Part
I: Introduction to stretching in the Lumbar Spine. Physiology and biomechanics.
This
series of papers aims to establish whether stretching techniques, as they
affect the soft tissues of the lower back, are conducive to promoting or
damaging its health and efficiency. Biomechanics
literature was surveyed in an attempt to link the process of stretching with
the physical response of soft tissues. A literature review was performed using
electronic databases to study the physiological and biomechanical principles of
stretching and to highlight ‘risks’ associated with stretching. Stretching, as an isolated activity, is
useful in treating pathology associated with tissue shortening and reduced
range of joint motion. A wider role in promoting the health and efficiency of
the lower back is possible when it is combined with strength and postural
training.
KEYWORDS:
Low back pain, stretching, cavitation,
spinal biomechanics.
It is important
to first consider the effects of stretching at a micro cellular level. ‘Cavitation’ and ‘streaming’ are principles usually associated with the
application of therapeutic ultrasound as mechanisms for inducing tissue changes
by influence the flow of chemical electrolytes into and out of tissue cells
(Low and Reed 1994). However, they may be implicated in tissue changes that
occur in response to tissue stretching.
Stretching and compressive forces imposed on soft tissue effect tissue
changes by stimulating extracellular activity that, in turn, influences
intracellular activity. Stretching produces new isoforms and
protein synthesis in muscle fibre modified at the level of gene transcription
(McComas, 1996). Stretching muscle fibre membrane releases soluble factors that
activate second messengers. These activate genes in the myonuclei that
transcribe coding for new isoforms of contractile proteins. In vivo research on
articular cartilage has claimed to demonstrate oscillatory compression
increases fluid flow, hydrostatic pressure and streaming potentials linked to
proteoglycan synthesis (Hall et al 1991; Kim et al 1994; Sah et al 1991 cited
in Alter p46). Research in this area has focused on cartilage but is logical to
extrapolate these finding to connective tissue in muscles, tendon and ligaments
being derived from the same substances, albeit in differing combinations (Alter
1996).
Cavitation accompanies the cracking noise of the
manipulative high velocity thrust of the chiropractic adjustment as gas bubbles
enter the joint. This chiropractic adjustment is comparable to Maitland’s
description of a ‘Grade V’ whereby the joint is taken beyond the elastic
barrier of resistance (Ayed 2003) and consequently remains unstable for up to
twenty minutes following an adjustment (Sandoz, 1976 cited in Alter 1996,
p204). The ostensive purpose in chiropractice for air entering the joint is for
it to distend the articulating surfaces to improve its range of movement
(Sandoz 1976 cited in Alter 1996, p204). Cavitation is associated with micro-streaming
or acoustic streaming manifest in the piezo electric effect that occurs when
connective tissue is deformed under pressure (Grodzinsky (1983). Streaming is
thought to change the local environment of a cell by modifying concentration
gradients around the extracellular membrane thereby influencing intracellular
activity. This may be a mechanism whereby mechanical forces of stretching and
compression effect gene expression and subsequent changes in protein synthesis
that determine the strength and flexibility of the tissues (Alter 1996). The chiropractic thrust that is associated
with cavitation applies an intensive stretch to the soft tissue (Ayed 2003).
Whether or not static stretching produces a similar cavitation response does
not appear to have been investigated. It is similarly unknown if the streaming
potentials that occur with oscillatory compression occur with static
stretching. Tissue compression will be accompanied by tissue elongation of
adjacent tissue and some oscillation is likely to accompany static stretching,
but the velocity required to produce streaming potentials or genetic
transcription changes is not known.
Stretching produces acute and
chronic adaptations. Acute adaptation is evident as viscoelastic stress
relaxation whereby passive tension in a muscle is reduced over time when held
in a stretched position but the original level of stiffness resumes in less
than one hour. Stretching soft tissue is therefore likely to only produce
transient increase in length unless it occurs repeatedly or excessively. The
fascia enclosing muscle fibres may increase in length semi-permanently and
tendons, ligaments and scar tissue may lengthen. Soft tissue may also adapt to
stretching by modifying gene expression of its contractile and extensile
elements (Alter 1998). Muscles fibres in vitro have demonstrated the production
of additional sarcomeres to facilitate extra muscle length in response to
chronic stretch but this has not been demonstrated in vivo (Alter 1996). Kline
(1997) reported little scientific evidence for chronic adaptations to
stretching or the mechanisms for long‑term changes in skeletal muscle
flexibility.
Stretching will raise the threshold
at which the stretch reflex responds, allowing muscle to tolerate greater
stretch intensities. Stretching soft
tissue may reduce the sensitivity of the stretch reflex (Evatt et al 1989;
Wilma 1983, both cited in Alter 1996 p 176). This can allow more extreme
stretching that may result in joint laxity (Alter 1996) and an increase in the
risk of injury. As stretching does not raise tissue temperature, it is unlikely
to be useful as a preparatory warm-up. Nor does it redirect blood flow away
from exercised muscles, so it will not aid the ‘cool-down’ (Murphy 1991).
Stretched soft tissue has a higher compliance and will rupture under less force
because it is less able to absorb energy. When muscles are stretched to the
extent that actin and myosin filaments no longer overlap the force is transmitted
to the cytoskeleton of the muscle fibre causing tissue damage (Shrier
1999).
Collagen provides soft tissue with
tensile strength and its main resistance to stretching. The annulus fibrosis of
the intervertebral disc is composed of concentric bands of collagen enabling
the disc to withstand high bending and torsion forces. Collagen fibres form a
matrix within a gel-like ground substance consisting of water and
glycosaminoglycans (GAGs). The water in the ground substance is bound to GAGs
by electrical charge enabling the tissue to deform and to withstand pressure as
evident in cartilage and the disc nucleus (Nordin and Frankel 2001).
The strength of the collagen fibres
depends on their arrangement within the matrix. They resist tensile stress in
the direction of their fibres and are strongest when fibres are aligned in
parallel. They derive strength from their mutual hydrogen bond cross-links, and
the forces imposed on them by skeletal movement determine the alignment and
cross-linkages. Whilst the cross linkages facilitated by hydrogen bonds
supplies tensile strength the bonding becomes more intense with age and the
tissues develop adhesions and stiffness. The ability of collagen fibres to
resist stretching and tensile force is proportional to the concentration of
cross-links (Nordin and Frankel 2000).
Chronic loss of tissue pliability will result from the molecular bonding
of collagen fibres being too tight. Stretching can break down hydrogen bonds
and this may therapeutically reverse chronic tissue shortening (Alter 1996;
Juhan 1987) but may also reduce tensile strength.
Ranges of joint movement
Estimates of ranges of joint
movement offer a working knowledge of relative values and provide some guidance
of how far it may be safe to stretch. However, people differ widely according
to age, sex, genetic disposition and ambient conditions (Lumbsden and Morris
1968 cited in Nordin and Frankel 2001, p260; Bogduk & Twomey 1991; Boyling
et al, 1994; Kapandji, 1974). There is also little agreement on the best method
of measuring ranges of joint movement in the spine (Troke 2002). Movements of the spine are guided by the
facet joints with the range of motion being determined by their orientation. In
the lumbar, the facet joints are at right angles to the transverse plane and 45
degrees to the frontal plane (White and Panjabi 1978 in Nordin and Frankel
2001, p260). In an average adult, this allows about 40 degrees of flexion, 30
degrees of extension, 20 to 30 degrees of lateral flexion per side and five
degrees of rotation to each side (Kapandji 1974). The facets at the
lumber-sacral joint are obliquely orientated allowing more rotation (Nordin and
Frankel 2001) Gliding takes place on both saggital and frontal planes giving
the spine a total of six degrees of freedom (Corrigan and Maitland 1998). Gleim and McHugh (1997) reported little
consensus in the literature about definitions and measurements of flexibility
and scant scientific understanding about the determinants of flexibility.
Rationale for stretching
Stretching
is performed to relax muscular tension (Coulter 2001) and is thereby
therapeutic as excessive muscular tension decreases sensory awareness,
increases blood pressure, wastes energy and is detrimental to circulation of
blood muscles and tissues. Impaired
blood circulation reduces the delivery of oxygen and nutrients to tissues and
the removal of toxic wastes leading to fatigue, aches and pain (Alter 1996).
There are acute and chronic adaptations to
stretching soft tissue. These can include reducing passive muscular tension,
altered posture and changes in the molecular tissue structure. These events may
increase or reduce pain, tissue strength and spinal stability, the outcome
dependent on the initial qualities of the tissue and the appropriateness of the
stretching technique (Alter 1996). There is no defined clinical assessment of
what constitutes sufficient mobility and the aims of stretching differ
according to the wider goals of the practitioner. Stretching is generally
thought useful in contributing to pre-exercise warm-up, post-exercise cool
down, enhancement of athletic performance and prevention of injury and
post-exercise soreness (Alter 1996; Anderson 1981). Stretching is accordingly
incorporated into sport and fitness training regimes and studio based classes
such as yoga, and pilates. Stretching of soft tissues is part of the processes
of spinal mobilization and manipulation employed in manual therapy to restore
normal range of movement to joints and relieve pain. Stretching may also be used
to alleviate muscle spasm and to break down tissue adhesions (Brukner and Khan
2000).
Maitland
et al (2001) prescribes stretching to restore range of movement. Evjenth (1988)
advises stretching before an activity and particularly after activity to return
shortened muscles to normal length. Calliet (1995) cites evidence that
stretching and strengthening exercises correct fascial, muscular, ligamentous
and capsular shortening and helps to restore proprioception following tissue
injury.
Risks of stretching
Stretching can break down hydrogen
bonds to the extent that the capsule and ligaments become lax, rendering the
joint incongruent and unstable. Collagen fibres will elongate if stretched by
1.5 % to 2% for over one hour. The tissue will return to pre -stretch length
unless stretched again within twenty-four hours. Elongation of 1.5% or more if
maintained more than one hour may cause permanent damage by tearing the
collagen fibres and disrupting the intermolecular bonds between the
tropocollagen units (Nordin and Frankel 2000). Exercises involving stretching
usually also impose compression on neighbouring tissues that may be damaged if
the force is excessive. Spinal flexion will stretch the posterior spine but
compress the anterior spine. Spinal extension will stretch the anterior spine
but compress the posterior spine. Similar contra lateral pressures occur in
lateral flexion and rotation. Compression can cause tissue damage including
torn cartilage or a herniated disc. Injury to the discs will lead to malfunction
of the facet joints that, in turn, will adversely affect their ability to
protect the discs from shear forces (Nordin and Frankel 2002). Excessive
compression of the facet joints can fracture the subchondral bone, the base of
the inferior or superior articular process and the vertebral lamina (Bogduk and
Twomey 1987). Some
authors consider the extremity of stretches inherent in some yoga postures, and
exercises resembling them, likely to produce soft tissue injury (Hillman 1991, Smith B 1994; Egger and Champion 1990; Eggar,
Champion and Hurst 1989; Donovan et al 1989; Fitness Professionals, circa
1990).
Risk reduction
Stretching is contraindicated in
cases of fracture; acute inflammation or infection around the joint;
osteoporosis; sharp or acute pain with joint movement or muscle elongation;
recent sprain or strain; some vascular or skin diseases; loss of function
resulting in decrease of range of motion (Alter 1998). Stretching needs to be
performed “gently, repeatedly, eliciting minimal and self limited discomfort
with no effort to exceed full range of motion, as determined by the patient
rather than the therapist using physiological standards” (Calliet 1995,
p279). BKS Iyengar (1966) is credited
with modifying and developing a system in the practice of yoga postures that
renders them safe and effective by prescribing postural alignment sympathetic
to the synovial joints and preparation to ensure adequate strength, flexibility
and correct technique (Mehta SM&S 1992; Mehta M 1992, 1994) reflecting
Western practice in physical exercise and physiotherapy. The method prescribes
a progression from elementary to advanced postures and warns that injury may
result from attempting advance postures prematurely (Mehta M 1992).
A selection of Iyengar yoga postures
are illustrated in Article III of this series.
Conclusion
The basic science suggests that
stretching may release muscle tension and balance strength and flexibility
across joints thereby improving posture and efficiency of the vertebral motion
segments. Prolonged stretching can weaken soft tissues by breaking down the
molecular structure but may also increase the cross sectional area thereby
increasing muscle force. The actual results of stretching depend on the
condition of the tissues, the duration, intensity and frequency of the
stretching. The role of stretching
exercises in treating low back pain depends on the cause of the problem.
Stretching may be a useful component in alleviating mechanical dysfunction of
musculo-skeletal origin that varies according to activity. Stretching can also
contribute to the remedy of poor posture that can contribute to mechanical
strain and pain. Specific stretches may contribute to structural rehabilitation
of the low back. However, it appears likely that stretching should accompany
strengthening exercises in order to be effective unless the pathology is
exclusively related to restrictions in joint movement caused by shortened
tissues.