Pressure biofeedback unit to assess and train lumbopelvic stability in supine individuals with chronic low back pain (2024)

Abstract

[Purpose] To determine if pressure biofeedback unit readings are related to abdominalmuscle activation and centre of pressure displacement as well as to test the effects ofusing it as a biofeedback tool to control lumbopelvic motion. [Participants and Methods]Eighteen volunteers with chronic nonspecific low back pain (21.28 ± 1.41 years old) whoperformed an active straight leg raising (dynamic postural challenge) with and withoutpressure biofeedback. Changes in the pressure biofeedback unit and on centre of pressuredisplacement were assessed, as well as bilateral electromyographic abdominal muscleactivity. Participants were not allowed to use a Valsalva manoeuvre. [Results] Pressurevariation was not significantly correlated with abdominal muscle activity or withmediolateral centre of pressure displacement. When used as a biofeedback instrument, therewas a significant increase in almost all abdominal muscles activity as well as asignificant decrease in pressure variation and in mediolateral centre of pressuredisplacement while performing an active straight leg raising with a normal breathingpattern. [Conclusion] Despite not being an indicator of abdominal muscle activity ormediolateral load transfer in the supine position, the pressure biofeedback unit couldhave great relevance when used in the clinic for biofeedback purposes in individuals withlow back pain.

Keywords: Active straight leg raising, Lumbopelvic instability, Lumbopelvic pain

INTRODUCTION

Low back pain has been associated with motor control dysfunction in the area. Indicators ofthis dysfunction include decreased contraction of the transversus abdominis and multifidusmuscles¹⁾ coincident with reducedthickness of the transversus at rest and during contraction²⁾, reduced cross-sectional area of the multifidus³⁾ and fat infiltration⁴⁾. An association with altered muscle recruitment patterns⁵⁾ and transversus abdominis activationdelay⁶⁾ was also found, as well asincreased back muscle fatigue⁷⁾ and alteredkinematic patterns in the hips and lumbar area⁸⁾.

Considering these indicators and taking into account the Panjabi spinal stabilizationmodel⁹⁾ it can be assumed that both theactive and neural subsystems are directly affected which in turn leads to stress in thepassive system^{1, 9)}.

This lack of stabilization may be clinically detected using an active straight leg raising(ASLR) test, as the inability to control lumbar rotation and the mediolateral load transfer(centre of pressure displacement in the same direction) are indicators of instability inthis region^10,11,12,13).

In order to reduce lumbar spine instability and therefore diminish pain, several studieshave suggested segmental stabilization training, involving a range of exercises from localsegmental control exercises to open-chain segmental control exercises^{1, 14)}. Given thedifficulty of perceiving an isolated contraction of the transversus abdominis required forthe local segmental control exercises, biofeedback strategies using electromyography andultrasound imaging have been used. However, because these strategies are difficult to applyin many clinical environments, the use of a pressure biofeedback unit (PBU) has beensuggested. This instrument consists of a non-elastic air bladder that when placed betweenthe supporting surface and the lumbar spine, allows detection of pressure fluctuationsinherent to movements in that region^{1, 15)}.

The PBU is by nature a biofeedback instrument and most of the scientific evidence isdirected for assessing transversus abdominis muscle function in a prone position^16,17,18,19).The relation between this instrument and the transversus abdominis has also been tested in asupine position by Grooms et al.¹⁵⁾ usingultrasound imaging, employing a selective contraction of this muscle (the drawing-inmanoeuvre). Yet this study found no relation between them, indicating the need for a moredemanding task.

Another clinical use for the PBU is to help train lumbopelvic stability in individuals withchronic low back pain during open-chain segmental control exercises. In a supine position,the objective is to help individuals maintain lumbopelvic neutral position while motorcontrol is challenged by active movements of the upper or lower limbs^{1, 20)}.

The immediate effect of biofeedback on abdominal muscle activity and pelvic rotation wasonly observed by Noh et al.²¹⁾ during anactive straight leg raising test in women with low back pain. Although not the focus of theinvestigation, they observed that using a PBU for biofeedback lead to a decrease in pelvicrotation with no significant changes in abdominal muscle activity. However no PBU data wascollected to see how much pressure variation differed from performing with and without PBUbiofeedback, as well as to test if a relation between PBU and abdominal muscle activityexists.

Therefore, the aim of the current study was to evaluate during a dynamic postural challengewith the lower limb (active straight leg raising) without biofeedback, if the PBU can be anindicator of abdominal muscle activity and mediolateral centre of pressure displacement(lumbopelvic stability indicators), and also to investigate the ability of the PBU to beused as a biofeedback tool to control lumbopelvic motion in individuals with chronic lowback pain.

PARTICIPANTS AND METHODS

Individuals from 18 to 30 years of age, with chronic nonspecific low back pain wererecruited from Campus University. Chronic nonspecific low back pain was defined as pain anddiscomfort, localized below the costal margin and above the inferior gluteal folds,persisting for at least 12 weeks, that is not attributable to a recognizable, known specificpathology (e.g. infection, tumour, osteoporosis, fracture, structural deformity,inflammatory disorder, radicular or cauda equina syndrome)²²⁾. A positive ASLR test was also considered an inclusion criterion.An ASLR test was deemed to be positive when the ASLR score (0–not difficult at all;1–minimally difficult; 2–somewhat difficult; 3–fairly difficult; 4–very difficult; 5–unableto do) reduced with the addition of manual pelvic compression^{23, 24)}. Exclusion criteria werelumbar, abdominal, or gynaecological surgery in the past year; disc herniation or spinalfracture; irradiated pain to the leg; neurological and respiratory pathologies; pregnancy orpostpartum (<6 months); body mass index (BMI) >25, as well as any other condition thatwould interfere with the ability to perform data collection^{11, 18, 23)}. Participants who experienced pain during the ASLR were excluded orrescheduled to prevent bias, since that pain could induce motor control changes differentfrom the ones attributed to pain chronicity.

All evaluations were performed at the Center for Rehabilitation Research, Center of HumanMovement and Activity in School. The study was approved by the university ethical reviewcommittee (approval no.: 0846), and each participant supplied signed informed consentaccording to the Declaration of Helsinki.

Thus, from 34 volunteers with low back pain, 18 individuals (14 females and 4 males) metthe participation criteria. Sample size was determined using G*Power (3.1.9.2; HeinrichHeine Universität Düsseldorf, DE) based on a pilot study with 6 individuals, with theobjective being the ability to decrease mediolateral centre of pressure (COP) displacement(an indicator of lumbopelvic stability) with the PBU as a biofeedback strategy. With aneffect size of 0.762, α=0.05, and a power of 0.80, the total sample size needed for thatobjective was 16 participants. Therefore, all individuals who meet the selection criteriawere included in the study and their mean age, height, weight, BMI, and numeric pain ratingscale were 21.28 ± 1.4 years, 1.65 ± 0.1 cm, 58.78 ± 6.9 kg, 21.66 ± 1.7 kg/m²and 3.78 ± 1.1, respectively.

Electromyographic signal (EMG) was collected using a portable electromyographer (biosignalsResearcher; PLUX Wireless biosignals SA, Portugal) with a sampling frequency of 1,000 Hz.Skin impedance was verified by an impedance checker (Noraxon USA Inc., Scottsdale, AZ, USA).The external oblique (EO), rectus abdominis (RA), and transversus abdominis/internal oblique(TrA/IO) were assessed from both sides, with the electrode locations halfway between theiliac crest and the ribs at a slightly oblique angle; 2 cm lateral to the umbilicus, overthe muscle mass; and 2 cm medially and below to the anterior superior iliac spine^25,26,27), respectively. Two digital filters withinfinite impulse response were used: a 30 Hz high-pass Butterworth 2nd order filter forcardiac signal removal and a 20–450 Hz band-pass Butterworth 2nd order filter for electricalnoise and/or cable movements. Root mean square (RMS) with 10 samples was also performed^{28, 29)}.

Mediolateral COP displacement (COPml) of the pelvis was evaluated through a force plate(FP4060-10; Bertec Corp, Columbus, OH USA) with a sampling rate of 100 Hz. A 7 Hz digitalinfinite impulse response filter −low-pass 2nd order Butterworth was applied to removeelectrical noise and/or cable movements²⁸⁾.

A PBU (Stabilizer; Chattanooga Group Inc., Hixson, TN, USA) was adapted and connected tothe electromyographer. Due to its placement under the lumbar spine, it can detect changes inlumbopelvic motion. A synchronization cable connected the electromyographer and the forceplate.

Participants were directed into a supine position with the pelvis over the force plate andtheir hands resting over their chest to avoid contact with the force plate. The PBU wasplaced under the lumbar spine (above the posterior superior iliac spines), with a pressureof 40 mm Hg¹⁵⁾. Participants performed anASLR, which consisted of raising the dominant leg 20 cm in height with the knee extended andmaintained that position for 6 s. The 20 cm height was controlled by a metal bar placed overthe ankle. During the task, a typical ventilatory pattern was requested to avoid a Valsalvamanoeuvre.

The ASLR was then performed with pressure biofeedback. The instrument display waspositioned right in front of the participant, and they were asked to perform an ASLR whiletrying to maintain the pressure steady at 40 mmHg. A training period was allowed until theywere able to perform the task with pressure disturbances bellow 5 mmHg, and without usingthe Valsalva manoeuvre. If a disturbance of more than 5 mmHg was detected, the procedure wasrepeated.

The order was not randomized to ensure that the normal ASLR was not influenced by thestrategies adopted in the ALSR with biofeedback. Each movement was repeated 3 times with a30 s rest period and 1 minute to rest between the normal ALSR and the ALSR withbiofeedback.

Participants also had to raise both legs simultaneously to a 20 cm height with the kneesextended and hold them for 6 s (submaximal activity) for EMG normalization. If pain wastriggered and did not pass immediately when returned to a resting position, the subject wasassessed, treated, and excluded from the study.

Data analysis was performed in a 3-s interval during the isometric phase with the ankle20 cm above the ground (maintenance phase). This period started 3 s after the initial changeon the ground reaction force vertical component, which corresponded to a stabilizationperiod of this component. A 2-s interval at the resting position was also analysed tocalculate mediolateral COP displacement (COPml) and PBU variations. These variables werecalculated as the mean difference between the rest and the maintenance phases, beingexpressed in millimetres (mm) and millimetres of mercury (mmHg), respectively.

Muscle activation intensity was considered as the average RMS followed by its normalizationto the average of three submaximal activity trials (RMS average), expressed as a percentage.Final data consisted of the average of the 3 trials.

Statistical analysis was performed using IBM SPSS Statistics (version 21.0; IBM Corp.,Armonk, NY, USA), with 0.05 as a significance level. A Pearson correlation test was used toidentify correlations between instruments, and a paired t-test to compare ASLR with andwithout pressure biofeedback. Mean and standard deviation were used as descriptivestatistics³⁰⁾.

RESULTS

Pressure variation detected by PBU was not significantly correlated with abdominal muscleactivity and with mediolateral COP displacement (p>0.05).

When ASLR was performed with a biofeedback strategy, there was a significant increase inall abdominal muscle activity (p<0.05), with the exception of TrA/IO, which despiteincreasing its muscle activity, did not reach statistical significance (p=0.068). Also, asignificant decrease in COPml (p<0.001) as well as PBU variation (p<0.001) wasobserved (Table 1).

DISCUSSION

Assuming that both abdominal muscle activity and mediolateral COP displacement during ASLRare key points for lumbopelvic stability^{9, 10, 31)}, the lack of correlations between them and the PBU variations couldindicate that this is not a valid instrument to indicate lumbopelvic stability inindividuals with low back pain. These results are supported by Grooms et al.¹⁵⁾ who found no association between PBUpressure and transversus abdominis activation (through ultrasound imaging) in a supinehook-lying position when performing a drawing-in manoeuvre. They mentioned the possibilitythat the drawing-in manoeuvre might not have been sufficiently demanding to test thisrelationship. Therefore, taking into account that an ASLR could be considered a moredemanding task as it imposes a challenge to the lumbopelvic region, it is correct toconclude that this relation may not exist in a supine position. The absent correlationbetween PBU and mediolateral COP displacement could indicate that this instrument might bemore appropriate for detecting changes in the sagittal plane, such as variations on lumbarlordosis or on pelvic tilt, rather than changes on mediolateral COP displacement, whichcould be interpreted as a surrogate variable for pelvic rotation or mediolateral loadtransfer¹⁰⁾.

However, this device in a clinical context is used mainly as a biofeedback instrument¹⁾ rather than a way to measure muscle activityand COP displacement. In that manner, when used as biofeedback during an ASLR, an increasein all abdominal muscle activity and a decrease in mediolateral COP displacement wereobserved, which could be interpreted as an increase in lumbopelvic stability^{9, 10, 31)}. These two results may actually beconnected, because it is possible that an increase in muscle activation could bettercounterbalance the torque imparted by the limb and prevent pelvic motion³²⁾. Also, the fact that all abdominal musclesincreased their activation may reinforce the idea that lumbopelvic stabilization is achievedthrough synergy between local and global muscles. Local muscles are more responsible forindividual spinal segment stabilization and global muscles have better mechanical advantagefor exerting control over lumbopelvic axial rotation in the transverse plane during activestraight-leg raise^{1, 32, 33)}.

Similar results were found by Noh et al.²¹⁾ during an ASLR with a PBU as a biofeedback tool. They observed adecrease in pelvic rotation but, unlike the present study, found no significant increases inabdominal muscle activity to support the decreased pelvic rotation, describing just a slightincrease in average muscle activation. The differences in the muscle activity results couldbe related to different EMG normalization methods; however, the values were quite similar inalmost every muscle at baseline. Another possible reason could be related to the magnitudeof PBU variation during biofeedback, which was not collected in this study. Hypothetically,to achieve a smaller PBU variation during an ASLR with biofeedback, a greater muscularactivation would be needed. The abdominal muscle activity increase together with a pelvicrotation reduction was found also during ASLR in a research conducted by Park et al.³²⁾, but instead of controlling pelvic motionwith a PBU as biofeedback, they used the pelvic control method that uses a self-biofeedbackscheme by palpating and controlling the position of the anterior iliac spines. Despitesimilar effects on lumbopelvic stability, the comparison between the two methods will needto be conducted in a future study since different outcomes were used.

Despite the adequate sample size to detect the effect of using the PBU as a biofeedbackstrategy, a bigger and more heterogeneous sample could have strengthened the correlationbetween PBU, abdominal muscle activity, and mediolateral COP displacement. As such, it isimportant to understand that these findings may not be generalized to individuals with lowback pain who do not meet the selection criteria. A kinematic analysis should also be addedfor a better understanding of the relationship between the kinetic and kinematic variables,especially the pelvic tilt motion. A more equal proportion of males and females would alsoimprove the study. Future research may study the effect of a therapeutic exerciseintervention on individuals with low back pain with and without a PBU as a biofeedbackstrategy.

In conclusion, despite not being an indicator of abdominal muscle activity or mediolateralload transfer in this position, clinicians should consider using this instrument forbiofeedback purposes to increase abdominal muscle activity and reduce COP displacement whenperforming an activity like an ASLR in individuals with low back pain.

Funding and Conflict of interest

None.

REFERENCES