Download comparison of different methods for improving hamstring flexibility

A COMPARISON OF DIFFERENT METHODS FOR IMPROVING HAMSTRING
FLEXIBILITY
by
Kenric Lai
A Thesis Submitted to the Faculty of
The College of Education
in Partial Fulfillment of the Requirements for the Degree of
Master of Science
Florida Atlantic University
Boca Raton, Florida
December 2003
A COMPARISON OF DIFFERENT METHODS FOR IMPROVING HAMSTRING
FLEXffiiLITY
by
Kenric Lai
This thesis was prepared under the direction of the candidate's thesis advisor, Dr. Joseph
A. O'Kroy, Department of Exercise Science and Health Promotion, and has been
approved by the members of his supervisory committee. It was submitted to the faculty
of the College of Education and was accepted in partial fulfillment of the requirements
for the degree of Master of Science.
SUPERVISORY COMMITTEE:
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Chairperson, Department of Exercise Science
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1
ABSTRACT
Author:
Kenric Lai
Title:
A Comparison of Different Methods for Improving Hamstring
Flexibility
Institution:
Florida Atlantic University
Thesis Advisor:
Dr. Joseph A. O'Kroy
Degree:
Master of Science
Year:
2003
Active-isolated (AI=ll) stretching was compared to static stretching (SS=8),
proprioceptive neuromuscular facilitation stretching (PNF= 10), and a control group
(C=9) at improving hamstring flexibility. Pre- and post-assessments of flexibility were
performed with a goniometer on the right leg. All subjects performed a warm-up on an
ergometer; after which, subjects in the stretching groups performed mode-specific
stretching of both hamstrings 4 days per week for 4 weeks. A significant increase was
found in flexibility after training for all stretching groups (p<0.05). However, there were
no significant differences in flexibility between groups (ANOV A: p>0.05). It is possible
the small number of subjects may have contributed to this finding.
iii
Table of Contents
List of Tables .................................................................................................. v
List of Figures ................................................................................................ vi
CHAPTER 1
INTRODUCTION .......................................................................................... 1
CHAPTER2
REVIEW OF LITERATURE ......................................................................... 3
CHAPTER3
METHODOLOGY ........................................................................................ 13
Subjects ....................................................................................................... 13
Study paradigm .............................................................................................. 13
Data analysis ................................................................................................. 16
CHAPTER4
RESULTS ...................................................................................................... 17
CHAPTERS
DISCUSSION ................................................................................................ 19
References ...................................................................................................... 30
iv
List of Tables
Table 1: Subject demographics ...................................................................... 23
v
List of Figures
Figure 1: Flexibility of the hamstrings before and after training .................. 24
Figure 2: Flexibility of the hamstrings before and after training ................... 25
Figure 3: AI subjects' initial flexibility versus changes in flexibility ........... 26
Figure 4: PNF subjects' initial flexibility versus changes in flexibility ........ 27
Figure 5: SS subjects' initial flexibility versus changes in flexibility ........... 28
Figure 6: C subjects' initial flexibility versus changes in flexibility ............. 29
CHAPTER!
INTRODUCTION
Flexibility is defined as a range of motion about a joint affected by bones,
tendons, muscles, and ligaments (3, 12, 15, 16). Other factors may influence
flexibility including age, gender, temperature, and type of activity performed (3). For
reasons unknown, flexibility has not been researched as thoroughly as other fitness
aspects, such as cardiovascular function and muscular strength even though flexibility is
also a health-related factor of fitness.
James Agre (1) was one of many researchers who have studied hamstring injuries.
He concluded that a lack of flexibility, in addition to the lack of strength and warm-up,
could lead to injuries. Many authors have agreed that injuries, such as muscle strains,
and lack of flexibility were closely related (3, 5, 6, 14, 24, 28, 32). Regardless of the
evidence illustrating the effectiveness of flexibility training, one group of researchers
disagreed with the notion that pre-exercise stretching decreased injury risk (25).
The flexibility/injury relationship is especially important for athletes who usually
require a greater range of motion than the average person (1 0). Unlike athletes, however,
naturally hyper-mobile individuals and pregnant females should not be allowed to stretch
excessively due to their increased risk of injury (9). Hyper-mobile individuals can easily
overstretch because of the natural laxity of their connective tissue. Pregnant females can
also overstretch as an increase in the hormone called relaxant increases the laxity of their
joints (9).
Results of studies focusing on flexibility have been inconsistent, but the
understanding of its importance has grown extensively (2, 3, 5, 6, 7, 8, 10, 19, 27, 29).
Regardless of the strength of the muscular system and efficiency of the cardiovascular
system, movement would not occur without flexibility. Flexibility can be improved and
maintained by stretching. The main types of stretching include static (SS), ballistic,
proprioceptive neuromuscular facilitation (PNF), and a more contemporary flexibility
training called active-isolated stretching (AI). Ballistic stretching has fallen out of favor
due to the associated high injury rate (5, 12, 30). SS, however, is easily taught and is
relatively safe. PNF has also been found to be quite effective, but is more complex and
requires more instruction. The AI stretching method, like PNF, includes muscle
contraction, but tries to prevent the activation of the stretch reflex. This study
hypothesized AI would be more effective than the other stretching methods in increasing
the flexibility of the hamstrings.
2
CHAPTER2
REVIEW OF LITERATURE
The understanding of the mechanics of flexibility is severely lacking. The
literature regarding flexibility is sparse compared to cardiovascular research. Flexibility
research also yields inconsistent results (2, 3, 5, 6, 7, 8, 10, 19, 27, 29). In addition, only
several theories are related to flexibility. The theory of muscular relaxation and muscular
elongation both attempt to explain how flexibility operates. Nevertheless, before
exploring the theories of flexibility, it is necessary to understand the fundamental
structures that are involved in flexibility.
The basic muscle fiber is composed of myofibrils. Myofibrils, in turn, are
composed of sarcomeres. A sarcomere can be described as a functional unit of a muscle.
The sarcomere has a boundary called a Z-line at each end. Within the sarcomere lies the
actin (thin filament) and the myosin (thick filament) (2). The actin and myosin are the
focus of the two-filament model, which is used to explain muscle contraction and
relaxation. The two-filament model has since given way to a three-filament model. The
third filament, called titin, had generally been ignored until the early 1990's because its
role in muscle function was unknown. Titin is now understood to attach the myosin
filament to the actin filament (18). Together, the titin filaments, actin filaments, and!bridges make up the 1-band and contribute to the striated appearance of muscle. The 1band, also known as the isotropic band, appears as a lighter section of a sarcomere and is
so named because light passes through at the same velocity in all directions.
3
Additionally, the A-band, also known as the anisotropic band, is a darker section
of the sarcomere and is composed of the myosin and actin filament overlap. The A-band
is so named because light does not pass through at the same velocity in all directions.
The H-zone is at the center of the A-band and appears as a lighter portion ofthe
sarcomere because it is found between the ends of the filaments. TheM-line is at the
center of the H-zone and, due to the parallel arrangement ofM-bridges, is a darker
portion of the sarcomere (2).
As mentioned previously, titin connects the myosin to the actin. Although the
existence oftitin has been suspected since the 1950's, it was not significantly
incorporated into the two-filament model because the function oftitin was unclear (18).
Currently, titin has two known functions, it produces resting tension for a muscle at its
relaxed length, and titin may also aid in centering the myosin to the middle of the
sarcomere (11 ). In addition, titin is flexible because it extends along with the sarcomere
when stretched. Thus, extensibility is not the exclusive domain of the actin-myosin
crossbridges. It has been proposed that, within a sarcomere, titin must be folded in some
manner. When a stretch is initiated, the portion oftitin between the Z-line and the
myosin unfolds and becomes a major contributor to the lengthening of a sarcomere (2).
The muscle' s ability to stretch lies in its viscoelastic properties. Taylor et al. (30)
explored the viscoelastic characteristics of the muscle-tendon unit. Since muscle behaves
viscoelastically, it has both viscous and elastic properties. Viscous properties are
described as having time and rate-changing characteristics. In other words, the relative
amount of deformation is directly proportional to utilized forces. Elastic properties can
be described as a change in length directly proportional to utilized forces.
4
by its own receptors. A muscle receives both excitatory and inhibitory impulses when
stretched. If a stretch is held for a longer period of time, the inhibitory impulses sent
from the GTOs eventually override the excitatory impulses causing relaxation. The
muscle spindles cause the initial reactions of the stretch reflex, but the GTOs' impulses
eventually dominate the weaker impulses sent by the muscle spindles. This protection
mechanism may prevent injury from reflex contractions that are caused by excessive
stretching (26).
On the other hand, reciprocal inhibition occurs when a stretch of the agonist
muscle causes the relaxation of the antagonist muscle. The antagonist muscles'
motomeurons are inhibited by afferent signals when the agonist muscles' motomeurons
receive excitatory signals (26). To clarify, autogenic inhibition and reciprocal inhibition
constantly receive both excitatory and inhibitory signals from the afferent nerves. The
ratio of the two types of signals determine whether motomeurons will be excited or
inhibited (26).
With knowledge of basic structures and functions of musculature, theories of
flexibility can now be explored. Currently, several theories attempt to elucidate the
function of flexibility.
The theory of muscular relaxation states that muscles will relax when there are no
longer any nerve impulses being received. Physically, the passive occurrence of
relaxation is the restoration of the elastic components of muscle that returns the
myofibrils to their initial lengths. However, chemical explanations of muscular
relaxation are not completely understood. It is hypothesized that during relaxation, the
calcium ions are returned to the sarcoplasmic reticulum as the calcium-troponin
7
Another concept pertaining to the flexibility properties of muscle-tendon units
included stress relaxation. Stress relaxation, also known as autogenic inhibition, occurs
when the force or stress declines in a viscoelastic substance that is stretched (30) and
held, such as that which occurs during SS. Stress relaxation occurs with the aid of stretch
receptors. Stretch receptors include muscle spindles and Golgi tendon organs (GTO).
Muscle spindles are located in the tendons and muscles and are considered to be the
primary stretch receptors. Muscle spindles are found in particularly high concentrations
in muscles that require fine motor control. The two types of muscle spindles are
intrafusal and extrafusal fibers. Intrafusal fibers are located within muscle fibers while
extrafusal fibers are located along the outside of the muscle fibers. Intrafusal fibers can
be further distinguished as nuclear bag and nuclear chain fibers. Nuclear bag fibers are
located near the center of the intrafusal fibers and have many nuclei bundled together.
Striated contractile filaments are located near the ends of the nuclear bag fibers. Nuclear
chain fibers are shorter than nuclear bag fibers and only have one row of nuclei laid out
in series. The ends of the nuclear chain fibers are also made of striated contractile
filaments, and all fibers insert into both ends of the connective tissue (2).
The innervation of muscle spindles is a complicated process. There are group Ia
afferent, called primary endings, and group II afferent, called secondary endings. In this
instance, afferent refers to the transduction of a signal from the spindles to the central
nervous system. Group Ia fibers, sensitive to stretch and rate, are branched into spiralshapes that connect nuclear bag and nuclear chain fibers. Group II fibers, sensitive
mainly to stretch, resemble small branch-like arms that mainly contact nuclear chain
fibers. The motor supply to muscle spindles consists of several types of y-motor axons.
5
Nuclear bag fibers have plate endings from dynamic y-motor axons while nuclear chain
fibers have plate endings from static y -motor axons (4).
GTOs are another type of stretch receptor. GTOs are almost always found in the
muscle-tendon (aponeuroses) junctions, and, in mammals, GTOs are encapsulated
possibly to increase sensitivity to stimuli (20). GTOs are arranged in series with muscle
fibers as opposed to the parallel orientation of the muscle spindles.
Muscle spindles have been studied more extensively than GTOs; hence, our
greater understanding of the former. Nevertheless, it is now understood that GTOs are
more responsive to muscular contraction forces rather than passively generated forces
(20). Both the muscle spindles and GTOs work together when a muscle is stretched.
Stretching causes a muscle to increase the frequency of impulses transmitted from the
muscle spindles to the spinal cord. The transmission causes the muscle spindles to
increase the frequency of motor impulses returning to that same muscle causing a
reflexive resistance to the stretch (26). Muscle spindles function by detecting a stretch as
muscles contract. The stretch causes the spindles to depolarize creating a generator
potential. Stronger generator potentials are produced with greater stretch. An action
potential results if the depolarization reaches threshold, which allows the amount and rate
of stretch to be detected (2). On the other hand, excessive tension within the muscle
activates the GTOs whose sensory impulses are carried back to the spinal cord. The
impulses from the GTOs have an inhibitory effect on the motor impulses returning to the
muscle, thus causing the muscle to relax (26).
The concept of autogenic inhibition is based on the function of the muscle
spindles and GTOs. Autogenic inhibition occurs when a muscle contraction is inhibited
6
structures separate. This results in the detachment of the actin-myosin structures, which
allows the fibers to return to their resting lengths (2).
A second theory, the theory of muscular elongation, states muscle fibers are
unable to stretch by themselves. Muscle fibers rely on outside forces such as the force of
antagonist muscles, movement, gravity, or a force provided by some other part of the
body or by another person to be stretched. Theoretically, the length to which a muscle
can be stretched can be determined by the extent the sarcomeres can be stretched. The
maximum length a sarcomere can be stretched without rupturing was found to be about
3.5 J..UD. A 50% or greater increase in length may occur from resting state since the
resting length of a sarcomere is 2.3 j..lm (2). The stretching portions of SS and PNF may
be explained by either of the two aforementioned theories. However, in-depth
hypotheses for the function ofPNF are few.
Several methods of PNF exist, but all of its incarnations include contraction and
relaxation of certain muscles preceding a stretch. PNF encompasses additional
techniques that aid in stretching, such as facilitatory and inhibitory techniques. Each
technique cannot exist without the other because as an antagonist muscle is relaxing, an
agonist muscle is facilitated. Facilitatory techniques are intended to increase
motomeuron excitability and recruitment. Inhibitory techniques, on the other hand,
commence hyperpolarization of motomeurons; in other words, fewer motomeurons are
actively discharged (26). Although it may not be as simple as reciprocal inhibition, an
additional hypothesis exists about how a stretch is achieved utilizing PNF. Moore and
Hutton (21) concluded the discomfort associated with the stretch of a muscle might be
masked by the contraction of the agonist muscle. In other words, the subjects did not
8
associate stretching discomfort with the hamstrings, but rather the quadriceps; thereby,
allowing subjects to increase the rate of stretch.
Even fewer explanations exist for the function of AI. Surprisingly, only one
published study investigating AI stretching was found. This study, by Mattes (19),
simply chronicled the different methods of flexibility and touted the advantages of AI.
For reasons unknown, AI has found more acceptance in the popular press and the
commercial fitness centers than in academia. The main difference between AI and the
other more established stretching techniques is the length of time a stretch is held. The
creator of this technique believed a stretch should only be held for 2 seconds; as opposed
to the 20-30 second hold for the other techniques, in order to bypass the stretch reflex
(19).
The stretch reflex is a protective mechanism governed by the central nervous
system. First, the stretch is received by the muscle spindles and golgi tendon organs.
Intemeurons, connecting the motomeurons with the sensory neurons, integrate the signals
in the spinal cord. The signals then proceed back to the muscles causing them to contract
to prevent overstretching. This is the primary reason why ballistic stretching is no longer
recommended. The stretch reflex can prevent a good stretch from occurring and cause
injury (4); however, it is unknown if the stretch reflex can be bypassed if the stretch is
only held for 2 seconds.
Static stretching has been the hallmark of flexibility training, but even this method
yields contrary results when studied. The following studies chronicle both the benefits
and the drawbacks ofSS. Two studies by DePino, Webright, and Arnold (6) and
Worrell, Smith, and Winegardner (33), explored the effectiveness of static stretching, but
9
they required that subjects lack 20 degrees of full extension in the hamstrings. The
limitation was imposed to ensure participants would respond well to their stretching
program. One group of investigators (6) consequently discovered SS was indeed
effective. However, the researchers also stated if an athlete waited more than three
minutes to begin the event, their gains in flexibility would be lost. Other studies
questioned the efficacy of flexibility to reduce injuries. Pope et al. (25) showed static
stretching did not reduce the risk of injury in new army recruits. Magnusson et al. (17)
utilized an isokinetic dynamometer to train subjects on hamstring flexibility. Their intent
was to investigate the long-term (three weeks) effects of stretching on human tissue
properties. The research group concluded the increased stretch tolerance allowed for the
increases in flexibility, but found no permanent changes in the viscoelastic properties of
human tissue. Additionally, Magnusson and another research team (16) suggested static
stretching caused no short-term effects on the viscoelastic properties of the hamstring
muscle group. Kubo et al. (14), however, declared static stretching increased elasticity
and decreases the viscosity of the muscles even though they did not study the time in
which gains in flexibility were maintained. The research group also concluded this effect
decreased the chance for injuries in athletes. Shellock and Prentice (28) listed injury
prevention as an effect of static stretching. They stated static stretching was indeed
effective based on previous research although they were unsure about the optimal amount
of time a static stretch should be held.
PNF has found many proponents. Sady, Wortman, and Blanke (27) found that
PNF was superior when compared to the other types of flexibility training, including
static and ballistic stretching. Nevertheless, they were unsure about the optimal amount
10
of time and sets necessary to obtain the best results. Although they only compared the
ballistic method against PNF, a Swedish research group confirmed these findings (31 ).
In terms offrequency, Wallin et al. (31) found stretching only once a week was enough to
maintain flexibility, while stretching more frequently increased flexibility. In addition,
Osternig et al. (23) revealed the Stretch-Relax method ofPNF was safer than the other
methods ofPNF even though it resulted in 3-6% less gains. Osternig et al. (1990)
conducted another study that observed the variances between the different methods of
PNF (24). Although they discovered the Agonist-Contract-Relax method increased
flexibility the most, they also stated that different athletic populations might benefit more
from the other PNF techniques. In addition, Moore and Kukulka (22) supported PNF' s
effectiveness, but stated it should be done quickly since post-contraction inhibition
ceased quickly. Moreover, Holt, Travis, and Okita (1 0) conducted a much earlier
investigation revealing the isometric contraction of the agonist soon followed with a
contraction of the antagonist (IA-CA), another method ofPNF, produced more
significant gains in flexibility than either ballistic or static stretching. Later, IA-CA PNF
came to be known as Contract-Relax-Antagonist-Contract (CRAC-PNF) and was studied
by Etnyre and Abraham (7). The researchers stated the CRAC method was the most
effective for increasing ankle dorsiflexion.
Correspondingly, Etnyre and Lee (8) agreed the CRAC-PNF method was more
effective in increasing shoulder extension and hip flexion than both the static and
contract-relax method ofPNF.
A few studies manifested results contrary to most flexibility studies. One of these
studies found static, dynamic, and PNF stretching could all produce significant increases
11
of flexibility in the hamstring-gastrocnemeus muscles (15). However, a later study
conducted with only the static and PNF methods manifested no significant improvements
in hamstring flexibility (33). More surprisingly, another group of researchers (29)
concluded pelvic positioning (anterior) was more important than either the static or PNF
stretching method. Hence, the various studies mentioned had just as many varied results.
This proposed study investigated the effectiveness of AI stretching against those of SS,
PNF stretching, and a control group.
12
placed vertically at the hips. The vertical PVC apparatus was used to guide the subject in
maintaining a vertical thigh position 90 degrees of flexion at the hip throughout the
flexibility measurement testing. The pretest flexibility measurement was then conducted
by taping a goniometer to the lateral side of the right knee. Subjects were instructed to
extend the right leg at the knee three times, to ascertain the greatest initial hamstring
flexibility. The left leg remained straight and flat on the mat throughout the assessment.
Subjects who qualified during the pretest were randomly assigned, using a
random number table, to one of the four following groups: control (C), static stretch (SS),
proprioceptive neuromuscular facilitation (PNF), and active-isolated stretch (AI). All
subjects were instructed to warm-up on a cycle ergometer for five minutes at 25 watts
prior to all stretching sessions. Additionally, all subjects were informed that no more
than 2 missed stretching days were allowed for the duration of the study to maintain
flexibility.
Subjects assigned to the control group were instructed not to perform any of the
following methods of stretching. However, if subjects in the control group were
stretching prior to participating in the study, they were instructed to maintain their
stretching program. Control subjects went to the testing lab 4 times per week for 4 weeks
to perform the cycle ergometer warm up only.
Those assigned to the SS group (n=8) were taught to stretch in a standing position
with the right leg extended. The heel of the right foot was positioned on the edge of a
table with approximately 90 degrees of hip flexion. Subjects then bent at the waist until
they felt a stretch in the hamstrings with a slight level of discomfort, but no pain.
14
Subjects performed 4 repetitions of 20 seconds duration for each stretch, with 5 seconds
rest between stretches. This procedure was performed 4 days per week for 4 weeks.
Subjects assigned to the PNF group were also taught to stretch in a standing
position with the right heel extended on the edge of a table with approximately 90
degrees of hip flexion. These subjects maximally contracted their right hamstring
isometrically for 5 seconds. They then relaxed for 5 seconds after which they contracted
their right quadriceps isometrically for 5 seconds. Afterwards, they bent at the waist until
they felt a stretch in the hamstring, but no pain, for 20 seconds. Subjects performed 4
repetitions of the above sequence. This procedure was performed 4 days per week for 4
weeks.
The AI group was instructed to lay supine on a stretching table with the left leg
bent at the knee and foot flat on the table. AI stretching was performed as instructed in
reference #19. During exhalation, the right leg was then raised as high as possible by
contracting the quadriceps muscles. The principal investigator (PI) assisted in the
maintenance of the extension of the right leg by placing one hand just above the patella
and the other behind the heel without assisting in elevating the leg. Each stretch lasted 2
seconds, after which the leg was returned to rest, flat on the table after each stretch. Four
sets often contractions with five second' s rest between sets were performed. The
frequency of stretching sessions was also 4 days per week for 4 weeks.
In order to observe changes in flexibility, a post-test was conducted in the same
manner as the pretest after the 4-week duration of flexibility training for all subjects in all
groups. Pre- and post-test measurements for flexibility were taken only on the right leg
even though the stretch training was conducted on both legs. Stretching the left leg was
15
an additional benefit for the subjects. In addition to the pre- and post-test measurements,
height, weight, age, gender, and activity level were recorded.
Data Analysis
After collecting the data from both pre- and post-testing, a repeated-measures
ANOV A (p>0.05) was utilized to observe for changes in flexibility of the hamstrings.
Significant differences were further analyzed using a dependent t-test to determine group
pre- and post-test differences. Further analyses were completed to observe correlations
between initial flexibility and changes in flexibility. All data were mean± standard
deviation (SD).
16
CHAPTER4
RESULTS
A total of 38 subjects qualified and were randomly assigned to one of 4 groups
(AI=11; SS=8; PNF=10; C=9). Twenty-four subjects did not qualify for the study due to
their hamstrings being too flexible and six subjects were dropped due to excessive
absences. The subjects who were dropped missed more than two stretching sessions and
could not continue in the study due to lack of consistent stretching. The data indicated a
significant increase in flexibility for all stretching groups after training (see Figure 1).
The following results are the changes in flexibility of the hamstrings for all groups after
their 4-week training sessions utilizing a paired T-test. Before stretching, the PNF
group's pre-test results were 144.5°±7.9. However, after their stretch training, their posttest results showed an increase to 157.9°±12.5, P=0.0007. Similarly, the AI group' s
initial flexibility measurements were 149.3°±8.7; while their post-test results showed a
small increase to 156.4°±7.6, P=0.0123. Correspondingly, the initial flexibility of the SS
group was 146.0°±8.8. This group showed an increase of flexibility to 156.6°±15.0,
P=0.0397. Finally, the control group's initial flexibility was 151.5°±3.9. The group's
changes of 154.3°±6.9, P=0.1710 were small. There was, however, no significant
difference between each of the flexibility training groups (see Figures 1 and 2).
Likewise, there were no significant differences in initial flexibility between groups
17
quadricep muscles caused a relaxation of the hamstrings which could have aided their
gains in flexibility (26).
Nevertheless, the fact that SS and PNF groups required subjects to stand and place
their heels on the edge of a table may have been a disadvantage. A few subjects were so
inflexible that the position made it difficult for them to acquire the anterior pelvic tilt
position. Sullivan, DeJulia, and Worrell (29) stated the anterior pelvic tilt position was
more important than the actual stretching method, but the effect of anterior pelvic tilt
position was not taken into account in the current study.
Moore and Hutton (21) discovered subjects in the PNF-CRAC group achieved the
greatest flexibility when compared toSS and another PNF group that only utilized the
contraction and relaxation portion of the technique. Their study, however, included the
use of electromyography (EMG) in order to observe the electrical activity that occurred
during stretching. It was revealed the PNF-CRAC group had the highest EMG activity;
yet, the method of stretching yielded the greatest increases in range-of-motion. A
popular notion of stretching is that a muscle has to be relaxed in order to stretch, but
Moore and Hutton's study (21) showed increases in flexibility were possible even during
high levels of muscle recruitment.
Similarly speaking, Magnusson et al. (17) detected no changes in the viscoelastic
properties of the hamstring muscles after being stretched for three weeks. A change in
tissue properties did not occur, because there was no decrease in force at the same joint
angle when measured with an isokinetic machine. The research also suggested the
increased range of motion was a result of increased stretch tolerance. However, the
20
(p>0.05). At the inception of the study, it was postulated that the less flexible subjects
would show greater increases in flexibility, while the more flexible subjects would show
less increase in flexibility. However, none of the groups manifested a significant
correlation between the initial hamstring flexibility and the increase in flexibility
measured after training (see Fig. 3-6).
18
CHAPTERS
DISCUSSION
The significant finding of this study was that an increase in the flexibility of the
hamstrings was found within each of the stretching groups after training. Although SS
and PNF manifested greater increases in flexibility than AI, no between-group
differences were indicated. A possibility for the lack of significance was the low number
of subjects in each group. The large individual differences in stretch performance also
possibly contributed to the lack of significance. An increase in the number of subjects
would probably also increase the power of the study. Although the study by Worrell,
Smith, and Winegardner (33) did not have the same test procedures, they had the similar
results. Utilizing both SS and PNF stretching with each of their 19 subjects, the
researchers found no significant increases in flexibility even though there was a trend
towards significance. They also acknowledged the small sample size and large subject
variation in stretch performance as the main obstacles to reaching significance.
In this current study, subjects randomly assigned to either the SS or PNF group
generally showed good improvement in their hamstring flexibility. SS takes advantage of
autogenic inhibition concept. It is possible the inhibitory impulses from the GTOs caused
the motomeurons to reduce their activity affecting relaxation when a stretch was held for
an extended period of time. The PNF group also took advantage of autogenic inhibition,
but the technique had an additional advantage of employing reciprocal inhibition when
subjects contracted their quadricep muscles. The contraction of the
19
source of the altered stretch tolerance remains unknown. The increased flexibility of the
hamstrings in this current study may also have been due to an increased stretch tolerance.
The effectiveness of increasing hamstring flexibility with AI stretching was minimal in
this study. The procedure for stretching the hamstrings utilizing AI was followed as it
was written in Mattes' article (19). Another factor that may have limited the efficacy of
AI was that the stretch did not utilize the bodyweight of the subject. In both the SS and
PNF groups, subjects were instructed to bend at the waist to attain a stretch in the
hamstrings. The bending action of the hips allowed the weight of the upper torso to
contribute to the stretch. Subjects stretching utilizing AI only had the weight of one leg
to contribute to the stretch.
Additionally, a type of stretching called Active-Assisted (AA) stretching existed
prior to the creation of AI. This type of stretching required that subjects contract their
quadriceps until they could no longer raise it further. As the range of motion was
achieved, the leg was further stretched by a partner. AA is very similar to AI, and
Iashvili' s study ( 13) observed a higher correlation of sports achievement and active
flexibility even though active range of motion (ROM) measurements were lower than
passive ROM' s.
The combination of passive and active ranges of motion makes up the total range
of motion. If active stretching is practiced, then active flexibility will be developed. The
same reasoning applies to passive stretching. The range in which the subject' s joint can
be pushed in a passive stretch is called the zone of passive adequacy. The range in which
their joint can no longer be pushed is called the zone of passive inadequacy. Active
stretching also has the same zones, but they are called the zones of active adequacy (e.g.
21
how high subjects can lift their leg utilizing their own strength) and active inadequacy
(e.g., the area in which subjects can not lift their leg on their own.) The chance for injury
increases if there is a great difference between the ranges of passive and active ranges of
motion. Strength exercises performed in the active inadequacy zone may aid in the
reduction of injuries (13). According to Mattes (19), AI is completely an active stretch so
subjects would not get the benefit of an outside force pushing their stretches further.
Since overall flexibility can be improved by both passive and active stretches, AI may be
more beneficial if a passive component is added.
The current study revealed stretching was indeed beneficial if increased flexibility
of the hamstrings is desired. However, choosing the most effective stretching method is
difficult since there was no difference between groups. At this juncture, AI stretching
was not shown to be superior to the other methods of stretching at increasing hamstring
flexibility. Nevertheless, stretching may indeed enhance the flexibility of the hamstrings
regardless of technique utilized. Moreover, most flexibility studies, including this study,
are further complicated by the fact that individual stretching effort cannot be measured.
As a result, some subjects in the same stretching group yielded very different results.
Future research should investigate methods to gauge individual stretching effort and
include a larger number of subjects.
22
Table 1 . Subject demographics
Group
Gender
Age
Height
Weight
Initial Flexibility
(years)
(em)
(kg)
(mean ±SO)
AI
M=4, F=7
23.3 ±3.8
172.5 ±8.6
72.5 ±15.5
7.1 ±7.8
PNF
M=5, F=5
25.6±6.9
172.7 ±8.9
77.1±16.1
13.4±8.4
ss
M=3, F=5
25.6±6.5
167.1 ±9.7
72.8±14.4
10.6±11.9
c
M=5 , F=4
25.6±4.3
172.5 ±6.6
74.2±13.4
2.7±5.5
N
w
All data are mean ± SO.
AI= Active-isolated stretching; PNF= Proprioceptive Neuromuscular Facilitation stretching; SS= Static stretching;
C= Control; None of the initial flexibility measures were significantly different between groups (p>0.05).
175
I
(B
ss
•
PNF
II
170
165
,.......
160
ell
Q)
Q)
II
II
&AI
II rn c
I
T
~ 155
j
*
*
*
B
N
~
·--·0'>l
,D
-
150
I
Q)
~
145
*
*
*
140
135
I
PreTraining
PostTraining
Figure 1. Flexibility of the hamstrings before and after training. All training groups (*) significantly increased
hamstring flexibility post training (p<0.05). The control group was not different post training (p>0.05). There
were no significant differences post training. SS= Static stretching; PNF= Proprioceptive Neuromuscular
Facilitation stretching; AI= Active-Isolated stretching; C= Control.
Figure 2. Flexibility of the hamstrings before and after training. All training groups significantly
increased flexibility after training(*; p<0.05). There was no significance between groups after
training. PNF= proprioceptive neuromuscular facilitation; C= control; SS= static stretching; AI=
active-isolated stretching
165
160 -1
fll
155
Go~
Go~
~
•
~
•
•
•
...............
•
a..
~
,e.
....Go~
"CC
I
150
....==
r=
,.Q
~
-.....-...
Go~
c
N
0'1
145
-
~
=
.....
•
...............
-0.58, P= 0.062
•
I
• AI subjects' initial
flexibility vs. changes in
flexibility
•
I
140
•
135
•
130
-15
-10
-5
0
5
10
15
20
Changes in flexibility (degrees)
Figure 3. AI subjects' initial flexibility versus changes in flexibility. Initial flexibility was not correlated to the changes
in flexibility with training for the AI group.
170
165
160
•
155
~
~
...
~
150
.....-...
--....
....
~
N
--l
,e.
145
.c
140
·~
•
•
•
~
~
•
•
•
•
•
~
~
135
~
....
=
~
130
r= 0.161 , p= 0.657
•
•
125
120
PNF subjects' initial
flexibility vs. changes in
flexibilitv
115
110
-5
0
5
15
10
Changes in Flexibility (degrees)
20
25
Figure 4. PNF subjects' initial flexibility versus changes in flexibility. Initial flexibility was not correlated to the
changes in flexibility with training for the PNF group.
30
170
165
-
160
~
•
•
~
~ 155
~
"CC
-t> 150
·-·- 145
·--" 140
..·-= 135
,.Q
•
~
~
N
QC
•
•
~
;:
•
•
130
r=
0.022, p= 0.96
•
SS subjects' initial
flexibility vs. changes in
flexibil!!Y_
125
120
-10
-5
0
5
10
15
20
25
30
35
Changes in Flexibility (degrees)
Figure 5. SS subjects' initial flexibility versus changes in flexibility. Initial flexibility was not correlated to the changes
in flexibility with training for the SS group.
165
I
r= 0.052, p= 0.894
•
160 '
-
C subjects' initial flexibility vs .
changes in flexibility
"'-!
•
~
i....
155
,e.
-........
-
j
•
•
I
,.Q
~
~
N
\0
-.....-...==
~
150
1
•
•
•
•
•
"""'
145
•
I
140+-------~--------~------~--------~------~~------~------~
-15
-10
-5
0
5
10
15
Changes in Flexibility (degrees)
Figure 6. C subjects' initial flexibility versus changes in flexibility. Initial flexibility was not correlated to the changes in
flexibility with training for the C group.
20
References
1. Agre, J. C. Hamstring injuries: Proposed aetiological factors, prevention, and treatment.
Sports Med. 2: 21-33, 1985.
2. Alter, M. J. Science ofFlexibility, (2nd ed.) Champaign, IL: Human Kinetics, 1996.
3. Anderson, B. and Burke, E. R. Scientific, medical, and practical aspects of stretching.
Clinics Sports Med. 10: 63-86, 1991.
4. Berne, R. M., and Levy, M. N. Physiology, (3rd ed.) St. Louis, MO: Mosby Year Book,
1993.
5. Brandy, W. D. and Irion, J. M. The effect of time on static stretch on the flexibility of the
hamstring muscles. Phys. Ther. 74: 845-850, 1994.
6. DePino, G. M., Webright, W. G., and Arnold, B. L. Duration of maintained hamstring
flexibility after cessation of acute static stretching protocol. J Athletic Training. 35: 56-59,
2000.
7. Etnyre, B. R. and Abraham, L. D. Gains in range of ankle dorsiflexion using three popular
stretching techniques. Am. J Phys. Med. 65: 189-196, 1986.
8. Etnyre, B. R. and Lee, E. J. Chronic and acute flexibility of men and women using three
different stretching techniques. Res. Q. Exerc. Sport. 59: 222-228, 1988.
9. Fredette, D. M. Exercise recommendations for flexibility and range of motion. ACSM's
Resource Manual for Guidelines for Exercise Testing and Prescription, (4th ed.) New York,
NY: Lippincott, Williams, and Wilkins, 2001.
30
10. Holt, L. E., Travis, T. M., and Okita, T. Comparative study ofthree stretching techniques.
Percept. Motor Skills. 31: 611-616, 1970.
11. Horowits, R. Passive force generation and titin isoforms in mammalian skeletal muscles.
Biophys. J. 61 : 392-3 98, 1992.
12. Hubley-Kozey, C. L. and Stanish, W. D. Can stretching prevent athletic injuries? J.
Musculoskeletal Med. 1: 25-32, 1984.
13. Iashvili, A. V. Active and passive flexibility in athletes specializing in different sports.
Soviet Sports Rev. 18: 30-32, 1983.
14. Kubo, K., Kanehisa, H., Kawakami, Y., and Fukunaga, T. Influence of static stretching on
viscoelastic properties of human tendon structures in vivo. J. Appl. Physiol. 90: 520-527,
2001.
15. Lucas, R. C. and Koslow, R. Comparative study of static, dynamic, and proprioceptive
neuromuscular facilitation stretching techniques on flexibility. Percept. Motor Skills. 58:
615-618, 1984.
16. Magnusson, S. P., Aagaard, P., and Nielson, J. J. Passive energy return after repeated
stretches ofthe hamstring muscle-tendon unit. Med. Sci. Sports Exerc. 32: 1160-1164, 2000.
17. Magnusson, S. P., Simonsen, E. B., Aagaard, P., Sorensen, H., and Kjaer, M. A mechanism
for altered flexibility in human skeletal muscle. J. Physiol. 497: 291-298, 1996.
18. Maruyama, K. Connectin, an elastic protein from myofibrils. Int. Rev. Cytology. 104: 81115, 1986.
19. Mattes, A. L. Active-isolated stretching. J. Bodywork Movement Therapies. 1:28-33, 1996.
20. Moore, J. C. The golgi tendon organ: A review and update. Am. J. Occup. Ther. 38:227236, 1984.
31
21. Moore, M.A. and Hutton, R. S. Electromyographic investigation of muscle stretching
techniques. Med. Sci. Sports Exerc.. 12: 322-329, 1980.
22. Moore, M.A. and Kukulka, C. G. Depression ofHoffinan reflexes following voluntary
contraction and implications for proprioceptive neuromuscular facilitation therapy. Phys.
Ther. 71 : 321-329, 1991.
23. Osternig, L. R., Robertson, R., Troxel, R., and Hansen, P. Muscle activation during
proprioceptive neuromuscular facilitation stretching techniques. Am. J Phys. Med. 66: 298307, 1987.
24. Osternig, L. R., Robertson, R., Troxel, R., and Hansen, P. Differential responses to
proprioceptive neuromuscular facilitation stretching techniques. Med. Sci. Sports Exerc. 22:
106-111 , 1990.
25. Pope, R. P, Herbert, R. D., Kirwan, J.D., and Graham, B. J. A randomized trial of
preexercise stretching for prevention of lower-limb injury. Med. Sci. Sports Exerc. 32: 271277, 2000.
26. Prentice, W. E. A comparison of static stretching and PNF stretching for improving hip joint
flexibility. Athletic Training. 18: 56-59, 1983.
27. Sady, S. P., Wortman, M., and Blanke, D. Flexibility training: Ballistic, static, or
proprioceptive neuromuscular facilitation? Arch. Phys. Medical Rehab. 63: 261-263, 1982.
28. Shellock, F. G. and Prentice, W. E. Warming-up and stretching for improved physical
performance and prevention of sports-related injuries. Sports Med. 2: 267-278, 1985.
12
29. Sullivan, M. K., DeJulia, J. J., and Worrell, T. W. Effect of pelvic positioning and stretching
method on hamstring muscle flexibility. Med Sci. Sports Exerc. 24: 1383-1389, 1992.
30. Taylor, D. C., Dalton, J. D., Seaber, A. V., and Garrett, W. E. Viscoelastic properties of
muscle- tendon units: The biomechanical effects of stretching. Am. J Sports Med. 18: 300309, 1990.
31. Wallin, D., Ekblom, B., Grahn, R., and Nordenborg, T. Improvement of muscle flexibility:
A comparison between two techniques. Am. J Sports Med. 13:263-268, 1985.
32. Worrell, T. W. and Perrin, D. H. Hamstring muscle injury: The influence of strength,
flexibility, warm-up, and fatigue. J Orthop. Sports Phys. Ther. 16: 12-18, 1992.
33. Worrell. T. W., Smith, T. L. , and Winegardner, J. Effect of hamstring stretching on
hamstring muscle performance. J Orthop. Sports Phys. Ther. 20: 154-159, 1994.
33