Muscle performance can be impaired for a variety of reasons, including:

1. Central or peripheral neurologic pathology decreases an individual’s ability to effectively recruit and functionally use their muscles;
2. Injury to the muscle from a strain or contusion decreases performance; and
3. Disuse or deconditioning for any reason.

The goal of examination/evaluation of muscle performance is to determine the cause of the impairment to develop the most efficient and comprehensive intervention plan.

This article discusses the potential factors that can cause impaired muscle performance, examination/evaluation results of each potential cause, and general intervention concepts for each specific cause.

1. Neurologic Pathology

Neurologic pathology can affect the contractile capacity of muscle as a result of pathology in the central or peripheral nervous system (CNS/PNS). The peripheral nervous system can be affected at the nerve root or peripheral nerve level.

Individuals with nerve root pathology may present with muscle performance impairments in the nerve root distribution. For example, nerve root compression at the L4-5 spinal level can produce quadriceps femoris weakness, while nerve root compression at the C5-6 spinal level can result in deltoid and biceps weakness.

Therapeutic exercise intervention depends on the prognosis for the nerve root involvement. If the changes are relatively recent and resolution of the nerve root compression is expected through conservative or surgical management, preventive and protective measures are taken.

The goal of therapeutic exercise intervention is not only to promote optimal muscle performance of the muscles innervated by the affected spinal segment (pending prognosis) but also promote spine stability and optimal movement patterns to alleviate any mechanical cause of nerve root pathology incurred by the spinal segment(s).

• Peripherally, use resistive exercise to maintain/improve current strength levels, while training inner lumbar or cervical core and girdle muscles to provide proximal stability.
• Centrally, use resistive exercise to train inner core muscles (i.e., longus coli, transversus abdominis, lumbar multifidus, pelvic floor) to effectively stabilise the spine and relieve mechanical nerve root irritants.

After the mechanical or chemical cause of nerve root injury is remediated, use specific, localised resistive exercise of the involved musculature to restore precise recruitment patterns.

Neurologic weakness may also result from a peripheral nerve injury. Compromise of the median nerve at the carpal tunnel, the radial nerve at the cubital tunnel, or the common peroneal nerve at the fibular head are examples of such injury. The pattern of sensory loss and weakness depends on which nerve and where along the nerve’s course the damage occurs.

Therapeutic exercise should be focused on remediating the mechanical cause of the peripheral nerve injury. For example, a depressed shoulder girdle may contribute to traction on the long thoracic nerve, causing motor changes in the serratus anterior.

Exercise and posture education to elevate the shoulder girdles may alleviate the traction on the long thoracic nerve and ultimately restore normal innervation to the serratus anterior.

Resistive exercise should also focus on maintaining and increasing the strength of the unaffected motor units in the involved musculature, and progressively strengthening motor units on reinnervation.

Take care to avoid excessive strengthening of intact muscles for fear of creating significant muscle imbalance. Exercise should try to maintain muscle balance and efficient movement patterns without developing a dominant muscle group. Splinting, bracing, taping, or other supportive measures may be necessary to maintain balance.

Other neurologic conditions include neuromuscular disease such as multiple sclerosis, postpolio syndrome, and Guillain-Barré syndrome, and muscular paralysis or paresis resulting from spinal cord injury or cerebral vascular accident (CVA).

Resistive exercise programmes must consider the prognosis and tailor the exercises appropriately. In situations such as Guillain-Barré syndrome, certain cases of spinal cord injury and CVA, and progressive stages of multiple sclerosis, some recovery is expected. Exercise programmes focus on maintaining strength in intact musculature and gently strengthening weakened muscles as recovery and remission advances.

Avoid fatiguing weakened muscles during strengthening exercises. Dosage parameters generally include several short exercise sessions of a few repetitions interspersed throughout the day.

During quiescent periods of diseases such as multiple sclerosis, a general conditioning programme of balanced strengthening and mobility exercises is appropriate. When recovery is not expected, resistive exercise programmes emphasise functional strength of remaining musculature. This includes strength for functional activities such as self-care, transfers, and mobility. Take care to avoid overworking these muscles.

Unlike those with full innervation who use their muscles efficiently, the individual with paralysis uses the few innervated muscles they have for nearly all their activities. Consequently, the potential for overuse injuries is very high.

2. Muscle Strain

Muscle strain occurs along a continuum from acute macrotraumatic
injury to chronic microtraumatic overuse injuries. Resistive exercise in the treatment of muscle strain injuries depends on where along this continuum the injury occurs. Resistive exercise that neither overloads nor underloads the tissue is optimal. Determining this resistance dosage is the challenge.

Acute traumatic injuries occur when a muscle is rapidly overloaded or overstretched and the tension generated exceeds the tensile capability of the musculotendinous unit. For example, the hamstring muscle is a common site of muscle strain injury. A combination of insufficient strength, reduced extensibility, inadequate warm-up, and fatigue has been implicated
in hamstring injuries. Strength, extensibility, and fatigue resistance protect a muscle from strain injury.

Eccentric loading is a common mechanism of muscle strain injury, and a muscle prepared for eccentric loading is less likely to sustain an injury. Eccentric loading should be an integral part of any resistance training programme. A programme to prevent muscle strain injuries should include dynamic resistive exercises with a strong eccentric component, flexibility exercises, an appropriate warm-up before activity, and attention to fatigue levels. A rehabilitation programme after injury should also focus on these factors.

Preventing Muscle Strain

Although some muscle strains are not preventable, precautions can reduce an individual’s risk of injury.

1. Warm-up before a vigorous activity; 5 to 7 minutes of a large muscle group activity such as walking, jogging, or cycling should suffice. This should be enough activity to break a sweat.

2. Stretch stiff and short muscles after your general warm-up. Stretch each muscle for 15 to 30 seconds for four repetitions.

3. Balance your sports or other leisure activities with strengthening exercises. Your clinician can help you focus on muscles susceptible to injury.

4. Avoid fatigue during the activity. Fatigue can increase your risk of injury.

5. Strengthen underused muscles to prevent overuse to susceptible muscles. Your medical/exercise professional can help you determine which muscles these are and what specific exercises you need to perform to maintain muscle balance.

Muscles may also be strained from chronic overuse. For example, extensor digitorum longus (EDL) strain is common in workers performing continuous repetitive elbow, wrist, and hand activities as a result of using the EDL for wrist extension and elbow flexion.

Training the individual to use the biceps for elbow flexion whenever possible (i.e. keep the hand supinated versus pronated during elbow flexion) can alleviate the overuse strain to the EDL.

A thorough evaluation can determine the cause of the overuse problem. Ergonomic assessment and appropriate work site modification are also necessary to prevent a recurrence of the strain if ergonomics are at the root of undesirable posture or movement patterns. If left untreated, this impairment can quickly lead to disability.

Strain resulting from muscle dominance overuse is managed by reducing the loads imposed on the strained muscle. When the tensor fasciae latae dominates over the iliopsoas during hip flexion and gluteus medius during abduction, the tensor fasciae latae is at risk for an overuse strain.

Improving the strength and recruitment patterns of the iliopsoas and gluteus medius can reduce the load on the tensor fasciae latae and allow it to recover. Postural habits (e.g. standing in medial rotation) and movement patterns (e.g. hip flexion or abduction with medial rotation) must also be modified to improve recruitment of the underused synergists.

A potential risk factor of muscle strain is gradual, continuous overstretching, which occurs when a muscle is continuously placed in a relatively lengthened, tension- producing position (see Length-Associated Changes). For example, the lower trapezius in an individual with forward shoulders is subjected to continuous tension and has adapted to a lengthened state.

It may not take much force to produce a strain injury in a muscle that is already overstretched. This type of strain puts the muscle at risk for two forms of muscle weakness, one from length-tension changes and the other from overstretch strain.

Education is a key component of a rehabilitation programme in the case of muscle strain associated with continuous overstretch. In the lower trapezius example, educate the individual about optimal postural habits to reduce tension on the lower trapezius. Improving postural habits and reducing tension on the lower trapezius with bracing or taping will allow the muscle to heal more rapidly. In addition, it will promote adaptive shortening and therefore ultimately achieve a more optimal length-tension relationship and reduce the risk for future reinjury.

3. Disuse and Deconditioning

Muscle performance may be impaired because of disuse or deconditioning for a variety of reasons. Illness, surgery, specific physical conditions (e.g. pregnancy with twins), or injury may necessitate a period of decreased activity. Subtle muscle imbalances can lead to overuse of one muscle and to disuse and deconditioning of another.

Illness and injury are common causes of deconditioning. For example, illness such as pneumonia or an injury such as a herniated disk can result in a period of decreased activity and subsequent deconditioning. In these situations, total-body deconditioning occurs, and general conditioning is necessary.

However, specific exercises also may be necessary to improve muscle performance and prevent secondary impairments. For example, an elderly individual may have relatively asymptomatic osteoarthritis until a bout with pneumonia produces general deconditioning. Subsequently, knee osteoarthritis becomes symptomatic because of impaired muscle performance in the lower extremity muscles involved in gait and other functional activities. Specific resistive exercises to recondition those muscles are necessary to restore proper biomechanics and prevent further disability.

Reduced activity levels can impair muscle performance in a similar manner. Multiparous pregnancies, exacerbation of a musculoskeletal injury, an episode of colitis, or social factors such as major life changes (e.g. job, school, divorce, family illness, or death) can reduce activity levels and result in impaired muscle performance.

For example, regular exercise may keep a woman’s patellofemoral malalignment from becoming symptomatic. When her activity level decreases in the late stages of pregnancy, the combination of decreased activity, weight gain, and hormonal changes produces symptoms at the patellofemoral joint. Selective resistive exercises combined with education can prevent this exacerbation.

Resistive exercises in the case of overall decreased activity must consider the muscles most likely to be affected, the individual’s desired activity level and preference, and any underlying or residual medical conditions.

An overlooked source of deconditioning or disuse is a subtle muscle imbalance. When activating muscles for a functional movement, the body chooses the most efficient muscular and motor unit activation pattern.

Certain motor units in a muscle may be preferentially recruited when a muscle is engaged in a particular task. For example, motor units in the lateral portion of the long head of the biceps are preferentially activated when this muscle is engaged in elbow flexion, whereas motor units in the medial portion are preferentially activated in forearm supination.

The recruitment thresholds of motor units in a muscle are also influenced by the type of muscle actions associated with a movement. In elbow flexion, biceps motor units have a lower threshold in slow concentric and eccentric actions than isometric actions; the reverse is true for the brachialis.

The recruitment thresholds of motor units of a muscle active in a movement may also be affected by changes in joint angle. Some muscles or portions of a muscle may be overused while other muscles or portions are disused, and a resistive rehabilitation programme must acknowledge this imbalance.

In the previous example, instruction in general resisted elbow flexion may exacerbate the imbalance whereas specific training of the weaker recruitment pattern can restore muscle balance.

Length-Associated Changes

The principle of the length-tension curve affects muscle performance when a muscle is adaptively lengthened from prolonged posture and repetitive movement patterns of the muscle in the lengthened state.

Examination of postural alignment controlled by the muscle suggests that the muscle is longer than ideal as in depressed shoulders or hip adduction and medial rotation.

Muscles will test weak in the:

• Short range when compared with synergists;
• Paired muscle of the other extremity;
• Other half of the axial skeleton, i.e.:
  • Posterior gluteus and tensor fasciae latae;
  • Right and left posterior gluteus medius; or
  • Right and left external oblique muscles (respectively).

This is referred to as positional weakness.

Intervention should focus on strengthening the muscle in the shortened range, optimising posture to reduce lengthening tension on the muscle, and altering movement patterns to recruit the muscle in the shortened range.