The effect of load on biomechanics of the back and upper limb in a bench to shoulder lift during the WorkHab Functional Capacity Evaluation

2.1 Study design

This was a within subject observational study where subjects act as their own control [33]. The repetitive lifting component of the WorkHab FCE was simulated for this experimental study, with video data of subjects completing the lifting component collected. This study used the video data to compare the joint angles of the lumbar and thoracic spine, shoulder and elbow at minimum load and safe maximum load weights during the bench to shoulder lift. The video analysis tool of Dartfish Pro-Suite (Dartfish, Lausanne, Switzerland) was used to view subjects in a sagittal plane in frames selected according to time during the bench to shoulder lift. Joint angles in these frames were then measured with the Dartfish angular measurement tool which allows calculation of angles using on screen drawing tools. Ethical approval was granted from the University Human Research Ethics Committee (H-2015-0010).

2.2 Participants

Staff and students from the Callaghan Campus of the University of Newcastle, Australia were recruited for the study. Participants were excluded if they had pre-existing musculoskeletal injuries or conditions which would impede them from completing a manual handling task. Thirty participants provided informed consent, however only data from 28 were eligible to be included in this study.

2.3 Equipment/procedures

Prior to commencing the assessment, all subjects completed a pre-screening general health questionnaire, blood pressure measurement and a three minute step test to determine heart rate recovery as per the WorkHab FCE guidelines [29]. Participants’ joints were then marked. A single researcher trained in physiotherapy joint palpation skills marked each participant to minimise variation. Markers were positioned on the right lateral aspect of the participant’s bodies to capture the sagittal motion of the shoulder, and elbow, and on the middle of the spinous processes of C7, T7, L3 and S2. Spinous processes were marked using foam balls applied with double sided tape so they could be viewed in a sagittal plane and black ink or white tape was used to mark the other landmarks. (See Fig. 1). Dartfish Pro Suite was used to view videos, capture still images, and measure distances and joint angles using the in-built software features [34] (Table 1).

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Fig.1

Position of joint markers.

The FCE was administered by a WorkHab accredited assessor [29]. The lifting task was completed in a controlled environment using WorkHab FCE protocols [35], with a standard bench height of 700 mm, and modular boxes aligned at the level of the subject’s waist (approximately navel) and shoulder (approximately acromion). All subjects wore heart rate monitors throughout the assessment. As per WorkHab FCE protocols [35], subjects completed three lifting repetitions before weight was increased. The lifted box, which has cut out handles on the side (weighing 5.5 kg) was lifted from beginning to end height and returned. This was repeated three times before additional weight was added. Initially subjects lifted the empty box as the minimum lift. After three lifts were completed, weight was increased by the WorkHab assessor as per the WorkHab protocol, amount being dependent upon the individual subject and closeness to maximum safe capacity, this was repeated until a safe maximum lift, as determined by the FCE assessor, was obtained. The three indications listed in the FCE guidelines for ceasing lifting tasks are the observation of compensatory or unsafe movements, participants reaching their maximum heart rate, or participants choosing to stop lifting [29]. Lifting style and lifting and lowering speed was self-selected by participants.

Video cameras were used to record the lifting – these were Sony Handycam Camcorders (Sony, Tokyo, Japan), which were positioned to view the rear coronal and right sagittal planes. For analysis of the bench to shoulder lift in this study, the right sagittal plane angle was utilized. Still images were collected at four points during the minimum and safe maximum lift. These points were determined by identifying the lowest vertical displacement of the box when all 4 corners of the box first lifted off the bench and ended when the box was at its highest during the lift. Both were marked with a horizontal line which were used to time the ascending and descending phases of the lift separately and then broken into thirds to attain three equal points across each lift when comparisons could be made. Joint angles were measured from still images at 0/3, 1/3, 2/3 and 3/3 of each lift. In the ascending lift, 0/3 is the lowest point and 3/3 is the highest point that the box reaches. In the descending lift, 0/3 is the highest point of the box and 3/3 is the lowest point of the box, when the lift is complete – see Fig. 2.

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Fig.2

The four points in a participant’s maximum lift: 0/3, 1/3, 2/3, 3/3.

Joint angles at each of the eight points in the ascending and descending lift (0/3, 1/3, 2/3, 3/3) were measured using the Dartfish angular measurement tool (Table 1). Each subject’s second lift of the three repetitions at minimum and maximum weight was chosen for analysis. The second or middle lift was chosen to allow for comparisons across all subjects up to the maximum weight. Data were entered into the statistical analysis program SPSS version 22 (IBM Corp. Released 2013. IBM SPSS Statistics for Windows, Version 22.0. Armonk, NY: IBM Corp.) to calculate descriptive statistics -means and 95% confidence intervals (CI), intra-class correlation coefficients (ICC), and paired t-tests, with alpha at 0.5. For reliability, measurements were independently repeated for ten subjects selected at random. Two raters selected the ascending and descending start and end points and took measurements from the still images at 0/3, 1/3, 2/3 and 3/3 of the ascending and descending lift phases as detailed above. Intra-rater and inter-rater reliability were determined from these measurements using intraclass correlation coefficients (Table 2).

3.1 Demographics

Twenty eight subjects participated in this study. The sample consisted of 19 females and 9 males, with an age range of 18–22 (mean age 20.9). The mean maximum weight lifted for females and males was 10.54 kg (range 8.5–13) and 16.56 (range 12–23.2) respectively.

3.2 Reliability

Both intra-rater and inter-rater reliability measures for all joint angles were considered excellent [36], with ICCs of greater than 0.75 for all measures (Table 2).

3.3 Ascending segment of lift

The mean joint angles, confidence intervals and p-values of the ascending bench to shoulder lift for all participants are shown in Table 3. Key findings in the ascending phase were significant differences between the minimum and safe maximum lift in joint angles in the thoracic spine, elbow and shoulder. These were found in the thoracic spine at the point 3/3 (p = 0.004, mean difference 2.292), where the thoracic spine was in greater extension during the maximum lift (Fig. 3); in the elbow at points 1/3 (p = 0.008, mean difference 6.405) and 3/3 (p = 0.009, mean difference 15.575) where the elbow was in greater flexion during the maximum lift (Fig. 4), and in the shoulder at point 3/3 (p = 0.000, mean difference 16.785) where the shoulder was in less extension during the maximum lift (Fig. 5). The lumbar spine presented slight joint angle differences between the minimum and maximum lifts however none of these were statistically significantly different with mean differences at all points being <1° (Fig. 6).

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Fig.3

Thoracic Spine results.

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Fig.4

Elbow results.

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Fig.5

Shoulder results.

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Fig.6

Lumbar Spine results.

In the descending segments of the bench to shoulder lift, three significant differences in joint angles between the minimum and safe maximum lift were observed (Table 4). These were found in the elbow at point 0/3 (p = 0.000, mean difference 18.446) and 3/3 (p = 0.035, mean difference 9.632) (Fig. 4), and in the shoulder at point 0/3 (p = 0.000, mean difference 16.647) (Fig. 5), where the joints were in greater flexion during the maximum lift. The lumbar and thoracic spine measurements produced slight joint angle differences between the minimum and maximum lifts however these had no statistical significance as mean differences at all points were <1.5°.

4.1 Elbow/shoulder

In the elbow joint, a significant difference at 1/3 of the ascending lift was identified where subject’s elbow joints were in less flexion during the maximum lift, shortly after the box is lifted from the bench. Research by Andersson and McLester shows that a 4 kg weight held at a distance of 50 centimeters from the body requires the same amount of muscle force as a 20 kg load kept close to the body, and that lifting a load at an arm’s length away causes a 10 times increase in the amount of force exerted on the spine [37, 38]. The safe maximum load (heavy) lifted during this study, caused the subject to keep the load low and close to the body for longer, prior to raising it to a higher surface. Significant differences were observed in the elbow (3/3 ascending, 0/3 descending) and in the shoulder (3/3 ascending, 0/3 descending). At these points the elbow and shoulder joints were in significantly greater flexion during the maximum lift. These are the highest points of both the ascending and descending lift, and occur shortly before the box is placed at shoulder height and shortly after the box is picked up from shoulder height respectively.

These results concur with a number of safe lifting guidelines that suggest the safest and most efficient way of moving a load is to keep it close to the body [39–42]. Heavy loads kept further away from the body require greater muscular force to lift [43]. By keeping loads close to the body, weight distribution becomes more even and large muscle groups are recruited [44]. This relates to FCE aims and highlights that through the use of correct manual handling techniques, muscle exertion and risk of injury can be significantly reduced. It also demonstrates that healthy people, without injury are naturally using safe lifting without prompting.

4.2 Lumbar

There was no significant difference in lumbar spine extension during the ascending or descending segment in the maximum lift when compared to the minimum lift, seen in Fig. 6. When lifting from a low to high surface (ascending), the force exerted on the lumbar spine is greater compared to beginning a lift at a high surface, as a result of an increased trunk angle putting strain on the posterior aspects of the spine [45]. In 2013, Rodriguez-Soto et al. investigated the effect of load on the lumbar spine and found that greater load increases superior lumbar spine lordosis (extension) [46]. Research shows that ligaments and muscles of the lumbar spine are strained when the lumbar spine is not in neutral lordosis [14]. Therefore, increased stability of the lumbar spine and neutral lordosis maintenance is required as load increases [47] in order to decrease risk of injury [48]. In the current study there was minimal change observed in lumbar extension which may have been the result of the specific style of lift conducted (bench to shoulder, as opposed to being from the floor).

4.3 Thoracic

Results from the current study indicate that the thoracic spine was in significantly greater extension (decreased kyphosis) at the highest point of the maximum ascending lift as opposed to the minimum ascending lift at the same point. As shown in Figs. 3–6, when the shoulder is in greater flexion and the elbow is in less flexion (the load is further away from the body), the thoracic spine is in greater extension suggesting that the muscles of the thoracic spine are under significantly more pressure. This is a result of the longer level at the shoulder joint being unable to produce the torque required. The thoracic spine shifts into extension, as a compensatory strategy, to allow the shoulder to stay at reduced range, and thus produce more force. This shift causes sheer, which can lead to injury [14]. The thoracic spine was also observed to be in slightly greater extension at point 0/3 (Fig. 3) of the maximum descending lift (mean difference 0.493), which is the highest point of the descending lift. It is interesting to note that significant differences in thoracic extension were noted during the ascending lift and at the highest point of the descending lift. This maybe related to the interactions between the lumbar and thoracic spine during a lift and the pattern of movement overall [49] and as the trunk compensates to keep postural stability [50]. Other than at these stages of the lift the thoracic spine was in less extension during the maximum lift, in both the ascending and descending components, as can be seen in Fig. 3.

Compared to the cervical and lumbar spine, the thoracic spine is considered to have a restricted range of motion in flexion and extension, caused by its spinous processes and attachment to the rib cage [51, 52], therefore significant joint angle change under load present a serious risk of injury [51]. The most common types of thoracic spine injuries occur in axial loading and are muscular irritation and costovertebral joint dysfunction [53, 54]. Ning, Zhou, Dai, and Jaridi studied trunk biomechanics and manual handling with sudden loading and found a higher load increased compression forces in the lumbar region of the spine and reduced the magnitude of trunk flexion [55]. This supports the findings in the current study, where the thoracic spine was at most risk when the lift is at its highest point as also noted by others [39, 42]. Keeping loads at lower heights when lifting is good practice and maybe a protective strategy, as recommended in safe lifting guidelines [41].