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TITLE
Acute effects of contract-relax (CR) stretch versus a modified CR technique
AUTHORS
Anthony D. Kay
1
, Steven Dods
1
& Anthony J. Blazevich
2
AFFILIATION
1
Sport, Exercise and Life Sciences, The University of Northampton, Northampton, United Kingdom
2
Centre for Exercise & Sport Science Research (CESSR), School of Exercise and Health Sciences, Edith Cowan
University, Joondalup, Australia
ADDRESS FOR CORRESPONDENCE
Anthony D. Kay
1
Sport, Exercise and Life Sciences
The University of Northampton
Boughton Green Road
Northampton
NN2 7AL
United Kingdom
Tel: 01604 892577
Fax: 01604 720636
tony.kay@northampton.ac.uk
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ABSTRACT
Purpose: Contract-relax (CR) stretching increases range of motion (ROM) substantively, however its use in
athletic environments is limited as the contractions performed in a highly stretched position require partner
assistance, are often painful, and may induce muscle damage. Therefore, the acute effects of performing the
contractions ‘off stretch’ in the anatomical position (stretch-return-contract [SRC]) were compared with
traditional CR stretching in 14 healthy human volunteers. Methods: Passive ankle joint moment and dorsiflexion
ROM were recorded on an isokinetic dynamometer with electromyographic monitoring of the triceps surae, whilst
simultaneous real-time motion analysis and ultrasound imaging recorded gastrocnemius medialis muscle and
Achilles tendon elongation. The subjects then performed CR or SRC stretches (4 ˟ 10-s stretches and 5-s
contractions) randomly on separate days before reassessment. Results: Significant increases in dorsiflexion ROM
(4.1-4.0°; P<0.01) and peak passive moment (10.9-15.1%; P<0.05) and decreases in the slope of the passive
moment curve (19.1-13.3%; P<0.05), muscle stiffness (21.7-21.3%; P<0.01) and tendon stiffness (20.4-15.7%;
P<0.01) were observed in CR and SRC, respectively. No between-condition differences were found in any
measure (P>0.05). Conclusions: Similar mechanical and neurological changes were observed between
conditions, indicating that identical mechanisms underpin the ROM improvements. These data have important
practical implications for the use of this stretching mode in athletic environments as performing the contractions
‘off stretch’ eliminates the pain response, reduces the risk of inducing muscle damage, and removes the need for
partner assistance. Thus, it represents an equally effective, simpler, and yet potentially safer, stretching paradigm.
Keywords: Proprioceptive neuromuscular facilitation, range of motion, tendon stiffness, ultrasound.
INTRODUCTION
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Both the maximal joint range of motion (ROM) and resistance to stretch during rotation (indicative of tissue
stiffness) are important functional parameters that may affect muscle strain injury risk (Witvrouw et al. 2003),
influence the capacity to perform activities of daily living (Mulholland et al. 2001), and are compromised with
aging (Bassey et al. 1989) and disease (Duffin et al. 1999). Static muscle stretching is a commonly used technique
to acutely improve ROM with these improvements thought to be attributable to several mechanisms, including
reductions in tissue stiffness (Kay et al. 2015; Morse et al. 2008), altered peripheral (afferent) output (Avela et al.
1999, 2004), and dampened pain, pressure or stretch perception increasing stretch tolerance (i.e. the capacity to
tolerate increased loading prior to terminating the stretch; Magnusson et al. 1996; Mitchell et al. 2007; Weppler
and Magnusson 2010). Despite the popularity of static stretching, proprioceptive neuromuscular facilitation
stretching (PNF) is regularly reported as being the most effective stretching technique for acute and chronic
improvements in ROM (Funk et al. 2003; Hindle et al. 2012). A common method of PNF stretching is the
contract-relax (CR) technique (Sharman et al. 2006), which includes a static stretching phase for a prescribed
duration, followed immediately by an intense, often maximal, isometric contraction performed in a fully stretched
position. Upon completion of the contraction the joint is rotated further to again stretch the target muscle, with
stretch intensity normally to the point of discomfort. While CR stretching is highly effective and often used in
clinical environments to achieve rapid increases in ROM, it is not commonly used in athletic warm-up routines
possibly because it normally requires an assisting partner, may be painful, and is thought to pose a greater muscle
strain injury risk compared with static stretching (Beaulieu 1981).
Few studies have examined the underlying mechanisms associated with increases in ROM following CR
stretching (Hindle et al. 2012; Kay et al. 2015), consequently these mechanisms remain essentially theoretical and
poorly understood. Two neuromuscular mechanisms (autogenic inhibition, gate control theory) have been
theorized (for review see Hindle et al. 2012). Regarding autogenic inhibition, a neuromuscular inhibition was
thought to occur as the loading of the tendon during the contraction phase of CR activated/stimulated type Ib
muscle afferent output from the golgi tendon organs, stimulating inhibitory spinal synapses and hyperpolarizing
the dendritic ends of spinal α-motoneurons of the stretched muscle. The Ib activity would likely diminish the
influence of homonymous Ia muscle afferents on the α-motoneuron pool of the stretched muscle, with the
diminished reflex activity thought to allow further increases in ROM (Prentice 1983). However, several original
studies have previously reported no change in electromyographic (EMG) magnitude at full ROM (Kay et al. 2015;
Mitchell et al. 2009; Osternig et al. 1990), with reviews concluding autogenic inhibition was unlikely to be an
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important mechanism underpinning the increase in ROM following CR stretching (Hindle et al. 2012; Sharman
et al. 2006). Gate control theory posits that pressure receptors (type III afferents) activated during the contraction
phase could inhibit pain perception (Mazzullo 1978), as pressure receptors are associated with larger myelinated
neurons that connect to the same spinal interneurons as un-myelinated nociceptive fibers (type IV afferents) within
the spinal horn (Melzack 1993). The increased activity of pressure receptors would theoretically diminish the
influence of homonymous IV afferent output and pain perception, thus enabling further increases in ROM. While
these neuromuscular pathways are theoretical, increased stretch tolerance (dampened pain perception) is
commonly reported following CR stretching (Kay et al. 2015; Mitchell et al. 2009). Thus although autogenic
inhibition has largely been discounted, a neurological contribution to the increased ROM following CR stretching
is at least partly supported.
The distinct muscle-tendon (and joint) loading characteristics of various stretching methods likely result in
different mechanical responses, with a key distinction between CR and other stretching techniques being the
inclusion of an intense, often maximal, isometric contraction performed following the stretching phase and
performed with the muscle remaining in a highly-stretched position. During passive ankle dorsiflexion stretches,
more flexible subjects demonstrate greater tendon elongation with no detectable differences in the onset or
magnitude of muscle activity toward the end of rotation or near full ROM (Blazevich et al. 2014), therefore tendon
properties may, at least partly, influence maximum ROM. While muscular and tendinous tissues experience
deformation during stretching (Blazevich et al. 2014; Morse et al. 2008), studies employing ultrasonography
techniques have found muscle stiffness to be reduced after an acute bout of static stretching, whereas tendon
stiffness remained unaltered (Kay & Blazevich 2009a; Morse et al. 2008). However, a recent study revealed that
CR stretching acutely reduced both muscle and tendon stiffness and elicited significantly greater increases in
ROM compared with a similar volume of static stretching after which only a reduction in muscle stiffness was
induced (Kay et al. 2015). This broader acute adaptive response, where both muscle and tendon stiffness are
influenced concurrently, offers a potentially important mechanism underpinning the superior efficacy of CR
stretching for acutely increasing ROM when compared to other stretching techniques.
CR stretching is implemented to the aim of increasing ROM, often in an attempt to reduce muscle strain injury
risk. However, paradoxically, performing intense muscular contractions in a highly-stretched position, where the
muscle is vulnerable to injury, increases the risk of inducing tissue damage (Beaulieu 1981; Butterfield and Herzog
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2006; Whitehead et al. 2003). Thus, the question should be asked whether the performance of isometric
contractions in a non-stretched position between each passive static stretching cycle is as effective as performing
the contractions during each passive static stretching cycle (i.e. contractions performed with the muscle in a
highly-stretched position). Interestingly, several studies have reported acute reductions in tendon stiffness
following maximal isometric contractions performed in the anatomical position (i.e. with the muscle off stretch;
Kay & Blazevich 2009b; Kay et al. 2015; Kubo et al. 2002). Furthermore, a recent study reported concomitant
increases in ROM and reductions in tendon stiffness following isometric contractions performed in the anatomical
position (Kay et al. 2015), with the acute increase in ROM being similar to that observed following static
stretching. Collectively, these findings suggest that substantial tendon loading, regardless of muscle length,
should influence tendon stiffness and ROM. Consequently, modification of the CR stretching technique to
perform the muscle contraction phase with the muscle ‘off stretch’ may provide a similar stimulus whilst reducing
injury risk. Therefore, the aims of the present study were to examine the influence of an acute bout of CR
stretching versus a modified CR technique (stretch-return-contract [SRC]; where the contractions are performed
‘off stretch’) on dorsiflexion ROM, maximal passive joint moment at full volitional ROM (stretch tolerance), the
slope of the passive moment curve (indicative of whole muscle-tendon complex [MTC] stiffness), gastrocnemius
medialis (GM) muscle stiffness and triceps surae EMG activity (measured during a passive joint stretch). The
acute effects of these interventions on Achilles tendon stiffness, maximal isometric plantar flexor joint moment
and peak triceps surae EMG activity during a maximal isometric contraction were then measured. We tested the
hypothesis that CR and SRC stretching techniques would produce similar increases in ROM and stretch tolerance
whilst reducing muscle and tendon stiffness.
METHODS
Subjects
Fourteen recreationally active participants (8 women, 6 men; age = 26.1 ± (SD) 9.6 yr, height = 1.7 ± 0.1 m, and
mass = 75.6 ± 13.3 kg) with no recent history of lower limb musculoskeletal injury or neurological deficit
volunteered for the study after completing a pre-test medical questionnaire and giving written and informed
consent. The subjects were asked to avoid any flexibility training, intense exercise and stimulant use for 48 h
prior to testing. All procedures performed in studies involving human participants were in accordance with the
ethical standards of the institutional research committee, and the study was completed in accordance with the
Declaration of Helsinki.