Estimated moments at L5/S1 level and muscular activation of back extensors for six prone back extension exercises in healthy individuals

Equivalent Weight: Connecting Exoskeleton Effectiveness with Ergonomic Risk during Manual Material Handling

Occupational exoskeletons are becoming a concrete solution to mitigate work-related musculoskeletal disorders associated with manual material handling activities. The rationale behind this study is to search for common ground for exoskeleton evaluators to engage in dialogue with corporate Health & Safety professionals while integrating exoskeletons with their workers. This study suggests an innovative interpretation of the effect of a lower-back assistive exoskeleton and related performances that are built on the benefit delivered through reduced activation of the erector spinae musculature. We introduce the concept of "equivalent weight" as the weight perceived by the wearer, and use this to explore the apparent reduced effort needed when assisted by the exoskeleton. Therefore, thanks to this assistance, the muscles experience a lower load. The results of the experimental testing on 12 subjects suggest a beneficial effect for the back that corresponds to an apparent reduction of the lifted weight by a factor of 37.5% (the perceived weight of the handled objects is reduced by over a third). Finally, this analytical method introduces an innovative approach to quantify the ergonomic benefit introduced by the exoskeletons' assistance. This aims to assess the ergonomic risk to support the adoption of exoskeletons in the workplace.

Variation in the spatial distribution of erector spinae activity during a lumbar endurance task in people with low back pain

This study aimed to investigate the spatial distribution and redistribution of lumbar erector spinae (ES) activity during a lumbar extension endurance task in pain-free participants and how this is modified in people with low back pain (LBP). High density surface electromyography (HDEMG) was recorded using 13 × 5 electrode grids placed over the lumbar ES in 13 LBP and 13 control participants while completing an Ito test to task failure. The root mean square of the HDEMG signals was computed, a topographical map of the EMG amplitude generated and the centre of the activity (centroid) determined throughout the task. The centroid of the EMG amplitude map was systematically more cranial (F = 6.09, P = 0.022) for the LBP participants compared with the control subjects. Regression analysis showed that the extent of redistribution of ES activity was associated with longer endurance. These results show that LBP participants utilised a different motor strategy to perform the endurance task, characterised by greater activation of more cranial regions of the ES and less redistribution of ES activity throughout the task. This study provides new insight into the functional activation of the lumbar ES and how it is modified when people have pain.

Loading conditions in the spine, hip and knee during different executions of back extension exercises

Background

Back extension (BE) is a strength exercise for training the dorsal trunk and hip muscles. To optimise training recommendations that avoid overloading and possible injury, the aim of this study was to determine the loading conditions and the influence of different execution forms of BE on spine, hip and knee ranges of motion (RoMs), joint moments and muscle activity.

Methods

The kinematics, kinetics and muscle activity (EMG) of two execution types (BE_h: dynamic hip, BE_s: dynamic spine) and two versions (one-legged and two-legged) of BE were measured in 16 subjects. RoMs and external joint moments were calculated using an inverse dynamics approach and analysed with a linear mixed model.

Results

Although lumbar spine flexion was observed in both execution types, thoracic spine flexion predominantly occurred during BE_s, whereas thoracic spine extension was observed during BE_h. Larger maximal back and hip moments were observed for BE_h than for BE_s. The activity of the dorsal back and hip muscles, as observed using EMG, was increased for one-legged executions.

Conclusion

To strengthen the hips and lower back, BE_h seem to be more efficient due to the higher moments, with higher or similar RoMs in the hip and lower back. One-legged BE_s seem to provide an effective training for the hamstrings and hip regions without subjecting the spine to excessive loading, possibly promoting this as an effective exercise during training and rehabilitation.

Posterior muscle chain activity during various extension exercises: an observational study

Background

Back extension exercises are often used in the rehabilitation of low back pain. However, at present it is not clear how the posterior muscles are recruited during different types of extension exercises. Therefore, the present study will evaluate the myoelectric activity of thoracic, lumbar and hip extensor muscles during different extension exercises in healthy persons. Based on these physiological observations we will make recommendations regarding the use of extensions exercises in clinical practice.

Methods

Fourteen healthy subjects performed four standardized extension exercises (dynamic trunk extension, dynamic-static trunk extension, dynamic leg extension, dynamic-static leg extension) in randomized order at an intensity of 60% of 1-RM (one repetition maximum). Surface EMG signals of Latissimus dorsi (LD), Longissimus thoracis pars thoracic (LTT) and lumborum (LTL), Iliocostalis lumborum pars thoracic (ILT) and lumborum (ILL), lumbar Multifidus (LM) and Gluteus Maximus (GM) were measured during the various exercises. Subsequently, EMG root mean square values were calculated and compared between trunk and leg extension exercises, as well as between a dynamic and dynamic-static performance using mixed model analysis. During the dynamic exercises a 2 second concentric contraction was followed by a 2 second eccentric contraction, whereas in the dynamic-static performance, a 5 second isometric interval was added in between the concentric and eccentric contraction phase.

Results

In general, the muscles of the posterior chain were recruited on a higher level during trunk extension (mean ± SD, 56.6 ± 30.8%MVC) compared to leg extension (47.4 ± 30.3%MVC) (p ≤ 0.001). No significant differences were found in mean muscle activity between dynamic and dynamic-static performances (p = 0.053). The thoracic muscles (LTT and ILT) were recruited more during trunk extension (64.9 ± 27.1%MVC) than during leg extension (54.2 ± 22.1%MVC) (p = 0.045) without significant differences in activity between both muscles (p = 0.138). There was no significant differences in thoracic muscle usage between the dynamic or dynamic-static performance of the extension exercises (p = 0.574).

Lumbar muscle activity (LTT, ILL, LM) was higher during trunk extension (70.6 ± 22.2%MVC) compared to leg extension (61.7 ± 27.0%MVC) (p = 0.047). No differences in myoelectric activity between the lumbar muscles could be demonstrated during the extension exercises (p = 0.574). During each exercise the LD (19.2 ± 13.9%MVC) and GM (28.2 ± 14.6%MVC) were recruited significantly less than the thoracic and lumbar muscles.

Conclusion

The recruitment of the posterior muscle chain during different types of extension exercises was influenced by the moving body part, but not by the type of contraction. All muscle groups were activated at a higher degree during trunk extension compared to leg extension. Based on the recruitment level of the different muscles, all exercises can be used to improve the endurance capacity of thoracic muscles, however for improvement of lumbar muscle endurance leg extension exercises seem to be more appropriate. To train the endurance capacity of the LD and GM extension exercises are not appropriate.

On the use of EMG-ratios to assess the coordination of back muscles

BACKGROUND

Electromyographic (EMG) amplitude ratios (EMG-ratios) have been proposed to assess back muscle coordination in chronic low back pain patients to avoid the normalization of EMG using maximal contractions. The aim of this study was to test the relevance of this type of EMG analysis.

METHODS

Healthy subjects (44 men and 13 women) and patients with chronic low back pain (57 men) performed three 7s static ramp extension contractions ranging from 0% to 100% of the maximal voluntary contraction while standing in a static dynamometer. A subgroup of 20 healthy men also performed 5s step contractions at 10%, 20%, 40%, 60% and 80% of the maximal voluntary contraction. Finally, to assess reliability, another subgroup (n=20 healthy and 20 men with chronic low back pain) performed the protocol three times, on different days. Surface EMG signals were collected from four pairs of back muscles and subcutaneous tissue thickness was measured at the corresponding electrode sites. EMG amplitude values were computed at each 5% force level from 10% to 80% of the maximal voluntary contraction. Then, EMG-ratios were computed between different electrode sites and averaged bilaterally.

FINDINGS

All EMG-ratios were affected by the force level and the contraction type (ramp vs step contractions). Statistically significant Pearson's correlations (r=-0.38 to -0.57) were obtained between some EMG-ratios and their corresponding subcutaneous tissue thickness ratios. The reliability of the EMG-ratio variables ranged from moderate to excellent (intra-class correlation coefficients between 0.50 and 0.91). Comparisons between 12 men and 13 women and between 24 healthy men and 57 men with chronic low back pain showed that EMG-ratios were sensitive to sex but not to pain status. Multivariate analyses applied on the EMG-ratios identified clusters of subjects but none of the main clinical variables were able to clearly characterize these clusters.

INTERPRETATION

Overall, even though additional research is warranted to further substantiate some important psychometric characteristics of the EMG-ratios as well as their biomechanical and clinical significance, these results support their use for assessing the coordination patterns of back muscles, provided that confounding variables such as the force level, the contraction type, and subcutaneous tissue thickness are accounted for.

Reliability of electromyographic and force measures during prone isometric back extension in subjects with and without low back pain

Maximal voluntary isometric activations (MVIA) are frequently used as inputs for models attempting to predict muscle force and as normalization values in studies assessing muscle function. However, pain may adversely affect maximal muscle activation. The purpose of this study was to assess reliability of MVIA force and electromyographic (EMG) activity during prone isometric back extension in subjects with and without low back pain (LBP). A novel sub-maximal method using the percentages of the estimated mass of the head–arms–trunk (HAT) segment was also investigated. Repeated measures on 20 male volunteers divided into an LBP (n = 10) and a control group (n = 10) were made on 4 occasions. Force and EMG activity were recorded bilaterally from upper lumbar erector spinae (ULES), lower lumbar erector spinae (LLES), and biceps femoris (BF). Subjects exerted a maximal extension effort against a harness assembly that was attached to a force transducer. Submaximal exertions were also performed with an additional resistance of 100%, 110%, 120%, 130%, 140%, 150%, 160%, and 170% of HAT. Mean MVIA forces were significantly (p ≤ 0.05) lower in LBP vs. control. Intraclass correlation coefficients (ICC) for MVIA force, right and left ULES, and LLES EMG indicated high reliability in controls (R > 0.90), but were significantly less in LBP (R = 0.36–0.80). EMG of BF demonstrated excellent reliability across both groups (R > 0.90). The resistance at 100% HAT demonstrated the highest reliability for LBP patients, whereas higher percentages of HAT showed either similar or higher reliability for controls. Force output and back EMG activity are less reliable with LBP individuals and should be taken into consideration when testing.

Trunk muscle activity in healthy subjects during bridging stabilization exercises

Background

Trunk bridging exercises are often used as therapeutic exercises for lumbopelvic stabilization. These exercises focus on the retraining of muscle coordination patterns in which optimal ratios between local segmental stabilizing and global torque producing muscle activity are assumed to be essential. However, a description of such ratios is lacking. The purpose of this study was to investigate both relative (as a percentage of maximal voluntary isometric contraction) muscle activity levels and ratios of local to global muscle activity, during bridging stabilization exercises.

Methods

Thirty healthy university students (15 men, 15 women) with a mean age of 19.6 year volunteered to perform 3 bridging exercises (single bridging, ball bridge and unilateral bridging). The surface electromyographic activity of different trunk muscles was evaluated on both sides.

Results

During all bridging exercises, the ratio of the internal oblique to the rectus abdominis was very high due to minimal relative activity of the rectus abdominis. In general, the ratio of the internal/external abdominal oblique activity was about 1. However, during the unilateral bridging exercise, the ipsilateral internal/external abdominal oblique activity ratio was 2.79 as a consequence of the significant higher relative activity of the internal oblique compared to the external oblique. The relative muscle activity and the ratios of the back muscles demonstrated similar activity levels for all back muscles, resulting in ratios about 1.

Conclusion

Both the minimal relative activity of the rectus abdominis and the high internal oblique to the rectus abdominis activity ratio reported in the present study are in accordance with results of other trunk stabilization exercises. The relative muscle activity and the ratio of the abdominal obliques seem to alter depending on the task and the presumable need for stability. The findings concerning the relative muscle activity and the ratios of the back muscles support the assumption that during these bridging exercises, all back muscles contribute in a similar way to control spine positions and movements in a healthy population.

Spinal muscle evaluation using the Sorensen test: a critical appraisal of the literature

The first test for evaluating the isometric endurance of trunk extensor muscles was described by Hansen in 1964. In 1984, following a study by Biering-Sorensen, this test became known as the "Sorensen test" and gained considerable popularity as a tool reported to predict low back pain within the next year in males. The test consists in measuring the amount of time a person can hold the unsupported upper body in a horizontal prone position with the lower body fixed to the examining table. This test has been used in many studies, either in its original version or as variants. Although its discriminative validity, reproducibility, and safety seem good, debate continues to surround its ability to predict low back pain; in addition, the gender-related difference in position-holding time remains unexplained and the influence of body weight unclear. A contribution of the hip extensor muscles to position holding has been established, but its magnitude remains unknown. The influence of personal factors such as motivation complicates the interpretation of the results. Despite these drawbacks, the Sorensen test has become the tool of reference for evaluating muscle performance in patients with low back pain, most notably before and after rehabilitation programs.

Back muscle strength and fatigue in healthy and chronic low back pain subjects: A comparative study of 3 assessment protocols

OBJECTIVE

To compare the sensitivity of 3 different back test protocols in measuring differences in strength and fatigue between subjects with and without chronic low back pain (CLBP).

DESIGN

Descriptive study using a repeated-measures design.

SETTING

A research laboratory within a rehabilitation center.

PARTICIPANTS

Eighteen healthy subjects and 13 subjects with CLBP were assessed in a single session to compare the 3 protocols. The protocols were an upright position test (UPP), a semicrouched lifting test (LIF), and the Sorensen fatigue test.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Moments of force and surface electromyography were recorded bilaterally from 4 homologous back muscles while the subjects performed static trunk extension efforts for each protocol. Fatigue was quantified by the slopes of the linear regression of electromyography time-series.

RESULTS

The back muscle fatigue and strength scores did not differ significantly for the 2 subject groups for any of the 3 protocols. The electromyography fatigue indices revealed that the Sorensen fatigue test and UPP produced more fatigue in the back muscles than the LIF.

CONCLUSIONS

It was impossible to specify which protocol is more sensitive to low back status because no between-group difference was observed for any of the 3 tests.

Back muscle fatigue during intermittent prone back extension exercise

The purpose of this study was to estimate the level of muscular activation and muscle fatigue of the low back muscles during the performance of an intermittent prone back extension (PBE) exercise. Forty-one healthy students (24 males and 17 females) lying prone on a bench with the legs fixed performed two maximal voluntary contractions (MVC) in extension, a maximum of 100 repetitions of an intermittent PBE exercise immediately followed by a final MVC in extension. In addition, 12 subjects (11 males and one female) repeated the PBE exercise but with the addition of a 45-N weight on the back. The PBE exercise consisted of a task broken into four 1-s segments, while lying prone on a bench (10 degrees below horizontal): (1) raising the trunk to a horizontal position; (2) holding the trunk in the static phase (10 degrees above horizontal); (3) returning to the original position; and (4) resting on the bench. Electromyography (EMG) was used to measure the level of muscle activity (erector spinae (ES), gluteus maximus (GM), hamstrings (HA)) relative to the maximum voluntary EMG (MVE).

RESULTS

Most of the subjects (34 out of 41) completed the 100 repetitions without excessive muscle fatigue according to the post-exercise MVC values. The intermittent PBE increased fatigue in the lumbar and hip extensor muscles in terms of: (1) a decrease in the MVC; (2) an increase in the level of muscle activation; and (3) a decline of the median frequency (MF). There was no gender difference in all EMG measurements. The level of muscle activation in the hip extensors (GM and HA) was associated with task failure (number of repetitions <100) for some subjects and the addition of a weight of 45 N had more impact on HA than ES. In conclusion, the PBE exercise as performed in the present study (including rest intervals), although not very strenuous for our healthy subjects, seems an adequate exercise to measure and train the aerobic capacity of the back muscles. However, to train specifically the back muscles, the exercise must be adjusted to avoid task failure due to possible hip extensor fatigue.

Activation of lumbar paraspinal and abdominal muscles during therapeutic exercises in chronic low back pain patients11No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors(s) or upon any organization with which the author(s) is/are associated

OBJECTIVES

To assess the activities of paraspinal and abdominal muscles during therapeutic exercises for the treatment of patients with nonspecific chronic low back pain (CLBP), and to study the effects of active physical rehabilitation on these activities.

DESIGN

A cross-sectional study comparing muscle activities during 18 stabilization exercises, and a prospective follow-up of patients with CLBP during rehabilitation.

SETTING

Rehabilitation clinic in university hospital in Finland.

PARTICIPANTS

Nine volunteers (5 men, 4 women) aged 27 to 58 years.

INTERVENTION

Three months of active outpatient rehabilitation (4 to 6 times in a rehabilitation clinic, supplemented with self-motivated exercise at home) supervised by a physiotherapist.

MAIN OUTCOME MEASURES

Surface electromyography was recorded bilaterally from L5 level paraspinal, rectus abdominis, and obliquus externus abdominis muscles. The recorded signal was averaged and normalized to the maximal electromyographic amplitude obtained during the maximal voluntary contraction. The measurements were taken before and after the exercise treatment period.

RESULTS

CLBP patients showed variable trunk muscle activity patterns during the different therapeutic exercises, similar to those that we reported earlier in healthy subjects. The maximal trunk isometric extension (pre, 147.3+/-75.9Nm; post, 170.1+/-72.3Nm) and flexion (pre, 72.0+/-37.9Nm; post, 93.5+/-42.5Nm) torques did not show a significant changes during the exercise period. However, trunk rotation-flexion torque (pre, 52.9+/-26.5Nm; post, 82.4+/-65.8Nm) increased significantly (35.8%) after the exercise period (P<.05). The corresponding maximal electromyographic amplitudes of back and abdominal muscles remained unchanged. Disability, as assessed by visual analog scale and Oswestry Disability Index, did not change.

CONCLUSIONS

The CLBP patients performed therapeutic exercises with similar abdominal and back extensor muscle activities in the same way as the healthy subjects in our earlier studies. In this study, active physical rehabilitation had no effect on the abdominal and back muscle activities or on pain and functional disability indices.