Proprioceptive
neuromuscular facilitation (PNF) is a form of stretching which relies on
reflexes to produce deeper stretches to improve flexibility. For this to occur,
the muscle must be stretched to its limit sending the inverse myotatic reflex, causing
the muscle to relax. There are three types of PNF stretch; hold-relax,
contract-relax, and hold-relax-contract. Hold-relax involves placing the muscle
in a stretched position and holding for a few seconds, isometric contraction
against the stretch and then relaxing before repeating the stretch deeper than
before. Contract-relax is similar to hold-relax expect the muscle is moving during
the contraction. Hold-relax-contract is also similar to hold-relax expect the
second stretch is active. the articles were found through a search using PubMed
and Embase under the search criteria of ‘proprioceptive neuromuscular facilitation’, ‘flexibility’, and ‘stretching’
being present in the title/ abstract and the study based on a clinical trial. A
total of 58 articles were yielded, articles were further excluded due to no
free access to the full article, the cohort of volunteers not being young
healthy adults and lack of relevance to the subject matter. A final total of
nine articles were selected for summarisation for the literature review.

Wicke
et al. (2013) compared self-administrated PNF to static stretching for changes
in range of motion. The subjects of the study were 19 college aged students
with no injuries to the hamstring muscle group, hip or back area in the past
year. The intervention lasted six weeks in total. A ten-minute warm up
consisting of walking at a self-directed pace before the outcome measures were
performed. Following the warm up the volunteers performed a sit-and-reach test,
following the standard protocols. The measurement was only recorded if the
stretch was maintained for five seconds. The angle of the hip was also measure
in the same position. A warm-up at the same pace as the first testing session
was performed at each training session. Both groups were asked to place a foot
on a chair approximately 50 cm in height, the static group held the stretch for
40 seconds and the self-PNF group performed a 15-second static stretch, a
10-second isometric hamstring contraction followed by a 15-second static re-stretch.
Both groups stretched twice a week performing 2 repetitions of stretches on
each leg. There was no statistical difference between the static stretching
group and the self-PNF stretching group, however the mean self-PNF group hip
range of motion increased by 6.2° v 0.6° in the
static stretching group. The results of the study imply that self-PNF can be
used in place of static stretching to improve flexibility of hamstrings and
that it is superior at increasing range of motion at the hip. As the study used
a small sample size and the volunteers were of college age the applications of
the study may not be able to be pasted onto an older person’s cohort of
patients.

The
previous year, Maddigan
et al. (2012) compared the effects of assisted and unassisted PNF to static
stretching. A group of 13 healthy adults volunteered to participant in the study
with no limitations of range of motion at the hip.

Puentedura
et al. (2011) compared the immediate effects of hold-relax PNF versus static
stretch on hamstring flexibility. The active knee extension (AKE) test was used
as an outcome measure for this study. A total of 30 subject participated in
this study and were randomly divided into two groups the PNF group or the
static stretching group. The volunteers left leg acted as the control for this
study. The subjects were excluded if they received a hamstring injury in the
past year, scored over 80° in the
AKE test or participated in a sport which involved hamstring stretches. The right
leg ROM was measured pre-stretch and post-stretch. The results of the study showed
there was no statistical significant difference between hold-relax PNF
stretching versus static stretching with p=0.782. However, both stretching
method did show a significant improvement in hamstring length (p=<0.05) immediately post stretching techniques. The study using the left leg as a control meant that the changes in the AKE test were internal within the exact same population. However it does not factor in innate differences of hamstring length previous to the participant volunteering in the study.   O'Hora et al. (2011) conducted a study in the same year as Puentedura et al. (2011) on the efficiency of static stretching versus PNF stretching on hamstring length. This study focused on the effect of one repetition of 30 seconds static stretching compared to one repetition of 6-second hamstring contract PNF stretching. A sample of 45 healthy university students with no known neurological or orthopaedic disorders participated in this study. The subjects were divided into three groups randomly, a static stretching group, a PNF group, and a control. The effeteness of the intervention was measured by passive ROM of knee extension in supine with the hip at 90°. The results show that there was a statistical difference (p=<0.05) in the increase of passive ROM for both stretching groups. The PNF stretching group demonstrated a significantly greater gain of passive knee extension over the static stretching group with the mean difference 4.27°. The results demonstrate the immediate positive effects of PNF compared to other stretching techniques in increasing passive knee extension. It also shows that repetitions are unnecessary to provide increased passive knee ROM.   Fasen et al. (2009)   Feland and Marin (2004) conducted a study to determine if the submaximal contractions in contract-relax PNF production similar levels of increase in hamstring flexibility as maximal voluntary isometric contractions (MVICs). A sample of 72 healthy college aged men volunteers for the study and were randomly divided into four groups. Group I; 20% of MVIC, group II; 60% of MVIC; group III; 100% of MVIC and group IV; the control group. Groups I-III performed three repetitions of contract-relax PNF at the relevant intensity for their group, with a ten-second rest between the contractions for five consecutive days. The subjects hamstring flexibility was measure before and after the intervention each day using a goniometer.  The results showed a significant change (p=<0.05) in hamstring flexibility in groups I-III compared to the control group, but insignificant differences between the treatment groups (p=0.06). This concludes that the use of maximal contracts is unnecessary to increase hamstring flexibly as using a submaximal force decreases the risk of injury to the hamstring with comparable gains in flexibility. The study does not inform the reader as to how the volunteers knew what effect the percentage of MVIC they were given was. This may have lead the volunteers to not completing their relative groups correctly.   Ozmen et al. (2017) studied the different effects of static stretching, proprioceptive neuromuscular facilitation (PNF), and kiniesio taping (KT) on muscle soreness and flexibility recoverly from Nordic hamstring exercise. The volunteers consisted of 65 female university students with no lower limb injury or neurological disorder. Participants were randomly assigned into four groups, PNF stretching, static stretching, KT and a control by being pulled out of a hat. Hamstring flexibility and muscle soreness measured at baseline, and at 24 hours and 48 hours' post intervention. Muscle soreness was tested by pressure algometry of the dominate leg and hamstring flexibility was measured by passive straight leg raises with a digital inclinometer. Static stretching was performed with five repetitions, for 30 seconds. The contract-relax method of PNF was used and repeated three times. The KT tape was applied to the hamstrings from origin to insertion at approximately 30% tension used. The study used the exercise protocol of Mediguchia et al. (2013) for Nordic hamstring exercise and preformed five sets of eight repetitions. The study showed that there was no difference (p>0.05)
in flexibility in any group from baseline to 48 hours’ post-exercise. The muscle
soreness was measured higher at 48 hours in the control group than the
intervention groups showing that it may influence muscle soreness.

Konrad et al. (2015)
investigated the influence of a six week PNF programme on ROM, passive
resistive torque (PRT) and maximum voluntary contraction (MVC) of gastrocnemius.
A cohort of 49 police cadets were involved in this study and were randomly
divided into a PNF stretching group and a control group. The volunteers were instructed
to maintain their current levels of activity. ROM was measured by an electric
goniometer, PRT was measure with an isometric dynamometer and MVC was measure
with the isometric dynamometer with the ankle in neutral. The PNF program was
under a contract-relax-antagonist-contract programme, five times a week, for
six weeks and performed independently. The results state that mean ROM increased
by 2° (p=0.02)  in the PNF group whereas MVC and PRT values
remained unchanged, this concludes that