Skeletal
muscle
The contractile elements of skeletal muscle are derived from myoblasts.
Each muscle is composed of several muscle bundles which in turn contain
muscles fibres, the basic unit of contraction. A muscle fibre is an elongated
cell and is composed of myofibrils (1-3 micron in diameter and 1-2 cm long)
which are collections of sarcomeres. Each sarcomere is composed of thick
filaments (myosin) and thin filaments (actin) arranged in an overlapping pattern
that allows the filaments
Muscle
fibres are divided into two types: fast twitch and slow twitch. Slow twitch
fibres are highly vascularised, and contract slowly and aerobically. They are
used in endurance (low-load, high-frequency) activities. Fast twitch fibres
contract more quickly and powerfully but they do so at the expense of efficiency
and are anaerobic such that they fatigue quickly. The distribution of fast and
slow twitch fibres is genetically determined but may be influenced by
selective training. Endurance training, typically increases the aerobic capacity
of the slow twitch fibres by increasing the number of mitochondria and capillary
density. Strength training involves using higher loads at lower repetitions and
leads to an increased number of myofibrils as well as hypertrophy of fast twitch
fibres.
Muscle
injury
During physical activity up to 70 per cent of the cardiac output may be
diverted to muscle blood flow. It is easy to see that whenever a muscle is
injured significant bleeding is an inevitable complication. The injury may be
direct, from a blow compressing the muscle against the underlying bone, or it
may be indirect due to excessive distraction of the muscle ends. The latter may
result in partial or complete rupture of the muscle.
When
bleeding occurs within a muscle the clinician must distinguish between
intramuscular bleeding, i.e. contained within a muscle sheath, and intermuscular
bleeding. Intramuscular haematoma is a more serious injury and the resultant
swelling usually persists beyond the first 48 hours, and is accompanied by
significant tenderness, pain and impaired muscle function. If the swelling
reaches a critical level then an acute compartment syndrome may result (vide
infra).
With
an intermuscular haematoma the dispersal of blood within the facial plains means
that after an initial period of increased pressure there is a relatively rapid
reduction in pressure and swelling with return of muscle function. Typically,
superficial bruising may be visible some distance from the site of injury
24—48 hours later.
A
number of factors predisposes muscles to rupture:
• inadequate warm-up prior to exercise;
• fatigue;
• previous injury, resulting in:
—
incomplete recovery of muscle strength,
—
inelastic scar tissue impeding muscle elongation and joint motion,
—
loss of muscle proprioceptive feedback;
• eccentric loading;
• muscle spanning two joints.
The
principles of treatment for any soft-tissue injury, and particularly for muscle
injuries, can be remembered using the acronym RICE.
• Rest
• Ice
•
Compression
• Elevation
Treatment should minimise the amount of bleeding within the tissues, and
cessation of activity as soon as possible after the injury will allow the fibrin
scaffold to develop on the damaged capillaries. A sportsman injured during the
course of a game presents the surgeon with a delicate judgement. The injured
player may be able to continue playing but the importance of the current match
must be weighed against the increasing damage that is being done with every
minute that he or she continues to play. However, players in the heat of battle
often require a little persuasion to see that curtailing the present game by 20
minutes can save them 2—3 weeks of rehabilitation in the future!
The application of ice and compression
vasoconstricts and tamponades the blood vessels, whilst elevation of the limb
hampers the player’s activity and improves venous drainage.
After
48 hours of this regime a more accurate assessment of the extent and nature of
the injury can be made. Inspection and palpation of damaged muscle is usually
sufficient to diagnose the majority of injuries. In cases of complete muscle
rupture the surgeon must decide whether exploration and apposition of the muscle
ends is required. If some continuity of the musculotendinous unit can be
demonstrated, for example active extension of the knee when a quadriceps rupture
is suspected, then surgery is not required.
Complications
of muscle injury
A small proportion of large muscle haematomas will fail to be
reabsorbed. The walls of the cyst become endothelialised and aspiration is of no
benefit. Resection of the cyst is then required with particular attention to
haemostasis to prevent recurrence.
Healing
deep intramuscular haematomas, most commonly of brachialis and vastus
intermedialis, may lead to the formation of bone rather than fibrous tissue, a process known as myositis
ossificans (MO). This condition should be suspected if pain, swelling and
restricted motion persist despite adequate rehabilitation. Over-vigorous
passive stretching of the muscle in the early phase of rehabilitation may
precipitate MO and should be avoided. Plain radiographs demonstrating hazy
callus formation confirm the diagnosis. Surgery for symptomatic cases must not
be performed until maturation of the ossified area is complete. Radiotherapy or
nonsteroidals should be given postoperatively to prevent recurrence.
Compartment
syndrome
Compartment syndrome is a condition in which high pressure within a
closed space bounded by fascia and/or bone reduces capillary blood perfusion
below a level necessary for tissue viability with subsequent neuromuscular
dysfunction. Acute compartment syndrome is a surgical emergency and is dealt
with more comprehensively elsewhere in this book. Chronic compartment syndrome
affects athletes and results from the vascular engorgement of muscles during
exercise superimposed on muscles hypertrophied by prolonged training. The
anterior and deep posterior compartments of the tibia are must commonly
affected, but the lateral compartment of the thigh and forearm compartments
may also be affected. The symptoms of a rapidly worsening ache or cramping pain
felt in the affected compartment characteristically occur at the same time
after the start of training. In the same way that a patient with vascular
insufficiency will describe cessation of symptoms upon ceasing the activity, so
will the symptoms recur if activity is resumed, usually after a shorter
interval. Clinical examination with the patient at rest is usually unhelpful,
and examination after the patient has exercised may be equally uninformative if
a deep muscle compartment is involved. The diagnosis can be made by using
intracompartmental pressure monitoring whilst the patient exercises. The resting
pressure is often 15—20 mmHg (normal 0—5 mmHg) and may rise with
exercise to
Assessment
of muscle function
The
inability of a patient to perform a satisfactory voluntary muscle contraction
should lead the surgeon to consider a potential site of injury anywhere from the
cerebral cortex to the muscle. All too often the focus is on the muscle itself,
forgetting the neural networks that are required to empower the muscle. The
patient may voluntarily or involuntarily inhibit the activity of the .muscle
because of pain; the central or peripheral nervous system may be damaged or
diseased or there may be a more generalised hereditary condition such as
Duchenne’s muscular
Muscle power is graded on the Medical Research
Council scale:
grade 0 — no movement;
grade I — only a flicker of movement;
grade 2 — movement with gravity eliminated;
grade 3 — movement
against gravity;
grade 4 — movement against resistance;
grade 5 — normal
power.
It is useful in diseased states, but
almost all sportsmen have muscle function in grade 5 and still complain
of weakness! Tests for muscle function in this situation are divided between
functional tests that quantify the ability of a muscle to perform a specific
task and objective tests of muscle strength. One example of a functional muscle
assessment is the single leg hop test. This tests principally the quadriceps
power of an injured knee compared with the control knee. From a single leg
stance the athlete jumps as far as he or she can and the distance is recorded.
This is then expressed as a percentage of the distance achieved by the control
leg. Inevitably, this test requires good function in several other muscle groups
as well as the joints above and below the knee, It is therefore less applicable
in the comparison of results between patients, but is useful as a means of
monitoring one patient’s recovery following injury or surgery. Objective tests
of muscle strength, such as grip strength using a hand-held dynamometer, are
readily performed but the results are often subject to great variation.
Furthermore, the relationship between results and functional outcome is often
not as close as we would wish. Task-specific outcomes such as running figures of
eight for assessment of the lower limbs and performing activities of daily
living for use with the upper limb are more time-consuming but have the
advantage that the task can he tailored to meet the requirements of the
individual.
Proprioception
Full recovery after injury to muscles and joints requires not only their anatomy to be restored but also the neuromuscular reflexes that control their motion. The word proprioception is derived from the Latin word ‘pro prius’ meaning self or own. It describes a process whereby an individual is aware of the position and movement of a joint. However, it is a rather vague term and its components are usefully considered under
three headings.
• Static joint position sense (knowing where the limb is held in
space), signalled by slow adapting Ruffini receptors found mainly in the
superficial layers of the fibrous capsule of joints.
• Kinesthetic sense (detection of displacement and or velocity)
detected by fast adapting Pacinian corpuscles located in the deeper layers of
the joint fibrous capsule.
• Unconscious closed loop afferent/efferent neural activity
required for reflex activity and regulation of muscle stiffness. This component
is signalled by Pacinian and Ruffini receptors but also by Golgi tendon organs
(slow adapting) and muscle spindle receptors (fast adapting).
These
receptors occur at multiple sites in the body and there is a complex, and poorly
understood, interplay between their signals in producing functional stability of
joints. Joint stability is maintained by the static mechanical restraints such
as the articular geometry and ligaments, as well as a dynamic element provided
by the muscles. The
It
is now well established that injury to a joint can impair the function of all
three of the components of proprioception. This impairment may be improved by
rehabilitation programmes that specifically address the restoration of sensory
feedback from injured joints. Reconstructive surgery, which one would expect to
address only the static mechanical restraints of an injured joint, also improves
the proprioception of the joints. Rehabilitation programmes that aim to improve
neuromuscular control begin by re-establishing joint position sense and
kinesthesia followed by dynamic joint stabilisation by using activities that
produce sudden alterations in joint position necessitating reflex neuromuscular
control. The final phase of these programmes addresses functionally specific
activities for return to the chosen sport.
Whilst
the importance of neuromuscular components in maintaining functional stability
of joints is universally accepted, the exact role of each of the components is
poorly understood and the relationship between loss of neuromuscular reflexes
and functional instability is not absolute.