–60 seconds between
supersets—2–3 minutes between sets is acceptable.
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.
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.
CONCLUSIONS
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
Table 4
An example week of training
and competition with a sled day
Day
Event
Monday
Practice
Tuesday
Competition
Wednesday
AM:
Sled training
PM:
Practice
Thursday
Practice
Friday
Competition
Saturday
Competition
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VOLUME 34 | NUMBER 2 | APRIL 2012
Nathaniel D.
M. Jenkins is
a graduate assistant in the
Department of
Health and Human Performance at
Oklahoma State University.
Ty Palmer is
a graduate assistant in the
Department of
Health and
Human Performance at Oklahoma State
University.
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