Download Eccentric Muscle Actions and How the Strength

Eccentric Muscle Actions
and How the Strength and
Conditioning Specialist
Might Use Them for a
Variety of Purposes
John F. Cowell, MS, John Cronin, PhD, and Matt Brughelli, PhD
Sports Performance Research Institute New Zealand (SPRINZ), Auckland University of Technology, Auckland,
New Zealand
SUMMARY
AN ECCENTRIC MUSCLE ACTION IS
DESCRIBED AS A MUSCULAR
CONTRACTION OCCURRING
WHILE THE MUSCLE IS SIMULTANEOUSLY LENGTHENING. ECCENTRIC MUSCLE ACTIONS CAN BE
IMPLEMENTED BY THE STRENGTH
AND CONDITIONING SPECIALIST IN
A VARIETY OF WAYS TO ACHIEVE A
NUMBER OF GOALS FROM
INCREASING STRENGTH AND
POWER PRODUCTION TO PREVENTION AND REHABILITATION OF
INJURY. THE PHYSIOLOGIC RATIONALE AS WELL AS THE LOADING
PARAMETERS FOR THESE VARIOUS
FORMS OF ‘‘ECCENTRIC TRAINING’’
WILL BE BRIEFLY DISCUSSED IN
THIS REVIEW.
INTRODUCTION
he job description of a strength
and condidioning specialist will
change with degrees of athlete
preparation, injury prevention, and
injury rehabilitation. For these requirements, the situational needs usually
involve a blending of certain muscle
actions—isometric, concentric, eccentric,
or a coupling of an eccentric–concentric
muscle action termed a stretch-
T
shortening cycle (SSC). Understanding
these muscle actions and how to program them into a resistance training
protocol is an essential component of
successful strength and conditioning
practice.
exhaustive review of each of these areas
but rather a brief introduction to the
many faces of eccentric training.
Most human motion involves this
eccentric–concentric coupling or SSC
muscle action. It is the eccentric phase
of this coupling that provides the focus
of this article. This article discusses how
we can modify the stress (load), strain
(length or amplitude of movement), and
velocity during the eccentric phase, to
impose a variety of mechanical stimuli
that have different adaptational and
functional effects. Specifically, we will
address the use of eccentric resistance
training for (a) tendon injury rehabilitation via tendinous remodeling; (b)
muscle injury prevention via shift in the
optimum length of muscle; (c) supramaximal and/or accentuated eccentric
loading (i.e., loads exceeding the 1 repetition maximum (1RM) and/or greater
than the concentric load) for strength,
performance, and hypertrophy; and (d)
improved sports performance via SSC
optimization. The physiologic rationale
for each form of eccentric training and
simple loading parameters are briefly
described. This is by no means an
Achilles tendinosis etiology is a degenerative process because of overuse
where there are no inflammatory cells,
but changes of the collagen fiber
structure result in the tendon’s inability
to adapt to changes in loading patterns
(15,17,23). In addition to a loss of
collagen fibers, there is also a loss
of fiber cross-links, which results in
a reduction in tendon strength (2).
Furthermore, as tendons are collagenous structures with limited blood
supplies, their slow metabolic rate
leads to an impaired healing response
(5). Many authors have considered
achilles tendinosis as an overuse injury
from repetitive loading, but others have
observed the condition in sedentary
populations (11,52). Depending on an
individual’s activity level, even short
walks could be enough loading to
cause overuse symptoms. Other
Copyright Ó National Strength and Conditioning Association
ECCENTRIC LOADING FOR
REHABILITATION OF ACHILLES
TENDINOSIS
KEY WORDS:
eccentric; rehabilitation; peak tension;
overload; stretch-shortening cycle
Strength and Conditioning Journal | www.nsca-scj.com
33
Multiple Purposes of Eccentric Muscle Actions
suggested etiologic factors include
aging, decreased blood supply,
decreased flexibility, muscle imbalance,
decreased concentric and eccentric calf
strength, faulty musculoskeletal alignment, and training errors (2).
One form of treatment that has gained
popularity in the rehabilitation of achilles tendinosis is eccentric strength
training. Eccentric training models were
based on the belief that tendon injuries
could result from tensile loads exceeding the tendon’s mechanical strength
(24). If these loads were repeated, as in
many activities and sports, then a symptomatic tendon condition could result.
The eccentric training theory promoted
the importance of structural adaptation
of the symptomatic tendon, so it could
cope with the increased repetitive loads
and prevent injury (24). Early researchers believed that because most tendon
injuries occurred during the eccentric
phase of muscle work, eccentric exercise would be a viable treatment
modality (75). Therefore, many earlier
researchers recommended eccentric
training programs to increase eccentric
strength in the tendons during running
and jumping (32). Currently, rehabilitation programs involving eccentric training using loads greater than body
weight have provided positive results
in the treatment of achilles tendinosis,
with a decrease in pain and a higher
percentage of patients returning to
preinjury levels of physical activity
(8,12,14,15,21,23,37,51,59).
There are many proposed reasons why
eccentric strength training is an effective
means of rehabilitation of achilles tendinosis. The most obvious is the increased
eccentric strength of the calf muscles
(2,6,52). Alfredson (1), and Alfredson
and Lorentzon (2), a leading researcher
in the area of achilles tendinosis, stated
that the effects of loading induced
hypertrophy, increased tensile strength,
and the effect of stretching the muscle
tendon unit were all positive adaptations
from eccentric training. Tension produces electric potentials that help organize
charged collagen fibrils (41). It has been
proposed that eccentric strengthening
can promote collagen production by
34
VOLUME 34 | NUMBER 3 | JUNE 2012
activating mechanoreceptors in tenocytes (41). The improved strength and
collagen production may be very
important in breaking the tendinosis
cycle. Lengthening of the muscle tendon
unit results in less load on the tendon
and better range of motion at the ankle
joint (2,27,38,52).
Eccentric training is thought to alter the
pain perceptions from the tendon (2).
Eccentric strengthening regimens have
resulted in decreased tendon volume
and improved healing with collagen
deposition and restoration of the matrix
component through cell to cell communication (1,57,70). Also sensory neuropeptides (53) have been found in the
achilles tendon after eccentric training,
which (9) could stimulate an inflammatory response, increase blood flow, and
thus promote tendon healing.
Despite the research interest in eccentric training, the exact loading parameters for eccentric strengthening of the
achilles tendon remain far from clear.
This is due principally to the research in
this area suffering a variety of methodologic limitations. With this in mind and
in an effort to provide some guidance as
to the optimal loading parameters for
eccentric rehabilitation for clinicians,
and strength and conditioning specialists, a review of some of the eccentric
loading parameters used in research for
achilles tendinosis rehabilitation are
detailed in Table 1.
Irrespective of the training programs
used by the authors in Table 1 and the
methodologic limitations of the studies, most studies reported positive
functional outcomes. From the studies
presented in Table 1, the following
guidelines and suggestions are recommended for eccentric training in the
rehabilitation of achilles tendinosis. A
standing straight leg calf extension
appeared to be the exercise of choice;
however, further research is needed to
determine if any benefit is gained from
seated calf extensions (where gastrocnemius contribution is minimized) and
other calf exercises. With regard to the
exercises used in rehabilitation, the
exercises appear easy to execute, can
be performed at home, require minimal
cost, and do not seem to suffer from
major technical requirements or associated complications.
A minimum duration of 12 weeks is
recommended for eccentric strengthening programs, given the research into
the time course of tendon remodeling
and regeneration. Three sets of 15 repetitions (reps), with increasing repetitions, appears the most common
loading progression. However, these
authors believe that it may be advantageous to decrease the repetitions and
increase the load in keeping with
traditional eccentric strength training
guidelines. This contention is supported by the finding that moderate
to heavy loads produced the most
significant changes in visual analog
scale (VAS) scores in those studies that
used the VAS scale as the variable of
interest, that is, indicator of pain
(27,73). In terms of pain tolerance, it
would seem that some pain is inevitable
with eccentric training and regulating
pain thresholds to no greater than 5/10
on the VAS scale would seem prudent.
Another manner in which the loading
could be progressed is via the use of
different training modalities. This could
involve progression from rubber tubing
training, body weight, body weight plus
load e.g., (backpack), and then the use
of resisted strength training equipment
whereby supramaximal loading can
occur. Thereafter, fast eccentric loading
through a full range of motion may be
attempted, for example, jump training
or drop jump training from progressive
heights. It is difficult to disentangle an
optimal frequency of training per week,
but it would seem safe to perform these
exercises daily and even twice daily
given the frequency statistics observed
in Table 1.
Adopting such exercises and loading
parameters may prevent surgical intervention for sufferers of achilles tendinosis (4). Furthermore, for normal
populations, standardized eccentric
training programs could be used as
preventative measures or as part of
a conditioning program. It should be
noted, however, that a great deal more
Table 1
Eccentric loading parameters for patients with chronic achilles tendinopathy According to the studies included
Author
Sample
size
Symptom
duration
Outcome
measures
NiesenVertommen
et al. (55)
17
.3
Ordinal scale
Alfredson
et al. (3)
13
.6
Strength
Alfredson
et al. (6)
11
.6
Strength/VAS
Alfredson
et al. (4)
14
17.8
Mafi
et al. (52)
22
Crosier
et al. (24)
Pain
level
Pain Strength Program
effect
effect
duration Frequency Frequency Set
size
size
(wks)
(wks)
(d)
(s)
Mild
Rep
(s)
Loading
intensity
12
6
1
5
10
L/M
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4
.16
42
7
4
3
20
L/M
Minor
3.4
.98
42
7
4
3
20
Progressive
Strength/VAS
Minor
3.1
12
7
2
3
15
M/H/VH
21
VAS
Minor
12
7
2
3
15
M/H/VH
9
663
Strength/VAS/
Minor
ultrasonography
3.3
10
3
3
1–5
30
L/M/H
Sibernagel
et al. (73)
40
20 6 25.4
VAS
Mild–
moderate
1.5
12
7
1–3
Roos et al.
(65)
44
5.5
FAOS
Mild
12
7
2
1–3
15
M/H
Fahlstrom
et al. (27)
101
19.2 6 28.6 VAS
Moderate–
high
12
7
2
3
15
M/H
Ohberg
et al. (57)
41
Ultrasonography/
Color Doppler
High
12
7
2
3
15
M/H/VH
Ohberg
et al. (57)
25
17.1
Ultrasonography
High
Shalabi
et al. (70)
25
.6
Ordinal scale
Moderate
Shalabi
et al. (70)
22
MRI
Knobloch
(43)
59
FAOS/VAS
High
1–3 10–100 M
Rate of
loading
30°/s–
120°/s
Slow–fast
H
.75
12
7
2
3
15
M/H
12
7
2
3
15
M
12
7
2
3
15
M
(continued)
35
36
FAOS = Foot and Ankle Outcome Score; H = heavy; L = light; M = medium; MRI = magnetic resonance imaging; VAS = visual analog scale; VH = very heavy; VISA-A = Victoria Institute of Sport
Assessment Achilles Questionnaire.
M/H
15
3
2
7
3.4
6
Langberg
et al. (48)
19 6 7
VAS
High
12
2
.48
High
34
Sayana
et al. (66)
13.9 6 8.2
VISA-A
Table 1
(continued )
12
7
3
15
M/H/VH
Slow–fast
Multiple Purposes of Eccentric Muscle Actions
VOLUME 34 | NUMBER 3 | JUNE 2012
research is needed in this area, particularly research that has an integrated
clinical/physiotherapeutic and strength
approach.
SHIFTING THE OPTIMUM LENGTH
WITH ECCENTRIC EXERCISE
All muscles have an optimum length for
producing peak tension. As the muscle
continues to lengthen beyond its optimum length, tension levels decrease.
This descending portion of the length–
tension curve is thought to be the region
of vulnerability in which muscle strain
injuries occur. Many believe that athletes
who produce peak tension at shorter
than normal muscle lengths are more
likely to suffer an acute muscle strain
injury (15,18,50,62). Brockett et al. (14)
explored this idea by measuring the
optimum lengths in athletes who had
previously injured their hamstrings. One
leg served as the experimental leg
(i.e., previously injured hamstring) and
the other leg served as the control leg
(i.e., noninjured hamstring). The previously injured hamstring produced peak
tension at 12.7° less than the noninjured
hamstring (i.e., shorter optimum length).
It was also reported that the difference
between eccentric and concentric hamstring strength was not different between
legs. The authors concluded that the
optimum length of peak tension was
a greater risk factor for future muscle
strain injuries than strength ratios.
It has been suggested that muscle strain
injuries could be reduced if the
optimum length is shifted to a longer
length as is shown in Figure 1 (14,15).
The only form of training that has been
shown to consistently increase the
optimum length of tension development has been eccentric exercise
(15,30,62). The shift has been shown
to occur in the elbow flexors, plantar
flexors, knee flexors, and knee extensors (12,14,21,60,76). The magnitude of
the shift depends on 3 variables: the
load of eccentric exercise, the volume
of eccentric exercise, and the length of
the muscle during eccentric muscle
actions. Shifts in optimum length have
varied from 3.9° (76) to 18° (61) after
eccentric exercise. The studies reporting the greatest shifts used protocols
with either high volume or high load at
long muscle lengths.
Since the exercise known as the
‘‘Nordic hamstring exercise,’’ shown
in Figure 2, was detailed in 2001 by
Brockett et al. (14), 5 studies have
documented the effects of eccentric
exercise on hamstring injuries in elite
soccer, rugby, and Australian rules
football players (7,10,18,30,62). However, none of these studies measured
the optimum length of the hamstrings;
thus, the shift in optimum length was
not quantified nor was its significance
investigated. Four of the studies used
the Nordic hamstring exercise, and
1 study used the Yo-Yo hamstring curl
exercise (10). The Yo-Yo exercise
involves the athlete performing eccentric leg curls in the prone position. The
Figure 1. Shift in muscle length where peak tension is produced.
hamstring exercise as it is a bilateral
and single joint exercise. Because hamstring injuries occur during unilateral
and multi-joint movements, a more
functional approach to exercise design
is needed (19). Exercises should be
developed that involve eccentric hip
flexion as well as eccentric knee extension. Despite these limitations, each of
the studies investigating the effects of
eccentric exercise on hamstring injury
rates has reported fewer injuries. Four of
these studies used similar protocols of
2–4 sets of 6–12 repetitions. Only Gabbe
et al. used a different protocol with very
high volume (i.e., 12 sets of 6 repetitions). It should be noted that the
compliance rate was very low in this
study. Therefore, to prevent low compliance rates and to reduce injury rates, it
is recommended that approximately 3
sets of approximately 8 repetitions be
performed twice a week. Furthermore, it
is recommended that it may be the best
practice to include ground-based eccentric exercise training (19) at the beginning or completion of practice (20).
ACCENTUATED AND
SUPRAMAXIMAL LOAD
ECCENTRIC TRAINING
Figure 2. Nordic hamstring exercise start (a) and finish (b).
Yo-Yo device is basically a flywheel that
is accelerated during the concentric
contraction and then decelerated during the eccentric muscle actions of the
hamstrings. Similar to the Nordic
hamstring exercise, the Yo-Yo leg curl
is a bilateral open chain exercise.
Despite the recent interest in eccentric
exercise for shifting the optimum
length and reducing hamstring injury
rates, loading patterns for athletic
populations are yet to be developed.
This is mostly because of the fact that
there is currently no research that has
reported the effects of both a shift in
optimum length and injury rates in the
same study. In addition, the majority of
studies reporting a shift in optimum
length have been acute muscle damage
studies and not training studies. Currently, there are only 2 training studies
in the literature. One of these was
a pilot study (21) and the other did not
use an athletic population (42).
All the studies in Table 2 reported
either a shift in optimum length or a
reduction in hamstring injury rates.
The following guidelines are presented
for shifting the optimum length to
longer lengths: eccentric muscle
actions should be performed at long
muscle lengths; muscle contraction
load should be moderate to high; the
combinations of long muscle lengths/
high load or long muscle length/
high volume result in the greatest acute
shifts; it is possible to maintain the shift
for over 4 weeks; and muscle damage is
not needed to induce a shift.
For reducing injury rates in chronic
studies, the Nordic hamstring exercise
appears to be the exercise of choice as
observed in Table 2. However, there are
a few limitations with the Nordic
It has been shown that humans are able
to recruit fewer motor units (with the
same force development) during an
eccentric muscle action than a concentric contraction at a given or absolute
load. Therefore, the neural efficiency of
eccentrics is greater, and it has been
suggested to maximize neural activation and subsequent strength adaptation; during eccentric muscle actions,
greater
loads
are
required
(40,45,64,83). It seems therefore that
the eccentric portion of a traditional
resistance exercise is underloaded even
when the concentric portion is at
maximum. Some research has suggested that subjects may be as much
as 20–60% stronger eccentrically than
concentrically (34). Given this information, a subject would be able to
lower (or yield to) a weight much
heavier than he/she can overcome
concentrically, as much as 120–130%
of the (concentric) 1RM (35,39,47).
Logically, if the same subject is
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VOLUME 34 | NUMBER 3 | JUNE 2012
Author
Sample
size
Jones
et al. (37)
9
Whitehead
et al. (76)
8
Brockett
et al. (14)
Program
duration
Frequency
(sessions/
wk)
Sets
Reps
Exercise
Muscle
length
Shift in
optimum
length
Muscle
group
1 session
2h
BW on
treadmill
SL
4.4°
Triceps
surae
10
1 session
1h
BW on
treadmill
SL
3.9°
Triceps
surae
Whitehead
et al. (76)
13
2 sessions
12
NH
LL
7.7–8.5°
Hamstrings
Askling
et al. (10)
30
1 session
1h
BW on
treadmill
SL
6.0–7.6°
Triceps
surae
Bowers
et al. (12)
9
10 wks
Philippou
et al. (60)
14
Proske
et al. (62)
4
8
NH
LL
2 sessions
12
20
Step-down
ML
10.5–15.4°
1 session
2
25
Biceps curl
LL
16–18°
NH
LL
Clark
et al. (21)
9
Pettitt
et al. (59)
9
4 wks
Brooks
et al. (18)
11
rugby
clubs
1 session
Gabbe
et al. (30)
220
Full
season
Prasartwuth
et al. (61)
8
12 wks
2
6
2
3
Injury
occurrence
Triceps
surae
3 injuries
Quadriceps
Biceps
brachii
2 injuries
2–3
5–8
NH
LL
6.5°
Hamstrings
3
25
Triceps
extension
ML
10.4°
Triceps
brachii
1
2–3
6–7
NH
LL
0.39
injuries/
1,000 h
5 total
sessions
12
6
NH
LL
4% of
players
injured
(continued)
(continued on next
page)
Multiple Purposes of Eccentric Muscle Actions
38
Table 2
Studies investigating the effects of eccentric exercise on either the optimum length or injury rates
1 session
BW = backward walking; LL = long muscle length; ML = moderate muscle length; NH = Nordic hamstring; SDL = single leg dead lift; SL = short muscle length; Yo-Yo = Yo-Yo hamstring curl.
Quadriceps
LL
Step-up
and stepdowns
10 min
15
steps/
min
4.0–7.0°
0.22
injuries/
1,000 h
LL
Yo-Yo
Full season
24
Yueng and
Yueng
(82)
2
3
8–12
Hamstrings
21°
SDL and
prone
hamstring
curl
3 wks
24
soccer
clubs
Arnason
et al.
(8)
7 total
sessions
2
8
LL
Biceps
brachii
14.0–16.7°
LL
Biceps curl
40–160
total
1 session
30
Kilgallon
et al.
(42)
Table 2
(continued )
attempting to improve strength or
hypertrophy using the overload principle (when a muscle increases in size
and/or strength when forced to contract at circa maximal tension), then
he/she would be able to increase the
overload substantially by implementing supramaximal eccentric loads
(SME) (16). Considering the potential
performance enhancement possibilities
of SME training, relatively little
research has been devoted to exploring
the possible benefits of SME training,
and although some of the results have
been positive, the benefits do not
appear to be exclusive to eccentric
training (22,74).
Some research has attempted to
exploit these potential benefits using
accentuated eccentric loads (AELs).
AEL is similar to SME, in that the load
used for the eccentric portion of the lift
is greater than that which is used
during the concentric. The fundamental difference between the 2 loading
protocols is that the eccentric load
used in AEL is not supramaximal.
The effects of SME or AEL on
concentric strength are shown in some
of the studies in Table 3. All studies
compared an SME or AEL approach
with a group who implemented a standard resistance training program (ST)
where the concentric and eccentric load
was the same. The greatest improvements using SME (110–120% 1RM)
were seen by Brandenburg and Docherty (13) who reported a statistically
significant (p , 0.05) increase in elbow
extensor strength of 24% compared
with 15% using ST, after 10 weeks of
training 2–3 times per week. Kaminski
et al. (39), while using an AEL
protocol, reported similar findings: the
AEL group significantly (p , 0.001)
improved overall hamstring strength
29% and the ST group improved 19%
after 6 weeks of training 2 times per
week. More recently, Sheppard et al.
(71) and Sheppard and Young (72)
demonstrated that AEL benefited not
only the bench throw exercise but also
peak power in the countermovement
jump.
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VOLUME 34 | NUMBER 3 | JUNE 2012
Sample size
Frequency
(sessions/wk)
Age (SD), y
Sets
Program
duration
(wks)
Gender
Author
Godard
et al. (31)
Training status
28
10
Eccentric load
(control)
Reps
Rest
23/wk
22.3 (3.1)
1
M and F
8–12
Exercise
Knee extensions
Muscle
group(s)
Quadriceps
Concentric
load (control)
(% concentric
1RM)
120 (80)
80 (80)
Physically active
Kaminski
et al. (39)
Hortobagyi
et al. (35)
27
6
23/wk
22.9 (3.1)
2
M
8
Healthy
60 between
sets
30
1
73/wk
20.9 (1.2)
5–6
F
10–12
Sedentary
180 s
between
sets
Prone leg curl
Hamstrings
100 (80)
40 (80)
Knee extensions
Quadriceps and 100–110
biceps femoris
(60)60 (60)
Dependent
variables
Result
Concentric knee
extensor
strength
Both groups
increased
concentric knee
extensor torque;
however, the
difference
between groups
was insignificant.
Concentric knee Concentric group
flexor strength
improved
concentric
strength 19% and
eccentric group
improved
strength 29%.
Eccentric 3RM
Maximum
increased 27% for
voluntary
AEL and 11% for ST.
isometric and
Concentric 3RM
isokinetic knee
increased 27% for
extensor
AEL and 26% for ST.
strength, 3RM,
AEL increased
quadriceps
isokinetic eccentric
EMG,
strength 149 versus
antagonist
64 N for ST.
muscle
Concentric
coactivity
isokinetic forces
increased 58 versus
53 N for AEL and
ST, respectively.
There was no
change in the ratio
of biceps femoris
EMG coactivity to
quadriceps EMG.
(continued on next page)
Multiple Purposes of Eccentric Muscle Actions
40
Table 3
Studies investigating the effects of supramaximal (.100% 1RM) or accentuated (#100% 1RM) eccentric exercise
Table 3
(continued )
Brandenburg
and Docherty
(13)
Friedmann
et al. (29)
23 (18)
9
2, 33/wk
University aged
3–4
M
10
Active in
resistance
training
120 between
sets
18
4
3–43/wk
24.5 (3.35)
3–6
M
25 reps in 45 s
Preacher curl,
supine elbow
extension
Biceps and
triceps
Knee extensions
Quadriceps
110–120 (75)
75 (75)
70 (30)
30 (30)
Active but
untrained
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Yarrow
et al. (80)
22
21.9 (0.8)
M
Untrained
1
100 (52.5)
Squat, bench press Pectorals,
anterior
deltoids,
4 (ST), 3 (AEL)
triceps, glutes, 40 (52.5)
hamstrings,
6 (ST and AEL)
and
60 s between
quadriceps
sets
2 exercise
bouts
Elbow flexor and Both groups (ST and
SME) increased
extensor
concentric
concentric
strength for both
1RM and
muscle groups
specific
significantly.
tension, MCSA
Elbow extensor
strength for SME
group increased
24 and 15% for ST
group. Neither
displayed
significant
changes in MCSA.
Strength
endurance
capacity,
maximal
strength, and
MCSA
Strength endurance
capacity improved
8% for ST and
insignificantly for
the AEL. Maximal
strength improved
5% for AEL and
insignificantly for
ST. MCSA
improved for both
groups; AEL =
2.4 6 3.3 cm, ST =
1.7 6 2.6 cm
Acute effects of Eccentric enhanced
group showed 15–
AEL versus ST
16% increase in BT
on TT, BT, GH,
immediately post
lactate, and
exercise as well as
RPE were all
increased lactate
tested
concentration and
immediately
RPE. Both groups
post exercise,
had 500–7,000%
and again 15,
above baseline GH
30, 45, and 60
concentrations. All
s post exercise
results came from
a single bout of
exercise.
41
(continued)
VOLUME 34 | NUMBER 3 | JUNE 2012
Yarrow
et al. (81)
22
5
100 (52.5)
Squat, bench press Pectorals,
anterior
4 (ST), 3 (AEL)
40% (52.5%)
deltoids,
triceps,
glutes,
6 (ST and AEL)
hamstrings,
60 s between
and
sets
quadriceps
Chronic effects of Both groups
experienced
AEL versus ST
increases in
on TT, BT, GH,
strength in both
lactate, and
exercises, TT, BT,
RPE were all
GH, and lactate.
tested
The AEL group
immediately
had significantly
post exercise
lower lactate
and again 15,
levels than ST
30, 45 , and 60
group 30 and 60
m post
m min post
exercise
exercise, and RPE
was higher for the
first 4 exercise
bouts.
5
2–33/wk
ST (WS)
versus AEL
(FW)
Strength (1RM),
hypertrophy,
load lifted
(task-specific
performance),
and average
work
WS and FW both
showed increases
in strength, load
lifted, and
hypertrophy. The
FW showed
greater changes
(p , 0.05, p ,
0.05, p , 0.025,
respectively).
BW: +20 kg
(M), BW:
+10 kg (F)
(BW)
Peak velocity,
peak force,
and peak
power
Both the BW and
BW+ jump
experienced
a nonsignificant
increase in peak
force and only the
BW+ group
experienced
a significant (p =
0.05) increase in
peak power.
21.9 (0.8)
M
Untrained
Norrbrand
et al. (56)
15
33/wk
Knee extensions
Quadriceps
4
M
7
Healthy
Sheppard
et al. (71)
16
5
33/wk
21.8 (4.9)
2
M and F
6–10
Active in
resistance
training
60 s between
jumps
Countermovement Quadriceps,
jump
hamstrings,
glutes,
and gastroc–
solues
complex
BW
(continued on next page)
Multiple Purposes of Eccentric Muscle Actions
42
Table 3
(continued )
Table 3
(continued )
Ojasto and
Häkkinen (58)
11
32.4 (4.3)
Strength and Conditioning Journal | www.nsca-scj.com
Sheppard and
Young (72)
4 testing
6 wks of
sessions
familiarization
training
before testing 4
Bench press
Pectorals,
anterior
deltoids, and
triceps
Test 1: 70%
(70%)
Test 2: 80%
(70%)
M
10
Test 3: 90%
(70%)
Healthy,
physically
active
120 s
Test 4: 100%
(70%)
14
1
4d
25.0 (1.0)
2
M
1
Highly familiar
with bench
press exercise
120 s
Bench press
Pectorals,
anterior
deltoids,
and triceps
40 kg (40 kg)
40 kg (60 kg)
EMG, eccentric/ Concentric and
isometric force pre
concentric/
to post was
isometric
reduced
force, GH, and
significantly (p ,
LA were
0.01–0.001). LA
measured pre
increased in all
and post
protocols but was
loading
insignificant
between groups;
a trend of increased
LA at higher loads
(90% [70%]) was
observed. Betweengroup GH levels
were insignificant,
and the highest GH
concentrations
were seen with the
90% (70%) group.
Optimal eccentric
load was
significantly
correlated to 1RM.
Concentric
output
(acceleration)
40 kg (70 kg)
40 kg (80 kg)
43
BT = bioavailable testosterone; FW = freewheel; LA = blood lactate; MCSA = muscle cross-sectional area; TT = total testosterone; WS = weight stack.
Concentric output in
the bench throw
was enhanced by
accentuated
eccentric load. The
magnitude or
highest peak
displacement was
individually specific.
Stronger subjects
benefited from
greater eccentric
loads (80 kg),
whereas weaker
subjects benefited
more from lesser
eccentric loads
(60 kg).
Multiple Purposes of Eccentric Muscle Actions
Whether comparing an SME or an AEL
approach with ST, all groups improved
concentric strength although not always
to the same degree as reported previously. Such was the case with Godard
et al. (31) when they studied the effect of
SME (120% 1RM) on quadriceps strength. They found that after 10 weeks of
training 2 times per week, participants
did not improve concentric strength in
one group significantly more than the
other. Brandenburg and Docherty (13)
reported similar findings with the elbow
flexors (SME increased 10% and ST
increased 9%).
One of the issues with some of the
current research is that despite an AEL
or SME approach, eccentric strength is
rarely assessed. Considering the welldocumented principle of training specificity, eccentric strength has been
shown to be best enhanced by eccentric-specific training (69). Hortobagyi
et al. (36) recognized this limitation and
addressed this in his study as well as any
potential neurologic benefits to AEL
training. As expected, eccentric strength
gains for the AEL (27%) group were
double that of the ST (11%) group; and
any increase in concentric 1RM was
accompanied by a directly proportional
increase in electromyographic activity.
Neither SME nor AEL has demonstrated any advantage over ST with
regard to hypertrophy. Ojasto and
Häkkinen (58) subjected healthy men
to a hypertrophy training protocol
using AEL and found that it was not
more favorable for hypertrophy when
compared with ST. Other studies have
shown that both AEL and SME present
no clear benefit in increasing muscle
cross-sectional area. Eccentric training
at high speeds (180° per second) has
been shown to be more effective for
strength and hypertrophy than comparable concentric training (13,31,56,81).
Higbie et al. (33) demonstrated that
eccentric strength was best developed
by eccentric-based training, whereas
concentric strength was best for
developing concentric strength.
It is very difficult to make conclusions as
to the benefit of AEL or SME for sport
44
VOLUME 34 | NUMBER 3 | JUNE 2012
performance enhancement. Based on the
current research as shown in Table 3,
there does appear to be certain advantages to an AEL or SME protocol for
untrained populations, those requiring
acute benefits from strength training,
training specific muscle groups and for
athletes required to perform at levels
above lactate threshold or for specific
muscle groups (13,36,39,80,81).
It should be noted that the research
tabulated and discussed in this brief
treatise is not without limitations. With
the exception of Brandenburg and
Docherty (13), all research used a nonathletic population. It is well known that
untrained subjects respond differently
to well-trained individuals. Furthermore,
all studies, with the exception of the
Yarrow et al. (80,81) and Ojasto and
Häkkinen (58), chose single joint exercises such as knee extension, leg curls,
and elbow flexion and extension.
Although these exercises have merit in
certain situations, increased performance
in them is not indicative of potential for
athletic performance, which requires
multi-joint movement. Furthermore,
the training protocols chosen for the
studies, such as training every day only
for 1 week or 1 set per day twice a week,
are often inconsistent with that which
are typically seen in sports performance–
based training programs.
There is a clear need for future research.
It is suggested that SME be used with
multi-joint exercises such as the squat
and dead lift on participants who are
actively involved in competitive sport
and/or resistance training. The program
chosen needs to be volume adjusted to
compensate for the increased eccentric
load, and the set/rep/rest/frequency
variables should be consistent with
a sports performance–based program.
ECCENTRICS FOR STRETCHSHORTENING CYCLE
PERFORMANCE
When a muscle is required to overcome
resistance, or contract concentrically, its
ability to do so may be determined by
whether that concentric contraction was
preceded by an eccentric muscle action.
Research has shown that concentric
force production in isolation is relatively
low compared with concentric contractions that are coupled with an initial
eccentric muscle action (44). This
pairing is termed the SSC as defined
previously. The SSC may have large or
small amounts of angular displacement
of the relative joints, and it is composed
of both voluntary and involuntary (stretch reflex [SR]) actions (46,68). For
optimal SSC potentiation (i.e., a more
forceful concentric contraction), a number of factors are thought critical:
Preactivation of musculature
before contact.
Little or no coupling times (i.e.,
time between the termination of
the eccentric phase and the onset
of the concentric phase).
Short-duration contractions.
High eccentric muscle action
velocities.
Relatively
small
amplitude
movements.
Preactivation. Dietz et al. (25) demonstrated that before ground contact in
a drop jump, the extensor muscles
were activated and it is this muscle
action that created muscle stiffness that
minimized the amount of lengthening
taking place in the muscle itself, and
most of the length change therefore
took place in the tendon of the
extensors upon contact with the
ground (28). The force that is created
upon ground contact produces energy,
which is stored principally in the
tendons and helps to create a more
powerful concentric contraction.
Coupling times: Short contraction
durations. Elasticity refers to the ability
of an object to return to form after it has
been altered, and elastic energy is the
work done during this process (26). A
number of tissues can store elastic
energy during an SSC including the
muscle’s connective tissues (e.g., perimysium, epimysium, endomysium),
structures in series with the muscle
fibers (e.g., tendon, titin), and the
contractile elements themselves. The
latter occurs within the cross-bridges
between filaments when the actual
muscle
lengthens
without
the
Table 4
Summary of typical loading parameters for different types of eccentric training
Type of eccentrics
Stress
Strain
Velocity
Rehabilitation
Low to moderate
Short to long length
Slow to moderate
Shift in optimum length
Low to moderate
Long lengths
Slow to fast
Accentuated
Moderate
Moderate
Slow to moderate
Supramaximal
High
Moderate
Slow
SSC
Moderate to high
Short lengths
Fast
SSC = stretch-shortening cycle.
‘‘popping’’ of the actin–myosin crossbridge. This energy stored in the various
tissues, however, is finite in duration
with a half-life of 0.85 second and a 55%
decrease by 1.0 second; therefore, to
make the most of the stored elastic
energy, the coupling times need to be
minimal and the SSC should last less
than 0.25 milliseconds (66,78). That
is, the force generated during the
concentric phase will tend to be higher
when the duration of the SSC is shorter.
As the SSC duration lengthens, the
benefits of stored energy dissipate (79).
High eccentric velocity: small
amplitude movements. When preactivation occurs and when the athlete
makes contact with the ground, a
reflexive action results called an SR
(54). The SR is a by-product of a signal
sent by the spindles in the muscles to
the central nervous system. Muscle
spindles are receptors in the muscles,
and they provide information about
length and velocity of length change. In
the drop jump, as the athlete makes
contact with the ground, the muscle
spindle senses the lengthening of the
affected muscles (ankle plantar flexors
and knee and hip extensors) and
a signal is sent to the spinal cord via
sensory motor neurons. A synapse
occurs in the spinal cord, and excitatory messages are sent to the muscles
via alpha motor neurons, which produce a concentric contraction in the
muscles (to return the spindle to its
initial length). The higher the velocity
of the stretch, the greater potentiation
of a reflexive forceful concentric contraction; this reflex is dependent on the
level of motor neuron excitation and
the amplitude of the movement, that is,
small relative joint motion.
Research has shown that the optimal
amount of energy stored during an
SSC is largely determined by the
amplitude of the relative joints. That
is, some joint movement is necessary;
however, too much angular displacement will decrease the number of actin
and myosin cross-bridges interacting
with each other and decrease reflex
potentiation, which ultimately affects
the storage and utilization of elastic
energy within the muscle, thereby
reducing its force production capability
(63). Rack and Chu (63) demonstrated
that drop jumps where the subjects
maintained knee angles of less than
75° allowed them to keep their foot
contact time to 416 6 41 milliseconds,
which created greater concentric force
production than jumps where the
subjects had knee angles greater than
85°. Longer contact times are indicative of larger stretch amplitudes of the
relative muscles, and when muscles are
stretched beyond a certain point, the
resulting concentric contraction no
longer benefits from the SR (49).
The training status of the athlete
determines the optimal peak eccentric
velocity or load of the exercise. If the
athletes are untrained, fatigued, or both,
then it will affect their ability to perform
the SSC task appropriately. In untrained
athletes, the peak eccentric velocity
will be less to keep the amplitude of
movement minimal. This athlete will
have coupling times that are longer in
duration and/or movement amplitudes
that are too great, if eccentric velocity
before contact is too high. That is, if
the stretch load is too great and there is
a large amplitude movement or the peak
eccentric velocity is too great for the
musculotendinous unit, the coupling
time could be substantial and the
benefits of the SR are therefore minimized or mitigated once more.
PRACTICAL APPLICATIONS
In summary, the stress, strain, and
movement velocity associated with
eccentric training can result in very
different adaptational and functional
outcomes, which are summarized in
Table 4. As the strength and conditioning specialist gains a greater understanding of the eccentric phase and
its defining characteristics, he or she
will be able to systematically implement
eccentric-based training for a variety of
goals. For example, injuries may be
rehabilitated or prevented by means of
tendon remodeling and/or injury prevention by shifting the optimal length of
a muscle to produce peak tension,
respectively. Performance may also
be enhanced with eccentrics by using
accentuated and/or supramaximal
loading as well as optimizing the SSC
by ensuring high-velocity eccentric
loading. Strength and conditioning for
many athletes/sports will involve
a blending of all these types of eccentric
Strength and Conditioning Journal | www.nsca-scj.com
45
Multiple Purposes of Eccentric Muscle Actions
loading, and the art will be in ensuring
that the effects sought with each type of
loading are optimized. Therefore, careful within-session, short-term, and longterm planning is needed.
Matt Brughelli
is a lecturer
in Sports
Biomechanics at
AUT University
and a strength and
conditioning coach.
John Cowell is
a postgraduate
student at AUT
University and
a strength and
conditioning
coach.
John Cronin is
a professor in
Strength and Conditioning at AUT
University and
adjunct professor
at Edith Cowan
University,
Australia.
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