Introduction

Low back pain (LBP) is a problem that not only affects the general population but is increasingly becoming a problem in athletes involved in endurance sports. Rowing in particular has been associated with a reported incidence of LBP as high as 38.4%¹. These subjects experienced repeated interruption of their training routine and reduced performance due to pain^2-5. In 15% of cases, symptoms have been sufficiently severe to cause subjects to give up rowing ¹.

Among young, athletic persons, pain frequently occurs in close temporal relation to physical exertion, either during practice or in competition. In such cases, degenerative changes of the lumbar spine do not provide a plausible explanation for the occurrence of LBP.  By contrast, similar sports-related pain has been previously reported in long-distance and marathon runners due to a chronic functional compartment syndrome (CFCS) of the anterior tibialis musculature^{6, 7}. According to the physiological model, this phenomenon is triggered by an increase in intracompartmental pressure during muscle activity^{8, 9}, which leads to vascular compression in the capillaries or an increased diffusion distance¹⁰. Finally, these changes may result in inadequate tissue perfusion, which in turn causes pain and loss of function in the affected muscle¹⁰.

CFCS has also been discussed as a cause of chronic LBP^11-14. In a previous study with young elite oarsmen, the diagnosis of CFCS was ruled out as the reason for the drop in tissue oxygenation (pO₂) during activity and the occurrence of localised fatigue in the multifidus muscle¹⁵. Other factors, such as pain-induced triggering of neural or humoral processes similar to those seen in reflex sympathetic dystrophy, have been suggested as a reason for the observed muscle performance changes during exercise in elite rowers.

Although several diagnostic studies have been performed to find the cause of the observed exercise-associated LBP, the exact diagnosis remains presently unclear. Nevertheless, in former training studies it has been recognised that training regimens improve the microcirculation in the affected muscle groups^{16, 17}. In addition, patients with LBP have experienced improvement in their symptoms as a result of exercise programmes designed to strengthen the low back musculature¹⁸. However, no training study has previously investigated the muscle performance parameters and their effects on sports-related LBP simultaneously.

Against this background, this present study hypothesises that a device-assisted training programme leads to an improvement of the microcirculation and muscle performance of the multifidus muscle, which contributes to the reduction of exercise-associated LBP. The study thus investigated the course of pain intensity and the microcirculation, and muscle performance of the multifidus muscle in a population of young elite oarsmen with exercise-associated LBP. Direct measurement techniques of the tissue microcirculation, such as photoplethysmography or laser-doppler flowmetry, show a maximum investigation depth of 1- 4mm, so that only an indirect in vivo measurement of the microcirculation and muscle performance is possible in the deep lumbar spine musculature. Simultaneous measurements of intramuscular pressure (IMP) were performed as the parameter for muscle volume increases during exercise, pO₂ as the parameter for tissue oxygenation, median frequency shift (MF shift) in the power spectrum of the surface electromyography (EMG) of the multifidus muscle as the parameter for muscle fatigue, and overall back muscle strength as the parameter of the training condition.

Methods

Patients

Thirteen athletes (five males, eight females; age of all patients: 15 years) from the Olympic Camp at Potsdam, Germany, who had trained for at least two years as high-performance oarsmen, were included in the present study. All athletes complained of exercise-dependent bilateral paravertebral LBP occurring in close temporal relation to training or competition events for a median duration of 12 months. The study was performed during a non-competitive period. Because of his condition, one oarsman had already started physiotherapy.

Study design

The study was approved by the ethics committee of the University of Ulm. After subjects had given their written consent, a clinical examination was performed to verify the diagnosis of exercise-dependent bilateral paravertebral LBP as an inclusion criterion. An X-ray examination in two planes was performed to rule out degenerative lumbar diseases. The current use of pain killers was also ruled out. Pain data were documented. Catheters were placed for measurement of IMP and pO_2, and EMG electrodes were affixed (details see below). The tests summarised in Table 1 were performed under continuous monitoring of IMP, pO₂ and EMG.

Table 1: Position, duration and trunk torque of the performed tests

MTT: maximum trunk torque

After baseline data had been obtained, subjects entered a three-month device-assisted training programme for strengthening the back musculature. Subjects continued with their usual rowing-specific training without change. Device-assisted training of the back musculature included dynamic exercises for all directions of spinal movement: lateral flexion (model R 17; mkb-Systems, 88481 Balzheim, Germany), rotation (model R 7; mkb-Systems, 88481 Balzheim, Germany), and extension of the lumbar spine (model R 14; mkb-Systems, 88481 Balzheim, Germany). A specially designed device was used to strengthen the thoracic extensor muscles (model R 18; mkb-Systems, 88481 Balzheim, Germany). During the first week, patients were instructed in the correct execution of the training programme using the exercise devices without application of weights (Figure 1).

Figure1: A. Model R7 – lumber rotation training. B. Model R14 – lumber extension training

C. Model R17 – lumbar lateral flexion training. D. Model R18 – thoracic extension training

During the next training phase (weeks 2 to 4), exercises were conducted with weights corresponding to 30% of the patients' maximum voluntary contraction (MVC). Each exercise was repeated between 30-40 times in two sets. Because of the patients' pain-related reduced capacity, exercises requiring numerous repetitions were performed with less applied weight. The exercise plan encompassed three training sessions per week, and was attended jointly by all patients. After four weeks (weeks 5 to 8), the applied weight was increased to 45% of the MVC with a simultaneous reduction to 20-30 repetitions per exercise. After another four weeks (weeks 9 to 12), patients reached the final level of 60% of the MVC and 10-20 repetitions per exercise. A final assessment of all parameters was conducted at the end of the exercise phase.

Pain analysis

Using a 10cm visual analogue scale (VAS), ranging from 0 (no pain) to 10 (excruciating pain), patients reported the frequency and the tolerable, average and maximum intensity of their pain.

Preparation for catheter implantation

Subjects were placed in the prone position on an examining table. Following careful cleansing, removal of fatty secretions and disinfection of the skin, the catheters and the surface electrodes were placed under ultrasound control (see Figure 2). Former studies showed side differences in paravertebral muscle performance depending on the patients’ hand dominance¹⁹. Therefore the contralateral side of the patient’s hand dominance was chosen for measurement. Catheters were fixed in place using strips of tape in order to prevent accidental dislocation.

Figure 2: Positioning of the probes and electrodes: 1) Temperature probe, 2) pO₂ probe, 3) IMP probe, 4) Electrodes for surface – EMG

Strength measurement

Intitially, resting values were determined in the supine position on an examination table. The subjects were then positioned and studied using fitness training equipment (model R 14; mkb-Systems, 88481 Balzheim, Germany). The unit permitted determination of maximum trunk torque. After the maximum trunk torque had been determined, we made parallel recordings of IMP, pO2, and the EMG signal of the multifidus muscle during sustained isometric exercise at 60% of maximum trunk torque. Measurement was stopped after the load resulted in flexion of the trunk of >10° due to muscle fatigue. Finally, the subjects were again placed on the examination table in the supine position and the same parameters were measured during the recovery phase (see Table 1).

Intramuscular pressure measurement

Local anaesthesia was induced by injection of a local anaesthetic agent (1ml) down to the fascia at a point 1cm lateral to the midline. Then a piezoelectric pressure catheter ²⁰ (ARGUS, MIPM GmbH, Mannendorf, Germany) was introduced through an indwelling venous catheter into the multifidus muscle in caudal direction, at an angle of 45°, at the L3 level of the lumbar spine (see Figure 2). The piezoresistive measuring method is based on the physical fact that semiconductors change their specific resistance under pressure. The analogue signal of the catheter was digitised at 10Hz and recorded for later evaluation. The IMP values were plotted against time and the maximum IMP value was noted and used for further calculation.

Measurement of tissue oxygen partial pressure

The same technique was used for implantation of the pO₂ catheter (LICOX C1-Sonde, GMS, Mielkendorf, Germany) into the multifidus musculature at the L2 level of the lumbar spine. Implantation was performed at a point 2cm lateral to the midline (see Figure 2). The technique and processing of the data are described in detail by Boekstegers et al¹. In addition, a temperature measurement catheter (LICOX C8-Sonde, GMS, Mielkendorf, Germany) was implanted into the multifidus muscle in order to correct temperature-dependent pO₂ drift. Data were digitised at 0.2Hz and recorded.

Electromyography

Surface electrodes were attached bilaterally to the skin above the multifidus muscle, caudal to the points of insertion of the catheters, at the L4 level of the lumbar spine (see Figure 2). Potentials were recorded in bipolar fashion with a reference electrode over the vertebra prominens. Self-adhesive 1.2cm silver-silver chloride surface electrodes with gel pads were used. The interelectrode distance was 2cm. The EMG raw signal was recorded with a bandwidth of 5-1000Hz and digitized at 2000Hz. The mean rectified amplitude and the MF shift of the power density spectrum of the EMG signal were calculated as parameters indicative of fatigue ^{22, 23}.

Data analysis

Evaluation of the parameters measured was performed by comparing the data sets obtained for each oarsman before and after the training programme. All parameters were evaluated descriptively. Statistical significance between the groups was assessed using the Wilcoxon matched-pairs test (statistical significance at p<0.05). Data are presented as rank order values since normal distribution of the values was not assured.

Results

Pain course

Three of 13 oarsmen (23.1%) reported complete resolution of pain as a result of training programme participation, while in seven other cases (53.8%) pain intensity was reduced below the patient’s individual tolerance threshold. Two oarsmen (15.4%), despite improvement in pain, reported that residual complaints remained above their individual tolerance threshold. One patient (7.7%), who ultimately withdrew from competitive rowing due to back pain, reported no improvement in pain as a result of the training programme.

Prior to the training programme, the median average pain intensity was 4.0 compared to 1.5 after the training programme. The reduction in median average pain intensity was 2.0 (p=0.002).  The median maximum pain intensity was 5.5 prior to the training programme compared to 3.0 after the training programme. The reduction in median average pain intensity was 2.5 (p=0.002) (see Table 2). After training programme participation, the reduction in the median frequency of pain was 2.3 days/week (p=0.002).

Table 2: Pain frequency, average pain intensity and maximum pain before and after training

VAS: visual analog scale

Maximum trunk torque and duration of exercise

Following the training programme, an increase in maximum trunk torque was documented in 12 out of the 13 oarsmen, while 8 out of the 13 showed improved endurance times. Following training, the oarsmen showed a median increase in isometric exercise duration of 44 seconds (p=0.096). The median increase in maximum trunk torque was 19N (p=0.002) (see Table 3).

Table 3: Comparison of maximum trunk torque (Test 2) and duration of exercise at 60% of the maximum voluntary trunk torque (Test 3) before and after training

Intramuscular pressure

Pre-training IMP values are available for all test subjects and post-training IMP values were obtained for 10 subjects. Failure to measure IMP in three cases was due to accidental dislocation of the IMP catheter. IMP at rest before the contraction tests was nearly identical before and after training. IMP values at the start and the end of contraction measurement increased significantly after training. At the beginning of endurance exercise, the median IMP was 60.2mmHg before training compared to a median IMP of 116.9mmHg after training (p=0.003). Measurements after endurance exercise showed medians of 77.1mmHg before and 134.0mmHg after training (p=0.013). The median increase in intramuscular pressure during endurance exercise was significantly greater before training (17.8mmHg) than after training (9.0mmHg) (p=0.039) (see Table 4).

Table 4: Comparison of intramuscular pressure (IMP) before and after training at rest, during endurance exercise and at rest following endurance exercise (all values in mmHg)

Oxygen partial pressure

Pre- and post-training pO₂ data are available for all oarsmen. Higher median pO₂ values at rest before contraction were measured after training. Median pO₂ prior to training was 37.7mmHg compared with 48.5mmHg after training (p=0.022). The median change in pO₂ during isometric endurance exercise was -6.6mmHg before training compared to 0.1mmHg after training (p=0.158) (see Table 5).

Table 5: Comparison of oxygen partial pressure (pO₂) before and after training at rest, during endurance exercise and at rest following endurance exercise (all values in mmHg)

Median frequency shift in the power spectrum of the EMG signal

Pre-training EMG values are available for all test subjects and post-training values were obtained for 10 subjects. In three cases, measurements could not be used due to the presence of 50Hz interference signals. Median frequencies at the start and end of the endurance exercise are practically identical. The median frequency shift prior to training was -11.5Hz, compared to -14.7Hz after training. Differences were not statistically significant (p=0.424) (see Table 6).

Table 6: Comparison of the median frequency [Hz] in the power spectrum of the electromyography before and after training at the beginning and ending of endurance exercise (Test 3)

Column 3 gives the MF shift during endurance exercise (all values in Hz)

Discussion

The present study investigates the effects of a device-assisted training programme on the microcirculation and muscle performance of the multifidus muscle as it was hypothesised that the improvement of these parameters contributes to a reduction of exercise-associated LBP. Simultaneous measurements of intramuscular pressure (IMP) were performed as the parameter for muscle volume increase during exercise, pO₂ as the parameter for tissue oxygenation, median frequency shift (MF shift) in the power spectrum of the surface electromyography (EMG) of the multifidus muscle as the parameter for muscle fatigue, and overall back muscle strength as the parameter of the training condition.

Exercise programmes aimed at strengthening the back musculature resulted in improvement in symptoms in patients with LBP¹⁸. To date, however, no findings have been published regarding the results of such training programmes in athletes. In this study’s population, 12 out of the 13 oarsmen reported reduced pain as a result of training programme participation. Because of their motivation, readiness to participate, and experience of daily goal-oriented physical exercise, these subjects exhibit important characteristics crucial to the therapeutic success of a training therapy. This may explain the significant reduction in pain in a population of young individuals not affected by degenerative changes.

Other results of the training programme were a significant increase in maximum trunk torque and prolongation of exercise duration in 8 out of the 13 oarsmen as a result of better training conditions. An increase in maximum trunk torque as a result of training programme participation has already been described in other studies ^24,25. In this present study, this effect was associated with a significant increase in baseline intramuscular pressure in the multifidus muscle, which has been shown to be dependent on the higher exercise load and the degree of muscular contraction ^{26, 27}. This may be due to hypertrophy of the musculature but could also be the result of a reduction in patients’ fear avoidance beliefs ²⁸.

Furthermore, it has been shown in studies of cardiopulmonary rehabilitation that regular training programme participation promotes microcirculation and pO₂ saturation in skeletal muscle. This effect has been demonstrated in young males ¹⁶, in older men with a life-long history of athletic activity ²⁹, and in patients with coronary insufficiency ^{17, 30, 31} compared to control groups. An improved microcirculation may involve an increase in arterial inflow and/or improved venous outflow. Improvements in arterial inflow have been shown in relation to an increase in peak flow in the large arteries as a result of exercise training^{17, 32, 33}. The higher pO₂ levels at rest before contraction (Test 1) and the lower drop in pO₂ during the endurance test (Test 3) indicate a better oxygen supply to the muscle at rest and during exercise.

Other studies have shown that an increase in vasodilatation in the resistance vessels ^{30, 31, 34} and enhanced venule density ³², resulting in increased muscular blood flow, are also effects of training participation. Increased venous blood flow may accelerate clearance of interstitial fluid and lead to a smaller interstitial fluid volume increase during isometric exercise. As the increase in the muscle volume within the constraints of a relatively inelastic fascial envelope is one cause of an increased IMP, this effect could be indirectly understood in this study. In the post-training measurement, the increase in IMP during endurance exercise was significantly smaller compared to the pre-training measurement. This speculation is also supported by the fact that, despite a longer exercise duration and a higher exercise load, unchanged MF shifts were measured post-training, implying better muscle performance post-training. Because the MF shift depends on the degree of exercise ³⁵ and is ascribed to the accumulation of metabolic products ²², an unchanged MF shift despite higher exercise levels suggests, furthermore, a more rapid clearance of metabolism products from the muscle.

The fact that no control group was investigated unavoidably limits the study’s conclusions. The investigation of a control group was not possible due to the limited number of healthy young elite oarsmen willing to participate in a study that involved “invasive” diagnostic measurements. Nevertheless, this study showed interesting changes and interactions of the measured parameters in the investigated study group after the performance of the device-assisted training programme. Firstly, longer exercise duration and a higher exercise load (Tests 2 and 3) have been observed after training participation, indicating a better training condition. This assessment was supported by the smaller increase in median IMP during the endurance exercise (Test 3) after training, which showed better muscle performance. Regarding the pO₂ levels, this present study found better rest values (Tests 1 and 4) and smaller changes during the endurance test (Test 3) after training, so that a better oxygen supply at rest and during exercise could be safely postulated. Despite a higher exercise load, the median frequency shift during the endurance exercise (Test 3) was not significantly different before and after training, confirming a better muscle performance and microcirculation of the multifidus muscle. As the pain course showed improvement in 12 out of the 13 oarsmen, a contribution of the improved muscle performance and microcirculation of the multifidus muscle to the pain course can be assumed.

Conclusions

In conclusion, training programme participation resulted in an increase in the exercise level of the multifidus muscle and an improvement in patients’ pain symptoms in 12 out of the 13 young elite oarsmen with exercise-dependent LBP. The smaller change in IMP and the smaller drop in tissue oxygenation during isometric endurance exercise indicate an improvement in muscle performance and microcirculation. The findings of the present study imply that muscle training focusing on the multifidus muscle should be taken into consideration in the planning of back training programmes for oarsmen.

Acknowledgements

The experiments comply with the current laws of Germany and were performed inclusive of ethics approval. No commercial party had a direct financial interest in the results of the research supporting this article or will confer a benefit upon the authors or upon any organisation with which the authors are associated.

Address for correspondence:

Dr Christoph Dehner, Department for Trauma, Hand, Plastic and Reconstructive Sugery, University of Ulm, Steinhövel Street 9, 89075 Ulm, Germany

Tel.: +49 (731) 500 54568

Fax: +49 (731) 500 27349

Email: christoph.dehner@uniklinik-ulm.de

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