A high-intensity lumbar extensor strengthening program is little better than a low-intensity program or a waiting list control group for chronic low back pain: a randomised clinical trial.

Introduction

Non-specific low back pain is, together with psychological overload, the disorder most frequently diagnosed during office hours by Dutch military company doctors (Royal Netherlands Army 1998). Hence, effective intervention for this disorder as well as prevention strategies are of great importance for army personnel and in particular for soldiers with physically demanding job tasks such as repetitive lifting and carrying, marching, and digging. Exercise therapy is currently the most popular treatment prescribed by doctors and physiotherapists of the Royal Netherlands Army for low back pain. An advantage of this treatment approach is that it matches well with the relatively active lifestyle habits of many soldiers.

Previous research has indicated that exercise therapy is beneficial for the treatment of chronic and recurrent low back pain (Koes et al 2001). Specific dynamic extension training of the lumbar back muscles is the exercise therapy commonly applied at our department in cases of subacute and chronic low back pain. We thereby aim at increasing the cross-sectional area, strength, and endurance of the back extensor muscles, improvement of co-ordination, and reduction of fear of movement in order to improve disability. Randomised prospective studies have shown that dynamic extension exercises may indeed cause physiological effects with regard to strength and endurance (Rissanen et al 1995, Storheim et al 2003, Verna et al 2002). In a comprehensive Finnish cohort study comprising 535 subjects, Rissanen et al (2002) also showed that poor dynamic trunk extension performance was associated with back-related permanent work disability. The working mechanisms of dynamic extension exercises do not involve merely physiological adaptations, but probably also psychological phenomena such as reduction in psychological distress, fear-avoidance beliefs, and fear of movement (Mannion et al 2001a, Mannion et al 2001c). The supposed benefits of dynamic extension exercises, however, need firmer scientific confirmation in randomised controlled trials which also address the optimal intensity of the dynamic extension exercises for low back pain.

In order to investigate the effectiveness of our dynamic extension exercise programs, we conducted a randomised controlled trial among Royal Netherlands Army personnel with chronic low back pain. Our research questions were:

1. Is eight weeks of high-intensity strengthening of the isolated lumbar extensors more effective than low-intensity strengthening or no strengthening (ie, waiting list control)?

2. Are any gains maintained 16 weeks after the cessation of intervention?

Method

Design

A three-arm observer-blinded randomised controlled trial was carried out among employees with chronic low back pain in the Royal Netherlands Army. The source population (n = 37 000) was informed about the planned study in the second half of 1998 by advertisements in military union media calling for volunteers to participate. After the informed consent procedure, all volunteers had their history taken by a physiotherapist, were physically examined by a sports physician, and had their isometric back strength measured by a human movement scientist. Subsequently, eligible participants were randomly assigned to the high-intensity training group, the low-intensity training group or the waiting list control group. Concealed randomisation was performed by means of a computer-generated table of random numbers with a block size of 6. A researcher who was not involved in the randomisation or measurement procedure provided the table of random numbers. Eight weeks after the initial randomisation, the participants in the waiting list group were randomised again, using the same concealment, to either the high-intensity training-program or the low-intensity training-program. Outcomes for the high-intensity training and low-intensity training groups were measured before randomisation (baseline), at 8 weeks (end of intervention), and at 24 weeks after randomisation (follow-up). For the waiting list-group, outcomes were measured before the first randomisation (baseline), at 8 weeks (waiting list period), at 16 weeks (end of intervention), and at 32 weeks after the first randomisation (follow-up). Measurement of outcomes was carried out by the principle investigator or research assistants who were not aware of group allocation. Both intervention and measurement were carried out at the Department of Training Medicine and Training Physiology in Utrecht, the Netherlands. The study protocol was reviewed and ethically approved by the inspectorate of the army medical services.

Participants

The inclusion criteria specified that participants should: be male employees of the Royal Netherlands Army in the age range of 18-54 years; have experienced low back pain for more than 12 weeks; be available to visit our department 1 to 2 times a week during eight consecutive weeks; and should be willing to abandon other treatment interventions for the lower back during the intervention period. Potential participants were excluded if they: had undergone spinal surgery in the last 2 years; reported severe back pain that was hindering them in performing maximal isometric strength efforts; or had radiation below the knee with signs of nerve root compression (Faas et al 1996).

Intervention

The high-intensity training group received an 8-week, progressive resistance exercise program for the isolated lumbar extensor muscle groups. The first two weeks provided two training sessions per week and the following six weeks one training session per week. The initial load was set at approximately 50% of the maximal isometric lumbar extension strength of the participant, as measured at baseline. The goal of every session was to perform 15 to 20 repetitions on the lower back machine. If the participant was able to perform more than 20 repetitions, a 2.5 kg weight was added at the next session. Conversely, the load was lowered by 2.5 kg if the participant was unable to perform 15 repetitions. This protocol was based partly on existing protocols (Pollock et al 1989, Graves et al 1990, Graves et al 1994), and partly on our own clinical experience (see Appendix 1 on the eAddenda for the complete trial method).

[FIGURE 1 OMITTED]

The low-intensity training group received an 8-week, non-progressive, low-intensity resistance exercise program. Throughout the eight weeks, the load was set at a maximum of 20% of the maximal isometric lumbar extension strength, as measured at baseline. Every session, one set of 15 or 20 repetitions was performed on the lower back machine. We assumed that resistance training on this load would not provide a physiological strength-training stimulus.

In both the high-intensity strengthening program and the low-intensity program, training was carried out on a modified lower back machine (a) (Figure 1). The main modifications consisted of a fixation of pelvis and hips to isolate the lower back, and a change in load curve during the training. Details of the modifications of the standard lower back machine have been published earlier (Helmhout et al 2004). A load curve adjuster was used as an extra modification. The load curve adjuster allows adjustment of the workload throughout the full range of motion of the lower back. In our previous study, we saw that some participants in the high-intensity training group were not able to make full extension, having inadequate strength in that particular part of the required range of motion of the back (Helmhout et al 2004). A load curve adjuster with a beginning support or an end support made it possible for them to move in the whole range of motion until exhaustion.

All sessions in both programs were supervised by the same physiotherapist, who was kept unaware of the results of the outcome measurements. Every training session was preceded by a 5-minute warm-up on an arm/leg ergometer (c). During the training, special attention was paid to the technique in terms of pace and movement. The flexion and extension of the lower back had to be executed in the full individual range of motion. Movements had to be slow and controlled: moving in two seconds from maximal flexion to maximal extension (concentric contraction and lifting the weight), and returning from maximal extension to maximal flexion in four seconds. The weight load used and the number of repetitions during each session were recorded.

Participants who were assigned to the waiting list-group received no intervention for their low back pain during the first eight weeks.

[FIGURE 2 OMITTED]

Outcome measures

Primary outcomes in this study were global perceived effect and disability. Global perceived effect was expressed as the self-perceived percentage increase or decrease in the degree of back symptoms. Disability was measured by the original Roland-Morris Disability Questionnaire, a 24-item scale with scores ranging from 0 to 24 points, where high scores indicate higher disability as a result of low back pain (Roland and Morris 1983). Secondary outcomes were: health-related quality of life, as measured by the Medical Outcome 36-item Short Form Health Survey (SF-36) (Shmueli 1998, van der Zee 1993), fear of movement/(re-) injury, as measured by the Tampa Scale for Kinesiophobia (Kori 1991, Vlaeyen et al 1995), and isometric back extension strength. The scores for the SF-36 range from 0 to 100% and indicate self-experienced health-related quality of life.

Items are grouped into eight domains from which an overall summary score, a physical component score, and a mental component score can be derived. A high score on each of the scales reflects a high level of self-experienced health. The Tampa Scale for Kinesiophobia is a 17-item scale with scores ranging from 17 to 68 points, measuring the extent to which a chronic back patient fears physical damage due to movement. High scores indicate a high degree of fear of movement/(re-)injury. Isometric back extension strength is evaluated with the same modified lower back training device as used in the exercise programs. A force transducer (b) was used for the measurement of net isometric extension strength in five angles throughout the range of motion. Modifications and protocols used for the isometric strength measurements have been described in detail elsewhere (Helmhout et al 2004).

Data analysis

In order to calculate sample size we judged a mean difference between the groups of 2 points (SD = 4) improvement on the Roland-Morris Disability Questionnaire as clinically important (Stratford et al 1998). Assuming a SD of 4, for power of 0.80 and a significance level of 0.05, a target population of 64 participants was needed for each group, ie, 192 participants in total.

Checks for missing and incorrect values were conducted prior to the analyses. All outcome measures were analysed by means of linear regression analysis (ie, analysis of covariance). Baseline values of the outcome measures were incorporated in the linear regression model as covariates, in order to correct for potential regression to the mean.

First, we compared the outcomes at 8 weeks of the high-intensity training group, the low-intensity training group and the waiting list group. Second, participants from the waiting list-group, who were randomly assigned to either a high-intensity strengthening program or a low-intensity strengthening program 8 weeks after initial randomisation, were analysed together with the initial high-intensity training group or the low-intensity training group. We compared the outcomes of these two newly-formed groups at 8 and 24 weeks follow-up. All analyses were carried out according to the intention-to-treat principle. Statistical significance was set at p values of less than 0.05.

Results

Flow of participants through the trial

Figure 2 shows the flow chart throughout the different phases of the trial. In the second half of 1998 and first half of 1999 a total of 71 volunteers responded to the advertisements and could be invited for the trial. Sixty-five of these 71 people met the inclusion criteria and were enrolled in the trial. For practical reasons we were not able to lengthen the time for inclusion and had to close recruitment to the trial in July 1999. Thus data were collected from March 1999 through to February 2000. In our view, the main reason for failing to recruit the targeted sample size, despite our great source population, was the travel distance to the research and training department (on average between 1 and 3 hours by car or train). The training location was in Utrecht, which is in the centre of the Netherlands. However, most Royal Netherlands Army employees work at locations in the countryside or even (temporarily) abroad on peace-keeping missions. Table 1 presents characteristics of the participants. All participants were still working, moderately disabled, and familiar with fitness and sport activities. There were only minor differences in demographic characteristics. According to these characteristics and the baseline values for the outcome measures (Table 2) we considered the three groups comparable at baseline. Six participants withdrew during the intervention period. One completed the first eight weeks of the waiting list period but could not be randomised due to a military mission; a further three withdrew during the follow-up so that 56 participants completed the trial.

Compliance with trial method

A total of 31 participants received the high-intensity strengthening program (the initial high-intensity training group and half of the waiting list control group). Only one member of this group missed one training session. The other 30 participants completed the whole program. A total of 28 participants took part in the low-intensity strengthening program (the initial low-intensity training group and half of the waiting list control group). Four of this group missed one session and only one missed three training sessions.

Twenty-three participants completed the whole program. No co-interventions were reported during both the intervention and the waiting list period.

Two participants who had attended the high-intensity strengthening program (the initial high-intensity training group and half of the waiting list control group) and three participants who had attended the low-intensity strengthening program (the initial low-intensity training group and half of the waiting list control group) sought professional medical help for their low back pain during follow-up. Further, 11 participants who had attended the high-intensity strengthening program and five participants who had attended the low-intensity strengthening program continued with exercise training, and some of them continued with the specific training. Seven participants of the high-intensity training group and nine participants of the low-intensity training group did not receive any intervention during follow-up.

Effect of intervention

Group data for the high-intensity training, low-intensity training and waiting list control groups at two measurement times (Week 0 and 8) as well as within- and between-group data are presented in Table 2. A mean difference in improvement of 7% (95% CI 1 to 13) in SF-36 overall score was found in favour of the high-intensity training group when compared to the low-intensity training group. Self-assessed decrease of back symptoms was on average 39% (95% CI 14 to 64) greater in the high-intensity training group when compared to the waiting list control- roup. A mean difference in improvement of 7% (95% CI 1 to 13) in overall SF-36 score was found in favour of the high-intensity training group when compared to the waiting list control group. No statistically significant differences in improvement between the three groups were found for any other outcome at 8 weeks.

In an additional analysis, we compared the high-intensity training and low-intensity training groups with the addition of the participants of the waiting list control group, who were randomly assigned to high-intensity strengthening or low-intensity strengthening 8 weeks after the initial randomisation. These newly-formed groups were not different with regard to demographic characteristics at baseline (data not shown). Group data for the high-intensity training and low-intensity training groups at 3 measurement times (Week 0, 8, and 24) as well as within- and between-group data are presented in Table 3. The differences in improvements between the high-intensity training and low-intensity training group at 8 and 24 weeks were not statistically significant.

Discussion

In this study we compared the effects of a high-intensity and a low-intensity strengthening program for the lumbar extensor muscles with a waiting list control group in a population of army personnel suffering from nonspecific low back pain. Most of the comparisons between the exercise groups and the waiting list control group for global perceived effect, disability, health-related quality of life, fear of movement/(re)injury, and isometric back strength showed point estimates which did not favour the two exercise groups. Immediately after intervention at 8 weeks, the high-intensity strengthening program produced a greater perceived effect than the low-intensity strengthening program. In addition, the high-intensity strengthening program produced a greater quality of life than the low-intensity strengthening program (in both analyses) but this difference had disappeared 16 weeks later.

Overall, the generally claimed beneficial influence of exercise for chronic low back pain, as suggested in systematic reviews and practitioner guidelines (Hayden et al 2005a; Koes et al 2001; Staal et al 2003), is only partly supported by the results of this trial. Significant differences in improvement were limited to global perceived effect and health-related quality of life in the short term (ie, 8 weeks) when comparing the groups with the highest contrast (high-intensity training group and waiting list control group).

Despite the robustness of the study design (ie, randomisation procedure, waiting list control group), our trial has some limitations. The main limitations of this study were the lack of statistical power and the relatively short follow-up period. The open voluntary recruitment of participants might further confine the generalisability of findings by limiting a direct comparison with a healthcare-seeking population. Voluntary recruitment might also be the cause of an overrepresentation of participants with only a moderate disability level. This is illustrated by the mean Roland-Morris Disability Questionnaire score at baseline for the total group, which ranged from 6.2 to 7.6 points (out of a maximum of 24 points). The moderate disability level may have elicited floor effects. The exercise programs may be more attractive for people with active lifestyle habits, which also means that they are less attractive and applicable to other populations.

The intensity of both exercise programs was based on existing protocols of the Royal Netherlands Army and on our own clinical experience. Given the lack of effects for strength gain, one might question whether the magnitude of the strength stimulus in the high-intensity training group (ie, one set of 15-20 repetitions twice weekly for two weeks, and once after this for another 6 weeks) was high enough to cause physiological training effects (ACSM 2007). On the other hand, the participants of this trial were not healthy adults and we expected them to be, to some extent, untrained due to their back pain, which legitimises a more careful approach. Another intensity issue concerns the total duration of all training sessions. A systematic review on strategies for using exercise therapy for chronic low back pain (Hayden et al 2005b) showed that high-dose (> 20 hours) exercise programs are more effective than low-dose (< 20 hours) exercise programs. The programs used in the present trial are low-dose programs according to the definitions of this systematic review which was published after our trial was designed (Hayden et al 2005b).

The application of exercise therapy for low back pain in clinical practice varies widely with regard to the type of exercises used, the intensity, the frequency, the duration, and whether the exercises are combined with other treatment modalities (Hayden et al 2005a, Hayden et al 2005b). Seven randomised controlled trials specifically investigated the effects of lumbar back muscle training for low back pain as we did in this study (Chok et al 1999, Hansen et al 1993, Manniche et al 1988, Manniche et al 1991, Kankaanpaa et al 1999, Mannion et al 1999, Rittweger et al 2002). Chok et al (1999) evaluated the effects of endurance training of the trunk extensor muscles 3 times a week for 6 weeks, compared to no exercises, and found no significant differences between the two groups at 6 weeks. Hansen et al (1993) compared 8 sessions of intensive dynamic back muscle exercises with conventional physiotherapy and placebo in a randomised controlled trial. They found that the intensive back exercises and physical therapy groups were significantly more effective over a 12-month period than the placebo-controlled group that had received traction and hot packs. In a trial of 105 subjects (Manniche et al 1988, Manniche et al 1991) 30 sessions of intensive dynamic back exercises over 3 months were compared with a similar program with 1/5th of the intensity and a program of mild exercises and passive modalities. The results for pain, disability and physical impairment (ie, back endurance and mobility) were in favor of the intensive exercise group. Kankaanpaa et al (1999) found that low back pain patients who underwent dynamic extensor training had significantly greater improvements in pain and disability than a group that underwent passive modalities. Other studies indicate that back muscle exercises are no more effective than other types of exercise treatments (Mannion et al 1999, Mannion et al 2001b, Rittweger et al 2002). The picture becomes even more complicated when differences in treatment intensity, treatment frequency, type of exercises, study population, follow-up period, and outcome measures between these studies are taken into account.

What emerges from these summarised study results is the tendency that strengthening of the back extensor muscles is more effective than passive modalities or doing nothing. This hypothesis is partially confirmed by the results of the present study. Further exploration is needed with regard to the optimal type, intensity, and frequency of isolated back extensor exercises, as well as to the mediating role of pain-related fear in exercise programs (Smeets et al 2006). There is still room for methodologically-sound randomised controlled trials with sufficient statistical power studying the effects of isolated back extensor exercises for low back pain.

eAddenda: Appendix 1 Trial Method available at www. physiotherapy.asn.au

Acknowledgements: The authors would like to thank Peter de Putter for the software of the measurement of isometric extension strength. Also, we would like to thank Kol. R Roelofs for authorising this study.

References

Beurskens AJ, de Vet HC, Koke AJ, van der Heijden GJ, Knipschild PG (1995) Measuring the functional status of patients with low back pain. Assessment of the quality of four disease-specific questionnaires. Spine 20: 1017-1028.

Chok B, Lee R, Latimer J, Tan SB (1999) Endurance training of the trunk extensor muscles in people with subacute low back pain. Physical Therapy 79: 1032-1042.

Graves JE, Pollock ML, Foster D, Leggett SH, Carpneter DM, Voso R, Jones A ((1990) Effect of training frequency and specificity on isometric lumbar extension strength. Spine 15: 504-509.

Graves JE, Webb DC, Pollock ML, Matkovich J, Leggett SH, Carpenter DM, Foster DN, Cirulli J (1994) Pelvic stabilization during resistance training: Its effect on the development of lumbar extension strength. Archives of Physical Medicine and Rehabilitation 75: 210-215.

Faas ACA, Koes BW, van den Hoogen JMM, Mens JMA, Smeele LJM, Romeijnders ACM, Van der Laan JR (1996) NHG-Standaard Lage-Rugpijn. Huisarts Wet 39: 18-31.

Fairbank JC, Couper J, Davies JB, O'Brien JP (1980) The Oswestry low back pain disability questionnaire. Physiotherapy 66: 271-273.

Hansen FR, Bendix T, Skov P, Jensen CV, Kristensen JH, Krohn L, Schioeler H (1993) Intensive, dynamic back-muscle exercises, conventional physiotherapy, or placebo-control treatment of low-back pain. A randomized, observer-blind trial. Spine 18: 98-108.

Hayden JA, van Tulder MW, Malmivaara A, Koes BW (2005a) Exercise therapy for treatment of non-specific low back pain. Cochrane Database of Systematic Reviews: CD000335.

Hayden JA, van Tulder MW, Tomlison G (2005b) Systematic review: strategies for using exercise therapy to improve outcomes in chronic low back pain. Annals of Internal Medicine 142: 776-785.

Helmhout PH, Harts CC, Staal JB, Candel MJ, de Bie RA (2004) Comparison of a high-intensity and a low-intensity lumbar extensor training program as minimal intervention treatment in low back pain: a randomised trial. European Spine Journal 13: 537-547.

Koes BW, van Tulder MW, Ostelo R, Burton KA, Waddell G (2001) Clinical guidelines for the management of low back pain in primary care: an international comparison. Spine 26: 2504-2513; discussion 2513-2514.

Kankaanpaa M, Taimela S, Airaksinen O, Hanninen O (1999) The efficacy of active rehabilitation in chronic low back pain. Effect on pain intensity, self-experienced disability, and lumbar fatigability. Spine 24: 1034-1042.

Kori SH, Miller RP, Todd DD (1990) Kinisiophobia: a new view of chronic pain behavior. Pain Management Jan/Feb: 35-43.

Manniche C, Hesselsoe G, Bentzen L, Christensen I, Lundberg E (1988) Clinical trial of intensive muscle training for chronic low back pain. Lancet 2: 1473-1476.

Manniche C, Lundberg E, Christensen I, Bentzen L, Hesselsoe G: (1991) Intensive dynamic back exercises for chronic low back pain: a clinical trial. Pain 47: 53-63.

Mannion AF, Muntener M, Taimela S, Dvorak J (1999) A randomized clinical trial of three active therapies for chronic low back pain. Spine 24: 2435-2448.

Mannion AF, Junge A, Taimela S, Muntener M., Lorenzo K, Dvorak J (2001a) Active therapy for chronic low back pain: part 3. Factors influencing self-rated disability and its change following therapy. Spine 26: 920-929.

Mannion AF, Muntener M, Taimela S, Dvorak J (2001b) Comparison of three active therapies for chronic low back pain: results of a randomized clinical trial with one-year follow-up. Rheumatology (Oxford) 40: 772-778.

Pollock ML, Leggett SH, Graves JE, Jones A, Fulton M, Cirulli J (1989) Effect of resistance training on lumbar extensor strength. American Journal of Sports Medicine 17: 624-629.

Rissanen A, Kalimo H, Alaranta H (1995) Effect of intensive training on the isokinetic strength and structure of the lumbar muscles in patients with chronic low back pain. Spine 20: 333-340.

Rittweger J, Just K, Kautzsch K, Reeg P, Felsenberg D (2002) Treatment of chronic lower back pain with lumbar extension and whole-body vibration exercise: a randomized controlled trial. Spine 27: 1829-1834.

RNLA (1998) Dokterbezoek en ziekteverzuim bij verschillende krijgsmachtonderdelen. Internal publication, RNLA, Utrecht.

Roland M, Morris R (1983) A study of the natural history of back pain. Part I: development of a reliable and sensitive measure of disability in low-back pain. Spine 8: 141-144.

Shmueli A (1998) The SF-36 profile and health-related quality of life: an interpretative analysis. Quality of Life Research 7: 187-195.

Smeets RJ, Vlaeyen JW, Kester AD, Knottnerus JA (2006) Reduction of pain catastrophizing mediates the outcome of both physical and cognitive-behavioral treatment in chronic low back pain. Journal of Pain 7(4) 261-271.

Staal JB, Hlobil H, van Tulder MW, Waddell G, Burton AK, Koes BW, van Mechelen W (2003) Occupational health guidelines for the management of low back pain: an international comparison. Occupational and Environmental Medicine 60: 618-626.

Storheim K, Holm I, Gunderson R, Brox JI, Bo K (2003) The effect of comprehensive group training on cross-sectional area, density, and strength of paraspinal muscles in patients sick-listed for subacute low back pain. Journal of Spinal Disorders and Techniques 16: 271-279.

Stratford PW, Binkley JM, Riddle DL, Guyatt GH (1998) Sensitivity to change of the Roland-Morris Back Pain Questionnaire: part 1. Physical Therapy 78: 1186-1196.

Van der Zee K, Sanderman R (1993) Het meten van de algemene gezondheidstoestand met de RAND-36: Een handleiding.

Noordelijk Centrum voor Gezondheidsvraagstukken, Groningen.

Van Tulder MW, Koes BW, Bouter LM (1995) A cost-of-illness study of low back pain in the Netherlands. Pain 62: 233-240.

Verna JL, Mayer JM, Mooney V, Pierra EA, Robertson VL, Graves JE (2002) Back extension endurance and strength: the effect of variable-angle roman chair exercise training. Spine 27: 1772-1777.

Vlaeyen JW, Kole-Snijders AM, Boeren RG, van Eek H (1995) Fear of movement/(re)injury in chronic low back pain and its relation to behavioral performance. Pain 62: 363-372.

Chris C Harts (1), Pieter H Helmhout (1), Rob A de Bie (2) and J Bart Staal (2)

(1) Department of Training Medicine and Training Physiology (2) University Maastricht The Netherlands

Correspondence: CC Harts, Department of Training Medicine and Training Physiology, PO Box 90000, 3509 AA, Utrecht, The Netherlands. Email: [e-mail]

Footnotes: (a) Lower Back ROM, Technogym Inc., Italy (b) Schwinn Airdyne Pro, Balans Inc., Nieuwegein, The Netherlands (c) Digimax RS 232, Interface Mechatronic Inc., Germany.

Table 1. Number (%) of characteristics of participants.

Characteristic                         HIT        LIT        WLC
                                     (n = 23)   (n = 21)   (n = 21)

Age (yr), mean (SD)                  44 (10)    42 (10)    41 (9)
Type of work
  sedentary                          19 (83)    18 (86)    19 (90)
  physical                            4 (17)     3 (14)     2 (10)
Time since first episode of low
back pain
  3-12 mth                            2 (9)      2 (10)     2 (10)
  1-5 yr                              5 (22)     6 (29)     4 (19)
  [greater than or equal to] 5 yr    16 (70)    13 (62)    15 (71)
Radiation                             7 (30)    11 (52)    11 (52)
Therapies for low back pain since
first episode
  no therapy                          4 (17)     4 (19)     3 (14)
  one therapy                         6 (26)     7 (33)     7 (33)
  more than one therapy              13 (57)    10 (48)    11 (52)
Work absenteeism due to low back
pain currently                        7 (30)     5 (24)     8 (38)
Work absenteeism due to low back
pain in last year
  < 1 wk                             19 (83)    19 (90)    15 (71)
  1-3 wk                              1 (4)      1 (5)      3 (14)
  [greater than or equal to] 3 wk     3 (13)     1 (5)      3 (14)

HIT = high-intensity training group, LIT = low-intensity training
group, WLC = waiting list control group

Table 2. Mean (SD) for each group, mean (SD) difference within groups,
and mean (95% CI) differences between groups for all outcomes at
baseline and 8 weeks.

Outcome                         Groups

                                Week 0

                        HIT      LIT      WLC
                       n = 23   n = 21   n = 22

GPE (%)                  --       --       --

Disability
  RDQ (0 to 24)         6.2      7.6      6.5
                       (4.4)    (4.6)    (3.9)
QoL
  SF-36 total (%)        74       72       69
                        (12)     (13)     (13)
  SF-36 physical (%)     70       72       69
                        (20)     (13)     (13)
  SF-36 mental (%)       88       88       81
                        (10)     (12)     (18)
Fear of movement
  TSK (17 to 68)         35       38       39
                        (8)      (8)      (7)
Strength ([dagger])
  Torque (Nm)           220      215      213
                        (67)     (52)     (64)

Outcome                         Groups

                                Week 8

                        HIT      LIT      WLC
                       n = 20   n = 20   n = 19

GPE (%)                  --       --       --

Disability
  RDQ (0 to 24)         3.4      6.1      5.2
                       (4.0)    (6.0)    (3.9)
QoL
  SF-36 total (%)        83       75       72
                        (12)     (13)     (13)
  SF-36 physical (%)     85       72       74
                        (15)     (23)     (19)
  SF-36 mental (%)       92       89       81
                        (10)     (11)     (21)
Fear of movement
  TSK (17 to 68)         33       33       37
                        (8)      (8)      (10)
Strength ([dagger])
  Torque (Nm)           222      227      208
                        (71)     (62)     (62)

Outcome                Difference within groups

                         Week 8 minus Week 0

                        HIT      LIT      WLC

GPE (%)                  39       22       0
                        (32)     (65)     (0)
Disability
  RDQ (0 to 24)         -3.0     -2.0     -1.8
                       (2.9)    (5.7)    (4.1)
QoL
  SF-36 total (%)        10       4        5
                        (8)      (11)     (10)
  SF-36 physical (%)     15       7        22
                        (17)     (18)     (11)
  SF-36 mental (%)       5        1        0
                        (8)      (14)     (10)
Fear of movement
  TSK (17 to 68)         -2       -5       -3
                        (5)      (5)      (7)
Strength ([dagger])
  Torque (Nm)            7        12       -4
                        (37)     (46)     (33)

Outcome                         Difference between groups

                                   Week 8 minus Week 0

                         HIT minus       HIT minus       LIT minus
                            LIT             WLC             WLC

GPE (%)                     17              39              22
                        (-9 to 43)      (14 to 64)      (-4 to 47)
Disability
  RDQ (0 to 24)            -1.7            -1.4             0.3
                       (-4.3 to 0.9)   (-4.0 to 1.1)   (-2.3 to 2.8)
QoL
  SF-36 total (%)            7               7               0
                         (1 to 13)       (1 to 13)       (-6 to 6)
  SF-36 physical (%)        11               8              -3
                         (0 to 21)      (-3 to 19)      (-13 to 8)
  SF-36 mental (%)           4               7               3
                        (-3 to 10)       (0 to 13)      (-4 to 10)
Fear of movement
  TSK (17 to 68)             3               0              -2
                         (-1 to 6)       (-3 to 4)       (-6 to 1)
Strength ([dagger])
  Torque (Nm)               -5              12              16
                        (-30 to 21)     (-12 to 36)     (-9 to 42)

HIT = high-intensity training group, LIT = low-intensity training
group, WLC = waiting-list control group; Results of linear regression
analysis corrected for baseline value; GPE = global perceived
improvement; RDQ = Roland-Morris Disability Questionnaire; QoL =
Quality of Life; TSK = Tampa Scale of Kinesiophobia, tMean isometric
net muscular torque of five angles

Table 3. Mean (SD) of combined groups, mean (SD) difference within
each group, and mean (95% CI) difference between groups for all
outcomes at baseline, 8 and 24 weeks.

                                    Groups

Outcome                    Week 0            Week 8

                        HIT      LIT      HIT      LIT
                       n = 35   n = 30   n = 32   n = 28

GPE (%)                  --       --       --       --

Disability
  RDQ (0 to 24)         5.3      7.4      3.7      5.8
                       (4.2)    (4.5)    (4.4)    (5.2)
QoL
  SF-36 total (%)        73       72       80       75
                        (12)     (13)     (13)     (12)
  SF-36 physical (%)     71       67       77       71
                        (20)     (20)     (18)     (22)
  SF-36 mental (%)       87       87       90       90
                        (12)     (13)     (12)     (10)
Fear of movement
  TSK (17 to 68)         36       37       35       35
                        (9)      (8)      (10)     (8)
Strength ([dagger])
  Torque (Nm)           222      210      232      220
                        (62)     (57)     (70)     (58)

                           Groups

Outcome                    Week 24

                        HIT      LIT
                       n = 26   n = 31

GPE (%)                  --       --

Disability
  RDQ (0 to 24)         3.0      3.2
                       (3.5)    (3.5)
QoL
  SF-36 total (%)        80       79
                        (14)     (12)
  SF-36 physical (%)     80       81
                        (20)     (19)
  SF-36 mental (%)       88       87
                        (16)     (14)
Fear of movement
  TSK (17 to 68)         33       33
                        (9)      (8)
Strength ([dagger])
  Torque (Nm)           248      230
                        (74)     (86)

                           Difference within groups

Outcome                 Week 8 minus      Week 24 minus
                           Week 0            Week 0

                        HIT      LIT      HIT      LIT

GPE (%)                  30       18       50       53
                        (51)     (55)     (32)     (37)
Disability
  RDQ (0 to 24)         -1.6     -1.9     -2.5     -4.5
                       (4.6)    (4.9)    (3.2)    (4.1)
QoL
  SF-36 total (%)        7        3        6        6
                        (10)     (11)     (14)     (13)
  SF-36 physical (%)     6        3        24       19
                        (20)     (15)     (8)      (3)
  SF-36 mental (%)       4        3        1        19
                        (8)      (14)     (12)     (13)
Fear of movement
  TSK (17 to 68)         -1       -3       -3       -3
                        (5)      (6)      (7)      (7)
Strength ([dagger])
  Torque (Nm)            13       11       30       22
                        (36)     (44)     (37)     (50)

                         Difference between groups

Outcome                Week 8 minus    Week 24 minus
                          Week 0          Week 0

                       HIT minus LIT   HIT minus LIT

GPE (%)                     13              -3
                        (-14 to 40)     (-22 to 16)
Disability
  RDQ (0 to 24)            -0.9             0.9
                       (-3.2 to 1.4)   (-0.7 to 2.4)
QoL
  SF-36 total (%)            5               0
                         (0 to 10)       (-7 to 7)
  SF-36 physical (%)         4              -3
                        (-4 to 13)      (-13 to 7)
  SF-36 mental (%)           0               1
                         (-5 to 5)       (-6 to 9)
Fear of movement
  TSK (17 to 68)             2               0
                         (-1 to 5)       (-3 to 4)
Strength ([dagger])
  Torque (Nm)               12              -15
                        (-13 to 37)     (-10 to 40)

HIT = high-intensity training group, LIT = low-intensity training
group, WLC = waiting-list control group; * Results of linear
regression analysis corrected for baseline value; GPE = global
perceived improvement, RDQ = Roland-Morris Disability Questionnaire,
QoL = Quality of Life, TSK = Tampa Scale of Kinesiophobia, ([dagger])
Mean isometric net muscular torque of five angles