Electromyography Analysis of Fatigue of the Paraspinal Muscles during a Squat and Stoop Lifting Task in Healthy Participants

1. Introduction

Repetitive movements are a common cause of musculoskeletal pain due to the increase in physical burden born by the tissues.¹ A common example is the association between repetitive lifting tasks and low back pain.^2-6 Lifting through bending of the lower limbs (squat-to-lift), rather than flexion of the low back while keeping knees straight (stoop-to-lift), has been promoted as the proper technique to reduce the risk of low back pain.^7,8 However, van Dieën et al.⁹ did not find sufficient evidence of a protective effect of the squat-to-lift technique in lessening the burden on the low back compared to the stoop-to-lift technique. Moreover, they reported that the stoop-to-lift technique might be safer than the squat-to-lift technique for maintaining balance during lifting.¹⁰ Biomechanically, the pressure exerted on the lumbar vertebrae is determined by multiple factors, including the weight of the box and the speed and acceleration of the movement, and, to a lesser extent, by the amount of bending at the waist.¹¹ Similarly, Hwang et al.¹² found no significant difference in the peak moment at the lumbar spine between the squat-to-lift and stoop-to-lift techniques for lifting of 5 kg and 10 kg boxes. Moreover, the peak moment was greater for the squat-to-lift than stoop-to-lift technique for 15 kg boxes. However, Kim¹³ reported more rapid fatiguing of the multifidus and iliocostalis lumborum muscles for the stoop-to-lift than squat-to-lift technique. Of note, however, Kim evaluated fatigue of the paraspinal muscles during an isometric holding task rather than a repetitive lifting task that is common in the workplace and everyday life. Moreover, analysis of only the multifidus and iliocostalis lumborum muscles may not be representative of all paraspinal muscles.

Muscle fatigue reduces strength and proprioception, decreasing the capacity of muscles to provide stability to the lumbar spine and increasing the risk of injury during lifting.^14,15 Therefore, preventing muscle fatigue during lifting would improve the stability the lumbar spine.¹⁰

The quadratus lumborum muscle attaches directly to the lumbar spine and plays a major role in the final phase of straightening the lumbar spine during lifting, as well as enhancing the stability of the lumbosacral spine, in conjunction with the lumbodorsal fascia.^16,17 The lumbodorsal fascia consists of three layers; the anterior layer lies on the anterior surface of the quadratus lumborum muscle, the middle layer between the quadratus lumborum and erector spinae muscles, with the posterior layer lying on the posterior surface of the erector spinae. The stabilizing effect of the lumbodorsal fascia is enhanced by the tension generated by the gluteus maximus muscle.¹⁸ As such, both the coordination and fatigue status of the paraspinal muscles, including the quadratus lumborum and gluteus maximus, will influence the stability of the lumbosacral spine during lifting. However, although various studies have evaluated muscle activation and fatigue of the erector spinae, multifidus and iliocostalis lumborum muscles, the activation and fatigue of the quadratus lumborum muscle and gluteus maximus muscles remain to be clarified. Therefore, the purpose of this study was to compare the profile of muscle fatigue of the erector spinae, rectus abdominis, quadratus lumborum, and gluteus maximus muscles for the squat- and stoop-to-lift techniques.

2. Participants and Methods

3. Results

3.1 General Characteristics of the Participants

The general characteristics of our participants are presented in Table 1.

Table 1

General characteristics of the participants

3.2 Number of Repetitions

The total number of repetitions was equivalent for the two lifting techniques (p > 0.05; Table 2).

Table 2

Total number of repetitions completed for each lifting technique

3.3 Change in the Median Frequency for Each Muscle

There was no difference in the median frequency of EMG signals between the two lifting techniques for all muscles (p > 0.05). With fatigue, the median frequency decreased, from baseline, in the quadratus lumborum and gluteus maximus muscles only with the squat-to-lift technique (p < 0.05; Table 3).

Table 3

Change in the median frequency of paraspinal muscles in the fatigue trial, from baseline

4. Discussion

The purpose of this study was to compare fatigue of the erector spinae, rectus abdominis, quadratus lumborum, and gluteus maximus muscles for two lifting techniques, the squat-to-lift and stoop-to-lift techniques. There was no significant difference in the number of repetitions between the two techniques. Moreover, there was no change in the median frequency of the EMG of the erector spinae and rectus abdominis muscles for either technique, with the sensation of fatigue likely being contributed specifically by the quadratus lumborum and gluteus maximum muscles.

As in our study, Park and Kim²¹ confirmed activation of the erector spinae muscles during lifting. As the erector spinae muscles are comprised of Type I fibers, they would be resistant to the effects of fatigue with repeated lifting maneuvers.²² By comparison, the absence of fatigue in the rectus abdominis muscle is explained by its functional role in supporting the weight of the object and body during the lifting task, rather than providing stability to the lumbar spine,²³ compared to the extensor muscles which maintain the erect posture of the spine, particularly in the final phase of the lift.

The quadratus lumborum and gluteus maximus muscles had a significantly lower median frequency, and hence fatigued to a greater extent,²⁴ during the squat-to-lift than the stoop-to-lift technique. This difference might be explained by differences in the amount of flexion at the hip between the two techniques, being greater for the stoop-to-lift than squat-to-lift technique.¹² Greater flexion of the hip joint would increase the passive tension in the lumbodorsal fascia during initial flexion phase of the lift to grab the box.¹⁷ The elastic force developed by the passive tension applied to the fascia would be available to assist with extension of the lumbar spine to return to a standing position. In contrast, for the squat-to-lift technique, this extension to return to a standing position is actively performed by the gluteus maximus muscle, with the decrease in median frequency of the EMG signal indicating that fatigue that develops over repeated cycles of the squat-to-lift technique.

The limitations of our study need to be acknowledged. First, we only included only healthy participants in their 20 s from the same geographic area. Therefore, future studies should be conducted with a wider variety of participants with regard to age, stature and physical fitness. As well, we did not evaluate the effect of past history of back injury and other health conditions on profiles of muscle fatigue. Lastly, we only considered fatigue, without evaluation of the underlying coordination between muscles. This aspect should be evaluated in future studies, including a larger set of muscles and lifting techniques.

5. Conclusions

Using an EMG-based analysis of muscle fatigue, we identified the quadratus lumborum and gluteus maximus to contribute to fatigue during repetitive squat-to-lift maneuvers. In contrast, fatigue did not develop when using the stoop-to-lift technique, indicating that the squat-to-lift technique should not always be considered as the preferred technique for lifting.

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