Stoop or squat: a review of biomechanical studies on lifting technique

Abstract

Objective. To assess the biomechanical evidence in support of advocating the squat lifting technique as an administrative control to prevent low back pain.

Background. Instruction with respect to lifting technique is commonly employed to prevent low back pain. The squat technique is the most widely advised lifting technique. Intervention studies failed to show health effects of this approach and consequently the rationale behind the advised lifting techniques has been questioned.

Methods. Biomechanical studies comparing the stoop and squat technique were systematically reviewed. The dependent variables used in these studies and the methods by which these were measured or estimated were ranked for validity as indicators of low back load.

Results. Spinal compression as indicated by intra-discal pressure and spinal shrinkage appeared not significantly different between both lifting techniques. Net moments and compression forces based on model estimates were found to be equal or somewhat higher in squat than in stoop lifting. Only when the load could be lifted from a position in between the feet did squat lifting cause lower net moments, although the studies reporting this finding had a marginal validity. Shear force and bending moments acting on the spine appeared lower in squat lifting. Net moments and compression forces during lifting reach magnitudes, that can probably cause injury, whereas shear forces and bending moments remained below injury threshold in both techniques.

Conclusion. The biomechanical literature does not provide support for advocating the squat technique as a means of preventing low back pain.
Relevance

Training in lifting technique is widely used in primary and secondary prevention of low back pain, though health effects have not been proven. The present review assesses the biomechanical evidence supporting the most widely advocated lifting technique.

Introduction

In view of the high costs associated with low back pain (LBP) and the high recurrence rate of the complaints, primary prevention has received considerable interest. Several recent review studies on the epidemiology of LBP conclude that lifting is the best documented risk factor for this disorder [1], [2], [3], [4]. In line with this, preventive strategies often involve measures aimed at reducing back load associated with lifting tasks. Next to engineering controls, administrative controls such as training and instruction in particular with respect to lifting technique are widely used [5], [6], [7], [8], [9], [10], [11]. Intervention studies have failed to demonstrate convincing effects of training and instruction with respect to lifting technique on musculoskeletal health. Health effects of training programmes with respect to lifting technique were either lacking or minimal [5], [8], [12], [13], [14], [15], [16], [17]. This may be due to a lack of skill or willingness of workers to apply the lifting techniques taught [5], [9], [10], [18], but also the rationale behind the principles taught has been questioned [9].

The most commonly advised lifting technique is the so-called squat technique or leg lift, in which the back remains as erect as possible and in which the knees are flexed [19]. It can easily be understood that compliance with this advise is often low, given the high energetic cost of this technique [20], [21], [22], causing higher perceived exertion and more rapid fatigue development [23], [24], as compared to its exact opposite the stoop technique. In repetitive lifting experiments, subjects have been shown to shift from a squat technique to a stoop technique, probably to avoid or diminish fatigue development [25], [26], [27], [28]. Better training programmes can possibly overcome this low compliance. However, if the rationale behind promoting the squat technique is dubious, more effort in improving methods of training and instruction does not seem warranted. The aim of the present review therefore was to evaluate the evidence that the lifting technique is an important determinant of the probability of contracting LBP. Since the premise behind training in lifting technique is that the mechanical load during lifting determines this probability, biomechanical studies on lifting technique were reviewed. This review was limited to studies comparing the stoop and squat techniques, as these are well defined and frequently studied techniques in manual materials handling. In addition, a limitation is made to symmetric lifting, since all the available data suggests that symmetric lifting is to be preferred over asymmetric lifting [29], [30], [31], [32], [33], [34], [35].