Lumbar Extensors

The term lumbar extensor is used colloquially to refer to the erector spinae muscle group, which is comprised of the iliocostalis lumborum, longissimus thoracis, and spinalis thoracis. These three long, narrow muscles are positioned lateral to the multifidus, have a common origin on the upper iliac crest and lumbar aponeurosis, and each inserts on several ribs superiorly.² The erector spinae allow for vertebral rotation in the sagittal plane (e.g., lumbar extension) and posterior vertebral translation when muscles contract bilaterally.³ The multifidus muscle is also involved with lumbar extension movements and is therefore a target of lumbar strengthening exercises. The fascicular arrangement of the multifidus suggests that this muscle assists the erector spinae group and acts on vertebral rotation in the sagittal plane without posterior translation. Both the multifidus and erector spinae are also capable of vertebral rotation in the coronal plane (e.g., lumbar lateral flexion) and transverse plane (e.g., axial rotation) with unilateral contraction.⁴

Strengthening Exercises

Strengthening exercises are also known as progressive resistance exercise (PRE), which is based on the principles of overload, specificity, reversibility, frequency, intensity, repetitions, volume, duration, and mode. Each of these concepts is briefly discussed here as it relates to lumbar extensor strengthening exercises.^5,6

Overload

For PRE to result in continued increases in muscular strength and endurance, training must progressively overload the targeted musculature, that is, make muscles function beyond their current capacity by increasing the intensity and volume of exercises.⁶ Overloading the intensity of exercise tends to produce gains in muscular strength, whereas overloading the volume of exercise tends to improve muscular endurance. Although important to achieve long-term goals, overload in intensity and volume should be gradual to avoid reinjury.⁷

Specificity

Specificity implies that the target muscle must be isolated during the prescribed exercises to achieve sufficient muscular activation.⁶ Trunk extension is a compound movement of the hip, pelvis, and lumbar spine through the action of the lumbar extensors, gluteals, and hamstrings.⁸ The relative contribution of individual muscle groups is unknown, but it is assumed that the larger gluteal and hamstring muscles generate the majority of the force.⁹ Therefore it has been suggested that to enhance specificity for the lumbar extensors, torque production from the gluteals and hamstrings should be minimized.⁸ This can be achieved through various types of equipment and techniques. The lumbar dynamometer, for example, incorporates restraint mechanisms to limit pelvic and hip rotation and maximize lumbar extension in the seated position.¹⁰ Similarly, prone back extensions on Roman chairs or benches improve specificity and isolation of the lumbar extensors by aligning the pelvis on the device, internally rotating the hips, and accentuating lumbar lordosis.¹¹

Reversibility

With all PRE training programs, much of the gains in muscular strength and endurance will be lost unless exercise is continued.⁶ To prevent this reversibility, it is recommended that therapeutic exercises be carried out long term. Once substantial improvement has been achieved, it may be possible to maintain physiologic gains with a reduced training frequency that may be as low as once per month.¹²

Frequency

The optimal frequency for PRE varies based on initial physical condition, duration of injury, and therapeutic goals. Generally, a frequency of one to three sessions per week of PRE is recommended.¹³ Patients who are most deconditioned may initially achieve gains with only one session per week, which must be gradually increased to exceed that improvement.^14,15 Studies have reported improvement in lumbar extensor strength in patients with CLBP who trained once per week; no differences were noted between two and three sessions per week.^16,17 Training frequency needs to also consider intensity and volume, because particularly challenging PRE sessions may require additional recovery time.⁶ It is often suggested that patients allow for at least 48 hours between sessions.

Intensity

The intensity of PRE is related to both the amount of resistance used (e.g., weight) and the number of repetitions. To increase muscle strength, high-intensity PRE consisting of higher resistance and lower repetitions is generally prescribed, whereas low-intensity PRE with lower resistance and higher repetition is used to improve muscular endurance.⁶ Depending on the therapeutic goals (e.g., strength vs. endurance), 6 to 25 repetitions at intensities of 30% to 85% of the theoretical maximum intensity can be used for PRE. In a randomized controlled trial (RCT) of PRE for CLBP, there were no differences in clinical outcomes between high- and low-intensity lumbar extensor strength training.¹⁸

Repetitions

In PRE, the number of repetitions is corollary to the amount of resistance: as resistance increases, repetitions decrease. Regardless of the number, each repetition of a particular exercise should be performed in a slow, controlled fashion. It has been suggested that, as a general rule, it should take two seconds for the concentric/shortening phase and four seconds for the eccentric/lengthening phase of a PRE.¹⁹

Volume

The volume of PRE refers to the number of sets performed at one time and is one of the elements that can be varied to maintain gains in strength and endurance once a certain level of resistance and repetitions can be comfortably achieved. Although the optimal volume of PRE is unknown, one to three sets of each exercise is typically recommended for each session.¹³ For asymptomatic individuals, lumbar extensor strength gains achieved with one set of exercises were equal to those achieved with three sets when using a dynamometer.¹⁹ However, in patients with CLBP, an RCT reported that clinical outcomes were maximized with an increased volume of prone back extension exercises on benches.²⁰

Duration

In the context of PRE, duration refers to the total length of the exercise program and not the length of individual sessions. Clinical improvement can be seen after only a few weeks of a resistance training program due to changes in specific physical function, whereas long-term improvements are attributable to physiologic changes as a result of PRE.⁶ A minimum of 10 to 12 weeks of PRE is generally required to achieve measurable physiologic changes (e.g., hypertrophy) in skeletal muscles.⁶ However, compliance with any therapeutic exercise program tends to decrease over time and it can be challenging to maintain patient participation for that period once functional improvements are achieved and symptoms diminish.

Mode

Mode refers to the specific type of exercises and equipment used during PRE.⁶ This can be defined according to muscular contractions, which are either isometric or dynamic. Isometric exercise implies that the targeted muscle contracts without changing its length, whereas dynamic exercise implies that the targeted muscle shortens (concentric phase) or lengthens (eccentric phase) while contracting, which also results in associated joint movement. The three categories of dynamic exercise are isotonic, variable resistance, and isokinetic.⁶ Isotonic dynamic exercise involves muscular contraction against a fixed resistance providing constant tension (e.g., dumbbells). Variable resistance dynamic exercise involves a muscular contraction against variable resistance that attempts to mimic the strength curve of the targeted muscle (e.g., Nautilus). Isokinetic dynamic exercise involves muscular contraction at fixed speed (e.g., treadmill). The five main modes of lumbar extensor strengthening exercise protocols use (1) machines (Figure 9-1), (2) benches (Figure 9-2), (3) Roman chairs (Figure 9-3), (4) free weights (Figure 9-4), and (5) stability balls (Figure 9-5). The characteristics, advantages, and disadvantages of each are highlighted in Table 9-1.

Figure 9-1 Lumbar extensor strengthening exercise with a machine.

Figure 9-2 Lumbar extensor strengthening exercise with a bench.

Figure 9-3 Lumbar extensor strengthening exercise with a variable-angle Roman chair.

Figure 9-4 Lumbar extensor strengthening exercise with stiff-legged dead-lifts.

Figure 9-5 Lumbar extensor strengthening exercise with a stability ball.

TABLE 9-1 Subtypes of Lumbar Extensor Strengthening Exercises

Lumbar Extension Dynamometer

The lumbar dynamometer allows for dynamic exercise through a 72-degree arc in the sagittal plane, with an adjustable weight stack to provide resistance from 9 kg to 364 kg in 0.5-kg increments.⁸ Variable resistance is accomplished through a cam with a flexion-extension ratio of 1.4:1. After securing the pelvic restraint mechanisms, the patient begins this exercise in the most flexed position that is pain free, and is instructed to extend their back into a position near terminal extension in a smooth, controlled manner, taking 2 seconds for the concentric phase (lifting the weight) and 4 seconds for the eccentric phase (lowering the weight). The supervising therapist typically provides encouragement to the patient to perform as many repetitions as possible. The machine itself also provides visual feedback on performance via a monitor that displays exercise load and position. When the patient is able to complete 20 or more repetitions with a particular resistance, the resistance is increased in 5% increments at the next training session.¹

Variable-Angle Roman Chair

The VARC is similar to fixed-angle Roman chairs, except that the angle can be adjusted from 75 degrees (nearly vertical) to 0 degrees (parallel to the ground) in 15-degree increments.²⁶ Patients typically begin the training program at a low resistance level with an angle setting of 60 degrees with their hands positioned on their sternum. After achieving the correct positioning on the device, the patients start each repetition of dynamic exercise in the extended position and lower their trunk in a smooth, controlled fashion, completing the eccentric phase in 4 seconds. Next, patients raise their torso during the concentric phase in 2 seconds and are verbally encouraged by the supervising therapist to perform as many repetitions as possible. When patients complete 20 to 25 or more repetitions at one angle, resistance is increased. Gradual PRE is achieved by altering the angle setting and the subject’s hand position.²⁵ Internally rotating the hips and accentuating lumbar intersegmental extension during lumbar extension further optimizes targeted muscle activity.²⁵

Conditioning Lumbar Musculature

The lumbar extensor muscles have long been considered the “weak link” in lower trunk function.⁹ For patients with CLBP, the lumbar extensors have been described as weak, atrophied, and highly fatigable, displaying abnormal activation patterns and excessive fatty infiltration.^15,28–30 It is therefore reasonable to focus on conditioning these muscles through PREs to improve their physiologic and structural integrity. Reversal of muscular dysfunction and structural abnormalities has been documented in patients with CLBP following lumbar extensor strengthening exercises.^16,31 Both isolation and progressive overload of the lumbar extensors are necessary to achieve these benefits.

Lumbar extensor PREs on machines, benches, or Roman chairs likely provide sufficient isolation and overload to improve lumbar muscular strength and endurance.^9,20,26 Whether or not appropriate isolation and overload actually occur from low load floor exercise (e.g., lumbar stabilization) or stability ball exercise is currently unknown. In a recent study by Sung,³² no change in the fatigue status of the lumbar multifidus was noted after a 4-week supervised stabilization exercise program. In another study, the effect of three different exercise-training modalities on the cross-sectional area of the lumbar extensor muscles of patients with CLBP was assessed.³³ Stabilization exercise alone was not sufficient to change lumbar muscle cross-sectional area but when combined with dynamic and isometric intensive resistance training, an increase in cross-sectional area of the musculature was noted.

Disc Metabolism

Disc metabolism could potentially be enhanced through repetitive exercise involving the spine’s full active range of motion, as performed with lumbar extensor strengthening exercises. Given that the absence of physical activity is known to reduce a healthy metabolism, it appears reasonable to believe that introducing exercise such as lumbar extensor strengthening may reverse such findings.¹ The association between biochemical abnormalities in the lumbar discs and CLBP has previously been documented.³⁴ Enhanced disc metabolism could improve repair and enhance reuptake of inflammatory and nociceptive mediators that may have migrated to an area of injury.

Psychological Mechanisms

In some patients with particularly severe or prolonged CLBP, or psychological comorbidities such as anxiety or depression, maladaptive illness behavior may become established. This type of behavior may manifest itself as fear of engaging in any activity or movement that has previously been associated with symptoms of CLBP. As time passes, virtually all activities gain this association, leading to a generalized fear of movement in an attempt to minimize exacerbations. Engaging in any form of supervised exercise therapy under the guidance of an experienced clinician able to gradually increase the type, dose, frequency, or intensity of movements can help break this cycle and demonstrate that not all movements are painful. This aspect is discussed further in other chapters of this text (e.g., Section 8—Behavioral Therapies).