Ergonomics Lifting Lab - Anatomy Essay


Ergonomics Lifting Lab - Anatomy Essay
Abstract - The purpose of the experiment was to determine the moment of force as well as the shear and compressive forces acting on the lumbar spine (L4/L5) during two different lifting techniques – crane and bent knee. The

experiment included photographing an 81.14 Kg male lifting a 20Kg mass 25 cm off the floor using each of the lifting techniques. The photographs were then used to calculate the moment of force of the erector spinae for the crane (250.3Nm) and bent knee (273.4Nm) lifts. The moment of force was used to calculate the force produced by the erector spinae in the crane (5006N) and bent knee (5470.2N) methods. The shear and compressive forces of the erector spinae were also calculated for the crane method – shear (530.49N), compressive (5312.3N) and the bent knee method – shear (351.35N, 158.32N) and compressive (5821.5, 6061.8N). The two measurements presented for the bent knee method indicate the two different angles of L4/L5.
The calculations indicate that the bent knee method would be preferred in preventing low back pain as it causes lower shear forces on the L4/L5 region of the spine.

Introduction:

Second only to the common cold, low back pain is the most prevalent affliction of man (Borenstein, 1995). Approximately 80% of the total population is affected by this problem sometime in their life, thus it cannot be ignored. With respect to the working community, back pain is the most common musculoskeletal condition with 25% of all working men being affected by low back problems, forcing one out of twenty-five to change jobs (Anderson, 1989). Work related risk factors include repetitive lifting; particularly in forward bent and twisting positions, exposure to vibration and predominantly static work posture. These problems peak at the age of 40 for men and 50-60 for women. Due to occupations requiring less physical work, women have a decreased incidence of low back pain in comparison with men, giving improper lifting of heavy weights (Anderson, 1989). However, a decrease in the quality of life and increase in the level of stress can have dramatic effects on the productivity of society.

Low back pain has been clearly established to be triggered by lifting (Moore, 1992). More importantly, improper lifting techniques have been implicated as the major cause of work-related low back pain (Fathallah et al., 1998). Recurrences are frequent, and three or more episodes have been reported in 30 to 70% of afflicted patients (Moore, 1992). The combination of lifting, bending and twisting is believed to be quite stressful to the spine because of the load moment. Weight, speed of the lift, location of the load and posture of the trunk are also important factors to consider (Anderson, 1989).

Compressive and shear forces are two primary forces that affect the lumbar spine (Garg, 1992). The vertebral body is the first structure to fail with compressive and shear forces causing the vertebrae to move forward and down relative to the vertebra below it (Adams and Dolan, 1995). With respect to this lab, the compressive force rises from 500 N during standing to 1900 N when stooping to lift a 10 kg weight (Adams and Dolan, 1995). Incidence rates of low back pain are nine times higher when compressive forces are greater than 650 kg (6500 N) (Anderson, 1989).

In this lab, we will examine the biomechanics of the crane and bent knee methods of lifting using a static model. We will determine the moment of force on the back extensor muscle group as well as the force acting on the lumbar spine. To find these values, the load on the lumbar spine (L4/L5) will need to be calculated. The weight of the body, arms, an external force (weight) and the erector spinae will also be considered. The erector spinae is the key extensor muscle of the vertebral column (Moore and Agur, 1996) while L4/L5 is critical for forward flexion and extension of the low back region (McGill and Norman, 1986). Due to the small moment arm (5 cm) between the erector spinae and L4/L5, small forces applied during lifting can produce large internal muscle forces (Garg, 1992), resulting in low back problems, and more specifically, chronic pain.
With the incidence of low back pain rising consistently, physiotherapists are at the forefront of creating techniques and treatments that relieve low back pain. Traditionally, these treatments have been aimed at “living with the pain” and recently more active mobilization techniques are being implemented (D’Orazio, 1993). The focus is now on understanding and teaching the mechanical principles involved in lifting, rather than simply teaching a particular technique. Because low back pain is often a result of repeated stress over time (Garg, 1992), and not merely a one time accident, physiotherapists have to understand the relationship between individuals and job-related factors and the resulting risk of injury to the worker (Moore, 1992). This understanding of the biomechanics of lifting will result in improved patient education and adherence to prevention and treatment plans.
Methods:

The subject of this experiment was a twenty year old, 81.14Kg male with no previous history of back pain. Anatomical landmarks were placed on the subject to define segments for the calculations. In order to have these markers visible in the photograph, the subject wore shorts and removed his shirt. These anatomical landmarks were positioned in specific areas; they included: the auditory canal, glenohumeral joint, ulnar styloid process, C7/T1 spinous process and the L4/L5 vertebrae at the anterior superior iliac spine. These landmarks were represented by florescent markers and were positioned in the sagital plane of the subject so that they were visible in the digital photograph.

Two separate photographs of the subject were taken. Each consisted of the subject being positioned parallel to a solid backdrop situated a few meters in front of the camera. Located on the backdrop were reference markers placed a meter apart (for scaling purposes). The subject was photographed lifting a 20 kg mass at a height of 0.25 m above of the ground using two separate techniques. The first photograph was of the subject exhibiting the “crane” method of lifting. In the subsequent photograph the subject utilized the “bent knee” technique. These digital recordings were then transferred to the computer where they were printed off for the purpose of static analysis (Appendix A & B).
The mass of each segment was determined by using values from Soderberg’s anthropometric data (Soderberg, 1986). The mass was calculated by multiplying the subject’s total mass (81.14Kg) by the percentage of total body weight of the particular segment. Thus, to determine each segmental force, the segmental masses were simply multiplied by the gravitational force.

The purpose of the aforementioned landmarks was to serve as a means of determining the segmental lengths of the subject. These body segments consisted of the head and neck, thorax and abdomen, the upper arm and the forearm. In order to determine the actual length of the body segments, they were first measured in the photograph (in centimeters) and then converted into actual length in meters using the scaling factor. We were then able to use these segmental lengths to determine each segment’s centre of mass. This was calculated by multiplying the segment length (from the photographs) by the proximal distance of the COM taken from Soderberg’s anthropometric tables (1986).

In determining the moment arm lengths of each segment and the load, we first drew a vertical line through the L4/ L5 axis on the photographs. By measuring the perpendicular distance from the centre of mass to the axis of rotation, we were then able to calculate the true moment arm lengths using our scaling factor. See pictures for drawing of the moment arms (Appendix A & B).

Prior to calculating the muscle moments of force, the muscle force, and the shear and compressive components of the Joint Reaction Force, several assumptions had to be made. These consisted of: 1) no acceleration (static equilibrium); 2) angle of vertebral body from horizontal during the crane lift was 60 degrees, and 35 & 15 for the bent knee lift; 3) no antagonistic muscle action; 4.) Moment Arm Length (MAL) of erector spinae (ES) muscle is 0.05m; 5) Force of ES acts perpendicular to the vertebral body, which is a compressive force; 6) Single muscle equivalents.

Results:
Table 1. shows the results of the calculations which can be seen in Appendix E. The results indicate that the moment of force of the erector spinae muscle is greater for the bent knee method of lifting than for that of the crane method by 23.2Nm. Table 1. also demonstrates the muscle force of the erector spinae when using the two different lifting techniques and shows clearly that the bent knee method causes a 464.16N larger force on the erector spinae than the crane method of lifting. The shear and compressive forces are also reported in Table

1. The crane method shows the lowest compressive force (5312N) and the highest shear force at 520.49N. The bent knee method (at both angles of L4/L5) shows higher compressive and lower shear forces. When the spine was bent at a 35º angle a compressive force of 5821.48N is seen and a shear force of 351.35N; when bent at a 15º angle the compressive force was 6061.8N and the shear force was 158.32N. Overall this demonstrates clearly that as the angle of the spine at L4/L5 increases as the compressive force decreases. The opposite is true for the shear force on L4/L5; as the angle increases the shear force also increases. Table 2. shows the moment of force for each body segment which was then used to calculate the moment of force of the erector spinae seen in Table 1. The calculations for the data found in these tables are presented in Appendix E.

Table 1. A comparison of the moment and muscle forces of the erector spinae muscle and the compressive and shear joint reaction forces on that muscle during two different lifting techniques – crane and bent knee.
Method of Lifting Moment Force of Erector Spinae (Nm) Muscle Force of Erector Spinae (N) JRF Compressive on L4/L5 (N) JRF Shear on L4/L5 (N) JRF Resultant (N)
Crane 250.3 5006 5312.3 530.49 5338.72
Bent Knee 15 273.5 5470.16 6061.8 158.32 6063.87
Bent Knee 35 273.5 5470.16 5821.48 351.35 5832.1

Table 2. Moment of force for each body segment, calculations in Appendix E
Segment Moment of Force (Nm)
Head & Neck 42.95
Thorax & Abdomen 47.13
Upper Arms 24.2
Lower Arms 12.62
Box 123.4

Discussion:
The purpose of this study was to determine and compare the moment of force of the erector spinae muscle group, as well as forces acting on the lumbar spine, between the crane method of lifting and the bent knee method of lifting.

In this study, the moment of force of the erector spinae muscle group was determined to be 23.2Nm greater during the bent knee method of lifting compared to the crane method of lifting (Table. 1). The bent knee method has a greater moment of force because all the segment moment arm lengths (MAL) are larger than they are for the crane method (Table 2 & Table 4). The definition of moment of force is the MAL of the segment(s) multiplied by the force acting on the segment(s) (Neumann, 2002). In addition, because the MAL for the erector spinae muscle group was 0.05m for both the crane and bent knee method, the MALs of the body segments plus the 20kg box were the only changing factors. Therefore, the extra moment of force of the erector spinae in the bent knee method is due solely to the increased distance to the center of mass of the box and body segments from the L4/L5 vertebrae.

The force of the erector spinae muscle group was also larger during the bent knee method compared to the crane method (Table. 1). The magnitude of the force generated by the erector spinae muscle group is directly proportional to the erector spinae’s moment of force. Therefore, because both the force and the moment of force of the erector spinae muscle group are directly proportional, the MALs are the only changing factors.

Potvin et al.’s study (1991) obtained similar results concerning the moment of force of the erector spinae muscle group. They implemented five trials of crane and bent knee lifts that involved increasing weights for each trial. In their trial with a 22kg weight the moment of force of the erector spinae muscles were 269.5Nm for the crane method and 275.6Nm for the bent knee method. Frankel and Nordin (1989) however, obtained data contrary to this study. Their results showed that the crane method had a larger moment of force of the erector spinae group than the bent knee method. Fortunately these contradictions can be explained by comparing MALs in both studies. Frankel and Nordin (1989) point out that the moment of force of the erector spinae muscles can be significantly increased during the bent knee method if the object’s distance from the body is increased. In their study, the moment of force of the erector spinae muscles increases from 151Nm when the object is close, to 212.5Nm when the object is farther away. Thus, the fact that the MAL in this study is longer in the bent knee method than the crane method suggests that the object in this study was farther away than in Frankel and Nordin’s study.

It is of note that there are a number of factors that contribute to spinal load during lifting and carrying. These factors are important for the physiotherapist and are as follows: 1) The objects position relative to the spine’s center of motion, 2) the spine’s degree of flexion and or rotation and 3) the size, shape, weight and density of the object (Frankel and Nordin, 1989).

The anatomical structure of the spine allows for the vertebrae to withstand a significantly larger compressive joint reaction force (JRF) compared to a shear JRF (McGill, 2002). More specifically, McGill (2002) states that the tissue tolerance in vitro for compressive forces is 10,000N while it is only 2800N for shear forces. In addition, according to NIOSH the maximum permissible limit (MPL) for lifting is a compressive force of 6400N and a shear force of 1000N. The significant difference between the MPL of compressive force compared to shear force on the lumbar vertebrae can be explained by the anatomy of the spine. The spinal cord is positioned in the body predominantly on a vertical axis. Furthermore, throughout the vertebral column each vertebra is separated by a vertebral disc containing vertebral fluid designed to cushion forces on the spinal cord. Therefore, because the definition of a compressive force is to push two ends of an object together the vertebrae are better able to withstand compressive forces than they are shear forces which act to slide ends apart.

The compressive JRF in this study was greater for the bent knee method compared to the crane method (Table. 1). The bent knee method had a compressive force of 5821.5N when the L4/L5 vertebrae had an angle of 35 degrees and 6061.8N when the angle was 15 degrees. The crane method had a compressive force of 5006N at an angle of 60 degrees (Table. 1). A higher compressive force for the bent knee method is expected due to the angle on the L4/L5 vertebrae being smaller than the crane method angle. The compressive JRF occurring at the L4/L5 vertebrae is measured by taking the Sine of the angle to which the segment weights are acting (Table. 3). In addition to the Sine component of all the segment weights the muscle force of the erector spinae muscles is added because we are assuming that the orientation of the erector spinae muscles allows for compressive JRF only. The erector spinae muscles are positioned practically perpendicular to the inferior and superior surfaces of the vertebral discs and parallel to the line of the vertebral bodies (McGill & Norman, 1987). Therefore, the greater the spinal angle, the larger the Sine component determining the compressive JRF will be. In other words, the more erect your posture during lifting, the greater the compressive force acting upon your vertebrae. Potvin et al’s study (1991) has similar findings on the compressive JRF during the two methods of lifting. In support of this study their findings were that the bent knee method creates greater compressive forces on the L4/L5 vertebrae than does the crane method.

The shear JRF in this study was greater for the crane method than the bent knee method (Table 1). Particularly, the shear JRF for the crane method, which has a spinal angle of 60 degrees, was 530.5N, while it was 158.3N for the bent knee method at 15 degrees and 351.35N for the bent knee method at 35 degrees (Table 1). Shear JRF is determined by the Cosine of the spinal angle multiplied by the sum of the external weights. This explains the shear JRF being greater for the crane method than the bent knee method. In other words the more you bend over when lifting the greater the shear forces on your L4/L5 vertebrae.

Shear JRF is considered to be more damaging than compressive JRF during lifting, in relation to low back pain (Potvin et al., 1991). The crane method of lifting has the greater shear force and, according to Potvin et al., is the more dangerous lifting method. However, it should be noted that the shear JRF during the crane lift was only 53% of the MPL, while the compressive force during the bent knee lift, at a spinal angle of 15 degrees, was 95% of the MPL. Therefore, even though shear forces are considered to be more dangerous in terms of lumbar vertebrae, the crane method only exerted a shear JRF that was half of the MPL. Thus, upon lifting a 20kg weight the shear JRFs are of no great concern. It should be noted that although the crane method is only producing a shear JRF that is half of the MPL, cumulative shear over a given period of time has been shown to be very important as a metric risk to injury (Norman et al., 1998).

A number of assumptions have been made during this study that could have affected the results. Two of the assumptions made for this study can be looked at together. 1) The erector spinae muscle group is studied as a single muscle equivalent, and 2) the MAL for the erector spinae is 0.05m. McGill and Norman’s study (1987) examined the erector spinae muscle group using the individual muscles and found that the MAL for the erector spinae should be 0.075m rather than the previous accepted 0.05m. This 50% increase of the MAL is determined by reassessing all the active extensor tissues that act under an equivalent MAL. In another study, the MAL of the erector spinae muscle group was 0.06m (Dennis and Barrett, 2002). Therefore, the implications to this study is that there is an under estimate of the erector spinae muscle group force. Increasing the MAL used would significantly lower the compressive JRFs of the crane and bent knee methods away from the MPL.

No antagonistic activity is another assumption made in this study. There has been some debate on the role of abdominal muscle activity during lifting. Zetterberg et al. (1987) found that there was above minimal antagonistic activity of the abdominals during lifting. It has been hypothesized that intra-abdominal pressure creates a tensile force as well as an extensor moment on the lumbar spine (Bartelink, 1957 and Morris et al., 1961). If this hypothesis is true it still doesn’t specify the force and extensor moment the abdominals produce at different positions of the spine. Therefore, in this study the antagonistic force of the abdominals would not be differentiated between the bent knee and crane method of lifting. As a result, if there was discrepancy because of antagonistic abdominal force it would not alter the results of the forces obtained on the lower back.

The last assumption that will be looked at is that the study was performed under static equilibrium. Static equilibrium has been used numerous times in the literature (Dennis & Barrett, 2002; Granata & Wilson, 2001; Kozey et al., 1990; McGill & Norman, 1985). Static equilibrium is used frequently when interpreting the forces acting on lumbar vertebrae during lifting. However, according to McGill and Norman et al.’s study (1985), using a static model gave lower lumbar moments of force as well as lower compressive JRFs compared to a dynamic model. The implications of the static model having values less than the dynamic model is that the actual stress the lower back is subjected to during lifting may be greater than the static model predicts. Therefore, lifting tasks that are under the MPL may actually be above the MPL, putting the subject in danger of lower back injuries.

The objective of the study is to determine the forces acting on the lower back during lifting. The crane method compared to the bent knee method is analogous to a flexed spine compared to a neutral spine. In accordance with our study, McGill (2002) states that the flexed spine creates a greater shear JRF than does the neutral spine. More specifically, McGill (2002) states that maintaining a more neutral lordotic posture, while lifting, will reduce the shear JRFs to about 200N from 1000N. The reduction in the shear force is due to the extensor moment created by the extensor musculature. The extensor moment creates a posterior shear force that supports the anterior shear forces produced by the upper body (McGill, 2002).

Additionally, the lumborum fibers help protect against anterior shear and the interspinous ligament helps resist posterior shear of the superior vertebrae, while maintaining a neutral spine. However, during lumbar flexion, the lumborum fibers cause a loss of the fiber’s oblique angle, causing loss of protection again anterior shear, while the interspinous ligament contributes to anterior shear on the anterior vertebrae (McGill, 2002). Furthermore, the risk of a shear injury during lumbar flexion is 10% versus a compressive injury being only 3% (McGill, 2002). Thus, it is plausible to deduce that a method of lifting that increases the shear JRF has a higher risk of injury than a method that has a lower shear JRF. In relation to this study, the crane method increases the shear JRF and therefore has a greater risk of inducing injury than the bent knee method.

Conclusion:
The purpose of this experiment is to determine which lifting technique (crane or bent knee) is the most effective in limiting the stress on the L4/L5 region of the spine. The results indicate that the bent knee method induces a lower shear force on this region of the spine and therefore is a better and safer lifting technique. Although the bent knee causes greater compressive forces than the crane method it is still safer because shear forces are what tend to cause injury and pain. The results indicate that the moment arm length is naturally shorter when an object is lifted using the bent knee method because of the natural position of the body. The shorter moment arm length leads to lower forces on the erector spinae. Also demonstrated is that a smaller angle of the vertebrae leads to lower shear forces. These results demonstrate clearly that a smaller vertebral angle and a smaller moment arm length will lead to lower forces on the erector spinae.

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Appendix D

Table 3. Presentation of mass, weight, length, moment arm length and calculation of moment for the crane lift.
Segment Mass (Kg) Weight (N) Length Measured (m) Actual Length (m) MAL Actual (m) Moment (Nm)
Head & Neck 6.57 64.386 0.024 0.178 0.667 42.95
Thorax & Abdomen 28.8 282.24 0.075 0.556 0.167 47.13
Upper Arms 4.54 44.492 0.0525 0.389 0.544 24.2
Lower Arms 2.6 25.44 0.046 0.341 0.496 12.62
Box 20 196 0.6296 123.4

Table 4. Presentation of data calculated in Appendix E showing the shear and compressive forces of the crane method with a spinal angle of 60º.
Segment Weight (N) Weight*Sin30 (Compressive N) Weight*Cos30 (Shear N)
Head & Neck 64.386 32.193 55.76
Thorax & Abdomen 282.24 141.12 244.43
Upper Arms 44.492 22.246 38.53
Lower Arms 25.44 12.72 22.03
Box 196 98 169.74

Table 5. Calculations of mass, weight, length, moment arm length and calculation of moment for the bent knee lift lift.
Segment Mass (Kg) Weight (N) Length Measured (m) Length Actual (m) MAL (m) Moment (Nm)
Head & Neck 6.57 64.386 0.023 0.177 0.654 42.108
Thorax & Abdomen 28.8 282.24 0.072 0.554 0.208 58.7
Upper Arms 4.54 44.492 0.052 0.4 0.546 24.29
Lower Arms 2.596 25.44 0.045 0.346 0.561 14.27
Box 20 196 0.715 140.14

Table 6. Presentation of data calculated in Appendix E showing the shear and compressive forces of the bent knee lifting method with a spinal angle of 35º.
Segment Weight (N) Weight*Sin55 (Compressive N) Weight*Cos55 (Shear N)
Head & Neck 64.386 52.74 36.93
Thorax & Abdomen 282.24 231.2 161.89
Upper Arms 44.492 36.45 25.52
Lower Arms 25.44 20.84 14.59
Box 196 160.55 112.42

Table 7. Presentation of data calculated in Appendix E showing the shear and compressive forces of the bent knee lifting method with a spinal angle of 15º.
Segment Weight (N) Weight*Sin75 (Compressive N) Weight*Cos75 (Shear N)
Head & Neck 64.386 62.192 16.66
Thorax & Abdomen 282.24 272.6 73.05
Upper Arms 444.492 42.98 11.52
Lower Arms 25.44 24.573 6.36
Box 196 189.3 50.73

Appendix E
Calculations:

Conversion factor Appendix A = 7.41
Conversion factor for Appendix B = 7.69

Sample calculation for conversion of measured length to actual length:

Crane method
1m = .135m
1/.135 = 7.41 = conversion factor
0.024m (measured length of head and neck) * 7.41 = 0.178m (actual length)

Sample calculation for Moment of Force

MAL * Weight of Segment = Moment of Force
0.667m * 64.386 = 42.95Nm

Sample Calculation for Moment of the Erector Spinae:

Crane method
0 = ?M
0 = -MES + MH + MT + MUP + MLA + MB
MES = 42.95 + 47.13 + 24.20 + 12.62 + 123.4
MES = 250.3 Nm
250.3Nm/0.05m = 5006N

Bent Knee method
0 = ?MES
0 = -MES + MH + MT + MA + MB
MES = 42.108 + 58.7 + 24.29 + 14.27 + 140.14
MES = 273.51Nm
273.51Nm/0.05m = 5470.16N

Sample Calculation for compressive component of individual body segments:
Crane method
Weight*Sin30
64.386*Sin30 = 32.193N

Sample Calculation for Compressive component of Joint Reaction Force (JRFC)
0 = JRFC – FH – FT – FA – FB - FES(c)
JRFC = 32.193 + 141.12 + 22.246 + 12.72 + 98 + 5006
JRFC = 5312.3N

Sample Calculation for shear component of individual body segments:
Crane method
Weight*Cos60
64.386*Cos30 = 55.76N

Sample Calculation for shear component of Joint Reaction Force (JRFS)
Crane method
0 = -JRFS + FH + FT + FA + FB
JRFS = 55.76 + 244.43 + 38.53 + 22.03 + 169.74
JRFS = 530.49N

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