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 to slide past each other. The force generated by a muscle is proportional to its functional cross-sectional area. A muscle contraction is concentric when the muscle fibres shorten and the tension within the muscle is proportional to the externally applied load. During an eccentric contraction the muscle lengthens (pays out) and the internal force is less than the external force. The latter have the greatest potential for high muscle tension and injury. During an isometric contraction tension is generated but the muscle does not shorten.

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 >100       mmHg. This abnormal elevation will often persist for 15 minutes or more after stopping the exercise. Treatment should aim to correct any abnormalities in the patient’s gait or training methods, but subcutaneous fasciotomy of the involved compartments has a good success rate.

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 dystrophy. Whilst it is important fur the surgeon to consider all possible causes of muscle dysfunction, this chapter will confine itself to the assessment of muscle function in athletes.

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 relative importance of damage to the neuromuscular, or proprioceptive’, feedback in maintaining stability of an injured joint has only recently received much attention. Damage to anatomical structures and the resultant change in stability of a joint is relatively easy to assess, whereas techniques for measuring the loss of neuromuscular reflexes that provided functional stability were only developed more recently.

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.