The number of lumbar spine fusions (LSFs) has increased considerably over the past few decades (21), but the best practice to apply during postoperative rehabilitation has remained unclear. In posterior lumbar spine surgery, dissection, retraction, denervation, and the fusion itself may result in changes in paraspinal muscle structure and function (10,14). Multifidus muscle atrophy and fatty infiltration have been reported in patients who have undergone LSF (7,11). Several studies have also reported low levels of trunk muscle strength and trunk extensor/flexor strength imbalance both preoperatively and postoperatively in LSF patients (13,25,28). These changes are associated with postoperative pain and disability (7,25). Hence, progressive and intensive training of the trunk muscles may be needed in postoperative rehabilitation after LSF to recover trunk muscle function.
After LSF, motion at the level adjacent to the fusion may be altered to compensate for changes caused by the fusion, an occurrence that must also be taken into account when planning postoperative rehabilitation programs. During the early postoperative phase, strengthening exercises should be performed while keeping the lumbar spine in a neutral position to minimize strain on the fused/adjacent segment and thereafter to avoid breakage of the fusion device or dislocation of the pedicle screws. In functional neutral spine control (NSC) exercises, a destabilizing force acts on the trunk through loading of the extremities, and therefore proper recruitment of the trunk muscles is required to stabilize the lumbar spine and lumbo-pelvic complex. Functional NSC exercises mimic the trunk muscle activity patterns that occur during activities of daily life, e.g., in lifting, pushing, or pulling movements (18,29). Neutral spine control exercises have been shown to decrease pain in patients with chronic low back pain (24).
The recommended resistance levels for increasing strength and obtaining training-induced muscle hypertrophy are 60–70% and 70–85% of the 1-repetition maximum (1RM), respectively (1); however, in the initial phase of training, lower-intensity exercises may also induce these changes in previously untrained persons (30). Electromyography (EMG) can be used as an indicator of the intensity of isometric exercises because a linear relationship between isometric trunk muscle force and EMG amplitude has been reported (5). It can be assumed that the level of EMG activation should be over 60% of maximal activity to stimulate muscle strength and structural adaptation (2).
Previous EMG studies indicated that the level of muscle activity needed in trunk muscle strength training might be achieved during functional NSC exercises performed in the sitting or standing position in healthy subjects (4,22,23,26,27). To our knowledge, the level of activation of the spinal muscles has not previously been studied in patients who have undergone LSF. The aim of this study was to investigate abdominal and back muscle activity during selected trunk-strengthening exercises that are performed with the lumbar spine in the neutral position. We hypothesized that these exercises would activate muscles to a level considered sufficient to lead to increased muscle strength.
Experimental Approach to the Problem
Trunk muscle activity was recorded during 4 dynamic upper limb exercises using 10-RM resistance, dynamic hip extension, and isometric trunk extension exercises. Muscle activity, measured through surface EMG, and pain intensity during exercise were used to evaluate the feasibility of the exercises for training after LSF. Electromyography activity during exercise was normalized to activity during maximal voluntary isometric contraction (MVIC), and normalized EMG amplitude was used as an approximate estimate of exercise intensity.
Twenty-two patients between 25 and 84 years of age (11 men and 11 women, mean age = 59 years) were recruited to the study (Table 1). All patients had undergone elective instrumented LSF, because of degenerative spondylolisthesis, spondylolysis, spinal stenosis, or degenerative disc disease, 3–11 months before the measurements. Other inclusion criteria were age >20 years; body mass index <30; and no neurologic, orthopedic, or cardiorespiratory problems that would prevent the physical exertion required for this study. All subjects were provided with information about the study protocol and possible risks and discomforts related to the measurements. Participants signed an informed consent form before measurements. This study was approved by the Ethics Committee of the Central Finland Health Care District.
The surgical procedure was decompression and instrumented posterolateral fusion, with or without posterior lumbar interbody fusion. Fusion levels were L2–S1 (n = 2), L3–L4 (n = 2), L3–L5 (n = 5), L4–L5 (n = 7), L4–S1 (n = 2), and L5–S1 (n = 4). During the first 3 postoperative months, patients were instructed to walk and perform light muscle endurance (abdominal crunch, supine bridge, and body weight squat) and stretching (hip extensor and flexor) exercises. The use of a bicycle ergometer was allowed 1 month after the operation. Other types of aerobic exercise, such as cross-country skiing, dancing, and water gymnastics, were permitted 2 months after surgery, but no specific program was provided for aerobic exercise. More strenuous loading of the lumbar spine and a gradual return to normal activities were allowed after the 3-month control visit.
Round, single-use silver/silver chloride surface electrodes (Ambu Blue Sensor M (M-00-S); Ambu, Ballerup, Denmark) were used to record muscle activity. Skin at the electrode attachment sites was shaved, abraded with sand paper, and cleansed with alcohol to decrease skin impedance. Pairs of electrodes were positioned at the rectus abdominis, external oblique, longissimus, and multifidus muscles on both sides of the body in the direction of the muscle fibers (Table 2) (27). The distance between the midpoints of the electrode pairs was 20 mm. Reference electrodes, to which a preamplifier was attached, were positioned in the area of the iliac spine. The measurements were done 10 minutes after attachment of the electrodes.
A bipolar ME6000 surface EMG device (Mega Electronics Ltd., Kuopio, Finland) was used for the measurements. Raw EMG data were recorded using a sampling frequency of 1,000 Hz and band-pass filtered using a bandwidth of 8–500 Hz (fourth-order Butterworth filter). A differential amplifier was used to amplify the measured signal and for filtering, as well as for the dampening of noise, with a common mode rejection ratio of >110 dB, a root mean square of noise <1.6 μV, and an amplification level of 305. The amplifier's feed impedance was >10 GΩ. The analog EMG signal was converted into digital format with a 14-bit Analog to Digital converter, after which it was saved on a computer for analysis. The raw digital EMG data were then rectified and averaged. The average amplitude level (in microvolts) of every exercise was calculated as the average of each of the analysis period's data segments (100 ms). Some EMG data were rejected because of poor EMG quality or lack of video data; out of 176 channels, data were rejected from bilateral shoulder flexion (n = 9), bilateral shoulder extension (n = 2), unilateral shoulder horizontal adduction (n = 2), unilateral shoulder horizontal abduction (n = 3), unilateral hip extension (n = 17), and modified Roman chair exercise (n = 9).
Each subject attended 2 sessions, on separate days. The first was a familiarization session, in which the patient had the opportunity to practice the exercises until performance technique was correct. During this session, the load to be used during 10RM was also evaluated individually for the 4 upper limb exercises. After a number of practice sets, load was added to the weight stack, and the subjects attempted 10 repetitions with proper movement technique. After each successful set of 10 repetitions, the weight was increased by 1–2 kg at a time until the subjects could no longer complete 10 repetitions. This was commonly achieved after 2–4 sets. Subjects rested for 2–3 minutes between each set. All EMG measurements were performed during the second session. The reference exercises were performed first and then the other exercises. Both were performed in a random order, except for the modified Roman chair exercise, which was always the final exercise. Patients had a 5-minute break after the reference exercises and a break of at least 1 minute between each exercise.
Abdominal muscle (rectus abdominis and external oblique) activities during maximal isometric trunk flexion and trunk extensor muscle (longissimus and multifidus) activities during maximal isometric trunk extension were used to normalize muscle activity levels during the actual exercises. Trunk flexion and extension measurements were performed in a standing position; the pelvis was fixed to the measurement frame at the level of the anterior superior iliac spine, and another support was placed below the knees. The feet were positioned 20 cm apart, measured from the medial border of the feet. A measurement harness was fastened around the shoulders and chest, and this was horizontally attached to a strain-gauge dynamometer (DS Europe, Milan, Italy). Two maximal efforts were performed. If the measured strength level increased more than 10% from the first effort, 1 additional effort would be performed. Muscle activity during the performance with the highest force value was used in the analysis.
The NSC exercises that we studied were: bilateral shoulder flexion and extension in a standing position, unilateral shoulder horizontal adduction and abduction in a sitting position, unilateral hip extension in a 4-point kneeling position, and modified Roman chair exercise in the Biering-Sorensen test position. The positions at the start and end of each exercise are shown in Figure 1. The subjects were advised to keep their lumbar spine in a neutral position during the performance of each exercise.
The upper limb exercises were performed against the resistance of a cable pulley machine (Lojer Ltd., Sastamala, Finland). Two separate cable handles were used in bilateral shoulder exercises. Mean loads (SD) on the weight stack were 6 (4) kg for bilateral shoulder flexion, 8 (4) kg for unilateral shoulder horizontal adduction, 12 (5) kg for bilateral shoulder extension, and 14 (9) kg for unilateral shoulder horizontal abduction. Unilateral upper and lower extremity exercises were done with the right limb. Body weight exercises were hip extension (10 repetitions) and modified Roman chair (30 seconds). The duration of each phase of the dynamic exercises was standardized by use of an acoustic metronome at 40 beeps per minute (3 seconds per repetition, 1.5 seconds concentric, and 1.5 seconds eccentric).
For the reference exercises, a 3-second time period was selected for analysis from the phase of effort in which the activity was the greatest. In addition, for the modified Roman chair exercise, a 3-second period from the beginning of exercise was used in the analysis. The starting and finishing points of dynamic upper and lower limb movements were determined from synchronized EMG and video (Legria HV40, Canon, Tokyo, Japan) data, and each repetition was divided into eccentric and concentric phases. Activity levels during the 2 phases were analyzed separately. The fifth repetition of the set was selected for the analysis to minimize the effects on performance technique of commencement of the set and neuromuscular fatigue. Relative loading of the trunk muscles was determined by comparing the ratio of EMG amplitude during exercises to the amplitude elicited during MVIC in the reference exercises.
Intensity of back and lower extremity pain was assessed by visual analog scale (VAS) (scale, 0–100) during the week before the measurements and during each exercise.
The results are presented as mean with SD. The activity levels of each muscle were normalized by being expressed as a percentage of the activity measured during the reference exercises (% of MVIC). Repeated measures analysis of variance was used to compare the surface EMG of each muscle, which was recorded during the different exercises. Paired t-tests were used to compare the EMG activities of the concentric and eccentric phases and between the left and right sides of the body. The alpha level was set at p ≤ 0.05 as the criterion for statistical significance. Statistical analyses were performed with the SPSS statistical analysis software program (version 19.0; SPSS Inc., Chicago, IL, USA).
Maximal isometric trunk extension and flexion forces (SD) were 342 (204) and 404 (198) N, respectively. The extension/flexion strength ratio was 0.86 (0.33). Electromyography activation in the dynamic exercises were higher during the concentric phase of exercise compared with the eccentric (p ≤ 0.05), with a few exceptions (Tables 3 and 4).
Electromyography activation of the abdominal muscles during the NSC exercises were significantly lower than the reference measurements of maximal isometric trunk flexion (p ≤ 0.05), with the exception of left side rectus abdominis activity during the concentric phase of bilateral shoulder extension (Table 3). The highest external oblique activity was measured during unilateral shoulder horizontal adduction and hip extension exercises.
Neural activation of the longissimus and multifidus during NSC exercises were significantly lower than during maximal isometric trunk extension (p ≤ 0.05), except for activity of longissimus during the concentric phase of bilateral shoulder flexion and modified Roman chair exercises. Multifidus activity was highest during the same exercises (Table 4).
As measured by the VAS, low back, and lower extremity pain was 12 (20) and 6 (18), during maximal isometric trunk extension, and 6 (14) and 5 (16), during maximal isometric trunk flexion. Intensity of back pain varied from 3 (7) during shoulder horizontal abduction to 16 (27) during Roman chair exercise. Corresponding VAS values of lower extremity pain varied from 3 (13) to 8 (19), respectively. There was no statistically significant difference between low back/lower extremity pain during the preceding week and pain during performance of the exercises, with the exception that low back pain during unilateral shoulder horizontal abduction and unilateral hip extension was lower than the average pain during the previous week (p ≤ 0.05).
In this study, the activity levels of the longissimus and multifidus muscles during bilateral shoulder flexion and modified Roman chair exercises were over 60% of MVIC, demonstrating the effectiveness of these exercises for the training of trunk extensor muscles. In addition, 50% of MVIC activity levels in the rectus abdominis and the external oblique muscles were measured during bilateral shoulder extension, unilateral shoulder horizontal adduction, and unilateral hip extension exercises. All NSC exercises caused minimal pain, supporting their feasibility for rehabilitation after LSF.
The hip extension and Roman chair exercises are traditional trunk-strengthening exercises that are widely used in low back pain rehabilitation. However, the exercise positions are nonfunctional, which may limit the transfer of training adaptations to daily activities. In this study, Roman chair exercise and bilateral shoulder extension achieved activity levels of the trunk extensor that were more than 60% of MVIC. Bilateral shoulder extension is a functional exercise, and based on our results, it can be assumed that this exercise can be used to correct low strength levels of the trunk extensor muscles and trunk extensor/flexor strength imbalance, as previously observed in LSF patients (25).
Previously, it has been shown that isometric and dynamic upper extremity pushing and pulling exercises with 1RM load are effective in loading the longissimus, multifidus, and external oblique muscles (>60% of MVIC) of healthy subjects in a standing position (26,27). However, the activities of the abdominal and back muscles are significantly lower if unilateral shoulder horizontal adduction and abduction exercises are performed without fixation of the pelvis (26). Because fixation of the pelvis cannot be used in home-based training, we assumed that the sitting posture would stabilize the pelvis and increase the base of support, allowing more intensive exercises. In this study, the highest average activity with 10-RM resistance was 48% of MVIC measured from the ipsilateral external oblique during the concentric phase of unilateral shoulder horizontal adduction. In other muscles, the average intensity varied between 7 and 37% during the concentric phase of unilateral upper limb exercises. It is probable that in functional exercises where loading takes place concurrently in several movement planes, muscle activity cannot be as high as in exercises where loading occurs in only 1 movement plane (18).
Previously, Konrad et al. (15) reported that trunk muscle activity is not necessarily similar during different phases of exercise and thus considering the mean activity level could underestimate the effect of exercise. Therefore, we divided exercise performance into concentric and eccentric phases. Interestingly, the differences in the activities of the trunk muscles between the concentric and eccentric phases of movement were significant in nearly all NSC exercises. Higher activities during the concentric phase vs. the eccentric phase were in particular observed in the rectus abdominis during bilateral shoulder extension and in the back muscles during bilateral shoulder flexion. The duration of the phases of the dynamic exercises was standardized with a metronome, but it is possible that the concentric phase was performed in a more controlled manner, which may explain the observed higher activity level.
The difference between left and right side activity in unilaterally performed limb exercises was noted in several previous studies (4,8,23,26). In unilateral exercises, loading induces rotational forces to the trunk, and control of these forces may cause contralateral differences in muscle activity. In this study, the largest differences between the right and left sides were recorded in the external oblique muscle during unilateral shoulder horizontal adduction and unilateral hip extension, which may be explained by the essential role of the external oblique muscle in controlling rotational forces (19). However, the activity levels of the right and left side longissimus were also different during modified Roman chair exercise, which cannot be explained by any biomechanical model. The most probable cause for this difference may lie in performance technique (e.g., rotational posture).
Many individual factors such as age, present health, and quality of the bone matrix may affect healing of the fusion. Increased loading of the spine is not possible until the fusion has been healed and strengthened. Although we suggest more intensive training after the early recovery phase, the risks associated with exercise should be minimized.
Maintenance of a neutral spine posture during loading increases the shear and compression tolerance of the lumbar spine (9,17) and avoiding bending probably improves the safety of the exercises. Lumbar fusion changes the mobility of the adjacent segment and may increase the probability of intervertebral disk degeneration (3). It has also been proposed that sagging of the thoracolumbar fascia may predispose patients to the development of adjacent segment disease after LSF (12). Therefore, training of the muscles that have attachments to the thoracolumbar fascia (e.g., latissimus dorsi and external oblique) may increase the stability of the lumbar region and the sacroiliac joints (20) and thus decrease the probability of adjacent level disorders.
Marshall et al. (16) and Danneels et al. (6) measured trunk muscle activity during trunk muscle exercises in patients with chronic low back pain. However, they did not report on pain intensity during exertion; thus, the feasibility of their studied exercises for rehabilitation purposes is difficult to evaluate. In this study, the average intensities of low back and lower extremity pain during the reference, upper extremity, and traditional trunk-strengthening exercises were all lower than average pain experienced during the previous week. The low levels of pain experienced during the exercises support the use of functional NSC exercises in postoperative training after LSF.
There are some factors that can be considered limitations of this study. The study population was quite heterogeneous, especially with regard to age. However, because definitions for the inclusion criteria were relatively loose, the study group is representative of patients who normally undergo LSF (25). The time between operation and measurement varied considerably (3–11 months) in the study group; shorter postoperative time may have some effect on effort level during MVIC and 10-RM measurements. However, after control radiography was carried out at 3 months, all patients were allowed to perform normal daily activities, an early activation that may hasten the restoration of the patient's confidence in using their back. In most previous research that evaluated muscle activation, the studied subjects were healthy young people; therefore, those results cannot be used directly to design postoperative rehabilitation protocols. Moreover, normalization of activity to maximal voluntary contraction may be difficult in patients who recently underwent LSF; therefore, we cannot completely exclude the possibility that pain or fear of pain may have had some effect on our results. However, since the pain intensity remained at relatively low levels during all of our measurements, this factor should not have a significant influence on our results. Most of the studied exercises were unfamiliar to the patients, and they may have had challenges in maintaining trunk posture and target speed of movement during the measurements. However, these difficulties were minimized by careful instruction, practicing, and monitoring of the performance.
Optimal functioning of the neuromuscular system is desirable for control and protection of the spinal segments after spinal fusion surgery, thereby decreasing the likelihood of disability. Attention should be paid to correcting detected changes in spinal structures and functions that will not spontaneously return to normal after surgery. The postoperative rehabilitation program should include functional exercises that can induce increases in trunk muscle strength and muscle mass, and that can be performed with the spine in the neutral position. The results of our study demonstrate that NSC exercises activate trunk muscles effectively without increasing pain and are therefore feasible exercises for improving muscle endurance and strength after LSF. In addition, functional upper limb exercises can be carried out at home by patients training with elastic bands with no other specific exercise equipment needed. The performance positions of exercise, the direction of loading, and whether upper limb exercises are performed bilaterally or unilaterally should be determined according to training goals. The results of this study may be used when planning progressive postoperative rehabilitation programs after LSF.
This study was funded by the Academy of Finland, Finnish Cultural Foundation, Pirkanmaa Regional fund, and by competitive state research financing of the Expert Responsibility Area of Tampere University Hospital, Tampere, Finland. The authors thank Laura Hilden PT, Tiina Kaistila MSc, Seija Rautiainen, Ari Salin PT, and Piia Tarnanen MSc for their help in data collection.
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Keywords:Copyright © 2014 by the National Strength & Conditioning Association.
electromyography; abdominal muscles; back; spinal fusion