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270 Carter et al
at least 1researcher reported that when stretching was incorporated as a
component of a fitness program, no attributable benefits were found.ll
Researchers agree that proprioceptive neuromuscular facilitation (PNF)
stretching techniques elicit maximal improvements in joint range of motion in the shortest amount of tirne.3,78,11,19,20
Three steps occur during PNF
stretching techniq~es*,~,~~:
The muscle is passively stretched, allowing the
stretch receptors of the muscle spindle to get used to the new, increased
range of muscle length; in the stretched position, the muscle contracts isometrically against resistance; this contraction activates the Golgi tendon
organs more than the passive stretch alone does and causes many of the
fast-twitch fibers of the muscle to fatigue. This fatigue makes it more difficult to produce a stretch reflex, and the muscle is passively stretched again
through the resulting increased range of motion. This passive stretch inhibits the muscle even more from causing a stretch reflex.
Based on the research on PNF stretching, many athletic teams have
adopted PNF stretching as part of their precompetition warm-up.21It is
believed that PNF stretching diminishes the stretched muscle's ability to
produce a stretch r e f l e ~ . ~Thus,
, ~ ~it~becomes
,~~
important to determine
whether PNF stretching inhibits involuntary movements that can be protective in nature.
The purpose of this investigation was to determine how a commonly
used PNF stretching technique affects the neuromuscular response of selected posterior thigh musculature during rapid elongation.
Materials and Methods
Subjects
Twenty-four women were recruited for participation (means:height = 167.27
cm; weight = 58.92 kg; age = 21.42 years; percent body fat = 18.41; body
mass index = 21.06 kg/m2). After an explanation of the study's purpose,
each subject signed an informed consent form approved by The University
of Mississippi's Institutional Review Board.
Preparatory Protocol
The subjects were prepared for electromyography (EMG) electrode placement. Any body hair in the area of electrode placement was removed.
Pregelled, silver-silver chloride bipolar surface electrodes (MedicotestA/
S, Rugmaken, Denmark) were placed according to the methods described
by Basmajian and D e L u ~ awith
, ~ ~ a center-to-center intereledrode distance
of 2.5 cm on the skin over the biceps femoris and semitendinosus muscle
bellies. A reference electrode was placed at the bony prominence of the
wrist. Muscle identification was achieved through manual muscle testing
using resistive knee flexion from the supine position. Once positioned
PNF and Rapid Muscle Lengthening 271
properly, the electrodes were not removed until the pretest and posttest
data had been collected. Subjectswere also fitted with standard stereo headphones, playing music from a radio, and a blindfold. The purpose of the
headphones and blindfold was to limit the subjects' awareness of their surroundings and to distract their attention so that the stretching stimulus
would be surprising.
Instrumentation
The analog signalswere converted to digital signals via an analog-to-digital
converter interfaced with a desktop computer. Both signals were collected
using Run Technologies DataPac 111Version 2.00 software (Laguna Hills,
Calif).Two channels of the Noraxon Telemyo (Scottsdale,Ariz) telemetered
EMG system (bandwidth = 10-2000 Hz; amplifier impedance < 10 flohm;
CMMR = 85 db; gain = 1000)were used to record the activity of the biceps
femoris and semitendinosus muscles. The EMG signals were sampled at
1000 Hz and processed, using passive demeaning and root-mean-square
smoothing (100-millisecond time constant), and notch filtered at 60 and
110 Hz. A third and separate time-matched channel was used to record
when the initiation of the stimulus occurred.
Involuntary Movement Assessment
As shown in Figure 1,the device used to create involuntary movement of
the lower leg was attached to the end of a treatment table. The device consisted of 2 two-by-fours extending the table by 17.25 in. A 1-in dowel rod
joined the 2 two-by-fours and could be adjusted at 3-in intervals, depending on each subject's femur length. The dowel rod was padded to assist in
subject comfort.An ankle cuff was attached to a cable, which fed through 2
frictionless pulleys and was attached to a circular 5-lb weight. The weight
rested in its initial position: on a pin attached to a solenoid that was stabilized and connected to 2 two-by-fours. The solenoid had a 20-ft-long
alternating-current (AC) cord with a flip switch and was fitted with an AC
plug. Just below the solenoid was a weight-drop trigger switch; when the
5-lb weight struck this trigger switch, a 5-V signal was sent to the computer. Using free-fall equations of motion, it was determined that the rapid
elongation of the posterior thigh muscles would be begin 0.482 seconds
after the trigger switch was activated by the falling weight.
Protocols
Testing consisted of 2 phases: a control group that participated in jerks
with no PNF stretching intervention and a treatment group that participated in jerks with the PNF intervention.
Each subject in the control group began by lying supine on the treatment table with her ischial tuberosity on the edge of the table. A padded,
272 Carter et a1
-
Figure 1
Subject in supine position with the weight supported by the solenoid.
adjustable dowel rod attached to an extensionbuilt on the table supported
the lower extremities. Both legs were placed over the dowel rod, which
was aligned with the popliteal space. The lower legs were then lowered to
form a 90" angle with the femur, measured by a handheld goniometer. The
ankle cuff was then attached to the subject's right leg. The weight was placed
on the pin, and the first trial was executed. After the weight was dropped
30 cm, and 5 seconds of baseline activity were recorded, the weight was
replaced on the pin and the procedure was repeated for the second and
third trials. The subject then rested approximately5 minutes, or the amount
that would be allowed for the treatment group to be stretched, before the
second bout of jerks was conducted.
The subjects in the treatment group experienced 3 jerks, a contract-relax
PNF stretch of the hamstring muscles, and then 3 more jerks. In order to be
stretched, each subject remained on the treatment table on which the instrumentation was built. The right leg was placed in a straight-leg knee
brace while the hip and left leg were secured to the treatment table using
seatbelts. Next, the right hamstring was passively stretched to the endpoint, shown through EMG activity, for 30 seconds to the count of a metronome. The subject was immediately instructed to try to push her leg back
to the starting position against resistance for 7 seconds. This procedure
was instantaneously repeated 2 more times, with no breaks between repetitions. Once the tests were completed, the seatbelts and straight-legbrace
were removed and the subject returned to the initial test position to coma1j
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274
Carter et al
that PNF stretching techniques diminish the muscular activity of the biceps femoris associated with the stretch reflex. These results are probably
best explained using the theories of muscle-spindle desensitization and
muscle size.
* PNF Group
u
I
I
I
Pre Test
Post Test
Figure 2 Average muscle activity of the semitendinosus between groups or across
pre- and post- values did not differ (F,,22 = .92; P = .02; +! = .04; P = .15).
"
1
0
Pre Test
Post Test
Figure 3 Average muscle activity of the biceps femoris. decreased as a result of PNF
stretching (F,,22 = 4.50; P = .04; +! = .17; P = .53). *Posttest PNF values were lower
PNF and Rapid Muscle Lengthening 275
Muscle-Spindle Desensitization
In order to understand this neurophysiologicalmechanism, it is important
to discuss the 2 proprioceptive bodies in the muscle: the muscle spindles
and the Golgi tendon organs (GTOs).Muscle spindles are found within the
muscle belly and provide information to the central nervous system (CNS)
regarding the absolute length and the velocity of the stretch in the skeletal
muscle^."^^,^^ The GTO is located in the tendon, close to the musculotendinous junction. The GTO responds as a feedback monitor and sends irnpulses to the CNS in response to t e n s i ~ n ? ~ , ~ ~
When stimulated, most commonlyvia a rapid stretch, the muscle spindle
signals the CNS to send excitatory efferent signals to the corresponding
muscle. The resulting contraction halts the intended muscle lengthening.
In contrast, via increased muscle tension, when the GTO is stimulated it
signals the CNS to send inhibitory signals to the corresponding muscles.10
Muscle spindles are desensitized when a muscle is held in a stretched position for a prolonged period of time. During this time the muscle spindles
habituate, signaling is reduced, and a greater muscle length can be obtained during the stretch. When desensitized,the muscle spindles' response
to a sudden stretch is diminished, as evidenced by little or no reflexive
contraction. This muscle-spindle desensitizationcould be beneficial because
it allows for the greatest gains in flexibility. It could also be detrimental
because it diminishes the protective stretch-reflex response. Researchers
have shown that muscle-spindle desensitizationoccurs during PNF stretching technique^?^,^^ Beyond that, Prentice1°demonstrated a reduced musclespindle response during the contract-relax PNF technique. This reduced
response is thought to be chiefly responsible for the immediate increase in
muscle length that is common during PNF stretches. The decrease in muscle
activity during the stretch reflex after a bout of contract-relax PNF found
in the current study is also indicative of muscle-spindle desensitization
and is congruent with these findings.
Other researchers have shown that average EMG muscle activity produced during PNF stretching techniques decreases, as we11.10,19,29,30
For example, researchers have assessed the H (Hoffman) and T (tendon tap) reflex responses after the isometric contractions of various PNF stretching
techniques and found diminished EMG activity during both response^.^,^^,^^
This body of research indicates that PNF causes a decreased excitability of
both the muscle spindle and the motor neurons immediately after an isometic contraction that lasts for approximately 10 seconds. Furthermore,
the depression of the reflexes is similar for voluntary contractions that vary
in duration from 1to 30 seconds.31
There is reason to believe that PNF stretching changes the functioning
of the intrafusal fibers.31There are 2 types of fiber that can modulate the
sensitivity of the muscle spindle. The dynamic fibers (primary effect on Ia
afferents) are larger and more prevalent in the muscle, transmit impulses
276 Carter et a1
quickly, and are strongly sensitized to dynamic stret~h.3~
These fibers are
called on when reflexive types of actions are req~ired.3l.~~
The static (types
Ia and 11)fibers regulate background Ia discharge and reduce the sensitivity of types Ia and I1fibers to dynamic stretch.32Assuming no independent
fusimotor activation, the 30-second passive stretch of the PNF technique
performed in this study should primarily alter the sensitivity of the static
However, because response to a stretch reflex depends on the dynamic fibers and a reduction in muscle activity was found, we conclude
that dynamic fibers were desensitized, as well?=
Muscle Size
The hamstrings, as a collective group of posterior thigh muscles, are capable of producing large amounts of force because they have long fibers
and intermediate physiologic cross-sectional
Nonetheless, the
muscles that make up the hamstrings are not all the same size. The long
head of the biceps femoris has a mass of 128zk 28 g and a length of 342 f 14
mm. The semitendinosus has a mass of 76.9 f 7.7 g and a length of 312 f 4
mm.33In addition, the biceps femoris has a greater cross-sectional area, a
smaller fiber length, and a lower fiber-length-to-muscle-lengthratio than
the semitendinosus does.33Depending on the size of a muscle, it can contain anywhere from 6 to 1300 muscle ~pindles.3~
There are more muscle
spindles to influence per unit area in the biceps femoris, because it is a
larger muscle than the ~emitendinosus.3~
Therefore, there is reason to believe that the sigrufrcant decrease in muscle activity between groups by
time shown in the biceps femoris and not the semitendinosus was because
the spindle desensitization had a greater effect.
Conclusion
The results of this study suggest that contract-relax PNF stretching techniques reduced the average EMG activity of the biceps femoris muscle associated with a stretch reflex. Explanations for these findings are rooted in
the process of muscle-spindle desensitization and the overall size of the
biceps femoris muscle. The desensitization of the muscle spindle that occurs
during the contract-relax PNF stretching technique can allow for a greater
risk of muscle and tendon injury to occur. Because the biceps femoris is a
larger muscle than the semitendinosus, there are more muscle spindles to
influence per unit area. This allows for greater effects of muscle-spindle
desensitization. This information is importantbecause it shows that stretching a large muscle, such as the biceps femoris, using the contract-relax
PNF stretching technique precompetition can be problematic, especially if
a reflexive adivity is warranted.
PNF and Rapid Muscle Lengthening 277
Acknowledgment
This research was part of the lead author's Master's thesis project, which was
completed at The University of Mississippi.
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