Physiotherapy - The Journal of Indian Association of Physiotherapists

REVIEW OF LITERATURE
Year
: 2017  |  Volume : 11  |  Issue : 1  |  Page : 12--16

Extensor trunk muscle activity during stabilization exercises: An update


Jayshree Shah, Zubia Veqar 
 Center for Physiotherapy and Rehabilitation Sciences, Jamia Millia Islamia, New Delhi, India

Correspondence Address:
Zubia Veqar
Center for Physiotherapy and Rehabilitation Sciences, Jamia Millia Islamia, New Delhi
India

Abstract

Stabilization exercises are commonly employed exercises and are popular due to reduction in spinal loads and for strengthening of trunk extensors. Different stabilization exercises are performed in various position and using different strategies. Local muscles of trunk as well as global muscles both are activated during these exercises. Activation of local muscle such as multifidus as compared to global muscles is now being emphasized for treating low back pain (LBP). Varying lumbar lordosis during stabilization exercises has shown to increase the activity of lumbar multifidus as compared to longissimus thoracis muscle which is a global muscle. In addition, a lordotic posture causes lesser reduction in moment arm of erector spinae and reduced muscle activation. The lumbar multifidus is an important stabilizer of the neutral zone, and dysfunction in these muscles is strongly associated with LBP. Lumbar multifidus has also shown to be selectively and bilaterally atrophied in LBP. Electromyography of trunk muscles during stabilization exercises will provide a means by which the muscle activation levels can be analyzed and can assist therapist in selecting appropriate exercise for the muscle which is commonly atrophied in chronic LBP. Exercises promoting targeted activation of atrophied muscles will result in effective rehabilitation for LBP. The extensor muscle activity during stabilization exercises will help providing a potential for progression and planning rehabilitation protocol for the specific muscle.



How to cite this article:
Shah J, Veqar Z. Extensor trunk muscle activity during stabilization exercises: An update.Physiother - J Indian Assoc Physiother 2017;11:12-16


How to cite this URL:
Shah J, Veqar Z. Extensor trunk muscle activity during stabilization exercises: An update. Physiother - J Indian Assoc Physiother [serial online] 2017 [cited 2020 Sep 25 ];11:12-16
Available from: http://www.pjiap.org/text.asp?2017/11/1/12/213277


Full Text



 Introduction



Chronic low back pain (CLBP) is characterized by atrophy of paraspinal muscle.[1] Cumulative evidences suggest that along with reduction in cross-sectional areas; there are fatty in filtration within the paraspinal muscles.[2] Studies also confirm that the function and cooperation of the stabilizing muscles of the lumbar spine are often impaired in patients with CLBP.[3] When compared to other paraspinal muscles, the morphological changes in the lumbar multifidus are specific and isolated.[4] A reduction of 18.1% in muscle volume compared to only 3.9% reduction in erector spinae muscle volume.[4] There have been evidences of atrophy and impairment of lumbar multifidus muscle in low back pain (LBP) but whether this atrophy is present at the site of pain or is more widespread is not clear as there are evidences for both propositions. Variable changes that are associated in paraspinal muscles in CLBP are changes in muscle fiber type, including reduction in Type I and II muscle fiber size, reduction in cross-sectional area of Type II muscle fiber. These changes in the muscle are being implicated as a contributing factor for the development as well as recurrence of LBP.[5] Lumbar multifidus muscle forms a part of local group of muscles with its primary role in controlling the curvature of the spine and giving lateral and sagittal stiffness to maintain mechanical stability at the lumbar spine.[6] For rehabilitating, CLBP stabilization exercise has been employed since they are thought to exert minimal loads on spinal column. These stabilization exercises are used to strengthen the muscles by coactivations pattern which involves holding the spine with minimal movement affecting.[7] Different strategies and positions are used for stabilization exercises such as in quadruped position, prone position, and supine position, bridge position, and use of labile surfaces. The level of activation of trunk muscles varies with each strategy and position. The level of trunk extensor muscle activation is also dependent on the lumbar posture as the neural control system appears to adjust the level of motor activities in the stabilizing muscles of the trunk.[8] Lumbar posture has shown to highly influences patterns of trunk muscle activity,[9],[10] but its influence during stabilization exercise has not been studied although the loading characteristic during these activity differ but the mechanics of cocontraction between local and global muscle are similar. Varying lumbar lordosis during exercise can be used as a progression and enhancing extensors activation by making them more active and in favorable position for exerting force.[9] Assessment of these trunk extensor muscles in CLBP is essential for not only planning a rehabilitation protocol but also for evaluating dysfunctions, for documentation of the efficacy of exercise programs, and for providing information to patients on their ability and potential for progressing. Use of surface electromyography (sEMG) for assessing objectively trunk muscle performance has been considered a best tool as it allows for comparison as well as investigate simultaneously different trunk extensors muscle.[11] For assessing trunk extensor muscle performance, sEMG is at times coupled to the endurance field test such as Sorensen and with dynamometric static as well as with dynamic tests (arch-up tests) and have been very frequently used in the literature.[12],[13],[14] EMG unable assessing trunk extensor muscle activation and understanding the role of individual muscle during stabilization exercise by placing the sEMG within the border of the muscle along the muscle orientation.

 Stabilization Exercises



According to McGill et al, “stabilization exercise is any exercise that challenges the spine stability while grooving trunk co-activation patterns that ensures a stable spine.”[15] For augmenting the extensor muscle performance strategies, such as use of unstable surface, exerting in prone lying, in quadruped position are mostly employed. One of such strategies is abdominal hollowing which increase local/global ratio and local muscles activity of multifidus as well as transversus abdominis during stair climbing when compared with stair climbing without abdominal hollowing.[16] This cocontraction between local muscles is necessary for proving stabilization before any movement.[16] Stabilization exercises in prone positions recruits not only the local muscle (lumbar multifidus) but also other global extensors that can potentially increase spinal loading. A study by Callaghan et al. determined that at L4/L5 anteroposterior joint shear force values on the lumbar spine prone exercises were approximately 250 N and was higher than other exercises.[17] such an increase in spinal loading could exacerbate LBP. Stabilization exercises in bridge position are frequently used for the stabilization of lumbopelvic in patients with LBP; the focus of this is on retraining the muscle coordination patterns, in which an optimal ratio is achieved between the local segmental stabilizers and global torque producing muscles.[18] When bridging exercises are performed on ring roller the activity ratio of multifidus to iliocostalis is increased in middle-aged women, indicating higher ratio of local muscle to global muscle.[19] There is a difference in muscle activation between supine and prone bridging. The prone bridging exercises recruit trunk extensors whereas the supine bridges recruit anterior trunk muscles.[20] Many studies have also employed labile surfaces for the activation of trunk extensors and the rationale behind the use of labile surfaces during stabilization exercises is based on the potential of maintaining sufficient control of spine and whole-body motion by increasing the demand on neuromuscular system. Although it increases trunk extensor activation is not clear because of difference in methodology used in various study. Use of labile surface such as exercise ball during stabilization exercise in healthy adults has also shown reduce to reduce muscle challenge because of reduced cocontraction between flexors and extensor muscles as well as reduced spinal loading that is compression and anterior-posterior shear forces when compared to exercising on flat surface, this is in contrast to usual assumption that using unstable surface challenges muscle activity and enhances muscle contraction,[21] a similar finding is also seen in chronic patients with LBP with no difference in their level of disability while performing exercises either on labile or stable surfaces which suggest that the intensity of exercise on labile surfaces is no difference in exercises performed on stable surface.[22] The use of the labile surface in CLBP during stabilization exercises has shown no increases trunk muscle activity or decreased whole-body balance in comparison to healthy adults but shows adaptations in recruitment patterns of muscles with higher and lower activity of trunk muscles which is task specific.[14] The addition of unstable devices shows no added benefit on recruitment of multifidus when compared to its counterpart stable surface instead their carry a risk of harm and its use is not advised.[10] Stabilization exercises when performed in quadruped position with trunk in horizontal and hands and knees touching the ground are used for the initial stages of rehabilitation and the most commonly employed. These positions reduce spinal loads and help in recruiting specific trunk muscles.[12] In a quadruped stabilization, exercise gravity plays a role by affecting its muscle activity and its sensitivity to stretch receptors increases as a result muscle contracts strongly in this position.[23] A study by Pirouzi et al. while comparing abdominal and lumbar muscle activation during quadruped stabilization exercise showed that showed that the mean activation of abdominal and lumbar muscles was different with the highest activity of transversus abdominis muscle and multifidus muscles showed the lowest activation pattern.[24] A higher activation potential of multifidus is seen in alternate arm and leg exercise in quadruped position, this is because of reduce base of support which creates a unstable status and a higher torque of arm and leg which in turns increases muscle activation.[25] Another potential benefit of using quadruped alternate arm and leg exercise is that the negative shear forces created at L4/L5 during the exercise is approximately − 200 N which is lesser than the commonly used prone trunk extension exercise which is approximately - 250 N.[17] Thus alleviating vertebral forces and spinal loads during the exercise. Use of leg extension during quadruped exercise reduces spinal load as only one side of the extensors at a time dominates the contraction level during the exercise[17] making the exercise safe in low back. Out of the different stabilization exercises, all exercises tend to activate global muscles along with local trunk muscle. Despite the existing evidence that only multifidus is atrophied in CLBP, the exercises used in rehabilitation tend to activate other extensors with similar level of activation.

 Effect of Lordosis on Trunk Extensor Activity



Lumbar posture (lordosis) is considered to play an important role in LBP and is of importance during rehabilitation as it affects the muscle activation pattern. There are evidence that lumbar lordosis can significantly alter the contribution of trunk extensors as shown in [Table 1]. The rationale behind increasing lumbar lordosis during stabilization exercises is based on improving muscle performance where muscle cross-sectional area and activation are most frequently impaired in persons with LBP.[26] Although its effect during stabilization exercises in CLBP has not been studied yet and is unclear. However, its effect during other position (sitting, standing, or while lifting) has been stated. The use of postural cueing to increase lumbar lordosis in healthy individuals during commonly prescribed stabilization exercises shows that there is an increase in local muscle (lumbar multifidus) activation without much increase in global muscle (longissimus thoracis) when compared to performing stabilization exercises in neutral position.[26] In a simulation study by Bae and Mun, 2010 during isokinetic exercise in healthy males showed that with increased lordosis, the joint torque was 16.6% larger than normal lordosis, the anterior-posterior shear forces was higher for hypolordosis than for increased lordosis, the lordosis did not affect the compressive intervertebral forces at each lumbar joint and for the required muscle force during isokinetic exercise for longissimus thoracis was reduced as lordosis was increased.[29] In LBP population, pain associated with sitting posture that involves thoracolumbar lordosis is said to produce excessive activation of the global muscle and minimal activation of local muscle such as transversus abdominis and multifidus, suggesting such postures to be used with precaution in such population.[30] Spinal curvatures at the lumbar and thoracolumbar region in a sagittal balance postures are one of the major determinants of regional extensor muscle activity as can be seen in healthy individual while maintaining four different spinal sitting postures, a flat posture which is a minimal curve at thoracolumbar and lumbar region, long lordosis which is lordotic curve at both region, short lordosis which flat/kyphotic at thoracolumbar and lordotic lumbar curve and a slump posture; there has been an incremental rise EMG activity of lumbar multifidus in these sagittal balance posture flat, long lordosis, and short lordosis, with highest activity observed in the short lordosis, whereas the longissimus thoracic muscle remained same between the postures which demonstrated the unique role of multifidus among other extensors for mild and subtle adjustment within the lumbar lordosis.[31] Stabilization exercises are used with different strategies and position (supine, prone, and quadruped position). Out of the different stabilization exercises, all exercises tend to activate global muscles along with local trunk muscle. Although exercises intended to enhance the activity of lumbar extensors, especially the lumbar multifidus produce similar level of activation across other global extensors. Hence, devising an exercise protocol that selectively recruits multifidus without concomitant increase in other extensor will reduce spinal loading and prevent structural damage as with higher activation of global muscles the spinal loading tends to increase.[31]{Table 1}

 Conclusion



Despite of the existing evidences that only multifidus is atrophied in CLBP, the exercises used in rehabilitation tend to activate other extensors with similar level of activation. Increased lumbar lordosis through postural cueing results in increase in lumbar multifidus muscle without much increase in other extensor. Although the effect of performing stabilization exercise with increased lumbar lordosis on multifidus activation and longissimus thoracis is unknown in CLBP. Moreover, future research on CLBP population is required to determine the efficacy of incorporating lumbar posture during stabilization exercises and to determine the level of extensor muscle recruitment and enhancing the finding of previous studies for providing a stimulus for the activation of lumbar multifidus.[62]

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

References

1Danneels LA, Vanderstraeten GG, Cambier DC, Witvrouw EE, De Cuyper HJ. CT imaging of trunk muscles in chronic low back pain patients and healthy control subjects. Eur Spine J 2000;9:266-72.
2Mengiardi B, Schmid MR, Boos N, Pfirrmann CW, Brunner F, Elfering A, et al. Fat content of lumbar paraspinal muscles in patients with chronic low back pain and in asymptomatic volunteers: Quantifi cation with MR spectroscopy. Radiology 2006;240:786-92.
3Panjabi MM. A hypothesis of chronic back pain: Ligament subfailure injuries lead to muscle control dysfunction. Eur Spine J 2006;15:668-76.
4Beneck GJ, Kulig K. Multifi dus atrophy is localized and bilateralin active persons with chronic unilateral low back pain. Arch Phys Med Rehabil 2012;93:300-6.
5Hodges P, Holm AK, Hansson T, Holm S. Rapid atrophy of thelumbar multifi dus follows experimental disc or nerve root injury. Spine (Phila Pa 1976) 2006;31:2926-33.
6Akuthota V, Ferreiro A, Moore T, Fredericson M. Core stabilityexercise principles. Curr Sports Med Rep 2008;7:39-44.
7Vera-Garcia FJ, Barbado D, Moya M. Trunk stabilization exercisesfor healthy individuals. Rev Bras Cineantropom Desempenho Hum 2014;16:200-11.
8O'Sullivan PB, Grahamslaw KM, Kendell M, Lapenskie SC, Möller NE, Richards KV. The effect of different standing andsitting postures on trunk muscle activity in a pain-free population. Spine (Phila Pa 1976) 2002;27:1238-44.
9Mayer JM, Verna JL, Manini TM, Mooney V, Graves JE. Electromyographic activity of the trunk extensor muscles: Effectof varying hip position and lumbar posture during Roman chair exercise. Arch Phys Med Rehabil 2002;83:1543-6.
10Martuscello JM, Nuzzo JL, Ashley CD, Campbell BI, Orriola JJ, Mayer JM. Systematic review of core muscle activity during physical fitness exercises. J Strength Cond Res 2013;27:1684-98.
11Konrad P. The ABC of EMG. Scottsdale, AZ: Noraxon; 2005. Available from: http://www.noraxon.com/wp-content/uploads/2014/12/ABC-EMG-ISBN.pdf. [Last retrieved on 2016 Dec 29].
12Stevens VK, Vleeming A, Bouche KG, Mahieu NN, Vanderstraeten GG, Danneels LA. Electromyographic activity oftrunk and hip muscles during stabilization exercises in four-point kneeling in healthy volunteers. Eur Spine J 2007;16:711-8.
13Demoulin C, Vanderthommen M, Duysens C, Crielaard JM. Spinalmuscle evaluation using the Sorensen test: A critical appraisal of the literature. Joint Bone Spine 2006;73:43-50.
14Desai I, Marshall PW. Acute effect of labile surfaces during corestability exercises in people with and without low back pain J Electromyogr Kinesiol 2010;20:1155-62.
15McGill SM, Grenier S, Kavcic N, Cholewicki J. Coordination of muscle activity to assure stability of the lumbar spine. J Electromyogr Kinesiol 2003;13:353-9.
16Lee AY, Kim EH, Cho YW, Kwon SO, Son SM, Ahn SH. Effects of abdominal hollowing during stair climbing on the activations of local trunk stabilizing muscles: A cross-sectional study. Ann Rehabil Med 2013;37:804-13.
17Callaghan JP, Gunning JL, McGill SM. The relationship betweenlumbar spine load and muscle activity during extensor exercises. Phys Ther 1998;78:8-18.
18Kang H, Jung J, Yu J. Comparison of trunk muscle activity duringbridging exercises using a sling in patients with low back pain. J Sports Sci Med 2012;11:510-5.
19Jeon JK, Lim HS, Shin SR, Kim BH, Lee SM. Effects of threebridging exercises on local and global muscles of middle agedwomen. J Phys Ther Sci 2013;25:853-6.
20Schellenberg KL, Lang JM, Chan KM, Burnham RS. A clinical tool for office assessment of lumbar spine stabilization endurance:Prone and supine bridge maneuvers. Am J Phys Med Rehabil 2007;86:380-6.
21Drake JD, Fischer SL, Brown SH, Callaghan JP. Do exerciseballs provide a training advantage for trunk extensor exercises?A biomechanical evaluation. J Manipulative Physiol Ther 2006;29:354-62.
22Marshall P, Murphy B. Self-report measures best explain changes in disability compared with physical measures after exercise rehabilitation for chronic low back pain. Spine (Phila Pa 1976) 2008;33:326-38.
23Emami F, Pirouzi S, Taghizadeh S. Comparison of abdominal and lumbar muscles electromyography activity during two types of stabilization exercises. J Res Med Sci 2015;17:4.
24Pirouzi S, Emami F, Taghizadeh S, Ghanbari A. Is abdominal muscle activity different from lumbar muscle activity during four-point kneeling? Iran J Med Sci 2013;38:327-33.
25Huang YH, Harn IC, Yang CH, Guo LY. The EMG activationselectivity of unilateral spinal extensor for different lumbar stabilization exercises. J Biomech 2007;40:S688.
26Beneck GJ, Story JW, Donald S. Postural cueing to increase lumbar lordosis increases lumbar multifi dus activation during trunk stabilization exercises: Electromyographic assessment using intramuscular electrodes. J Orthop Sports Phys Ther 2016;46:293-9.
27Claus AP, Hides JA, Moseley GL, Hodges PW. Different ways tobalance the spine: Subtle changes in sagittal spinal curves affect regional muscle activity. Spine (Phila Pa 1976) 2009;34:E208-14.
28O'Sullivan PB, Dankaerts W, Burnett AF, Farrell GT, Jefford E, Naylor CS, et al. Effect of different upright sitting postures on spinal-pelvic curvature and trunk muscle activation in a pain-free population. Spine (Phila Pa 1976) 2006;31:E707-12.
29Bae TS, Mun M. Effect of lumbar lordotic angle on lumbosacral joint during isokinetic exercise: A simulation study. Clin Biomech (Bristol, Avon) 2010;25:628-35.
30O'Sullivan PB. Lumbar segmental 'instability': Clinicalpresentation and specific stabilizing exercise management. Man Ther 2000;5:2-12.
31McGill SM. Low back exercises: Evidence for improving exercise regimens. Phys Ther 1998;78:754-65.
32Danneels LA, Vanderstraeten GG, Cambier DC, Witvrouw EE, De Cuyper HJ. CT imaging of trunk muscles in chronic low back pain patients and healthy control subjects. Eur Spine J 2000;9:266-72.
33Mengiardi B, Schmid MR, Boos N, Pfirrmann CW, Brunner F, Elfering A, et al. Fat content of lumbar paraspinal muscles in patients with chronic low back pain and in asymptomatic volunteers: Quantification with MR spectroscopy. Radiology 2006;240:786-92.
34Panjabi MM. A hypothesis of chronic back pain: Ligament subfailure injuries lead to muscle control dysfunction. Eur Spine J 2006;15:668-76.
35Beneck GJ, Kulig K. Multifidus atrophy is localized and bilateral in active persons with chronic unilateral low back pain. Arch Phys Med Rehabil 2012;93:300-6.
36Hodges P, Holm AK, Hansson T, Holm S. Rapid atrophy of the lumbar multifidus follows experimental disc or nerve root injury. Spine (Phila Pa 1976) 2006;31:2926-33.
37Akuthota V, Ferreiro A, Moore T, Fredericson M. Core stability exercise principles. Curr Sports Med Rep 2008;7:39-44.
38Vera-Garcia FJ, Barbado D, Moya M. Trunk stabilization exercises for healthy individuals. Rev Bras Cineantropom Desempenho Hum 2014;16:200-11.
39O'Sullivan PB, Grahamslaw KM, Kendell M, Lapenskie SC, Möller NE, Richards KV. The effect of different standing and sitting postures on trunk muscle activity in a pain-free population. Spine (Phila Pa 1976) 2002;27:1238-44.
40Mayer JM, Verna JL, Manini TM, Mooney V, Graves JE. Electromyographic activity of the trunk extensor muscles: Effect of varying hip position and lumbar posture during Roman chair exercise. Arch Phys Med Rehabil 2002;83:1543-6.
41Martuscello JM, Nuzzo JL, Ashley CD, Campbell BI, Orriola JJ, Mayer JM. Systematic review of core muscle activity during physical fitness exercises. J Strength Cond Res 2013;27:1684-98.
42Konrad P. The ABC of EMG. Scottsdale, AZ: Noraxon; 2005. Available from: http://www.noraxon.com/wp-content/uploads/2014/12/ABC-EMG-ISBN.pdf. [Last retrieved on 2016 Dec 29].
43Stevens VK, Vleeming A, Bouche KG, Mahieu NN, Vanderstraeten GG, Danneels LA. Electromyographic activity of trunk and hip muscles during stabilization exercises in four-point kneeling in healthy volunteers. Eur Spine J 2007;16:711-8.
44Demoulin C, Vanderthommen M, Duysens C, Crielaard JM. Spinal muscle evaluation using the Sorensen test: A critical appraisal of the literature. Joint Bone Spine 2006;73:43-50.
45Desai I, Marshall PW. Acute effect of labile surfaces during core stability exercises in people with and without low back pain. J Electromyogr Kinesiol 2010;20:1155-62.
46McGill SM, Grenier S, Kavcic N, Cholewicki J. Coordination of muscle activity to assure stability of the lumbar spine. J Electromyogr Kinesiol 2003;13:353-9.
47Lee AY, Kim EH, Cho YW, Kwon SO, Son SM, Ahn SH. Effects of abdominal hollowing during stair climbing on the activations of local trunk stabilizing muscles: A cross-sectional study. Ann Rehabil Med 2013;37:804-13.
48Callaghan JP, Gunning JL, McGill SM. The relationship between lumbar spine load and muscle activity during extensor exercises. Phys Ther 1998;78:8-18.
49Kang H, Jung J, Yu J. Comparison of trunk muscle activity during bridging exercises using a sling in patients with low back pain. J Sports Sci Med 2012;11:510-5.
50Jeon JK, Lim HS, Shin SR, Kim BH, Lee SM. Effects of three bridging exercises on local and global muscles of middle aged women. J Phys Ther Sci 2013;25:853-6.
51Schellenberg KL, Lang JM, Chan KM, Burnham RS. A clinical tool for office assessment of lumbar spine stabilization endurance: Prone and supine bridge maneuvers. Am J Phys Med Rehabil 2007;86:380-6.
52Drake JD, Fischer SL, Brown SH, Callaghan JP. Do exercise balls provide a training advantage for trunk extensor exercises? A biomechanical evaluation. J Manipulative Physiol Ther 2006;29:354-62.
53Marshall P, Murphy B. Self-report measures best explain changes in disability compared with physical measures after exercise rehabilitation for chronic low back pain. Spine (Phila Pa 1976) 2008;33:326-38.
54Emami F, Pirouzi S, Taghizadeh S. Comparison of abdominal and lumbar muscles electromyography activity during two types of stabilization exercises. J Res Med Sci 2015;17:4.
55Pirouzi S, Emami F, Taghizadeh S, Ghanbari A. Is abdominal muscle activity different from lumbar muscle activity during four-point kneeling? Iran J Med Sci 2013;38:327-33.
56Huang YH, Harn IC, Yang CH, Guo LY. The EMG activation selectivity of unilateral spinal extensor for different lumbar stabilization exercises. J Biomech 2007;40:S688.
57Beneck GJ, Story JW, Donald S. Postural cueing to increase lumbar lordosis increases lumbar multifidus activation during trunk stabilization exercises: Electromyographic assessment using intramuscular electrodes. J Orthop Sports Phys Ther 2016;46:293-9.
58Claus AP, Hides JA, Moseley GL, Hodges PW. Different ways to balance the spine: Subtle changes in sagittal spinal curves affect regional muscle activity. Spine (Phila Pa 1976) 2009;34:E208-14.
59O'Sullivan PB, Dankaerts W, Burnett AF, Farrell GT, Jefford E, Naylor CS, et al. Effect of different upright sitting postures on spinal-pelvic curvature and trunk muscle activation in a pain-free population. Spine (Phila Pa 1976) 2006;31:E707-12.
60Bae TS, Mun M. Effect of lumbar lordotic angle on lumbosacral joint during isokinetic exercise: A simulation study. Clin Biomech (Bristol, Avon) 2010;25:628-35.
61O'Sullivan PB. Lumbar segmental 'instability': Clinical presentation and specific stabilizing exercise management. Man Ther 2000;5:2-12.
62McGill SM. Low back exercises: Evidence for improving exercise regimens. Phys Ther 1998;78:754-65.