Archives of Physical Medicine and Rehabilitation
Volume 89, Issue 7 , Pages 1314-1322, July 2008

Activation Characteristics of Trunk Muscles During Cyclic Upper-Body Perturbations Caused by an Oscillating Pole

Presented in part as a poster to the Deutsche Physiologische Gesellschaft, March 29, 2006, Munich, Germany.

  • Christoph Anders, PhD

      Affiliations

    • Corresponding Author InformationReprint requests to Christoph Anders, PhD, Clinic for Trauma-, Hand- and Reconstructive Surgery, Division for Motor Research, Pathophysiology and Biomechanics, Friedrich-Schiller-University Jena, D-07740 Jena, Germany
    • ,
  • Beatrix Wenzel, MD
    • ,
  • Hans C. Scholle, PhD

Clinic for Trauma-, Hand- and Reconstructive Surgery, Division for Motor Research, Pathophysiology and Biomechanics, University Hospital Jena, Jena, Germany.

Article Outline

Abstract 

Anders C, Wenzel B, Scholle HC. Activation characteristics of trunk muscles during cyclic upper-body perturbations caused by an oscillating pole.

Objective

To evaluate the effect of a new device on trunk muscle activation.

Design

Cross-sectional survey of trunk muscle activation characteristics.

Setting

Physiologic laboratory at university institute.

Participants

Thirty healthy subjects (15 men, 15 women) were recruited from a university campus.

Interventions

A simple flexible pole that applies rapidly alternating forces on the trunk when set into motion was used. The device was held horizontally in both hands, in front of the body. It was used at 3 different oscillation frequencies (3, 3.5, 4.5Hz), in horizontal and vertical plane, respectively.

Main Outcome Measures

Surface electromyography of 5 trunk muscles was measured and the data were normalized according to relative cycle time. Time dependent (amplitude curve) and time independent (mean amplitude over cycle) parameters were used for analysis.

Results

Rectus abdominis and external oblique muscle amplitudes were directly proportional with oscillation frequency (analysis of variance), and these effects were independent of sex. Multifidus amplitude levels were subject to oscillation plane with increased levels for vertical oscillation in men but not in the women. All abdominal muscles exhibited continuous activation pattern, independent of oscillation plane. Back muscles changed from a continuous activation in horizontal plane into similarly phasic patterns in vertical oscillation plane. The occurring amplitude peak moved forward in relative cycle with increasing oscillation frequency.

Conclusions

Back muscle activation patterns were subject to oscillation plane. Abdominal muscle activation was independent from oscillation frequency and oscillation plane. These normative data may be used to identify disturbed trunk muscle coordination patterns and to control success of functional restoration during rehabilitation interventions of back pain patients.

Key Words: Electromyography, Muscle coordination, Rehabilitation, Task performance and analysis

List of Abbreviations: ANOVA, analysis of variance, LBP, low back pain, MVC, maximum voluntary contraction, NS, not significant

 

LOCALIZED LBP IS OFTEN related to muscular problems. This is the main message conveyed in summarizing the physiologic research of chronic nonspecific LBP during the last 2 decades.1, 2, 3, 4, 5, 6, 7, 8 Two different kinds of disturbances in patients with LBP have been identified: reduced endurance of their back extensors,9, 10 and delayed essential feed-forward postural responses of deep abdominal muscles.6

Reduced endurance of back extensors may be due to chronic underuse of the respective muscles due to a lack of appropriate physical activity. At present, the question remains unanswered whether the pain is generally a result of physical deconditioning or vice versa. Because pain usually reduces physical activity it is hard for these patients to get out of the vicious circle of pain, leading to reduced mobility, which leads to even more reduced physical activity. Either way, intense physical training of any kind11 is often able to reduce pain, but long-term success remains elusive if training is not maintained.12, 13

Delayed postural responses in trunk muscles6 may lead to impaired coordination of trunk muscles in patients with LBP. One possible reason for this might be a cumulative effect of microlesions of proprioceptors at the spine.14 This hypothesis is supported by other results that could prove impaired movement perception of both patients with LBP15 and patients with lumbar stenosis.16

The identification of disturbed coordination patterns led to the development of specific training programs targeting deep abdominal as well as back muscles.17 One of these programs was able to reduce LBP, lasting for a critical observation period of 2 years.18 This spinal segmental stabilization program involves a combination of biofeedback guided voluntary activation of the deep transverse abdominal muscle and therapist-assisted voluntary activation of the multifidus muscle.17 The basic principle for this training originates from the assignment of all trunk muscles to either the local or the global muscle systems.19 These muscle systems are believed to behave in a predictable manner defined by their function: local muscles are thought to constantly remain active at low levels, providing segmental stability; global muscles should activate in a phasic manner, to initiate or limit movements.20

Results from our own studies could not completely confirm these different classifications. For the back muscles, no substantial differences of fiber type distribution patterns have been identified between mobilizing (erector spinae muscle) and stabilizing (multifidus muscle) muscles.21 Perhaps this explains why during static tasks both muscles acted in an identical manner.22 It does not suggest, however, why activation characteristics during dynamic requirements (treadmill walking) differ, with eccentric and concentric activations for the multifidus muscle and more pronounced eccentric activations for the erector spinae muscle.23 Analogous findings during static tasks could be identified for abdominal muscles: the amplitude-force relationship of several abdominal muscles was virtually identical,22 independent of whether they were assigned to the mobilizing (rectus abdominis muscle) or to the stabilizing muscle systems (both oblique abdominal muscles).

Overall, these results seem to conflict, but, from our point of view, they simply indicate different aspects of functional pathology, which can be found to varying degrees among the patients with LBP. Patients with LBP most probably are characterized by inadequate physical condition and impaired trunk muscle coordination. Previously, it has not been possible to definitely diagnose the degree of either one.

To summarize, patients with LBP exhibit 2 kinds of muscular disturbances: inadequate force capacity, supposedly caused by chronic underuse, and disturbed coordination, most probably due to impaired proprioception. For the first problem, any kind of physical training can improve status short term, but for long-term effects adequate muscular coordination is probably a critical factor.

The Spinal Segmental Stabilization Program17 involves high labor costs because it requires a 2-step training process: the initial strengthening and training and then a subsequent training, because the voluntary activation of the trained muscles has to be transferred back into involuntary muscle function for performing everyday activities. Therefore, there is still a need for therapeutic techniques that activate the target muscles involuntarily and are robust against external influences.

A better solution would incorporate techniques that automatically train the stabilizing muscles. We received notice of a new device that can roughly be described as a simple flexible pole. This pole can be set into oscillation, resulting in tunable, rapidly alternating forces. By holding this device in both hands in front of the body, forces act on the whole trunk. Hypothetically, this should trigger activity of all trunk muscles.

The study was carried out to determine the effect of using this device on trunk muscle activity and, furthermore, to determine the influence of changes in oscillation frequency and oscillation plane of the device. Local muscles are expected to act at continuous activation level (tonic activation), mostly independent from oscillation plane and oscillation frequency. On the other hand, global muscles are expected to show alternating activation patterns (phasic activation), which should clearly be influenced by oscillation plane and oscillation frequency of the test device.

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Methods 

In this study, 30 healthy subjects participated voluntarily. Prior to the investigation, all persons gave written informed consent. The study was part of a larger experimental setup and was approved by the Jena University ethics board (0558-11/00). The group consisted of 15 men and 15 women (for anthropometric data, see table 1).

Table 1. Anthropometric Data of the Investigated Subjects
CharacteristicsWomenMenP⁎
Height (cm)169.9±5.8179.7±3.9.000
Weight (kg)58.8±5.172.7±7.8.000
Body mass index (kg/m2)20.4±1.522.5±2.5.008
Age (y)23.1±2.025.5±5.7.154

NOTE. Values are mean ± SD.

⁎Student t test for independent samples.

The exercise involved initiating and maintaining the oscillation of the device (Propriomed),a which is a pole held horizontally in both hands in front of the body (fig 1). The grip located in the middle of the device has a width of about 20cm that enabled a secure hand hold for both hands. Attention was paid to correct task execution of the evoked oscillations with only 1 node, located in the middle of the pole (see fig 1). After oscillation initiation virtually no further movements of the handle were made, except for the small impulses to maintain the oscillations. The device has no active parts. Therefore, every movement of the device is a result of the interaction between subject and device.

  • View full-size image.
  • Fig 1. 

    Subject using the device. (A) Horizontal oscillation, (B) vertical oscillation, and (C) positions of the adjustable weights. Oscillation frequencies can be tuned by moving the adjustable weights. Weight positions and their respective oscillation frequencies are given.

The device had a total length of 170cm and a weight of 1035g, equipped with 2 adjustable weights at each side. The applied oscillation frequencies resulted from the positions of the adjustable weights: (1) two each positioned near the enveloped ends of the pole (3.0Hz, slow), (2) one each positioned near the hand grip while the other one remained near the pole ends (3.5Hz, moderate), and (3) two each being positioned near the hand grip (4.5Hz, fast) (see fig 1). The recommended oscillation amplitude of about 50cm at the pole ends was maintained for all trials. Unlike a similar device (BodyBlade),24 which consists of a flexible foil blade, the Propriomed consists of a flexible rod, enabling oscillations in all possible directions of the sagittal plane. Therefore, in addition to maintain the requested oscillation amplitude, subjects had to pay attention to oscillation plane. Two oscillation planes were applied: the horizontal plane and vertical plane. The order of the 6 possible plane-oscillation combinations was randomized separately for every volunteer. Each task was performed 3 times for each plane and frequency combination. For measurement, each trial lasted for at least 10 seconds of stationary oscillation action. Sufficient breaks between the trials were maintained to prevent muscle fatigue (60s between trial repetitions, 90s between different tasks).

We evaluated muscle response by surface electromyography. Five different trunk muscles were investigated on both sides simultaneously (fig 2). The muscles and the respective electrode locations are detailed in table 2. Surface electromyography was measured using a common bipolar montage with an interelectrode distance of 2.5cm. The circular uptake area of the disposable Ag/AgCl solid gel electrodesb had a diameter of 1cm. Signals were amplifiedc with a gain of 2500 (−3dB between 5 and 700Hz). Analog-to-digital conversion was done at 2000 samples per second with an accuracy of 1μV/bit (DAQCard -AI-16E-4d; 12 bit). A bidirectional accelerationa sensor was fixed on the right end of the pole to enable identification of correct oscillations with respect to plane. Only oscillation cycles that could be identified to match the required plane were used for further analysis (criterion: maximum time difference of 25ms between both peaks). Therefore, the mean proportion of cycles for analysis varied between 78% (horizontal plane, fast) and 94% (vertical plane, moderate) of all performed cycles. Furthermore, only cycles with a temporal deviation of less than 10% from the median cycle time of every single trial were used for calculation. Raw surface electromyographic signals were band-pass filtered between 20 and 300Hz and a moving root mean square (window, 15ms) was calculated. To reduce slight differences in cycle duration and to compare the different oscillation frequencies, all cycles were time normalized with an accuracy of 0.5% (201 time points). The normalized cycle starts at the rear inversion point (vertical oscillation: top inversion point), at 50% the front inversion point is reached (vertical oscillation: bottom inversion point).

Table 2. Investigated Muscles and Electrode Locations, According to Hermens48 and Ng49 and Colleagues
MuscleElectrode Orientation and Position
Rectus abdominis (upper part)4cm lateral umbilicus, vertical, caudal electrode at level of umbilicus
Obliquus internus abdominisAlong horizontal line between both ASISs, medial from inguinal ligament
Obliquus externus abdominisCranial electrode directly below most inferior point of costal margin on line to opposite pubic tubercle
Multifidus (lumbalis, multifidus muscle)1cm medial from line between PSISs and first palpable spinous process, caudal electrode at L4 level
Erector spinae (longissimus, erector spinae)Vertical, over palpable bulge of muscle (≈3cm lateral midline) caudal electrode at L1 level

NOTE. Muscles from both sides were investigated simultaneously.

Abbreviations: ASIS, anterior superior iliac spine; PSIS, posterior superior iliac spine.

For each subject, we calculated a mean surface electromyography curve of each trial, separately for all muscles. A grand average was calculated by averaging the 3 respective single trials per plane and oscillation frequency. From these grand averaged data, time independent parameters (mean amplitude level) and time dependent parameters (muscle coordination patterns during oscillation cycle) were used for analysis. To identify coordination patterns, amplitude patterns were normalized as well according to the maximum amplitude within cycle.

All statistical calculations were performed using the SPSS package.e To test interactions of oscillation plane, oscillation frequency, and sex for mean surface electromyographic amplitudes a repeated-measures ANOVA was used with sex as intersubject factor.

Nonparametric tests were chosen for statistics of surface electromyographic parameters. Influence of the 2 oscillation planes was tested using the Wilcoxon test for dependent samples. Comparisons between the 3 oscillation frequencies were calculated using the Friedman 1-way ANOVA by ranks for dependent samples. Sex differences were tested using the Mann-Whitney U test for independent samples.

To test differences of the time dependent data, we tested average values of sections of 10% of the normalized cycle using the corresponding tests according to the time independent data.

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Results 

Time Independent Data 

ANOVA revealed no interactions between the 2 intrasubject and the intersubject factors (table 3). Therefore, differences between sexes, influence of oscillation frequency, and differences between oscillation planes could be calculated separately.

Table 3. P Values of the Repeated-Measures ANOVA for Mean Amplitudes Over a Normalized Cycle
MusclePlanePlane by SexFrequencyFrequency by SexPlane by FrequencyPlane by Frequency by SexSex
Rectus abdominis leftNSNS.018⁎NSNSNSNS
Rectus abdominis rightNSNS.019⁎NSNSNSNS
External oblique leftNSNS.024⁎NSNSNS.010⁎
External oblique rightNSNS.010⁎NSNSNS.038⁎
Multifidus muscle left.010⁎NSNSNSNSNS.017⁎
Multifidus muscle right.002†NSNSNSNSNS.017⁎

NOTE. The internal oblique and erector spinae muscles did not show any significant difference.

⁎P<.05;

†P<.01.

Significantly higher mean surface electromyographic amplitudes were seen in the multifidus muscle in vertical plane when compared with horizontal plane at slow and moderate oscillation frequencies (table 4). Influence of oscillation frequency on mean surface electromyographic amplitude level was only detectable in male subjects. Surface electromyographic amplitudes for external oblique (both sides, P<.05) and internal oblique (left: P<.05; right: P=NS) muscles increased together with increasing oscillation frequency in vertical plane (see table 4). Mean back muscle surface electromyographic amplitudes increased with oscillation frequency in horizontal plane, significant for left multifidus and right erector spinae muscles. Also in horizontal plane, the rectus abdominis muscle showed highest mean surface electromyographic amplitude levels for moderate oscillation frequency (left: P<.05; right: P=NS). The right external oblique showed highest mean surface electromyographic amplitude levels for moderate oscillation frequency.

Table 4. Mean Surface Electromyographic Amplitudes for All Investigated Trunk Muscles
MuscleHorizontalVertical
SlowModerateFastSlowModerateFast
Rectus abdominis left
Male5.0(3.9–1.9)6.5(2.3–3.0)5.6(8.2–2.4)3.9(2.5–0.4)6.0(3.4–1.9)4.8(3.1–0.9)
Female4.0(3.1–0.4)4.2(5.1–0.7)4.4(3.4–0.3)4.5(2.6–0.8)5.4(3.2–1.7)5.9(1.3–1.1)
Rectus abdominis right
Male4.9(3.5–0.5)7.3(2.1–3.0)5.0(6.7–0.5)6.8(1.2–1.7)7.0(2.5–1.1)6.6(2.6–0.6)
Female5.3(2.4–0.5)5.9(3.8–0.8)6.1(3.1–1.2)5.4(2.5–0.4)6.0(3.9–0.4)6.7(2.6–0.9)
Internal oblique left
Male29.8(8.1–17.1)28.7(4.8–16.0)31.4(15.3–19.7)28.0(4.1–13.9)29.3(8.2–13.1)31.2(6.8–11.8)†
Female22.4(2.5–11.7)22.1(2.8–9.9)21.0(6.0–7.7)23.1(2.3–11.1)21.3(6.6–9.1)20.1(6.7–7.4)†
Internal oblique right
Male23.6(11.2–9.8)25.4(8.1–15.7)22.8(16.2–13.6)23.7(6.1–12.3)23.0(12.7–11.5)28.1(7.2–13.4)
Female13.5(11.8–4.0)10.9(16.9–1.5)12.5(11.5–3.1)13.5(9.6–4.5)14.3(13.2–5.0)16.4(12–6.4)
External oblique left
Male7.0(1.7–1.5)†6.5(5.0–0.9)†7.3(2.9–1.1)7.2(1.1–1.2)†7.7(1.8–1.4)†7.9(3.3–2.2)†
Female11.2(4.7–2.6)†11.1(5.2–3.1)†11.4(8.0–3.0)11.2(3.3–2.3)†10.7(6.2–2.4)†13.0(6.8–5.0)†
External oblique right
Male8.4(3.3–2.0)†9.6(3.2–3.6)9.1(8.0–1.9)7.3(3.1–1.1)†9.2(3.8–2.7)9.4(4.0–1.8)
Female11.0(5.1–1.4)†12.0(8.5–2.6)12.5(8.7–2.8)12.3(5.6–3.1)†11.3(7.9–2.7)11.8(5.9–1.2)
Multifidus left
Male32.9(11.9–15.1)⁎31.0(12.5–8.6)34.1(14.4–11.2)†36.6(7.3–7.5)⁎†33.8(10.7–5.1)†33.6(6.9–5.3)
Female19.8(11.4–5.6)19.5(7.4–4.5)20.0(13.7–6.6)†27.3(3.6–11.2)†24.6(8.3–6.6)†23.5(10.6–6.6)
Multifidus right
Male30.5(16.4–6.1)†32.5(11.6–10.2)⁎†36.1(11.1–12.0)†35.0(12.2–7.5)†38.0(7.5–9.2)⁎†32.3(10.7–5.3)
Female20.5(4.7–5.5)†19.1(6.5–4.0)⁎†19.8(6.6–7.3)⁎†21.0(10.9–5.8)†20.4(15.6–3.5)⁎†19.4(16.6–3.3)⁎
Erector spinae left
Male26.9(9.0–8.5)27.9(8.3–9.2)28.0(11.5–9.3)25.2(11.3–5.4)27.3(6.9–7.3)26.9(6.4–10.8)
Female25.1(14.4–9.4)27.0(9.5–10.1)30.5(7.9–10.9)22.1(14.5–1.0)25.1(7.9–5.0)25.5(7.0–2.6)
Erector spinae right
Male35.3(3.9–15.4)33.3(7.4–11.8)38.6(3.1–19.4)⁎27.9(7.0–6.3)29.7(9.1–4.4)29.8(6.9–10.8)⁎
Female22.1(12.2–3.6)24.5(8.7–8.3)25.2(15.4–7.4)25.8(11.9–6.9)24.8(11.0–5.1)28.5(6.9–5.5)

NOTE. Values are median and upper and lower quartile range. Boldface denotes significant differences between oscillation frequencies (Friedman ANOVA by ranks).

⁎Significant differences between oscillation planes (Wilcoxon).

†Significant differences between the sexes (Mann-Whitney).

Most mean multifidus surface electromyographic amplitude levels were significantly higher in men than women (exceptions being fast frequency in vertical plane and left multifidus muscle at slow and moderate frequency in horizontal plane). In contrast, women showed significantly higher external oblique amplitudes, except the right-sided external oblique for moderate and fast oscillation frequencies.

See table 4 for mean surface electromyographic data. Results of the statistical calculations are indicated.

Time Dependent Data 

Activation characteristics differed considerably between oscillation planes: horizontal oscillation evoked continuous activities in all muscles, whereas in vertical plane both back muscles exhibited clear phasic patterns (fig 3). Their highest amplitudes were reached right after top inversion point during downward movement of the ends of the pole. For the highest oscillation frequency this amplitude peak moved forward in terms of relative cycle time, more near the top inversion point (fig 4). Abdominal muscles, independent of oscillation plane, all exhibited continuous activation patterns that became only slightly phasic with increasing oscillation frequency. A clear order of amplitude levels could be identified: the rectus abdominis and external oblique with the lowest levels, followed by the internal oblique, multifidus, and erector spinae in horizontal plane. Vertical oscillations exhibited slightly higher amplitude peaks in the multifidus than in the erector spinae, but minimum amplitudes were lower for the multifidus.

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  • Fig 3. 

    Grand average surface electromyographic (SEMG) curves of trunk muscles during the applied oscillation frequencies in horizontal (left column) and vertical (right column) planes. Data from both sexes were pooled. Positions of the pole ends during the normalized oscillation cycle are indicated. Abbreviations: EO, external oblique; ES, erector spinae; IO, internal oblique; MF, multifidus; RA, rectus abdominis.

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  • Fig 4. 

    Coordination patterns of back muscles for oscillations in horizontal (left column) and vertical (right column) planes. Influence of oscillation frequency was tested by using the nonparametric Friedman ANOVA by ranks. *Significant differences of P<.05. Data from both sides and sexes were pooled. Positions of the pole ends during the normalized oscillation cycle are indicated.

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Discussion 

Time Independent Data 

The study was performed to test the influence of using the Propriomed device on trunk muscles, regarding their relationship to the biomechanically19 and functionally defined muscle systems.20 We expected continuous activation of the local multifidus muscle with only little alteration from task characteristics, and strongly task-related adaptive changes for the other global stabilizing and mobilizing trunk muscles. In this context, the results have to be considered as inconsistent with respect to the evoked activation characteristics. The role of the rectus abdominis, which belongs to the global mobilizer group, remained marginal for most situations. The external oblique, which is considered to belong to the global stabilizer group, also remained at insignificant amplitude levels that hardly cross 10μV if considering the whole group.

Most mean multifidus surface electromyographic amplitude levels were significantly higher in men than women (exceptions being fast frequency in the vertical plane and left multifidus at slow and moderate frequency in horizontal plane). In contrast, women showed significantly higher external oblique amplitudes, except right-sided external oblique for moderate and fast oscillation frequencies. Together with the multifidus, here sex differences were obvious: external oblique amplitudes of the female subjects reached significantly higher levels than in the investigated men. The mean difference was rather small, however. Surface electromyographic amplitudes are subject to several technical25, 26, 27 and physiologic alterations. The distance between electromyography source and detection site plays an important role: in general surface electromyographic amplitudes are considered to be attenuated with increasing skinfold thickness.28 Therefore, this slight shift might be interpreted as sex related in terms of different amplitude attenuation because the average body mass index was significantly higher for the investigated men. However, if so this effect should be a general one, observable for all investigated trunk muscles. In contrast, ANOVA revealed higher amplitudes of the multifidus muscle in men than in women. Also for the internal oblique a slight but insignificantly higher level could be found for the male group. Assuming the comparably small number of investigated subjects, this observation likely could be verified with a higher number of cases (for details, see the Statistical Considerations section). Therefore, for the found mean surface electromyographic amplitude differences between both sexes a general “skinfold related effect” can be excluded. Differences in fiber type composition can be excluded also, because of the same proportions in both sexes.21 Fiber type proportion and functional cross sectional area of the respective fiber types are different issues, however. It is known that men have generally larger muscle fibers than women.29 Furthermore, in contrast to men, women's type I fibers are larger than their type II fibers.21 Moreover, these differences are not able to evoke the observed differences, because both back and abdominal muscles are comparable in fiber type composition between sexes. Differences on fiber type composition between front and back regions are known: abdominal muscles have higher proportions of type II fibers than do back muscles.30, 31 All these variations should cause regionally related differences, but the observed distinctions between sexes were related to certain muscles without regional association. Consequently, these differences have to be considered as sex related with respect to differently organized coordination patterns of trunk muscles. Although not mentioned as a result, obviously, side differences occurred for the abdominal muscles with right-sided predominance of the rectus abdominis and external oblique and left-sided predominance of the internal oblique. Back muscle mean amplitudes were virtually symmetric. Because almost all subjects were right handed (1 female ambidextrous, 1 male left-handed) these differences are most probably related to handedness. Data about trunk muscle amplitude levels in relation to handedness are not available from the literature, but other investigations also revealed asymmetrical amplitudes of shoulder muscles in relation to handedness.32 The apparent opposite predominance of the 2 oblique abdominal muscles may be subject to measurement site: the internal oblique at the position in which it was actually measured shares fiber direction with the deep transverse abdominal muscle, which belongs to the local muscle group. Because cross-talk cannot be eliminated completely,33 internal oblique data are expected to partly reflect local muscle activation characteristics. During predominantly right sided hand use, pronounced left sided compensation might be necessary. The known crossing of the thoracolumbar fascia34 supports this hypothesis, but further investigations are necessary to highlight this specific issue.

Time Dependent Data 

Oscillation plane did not affect abdominal muscle activation characteristics, but back muscle activation patterns differed considerably between both planes: In horizontal plane, mean amplitude appeared to be slightly higher for the erector spinae. In vertical plane, the multifidus exhibited slightly higher mean amplitudes. In horizontal plane, both muscles were characterized by continuous activation, like all other investigated muscles. In vertical plane, this activation changed into a clear phasic pattern: just after the top inversion point, during the initial downward movement of the pole ends an amplitude peak was exhibited. Therefore, the change of oscillation plane was able to evoke completely different activation characteristics, which were found to be similar for both back muscles. Furthermore, relative time of the amplitude peak shifted forward with increasing frequency. Therefore, reaction times of the multifidus and erector spinae muscles also decreased, from 60ms at 3Hz via 45ms at 3.5Hz toward 13ms at 4.5Hz after the top inversion point. Most probably, this is a compensatory mechanism for the electromechanic delay, that is known to be invariant in time for repetitive muscle activities.35, 36 The remaining time interval from the back muscles amplitude peak to the bottom inversion point stayed constant likewise: The resulting intervals were found to be 107ms at 3Hz, 97ms at 3.5Hz, and 98ms at 4.5Hz. These data are in accordance with data from the literature.37

Because the multifidus and erector spinae muscles are assigned to either the local (multifidus) or the global (erector spinae) systems,19 the similarities of multifidus and erector spinae coordination patterns are surprising. As a local muscle, basically, a more continuous characteristic would have been expected for the multifidus, but its peak amplitudes reached even higher amplitude levels when compared with the erector spinae. The amplitude normalized data also showed a greater range for the multifidus (see fig 4). Even though one cannot expect to exactly measure activation characteristics of the deep multifidus at the surface, a virtually identical pattern for the erector spinae during the downward moving phase of the pole ends could be observed.38 During this phase, most likely, an eccentric contraction is evoked to compensate for the following downward directed force on the muscles at the bottom inversion of the pole. Therefore, it is not the multifidus activation pattern, but the erector spinae activation pattern, that is the most surprising result. Functionally, the concordant pattern qualifies the erector spinae to act also with the eccentric activation that is usually assigned to global stabilizers.20 In other words, as was stated already by other authors, there is no “key muscle”39, 40; rather, proper coordination of all involved trunk muscles is an important feature for adequate trunk stability.

The cyclic activation enforces feed-forward activation mechanisms to compensate for the trunk flexing moment applied by the device. Delayed feed-forward activation of local muscles6 is accepted as being a key factor in the pathogenesis of LBP. Their reaction time could be shortened as a direct result of repeated voluntary abdominal activation, independently from whether specific transverse abdominal training or general abdominal activation was performed.41

Therefore, the application of the device can be a contributing factor for the diagnosis, prevention, and, possibly, the therapy of LBP. The variation of oscillation planes and frequencies has considerable effect, most notably on back muscle coordination patterns.

The device comes in 4 different lengths, from 130cm up to 190cm in 20-cm steps, resulting in a frequency range from 2.5 up to 7.5Hz. This enables a wide range of applications from rehabilitation to fitness. Compared with other training devices the Propriomed offers a reasonable alternative to train trunk muscle coordination.

Study Limitations 

Physiologic considerations 

With the presented data we cannot evaluate if any strengthening effect of trunk muscles might be expected by using the device, because prior to the investigation no MVC test was performed. We chose not to use the MVC test for 2 reasons: (1) it is extremely strenuous for the subjects to produce MVC data for all investigated trunk muscles separately; and (2) encouragement42 and other motivational influences43 have impact on the measured root mean square values during MVC. This becomes even more important if LBP patients have to be investigated, because their data likely will not reflect reliable MVC levels.44 Second, investigations during gait, which is a common cyclic activity, revealed reduced reliability if data were normalized according to previously determined MVC levels.45 Investigations during running, that is, maximum cyclic effort, could prove peak amplitudes which exceed static MVC levels by far.46

A recently published article by Moreside et al24 investigating a similar but different device (BodyBlade; natural frequency, 4.5Hz) revealed mean trunk muscle amplitudes that reached 10% to 60% of previously determined MVC levels of trunk muscles if oscillation amplitude was increased. One test situation was similar to vertical oscillations of our setup, but differed in detail. Therefore it is difficult to directly compare both results. In our investigation, only the oscillation frequency was changed, with only little effect on amplitude levels.

The investigation was performed with healthy volunteers. Therefore, the question still remains unanswered if the observed patterns can be evoked also in patients with LBP. If the application of different oscillation frequencies is suited to improve spinal stability also needs separate investigations.

Statistical considerations 

In general, small sample sizes require larger effect sizes to be detected. This effect is independent from what kind of statistics is applied. Power analysis is not available for nonparametric statistical methods for dependent variables; therefore, we approximated the necessary effect size of the data by calculating the power for dependent variables while using the paired t test. Power analysis47 revealed a required effect size (difference between mean values over variance of the differences) of .53 between groups, according to the given sample size of 30 subjects for a common power level of 80%. With only 15 subjects per group, as was the case for testing sex-related differences, the necessary effect size to be detected increases to 1.36. Therefore, considering the limited number of investigated subjects, the probability to interpret false positive results (second-order statistical error) has to be regarded as very unlikely.

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Conclusions 

In healthy subjects, the device is able to induce predictable activation of all investigated trunk muscles. Oscillation frequency and oscillation plane had only little to negligible effect on mean trunk muscle amplitudes. Oscillation planes evoke different activation characteristics for the back muscles but not for the abdominal muscles. Abdominal muscles all showed continuous activation patterns. For the back muscles, during vertical oscillations, clear phasic pattern could be observed; whereas in horizontal plane a continuous activation existed. The kind of cyclic alternating force application on the trunk manifested by holding an oscillating object might be used for diagnosis, prevention, and treatment of impaired back muscle function.

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Acknowledgments 

We thank Elke Mey for technical assistance and Marcie Matthews, MSc, for language correction of the manuscript.

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  • a Haider-Bioswing, Gesundheitssitz- und Therapiesysteme GmbH, Dechantseeser Str 4, 95704 Pullenreuth, Germany.
  • b Tyco Healthcare Deutschland GmbH, Gewerbepark 1, 93333 Neustadt (Donau), Germany.
  • c Biovision GmbH, Bleichstr 6a, D-61273 Wehrheim, Germany.
  • d National Instruments Corp, 11500 N Mopac Expwy, Austin, TX 78759-3504.
  • e SPSS Inc, 233 S Wacker Dr, 11th Fl, Chicago, IL 60606.

 No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the authors or on any organization with which the authors are associated.

PII: S0003-9993(08)00271-2

doi:10.1016/j.apmr.2007.11.047

Archives of Physical Medicine and Rehabilitation
Volume 89, Issue 7 , Pages 1314-1322, July 2008

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