Guex SportsMed13 Hamstring Framework

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CURRENT OPINION
Conceptual Framework for Strengthening Exercises to Prevent
Hamstring Strains
Kenny Guex •Gre
´goire P. Millet
ÓSpringer International Publishing Switzerland 2013
Abstract High-speed running accounts for the majority
of hamstring strains in many sports. The terminal swing
phase is believed to be the most hazardous as the ham-
strings are undergoing an active lengthening contraction in
a long muscle length position. Prevention-based strength
training mainly focuses on eccentric exercises. However, it
appears crucial to integrate other parameters than the
contraction type. Therefore, the aim of this study is to
present a conceptual framework based on six key param-
eters (contraction type, load, range of motion, angular
velocity, uni-/bilateral exercises, kinetic chain) for the
hamstring’s strength exercise for strain prevention. Based
on the biomechanical parameters of sprinting, it is pro-
posed to use high-load eccentric contractions. The move-
ment should be performed at a slow to moderate angular
velocity and focused at the knee joint, while the hip is kept
in a large flexion position in order to reach a greater
elongation stress of the hamstrings than in the terminal
swing phase. In this way, we believe that, during sprinting,
athletes would be better trained to brake the knee extension
effectively in the whole range of motion without over-
stretch of the hamstrings. Finally, based on its functional
application, unilateral open kinetic chain should be
preferred.
1 Introduction
Hamstring strain injuries have a high prevalence in many
sports, including soccer [1,2], rugby [3,4], Australian
football [5], American football [6], Gaelic football [7], and
sprinting [8–11]. High-speed running—the common point
between these activities—accounts for the majority of
hamstring strains [2,7,12,13]. A good understanding of
the biomechanical characteristics responsible for this injury
during running is required to optimize the preventive
intervention.
The running cycle begins when one foot comes in
contact with the ground (0 %) and ends when the same foot
contacts the ground again (100 %). It is split into two
separate phases: the stance and the swing. Toe off marks
the end of stance and the beginning of the swing phase. It
occurs at *39 % of the running cycle when running [14]
and at *22 % when sprinting [15]. Terminal swing and
early stance phases have both been proposed to be the main
periods for hamstring strain [16,17]. However, the termi-
nal swing is believed to be the most hazardous for many
reasons [2,18–23]. First, between 75 and 85 % of the
running cycle, the hamstrings are undergoing an active
lengthening contraction (i.e., eccentric) [19]. The repetition
of eccentric contractions, as is the case during sprinting,
has the potential to induce damage within the muscle [24,
25]. Furthermore, the musculotendon peak force is reached
at *85 % of the running cycle and it was shown that the
loads during the terminal swings exceed those during
stance [19]. Second, during this phase, the combination of
hip flexion and knee extension induces a substantial elon-
gation stress on the biarticular hamstrings (Fig. 1)[19].
At *85 % of the running cycle, the semimembranosus,
semitendinosus, and long head of the biceps femoris are
stretched by 9.8, 8.7, and 12.0 %, respectively, beyond
K. Guex (&)
Department of Physiotherapy, School of Health Sciences,
University of Applied Sciences Western Switzerland,
Av. de Beaumont 21, 1011 Lausanne, Switzerland
e-mail: kenny.guex@hesav.ch
K. Guex G. P. Millet
Department of Physiology, Faculty of Biology and Medicine,
ISSUL Institute of Sport Sciences, University of Lausanne,
Lausanne, Switzerland
Sports Med
DOI 10.1007/s40279-013-0097-y

their upright lengths (i.e., hip and knee at 0°of flexion)
[19]. Due to the stretch-shortening cycle—the transition
between negative and positive power takes place at *85 %
of the running cycle [19]—the local elongation stress on
the myotendinous junctions and tendons (i.e., the most
sensitive structures to strain [26,27]) is increased. Finally,
two case studies have reported biomechanical data when
running athletes incurred acute hamstring strain injuries
[20,21]. Both studies described the terminal swing rather
than stance as the most likely time of injury.
Unmodifiable risk factors for hamstring strain injuries,
such as age, ethnic origin, and past history of posterior
thigh injury have been described [28]. On the other hand,
strength imbalances of the hamstrings, such as a weakness
[29–31], bilateral asymmetry [32], lower hamstrings to
quadriceps ratio [33], and a decrease in the optimum length
[34–36], have been suggested as modifiable risk factors of
hamstring strain injuries. In order to address these modi-
fiable risk factors, six intervention studies have docu-
mented the effects of eccentric exercises on hamstring
strain prevention in soccer, rugby, and Australian football
[12,35,37–40]. Five of the studies used the Nordic ham-
string exercise (NHE), while one used the yo-yo hamstring
curl exercise [38]. The six studies reported 60–70 %
reduction in injury rate compared to control groups after
the eccentric exercise protocol. This could be explained by
the improvement that was observed after this type of
exercise of some of the risk factors such as strength
[41–43], the hamstrings to quadriceps ratio [42], or the
optimum length [43–45].
Although the reduction in the injury rate observed in
these studies is meaningful, a substantial number of ham-
string strains still occur. In regard to the stress on the
hamstrings during the late swing phase of sprinting, it
appears crucial to integrate parameters other than the
contraction type (i.e., eccentric) for the conception of
strength exercises. Particularly, the load, range of motion,
and angular velocity are of great importance. The fact that
exercises are uni- versus bilateral and in open versus closed
kinetic chain should, in our opinion, also be taken into
account. The aim of this study is, therefore, to present a
conceptual framework for strengthening exercises to pre-
vent hamstring strains, focusing on six key parameters: (1)
contraction type; (2) load; (3) range of motion; (4) angular
velocity; (5) uni-/bilateral exercises; and (6) kinetic chain.
2 Contraction Type
Contraction type is the most commonly monitored
parameter in strength training to prevent hamstring strains.
Since the NHE was described in 2001 [44], all researchers
[12,35,37–40] in the field of hamstring strain prevention
have used eccentric exercises, with encouraging results as
stated above. The improvement of many risk factors could
explain the decrease in injury rate. First, the NHE was
shown to be effective in improving the eccentric ham-
strings strength [41–43,46]. Consequently, after 10 weeks
of NHE, the ratio between eccentric hamstrings torque and
concentric quadriceps torque (mixed H/Q ratio) was shown
to be increased from 0.89 to 0.98 [42], which corresponds
to the strength status of the normalized muscle [33]. Given
that strength training is largely mode specific [42,47,48],
and that the strains mainly occur when the hamstrings act
eccentrically to brake the knee extension at the end of the
running swing phase [16], it seems relevant to prioritize
this contraction mode to increase the special strength of the
hamstrings. Second, the hamstrings’ optimum length—the
knee angle at which hamstrings’ peak torque occurs
(Fig. 2)—was found to be shifted in the direction of longer
muscle length directly after a single bout of eccentric
strength training [44]. Many studies have also observed a
shift in the same direction after 3–8 weeks of hamstrings
eccentric strength training [41,43,45,49]. However, one
Fig. 1 Terminal swing phase of
the right leg of a world-class
sprinter, corresponding to 75,
80, 85, 90, and 95 % of the
running cycle. At *85 %, the
hamstrings reach their
elongation peak. The
‘‘elongation stress’’ on
hamstrings is determined by
subtracting the knee angle from
the hip angle. The left leg is in
the early swing phase,
corresponding to 25 % (i.e.,
*toe off), 30, 35, 40, and 45 %
of the running cycle
K. Guex, G. P. Millet

study [46] with a high variability in results did not report
this shift.
The shift of the optimum length was found to be similar
after an acute and a chronic intervention, although the
mechanisms are different. After a single bout of eccentric
exercise, this shift was believed to be the result of muscle
fibers damage, an early stage in the process leading to
delayed-onset muscle soreness (DOMS), which is largely
specific to eccentric contraction type [50–52]. The dis-
rupted sarcomeres increase the muscle’s series compliance
(i.e., less force is required to elongate the muscle), leading
to a shift in optimum length. After a chronic intervention,
the observed shift was attributed to an increase in fascicle
length that suggests an addition of sarcomeres in series
within the muscle [41,53,54]. This allows the muscle to
produce its peak force in a longer position without having
an overstretch of the sarcomeres. To prevent injury, this
chronic adaptation seems particularly relevant regarding
the substantial elongation stress on the biarticular ham-
strings during the terminal swing phase of the running
cycle [19].
3 Load
The hamstrings’ weakness has been suggested as a modi-
fiable risk factor of strain injury [29–31] and it was stated
that stronger muscles would provide greater protection
against strain injury [55]. Muscular strength development
involves the coordinated functioning of several processes
[56], with the ability to produce maximal force attributed to
both neural and muscular components [57]. In the early
phases of resistance training, the increases in strength are
mainly associated with neural adaptations [56,57], while
hypertrophic responses contribute to the strength gains in
the later phases [58].
The load refers to the amount of weight assigned to an
exercise set and is probably the most important variable in
resistance training program design [59]. It was shown that a
load greater than 80–85 % of 1 repetition maximum (RM)
is needed to produce further neural adaptations and
strength in experienced lifters [60]. This load range appears
to maximally recruit muscle fibers and will specifically
increase dynamic 1 RM strength [60]. This is relevant in
the field of injury prevention, since it was shown that fully
stimulated muscles are able to tolerate greater stress before
strain than partially activated muscles [55]. Strength
increases have been shown to be greater when using heavy
weights for 3–5 RM than with 9–11 and 20–28 RM [61].
Therefore, it is recommended that novice to intermediate
individuals train with loads corresponding to 60–70 % of 1
RM, while advanced individuals use loads of 80–100 % of
1 RM to maximize muscular strength [62]. So, to optimize
strength exercises in the field of hamstring strain preven-
tion, the load should be monitored and be at 80 % of the 1
RM at least in athletes. However, it is suggested that
strength developed eccentrically should be 20–60 % higher
than concentrically [63]. Kaminski et al. [64], have
reported better improvement (?29 vs. ?19 %) in ham-
string strength after 6 weeks of training twice a week
(2 98 repetitions) in eccentric at 100 % than in concentric
at 80 % of 1 RM. Therefore, using loads corresponding to
Fig. 2 Example of an unpublished hamstrings’ force–length rela-
tionship of a national-level sprinter. The arrows represent the
optimum length (i.e., knee angle at which hamstrings’ peak torque
occurs). The test was performed on an isokinetic device at a80°and
b0°of hip flexion. In both positions, the hamstrings of the left leg
have a lower optimum length than the right leg. The risk of injury is,
then, higher for the left hamstring than for the right
Hamstring Strain Prevention

100 % of 1 RM at least seems relevant when the resistance
exercise is performed in eccentric. In the case of the NHE,
for example, it is difficult to adjust the load. The correct
and complete execution of the movement is often too
arduous for beginners, while it could become too easy for
advanced athletes. However, even if the untrained athletes
can only brake the knee extension a few degrees of the
range of motion, they still do the eccentric contraction for
the rest of the movement [46]. For advanced athletes, the
load could be increased by pushing on their back.
4 Range of Motion
As mentioned in Sect. 2, eccentric resistance training has
been shown to shift the hamstrings’ optimum angle in the
direction of longer muscle length [41,43,44,49]. How-
ever, it was reported that the fascicle length adaptations
that explain this shift were strongly influenced by factors
other than contraction mode [53]. In fact, Blazevich et al.
[53] have reported a similar increase in fascicle length of
the vastus lateralis after 10 weeks of concentric versus
eccentric training at long muscle length. This supports the
idea that the training range of motion (i.e., muscle excur-
sion range during loading) is the dominant stimulus for
fascicle length adaptation [53]. In line with this result, a
recent study showed that 30 concentric contractions of the
hamstrings at a long musculotendinous length (at 80°of hip
flexion, in a seated position) immediately induced a sig-
nificant shift of 15°of the optimum knee angle, while 30
concentric contractions of the hamstrings at a short mus-
culotendinous length (at 0°of hip flexion, in a supine
position) did not lead to any shift of the optimum angle
[65]. Moreover, during eccentric contractions, the magni-
tude of the shift was shown to be influenced by the muscle
length during contractions. Indeed, a larger shift of the
optimum length in the direction to longer muscle length
was observed after 50 eccentric contractions performed on
the descending part of the length–tension relationship
(beyond optimal length) than on the ascending part (below
optimal length) [51].
From an anatomical point of view, the hamstrings are
mainly biarticular muscles (except the short head of the
biceps femoris). Therefore, both hip flexion and knee
extension are able to stretch them, which influences their
strength production. On an isokinetic dynamometer, it was
shown that, as hip flexion increased, hamstrings’ peak
torque increased [66]. The suggested mechanism was that
with a flexed hip, the hamstrings’ passive structures are
likely more stretched and would contribute to a greater
extent to their strength production. During the swing phase
of sprinting, these two joints have a large range of motion
(Fig. 1).
4.1 Hip Position
The hip switches from a position of extension at the end of
the stance phase to more than 70°of flexion at *80 % of
the running cycle [67]. The hip is still between 60°and 70°
of flexion at 85 % of the running cycle when the ham-
strings reach their elongation peak. It is, then, extending
gradually up to *45°of flexion before the following
stance phase [14,67].
4.2 Knee Position
During the early swing phase, the knee reaches 130°of
flexion at *55 % of the running cycle [14]. The knee is
quickly extending to less than 30°at *95 % of the running
cycle [67]. Between 95 and 100 %, the knee is slightly
flexing [14,67].
4.3 Elongation Stress
The combination of hip flexion and knee extension induces
a substantial elongation stress on the biarticular hamstrings
at the end of the swing phase [19]. We propose to assess
the amount of elongation by subtracting the knee flexion
angle from the hip flexion angle (Fig. 1). If the result is
positive, the hamstrings are stretched beyond their optimal
length, while if the result is negative, they are placed below
optimal length. Zero corresponds to the upright position
(i.e., optimal length). The ‘‘elongation stress’’ values
obtained with this method are in line with the observations
of Chleboun et al. [68] on the fascicle length of the biceps
femoris at different hip and knee angles. However, it does
not take into account the greater moment arm of the
hamstrings at the hip than at the knee [68,69]. In fact,
changing the hip angle had a larger effect on the long head
of the biceps’ femoris length than changing the knee angle
[68,69]. One may argue that the ‘‘elongation stress’’ values
obtained with our method should be even greater when the
hip is flexed to a larger extent during sprinting (i.e.,
between 70 and 90 % of the running cycle).
One may hypothesize that performing hamstring
strengthening exercises at a long musculotendinous length
with a high hip angle may be more efficient to prevent injury
by inducing a shift of the optimum angle to a longer muscle
length. Some authors [70–73] have proposed relevant
exercises that include hip flexion (e.g., eccentric stiff-leg
deadlift, eccentric single leg deadlifts, hamstring catapult,
sprinter eccentric leg curl, eccentric loaded lunge drops,
barbell leg curl, eccentric box drop, razor curl, lengthened
state eccentric training on cable column). This requires
further investigation, but these exercises may be more effi-
cient than strengthening with a lower (or without) hip flex-
ion as in most of the current ‘classical’ eccentric hamstrings
K. Guex, G. P. Millet

exercises, e.g., the lying hamstring curl or the NHE. During
the latter exercise, the ‘‘elongation stress’’ on hamstrings is
non-existent: the exercise starts at *90°of knee flexion and
0°of hip flexion (elongation stress =-90). At the end of
the movement, the knee is, in the best case, at 0°and the hip
still at 0°(elongation stress =0). Moreover, only a few
athletes are able to perform this exercise with the whole
range of motion. Therefore, in our view, the NHE does not
provide a sufficient ‘‘elongation stress’’ on hamstrings in
regard to the sprinting biomechanics.
5 Angular Velocity
Between *25 and 80 % of the running cycle, the hip is
flexing with a peak velocity greater than 700°/s [74].
Between *55 and 95 % of the running cycle, the knee is
extending with a peak angular velocity greater than 1,000°/s
[74]. In terms of hamstrings’ elongation velocity, the peak
(*1 m/s) is reached in the early swing phase at about
*60 % of the running cycle [19,75]. At this point the
elongation stress on the biarticular hamstrings is negative.
In fact, the hip and the knee are flexed at *50°and 130°,
respectively [67]. From this moment, the hamstrings begin
to contract eccentrically to brake the knee extension [75].
Therefore, their elongation velocity gradually decreases
until *85 % of the running cycle, where the transition
between negative and positive velocity takes place [19]. As
previously mentioned in Sect. 1, this period of stretch-
shortening cycle is thought to be the most hazardous for
hamstring strain injuries.
It is very complicated to develop strength exercises that
reproduce the knee extension velocity (i.e., [1,000°/s)
found during the terminal swing of sprinting. However, it
was shown that the training adaptations observed after
eccentric training were independent from the velocity of
exercise [46,65,76,77]. For example, after 4 weeks of
NHE—a low movement velocity exercise—the same gain
in peak torque was observed, on an isokinetic device, at 60,
120, and 240°/s [46]. This is a promising finding for
hamstring strain prevention. In fact, the adaptations
observed after an eccentric strength program performed at
slow angular velocity may protect the hamstrings’ muscle–
tendon complex from the fast elongation occurring during
the swing phase of sprinting. So, for optimizing strength
exercises in the field of hamstring strain prevention, the
exercises should be performed at a slow or moderate
angular velocity.
6 Uni-/Bilateral Exercises
As stated above, at *85 % of the running cycle, the semi-
membranosus, semitendinosus, and long head of the biceps
femoris are stretched by 9.8, 8.7, and 12.0 %, respectively,
beyond their upright lengths [19]. This stretch is even
greater due to the contralateral hip extending at the same
time, which allows only a slight pelvis oscillation around its
average angular position during the running cycle [78],
while a posterior tilt would have decreased the hamstring
stretch. Bilateral hamstrings strength exercises that include
hip flexion (e.g., eccentric stiff-leg deadlift, barbell leg curl,
eccentric box drop, razor curl, seated hamstring curl),
should, then, be performed carefully because the pelvis
could easily be tilted in a posterior position. Using unilateral
exercises would ensure only a slight pelvis oscillation and,
then, a more specific hamstrings ‘‘elongation stress’’.
Since bilateral asymmetry has been suggested as a
modifiable risk factor of hamstring strain injuries [32], it
seems relevant to propose exercises that are able to
strengthen both legs to same extent. In this context, bilat-
eral exercises could allow one limb to support more load
than the other one. Clark et al. [45] have observed an
increase in optimum length asymmetry between the dom-
inant and non-dominant legs after 4 weeks of training with
the NHE. It was hypothesized that the limb with the higher
knee extension optimum angle of the hamstrings may be
required to take over control of the movement towards the
Fig. 3 Example of some of the hamstring strengthening exercises currently most frequently used to prevent strain injuries: athe Nordic
hamstring; bthe eccentric box drop; and cthe single leg deadlift
Hamstring Strain Prevention

end of the repetition [45]. However, this hypothesis was
refuted by Iga et al. [46], who recorded similar electro-
myographic activity between the dominant and non-domi-
nant legs, suggesting that both hamstrings were recruited to
the same extent during NHE. Overall, in regard to the
sprinting biomechanics, a unilateral exercise seems more
specific.
7 Kinetic Chain
The final key parameter is the type of kinetic chain of the
exercise. Two types of kinetic chain have been described:
the open and the closed kinetic chain. The open one allows
a free motion of the distal segment, while in the closed one,
the terminal joint meets considerable external resistance
that restrains free motion (i.e., generally, the proximal
segment moves relative to the distal segment) [79]. In open
kinetic chain exercises, the proportion of shear force may
be greater, whereas compression forces are greater in
closed kinetic chain exercises [80]. From a neurophysio-
logic point of view, another distinction exists between
these two types of kinetic chain. The open kinetic chain
motion generally consists of one muscle group acting on a
single joint, whereas the closed kinetic chain motion
involves multiple joints and controlled contractions of
synergistic and antagonistic muscles [81]. In practice,
closed-chain exercises are often thought to be benefic due
to their more intimate relationship to functional move-
ments, while open-chain exercises are rated advanta-
geously in therapy and training in isolated arthromuscular
queries [81].
When sprinting, the swing phase consists of one open
kinetic chain activity where the hamstrings’ role is to brake
the knee extension up to *85 % of the running cycle (i.e.,
at the ‘‘elongation stress’’ peak) [19]. One may, then, argue
that performing open-chain kinetic exercises would be
more specific to this phase of sprinting, although the
influence of this latter parameter remains unclear.
The lying hamstring curl is clearly an open kinetic chain
exercise but the NHE is difficult to classify. Some authors
[70,71] consider the NHE to be an open-chain exercise,
probably due to the single-joint movement at the knee,
whereas it might also be considered to be a closed-chain
exercise due to the considerable external resistance that
restrains free motion of the distal segment. The same
Table 1 Proposal for a new framework for hamstring strain pre-
ventive exercises
Parameters Start position Range of motion End
position
Contraction type Eccentric
Load (%) C100 % of 1 RM
Range of motion
Hip position (°)80 0 80
Knee position (°) 130 110 20
Elongation stress -50 ?60
Angular velocity Slow to moderate
Uni/bilateral exercise Unilateral
Kinetic chain Open
RM repetition maximum
Table 2 Analysis of some of the currently most frequently used hamstring strengthening exercises (Fig. 3) to prevent strain injuries
Parameters Exercises
Nordic hamstring Eccentric box drop Single leg deadlift
Start
position
Range of
motion
End
position
Start
position
Range of
motion
End
position
Start
position
Range of
motion
End
position
Contraction type Eccentric (1) Eccentric (1) Eccentric (1)
Load (%) ~Monitorable (–) Non-monitorable (2)*Monitorable (–)
Range of motion
Hip position (°)0(2)0(–)0(2)0(2) 130 (2) 130 (1)0(2)90(2)90(1)
Knee position (°)90(–)90(–)0(1)0(2)-130 (2) 130 (2)0(2)0(2)0(1)
Elongation stress -90 (–)0(2)0(2)0(2)0(2)90(1)
Angular velocity Slow to moderate (1) Moderate (1) Slow to moderate (1)
Uni-/bilateral exercise Bilateral (2) Bilateral (2) Unilateral (1)
Kinetic chain *Open (–) Closed (2)*Closed (–)
*indicates the parameter characteristic is not optimally definable/applicable; no sign before a value indicates that the value is positive (i.e., the
range of motion is in the direction of more elongation stress or the elongation stress is positive); -before a value indicates the value is negative
(i.e., the range of motion is in the direction of less elongation stress or the elongation stress is negative); 1after the value indicates the parameter
corresponds to the new framework; –after the value indicates the parameter is not optimal in regard to the new framework; 2after the value
indicates the parameter is non-specific to the new framework
K. Guex, G. P. Millet

confusion exists for other exercises such as the single leg
deadlift. In fact, this exercise is described as a closed-chain
exercise [70], while it might also be considered an open-
chain exercise due to the single-joint movement at the hip.
8 Conclusion
In our view, hamstring strength exercises should be more
specific to the terminal swing phase of sprinting. Based on
the biomechanical parameters of sprinting, we therefore
propose a conceptual framework for analyzing the current
exercises and designing new ones (Table 1). It is proposed
that the focus should be on high-load eccentric contractions
at the knee joint and to keep the hip in a large flexion
position (80°) in order to reach a greater ‘‘elongation
stress’’ than in the terminal swing. In this way, we believe
that, during sprinting, athletes would be better trained to
brake the knee extension effectively in the whole range of
motion without overstretch of the hamstrings. In Table 2,
some of the hamstring strength exercises most frequently
used currently are analyzed. This article suggests clearly
that the ‘optimal’ exercise has not been designed yet.
Therefore, further investigations on hamstring strain pre-
vention should focus on this point.
Finally, it should be noted that strain prevention is not
only a question of strength but also depends on the timing
of contraction, or a combination of both [11]. Neuromus-
cular control exercises targeting the lower extremities (e.g.,
running drills) and the lumbo-pelvic region (e.g., variations
of trunk movements during running) have not been dis-
cussed in the present study, although two studies [13,82]
have suggested their potential contribution to hamstring
strain injury prevention. However, further studies are
needed to determine the exact role of the muscle contrac-
tion timing in the process of muscle strain.
Acknowledgments The authors would like to thank Mrs. Weibel-
Pache for her contribution during the preparation of this manuscript.
No funding was used to assist in the preparation of this article. The
authors have no conflicts of interest that are directly relevant to the
content of this article.
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