Information for doctors:
Technical requirements for precise diagnosis of lumbar muscle function

It is only possible to diagnose the cause of chronic low back pain on the basis of symptoms with 10%–15% of patients. In recent years, weak lumbar extensors have been recognised as a particular primary risk factor for low back pain. It is important, therefore that we can diagnose the functional ability of these muscles. A diagnosis provides objective evidence of functional deficits and can be used for planning therapy and documenting its effectiveness.

The following is required for an accurate diagnosis:

1. Isolation of lumbar extensors by immobilising the pelvis
2. Effects of gravity compensated (upper body)
3. Measurement of soft-tissue tension and net muscle strength
   
4. Isometric testing through entire range of motion (ROM)
5. Identification of muscle fibre by analysing fatigue response
   

1. Isolation of lumbar extensors by immobilising the pelvis 

Straightening the upper body requires a complex movement in which the lumbar extensors work with larger and stronger muscles: it is the gluteal and ischio-crural muscles that straighten the pelvis. The full range of motion for this complex movement is 182° (Diagrams 1 and 2). The lumbar extensors only account for 72° of the movement to straighten the spine (Diagram 3).

2. Compensating the effects of gravity

The upper body is subject to gravity and this distorts the test results. As a result, we need to measure the weight of the upper body and then offset it by means of a counterweight.

According to Winter, the lack of a counterweight to offset upper body weight can produce an error of up to 500% in the measurement of lumbar extensor strength. Diagram 5 shows the results of a measurement with and without a counterweight to offset upper-body weight.




Diagram 5: Errors in measurement as a result of gravity

3. Measurement of soft-tissue tension and net muscle strength 

Measurable strength (functional strength) consists of the strength from pure muscle contraction (net muscle strength) and soft-tissue tension. The latter is a non-muscular torque and occurs particularly when we bend the upper body forward (flexion). It comes from elastic tension in the lumbar extensors combined with counter-pressure from abdominal organs. Curve 3 in Diagram 6 shows the non-muscular torque, Curve 2 the net muscular strength (torque) and Curve 1 the functional strength. The area in red is the measurement error if no account is taken of non-muscular torque.



Diagram 6: Errors (red area) if non-muscular torque (yellow area) is ignored, see text

4. Isometric testing through entire range of motion

When measuring dynamic strength, intramuscular friction can be a source of error. The magnitude of the error increases as the speed of movement increases. Dynamic strength curves (isokinetics)) can be divided into acceleration phase, isotonic phase and deceleration phase. The only relevant phase in this context is the isotonic phase.

With an isokinetic strength curve, the amount of data lost depends upon the speed of the movement. In a test of the lumbar extensors, it may be as much as 50% of the entire range of motion. In addition, dynamic strength tests trigger accelerating forces (impact forces), which can be several times the weight used and so the risk of injury is considerable. Isometric testing is safe because there are no significant impact forces. It avoids the error associated with intramuscular friction and allows am accurate measurement of strength through the entire range of motion. Diagram 8 shows the test positions in a MedX machine and the strength measured at each angle.

The normal isometric strength curve of the lumbar extensors decreases lineally from flexion to extension and describes a range of motion of 72°. Patients with low back pain often have restricted mobility and an abnormal strength curve. This may indicate that overall strength is low or that strength is reduced at certain angles (test positions).

If we test strength at only one angle or only in complete flexion or extension, we fail to identify abnormalities, e.g. a fall-off at 36°. Similarly, we are unable to identify whether therapy changes the curve. In other words, for an accurate diagnosis, we need to measure the strength of lumbar extensors at various closely-positioned angles over the entire range of motion.

A study by Pollock et al. involving 136 healthy patients looked at the reliability of isometric tests using MedX lumbar extension machines: the result for all angles was r > 0.9, i.e. this functional diagnosis is extremely reliable.

Diagram 8: Computer representation of strength measured at individual positions (angles)

5. Identification of muscle fibre type by fatigue response analysis

Every muscle consists of different types of fibres. We need to know the composition of muscle fibres (e.g. to deal with the absence of progress with therapy) so that we can determine the right interval between individual therapy sessions. The relationship between the composition of muscle fibre types and fatigue response is well researched and so we can use the following 3-part test procedure:

First of all, we test strength when the patient is fresh. This is followed by a dynamic test using a weight equal to 50% of the maximum strength measured in this initial test. Finally, we measure residual strength after the patient has exercised to temporary fatigue. The difference between the two curves is the fatigue response. The red area in Diagram 11 is the distinctive fatigue-response pattern of a person with a dominance of fast-twitch fibres (30% of all test persons), the area in yellow is the fatigue-response pattern of a person with a dominance of slow-twitch fibres (10% of all test persons). 60% of all test persons have a mixed fibre type.

Diagram 11: Different fatigue responses (see text):
Red: dominance of fast-twitch fibres
Yellow: dominance of slow-twitch fibres

To maximise the strengthening effect, patients with different fatigue responses require a different training intensity and a different interval between training/therapy.

Muscle physiology and principles of resistance training

An understanding of the principles of physiology and training is essential if we are to improve the strength and endurance of lumbar muscles. The role of the muscle is to produce tension. Muscle strength is defined as the maximum possible tension generated during the contraction of a muscle. Muscle endurance is defined as the ability of a muscle to do repeated contractions at a sub-maximum load.

Both factors play an essential role in the prevention and treatment of low back pain and can be improved by progressive resistance training.

The physiology of the strengthening process is complex; it includes neurological, morphological and biochemical adaptations. The training of muscle strength and endurance improves the recruitment of motor units, increases the intra-muscular storage of aerobic and anaerobic metabolites and enzymes, increases muscle and bone mass and thickens connective tissue. The key to the production of muscular hypertrophy is the tension or strength developed by a muscle when faced with a resistance. This tension also encourages the proliferation of bone cells and connective tissue cells.

The composition of muscle-fibre types is also relevant to the development of strength and endurance. The strength potential of a muscle with mainly Type 1 fibres (slow-twitch) is limited but its endurance potential is good.

The endurance potential of a muscle with mainly Type 2a and 2b fibres (fast-twitch) is limited but its strength potential is good.  Type 2a fibres seem to be capable of adapting to the type of training, i.e. whether trained for endurance or strength. A muscle with approximately equal percentages of Type 1 and Type 2 fibres has an average potential to develop both strength and endurance.

When a muscle works against a low resistance, it is the endurance fibres that are recruited last. Strength fibres are only recruited as well if the resistance is of high intensity and in this case the largest and strongest motor units are recruited last. To develop the optimum level of strength, a muscle must work against a resistance of high intensity.

Norm data and therapy aims

During the process to rehabilitate an injured extremity, one of the aims of therapy is to maintain the strength ratio of the uninjured side. However, with the spine, comparisons are not possible and so the University of Florida in Gainesville produced norm data corrected for age, gender and weight by doing strength tests on healthy but untrained subjects. In terms of function, therapy is designed to improve the function of lumbar muscles (isometric and dynamic strength through complete ROM, endurance and mobility) and to develop a norm strength curve that is in balance. The required increase in strength is achieved by training with an adequate weight both concentrically and eccentrically and in maximum extension (contraction), tension is held in each case for 2 seconds. Using this method of training, a patient can expect to achieve the greatest increase in strength. The resistance is variable and adapts to the biomechanics of the lumbar spine by the use of a cam. Normally, the resistance (weight) is increased by 5% at each subsequent therapy session (progressive resistance training) provided that the patient has reported no abnormal reaction to the previous session. To achieve optimum strength, 60% of patients only require one therapy session per week. As a rule, 12-18 therapy sessions within 2-3 months are enough to achieve the optimum improvement in spinal function.

Supplementary measures

It is generally recognised that patients with chronic low back pain need to improve the strength of every trunk muscles. The lumbar extensors are the weakest link in the group of muscles that stabilises the lumbar region and so the strengthening of these muscles should be the primary aim of therapy. As already mentioned, the auxiliary support muscles play an important role in the stabilising spinal movements. As a result, the abdominals, gluteals, ischio-crural muscles, lateral trunk muscles, pectoral muscles and shoulder muscles should all be trained during any medical training therapy based on Gustavsen. Training should be done on training machines that provide muscle isolation and a variable resistance, e.g. Nautilus or MedX Exercise machines.

Physiotherapy (coordination training, stretching exercises, posture training) may be required. In addition, passive measures are sometimes necessary for regeneration purposes or to combat stubborn pain but they should be kept to the absolute minimum. They only have an indirect relationship with the actual aim of therapy, i.e. the improvement of spinal function. However, they can sometimes help (as can NSAR medication) to ensure that patients can tolerate therapy or to prevent them from stopping.