A Kinematic and Dynamic Analysis of Shoveling Snow
by
José Andrés DeFaria
An Engineering Project Submitted to the Graduate
Faculty of Rensselaer Polytechnic Institute
in Partial Fulfillment of the
Requirements for the degree of
Master of Engineering
Major Subject: Mechanical Engineering
Approved:
_________________________________________
Ernesto Gutierrez-Miravete, Thesis Adviser
Rensselaer Polytechnic Institute
Hartford, Connecticut
May 2016 (Third Progress Report: April 2016)
i
© Copyright 2016
by
José Andres DeFaria
All Rights Reserved
ii
CONTENTS
A Kinematic and Dynamic Analysis of Shoveling Snow ................................................... i
LIST OF TABLES ............................................................................................................ iv
LIST OF FIGURES ........................................................................................................... v
ACKNOWLEDGMENT .................................................................................................. vi
ABSTRACT .................................................................................................................... vii
1. Introduction.................................................................................................................. 1
2. Background .................................................................................................................. 2
3. Methodology ................................................................................................................ 6
3.1
Initial Phase ........................................................................................................ 7
3.2
Second Step ........................................................................................................ 9
3.3
Third Step ......................................................................................................... 10
3.4
Fourth Step ....................................................................................................... 11
3.5
Fifth, Sixth, and Seventh Step .......................................................................... 12
4. Results........................................................................................................................ 13
5. Discussion .................................................................................................................. 16
6. Conclusions................................................................................................................ 20
7. References.................................................................................................................. 21
8. Appendices ................................................................................................................ 22
8.1
Appendix A - Determination of Body Segment Weights ................................ 22
8.2
Appendix B - Maple Worksheet for the Traditional (straight-shaft) shovel .... 24
8.3
Appendix C - Maple Worksheet for the Ergonomic (bent-shaft) shovel ......... 25
iii
LIST OF TABLES
Table 1: Errors associated with estimation in step three ................................................. 10
Table 2: Offset angles applied and resultant errors and percent reduction in step three . 11
Table 3: Maximum Moments for the five subjects .......................................................... 14
Table 4: Maximum Moments for the five subjects, normalized by height and weight ... 15
Table 5: Extension angular impulse for all five subjects ................................................. 15
Table 6: Predicted maximum moments based on [6] ...................................................... 16
Table 7: Peak Flexion Angle reported by [5] & [6]......................................................... 17
Table 8: Normalization of study data using the [6] method for normalization ............... 18
Table 9: Accuracy of different normalization methods ................................................... 19
Table 10: Average body heights and lengths [8] ............................................................. 22
Table 11: Average body segment weights [8] ................................................................. 22
Table 12: Average heights and weights for across the adult human population ............. 23
Table 13: Body segment lengths and heights across the human adult population .......... 23
Table 14: Body segment weights across the human adult population ............................. 23
iv
LIST OF FIGURES
Figure 1: A typical straight-shaft shovel and an ergonomic bent-shaft shovel [3] ............ 1
Figure 2: Regular and modified shovel design for dirt used in study [4] .......................... 2
Figure 3: Flexion angle and loads which induce moments in the lower back [5] ............. 3
Figure 4: Dimensions of the snow shovels used in [6] ...................................................... 4
Figure 5: Mean L5/S1 extension moment-time curves (sagittal plane) [6] ....................... 5
Figure 6: Mean upper body flexion-time curves (sagittal plane) [6] ................................. 5
Figure 7: Heaviest and lightest weights for subjects of given heights [7] ......................... 6
Figure 8: Initial positions of upper arm (green) and lower arm (blue) for the Lewinson
Average .............................................................................................................................. 8
Figure 9: Calculation of moment arm ................................................................................ 9
Figure 10: Position of back and arms at the end of the second step for the straight-shaft
(left) and the bent-shaft (right) shovels ........................................................................... 10
Figure 11: Graph of the moment for the Lewinson average individual with both shovels
......................................................................................................................................... 13
Figure 12: Graphs for all five subjects with straight shovel and bent shovel .................. 14
v
ACKNOWLEDGMENT
Thank you to my family: my wife, Shelby, my parents, Anne and Joe, and my sister,
Christina for supporting me throughout the years. Thank you to my high school physics
teacher, Mr. Hurley, for teaching physics in an interesting and engaging way, that led me
to choose mechanical engineering as a major, and inevitably, a career. Thank you to my
employer, for allowing to me to further my education. Thank you to my adviser,
Professor Gutierrez-Miravete, for his guidance with this project and during my time at
Rensselaer. Thank you to the cold New England winters, which like to blanket us in
snow over and over again, for inspiring me to complete this project; however, now that
this project is complete, winter is kindly advised to go away and never return.
vi
ABSTRACT
Snow shoveling is a strenuous physical activity that many Americans participate in
without formal training. Improper usage of a traditional snow shovel induces large
moments at the base of the spine leading to injury. This paper evaluated an ergonomic
snow shovel and a traditional snow shovel to determine if the ergonomic shovel reduced
loading at the base of the spine. This kinematic analysis was completed using computer
algebra software and data from previous similar evaluations. Algebra software permitted
evaluation for varying height and weights including 5th percentile females and 95th
percentile males. This paper concluded an ergonomic shovel reduces the bending
moment at the base of the spine when compared to a traditional snow shovel across the
adult human population.
vii
1. Introduction
Snow shoveling is a routine winter task for many Americans, especially those living in
southern New England. Boston and Hartford both receive in excess of 30 inches of
snowfall per winter [1]. While corporations and municipalities rely on plowing or
snowblowers for snow removal, private residences are typically shoveled by the owners
or occupants [5]. Since snow shoveling is not the primary occupation of many of the
participants, they do not receive training as a full-time laborer might. As a result, many
of these residents may incorrectly use the snow shovels they have, such as lifting with
their back, rather than with their knees.
This type of misuse is likely the reason why approximately 11,500 individuals are
treated in US emergency rooms each year due to injuries sustained while shoveling snow
[2]. Many of these injuries occur in the lower back. Ergonomic snow shovels exist which
are purported to reduce stress in the back due to their bent-shaft design, but many
households continue to use straight-shaft shovels.
Figure 1: A typical straight-shaft shovel and an ergonomic bent-shaft shovel [3]
1
2. Background
There have been some previous studies into the impact of ergonomic shovel use. Some
of these studies relied on qualitative surveys to determine the effectiveness of each
shovel design. Other studies used mechanical means in an attempt to measure lifting
forces. A select few studies used data collection equipment to take quantitative
information into the bending of the back during shoveling tasks.
In one of the earlier studies found, a different style of ergonomic shovel was evaluated
[4]. This shovel was designed for dirt rather than snow. Instead of using a bent shaft
design, similar to that depicted in Figure 1, it used a handle mounted to a secondary
shaft, as shown in Figure 2. Only qualitative data was collected via surveying study
participants; however, the results indicated that the modified shovel did reduce perceived
pain in the participants. The authors also noticed that the modified shovel appeared to
reduce the tendency for the participants to stoop. It is these extended periods of bending
that are considered the primary cause of muscle fatigue and back pain or injury. It should
be noted all of the study participants were male, and all were industrial workers who
completed shoveling or digging tasks in their daily work.
Figure 2: Regular and modified shovel design for dirt used in study [4]
In another study, published ten years later, shovels similar to those depicted in Figure 1
were used exclusively for snow [5]. Quantitative data was collected for this study,
including the trunk flexion angle, lateral bending angle, and rotation angle. The results
2
showed usage of the bent shaft shovel significantly reduced the trunk flexion angle. The
average trunk flexion angle was 41.4° with the bent shaft shovel compared to 49.2° for
the straight shaft. This difference in bending angle is significant because it decreases the
moment placed on the lower back by the upper body as shown in Figure 3. It should be
noted that all study participants were male.
Figure 3: Flexion angle and loads which induce moments in the lower back [5]
Despite the fact that ergonomic shovels are marketed as such, as recent as 2014, no
scientific evidence supported claims that the shovel would reduce mechanical loading on
the lower back [6]. Another study, [6], continued the work from [5], with an attempt to
determine the reaction moment in the lower back, referred to as the L5/S1 extension
moment. L5 and S1 refer to a particular location within the vertebrae, between the
lumbar and the sacrum, essentially the base of the spine.
3
Figure 4: Dimensions of the snow shovels used in [6]
The dimensions of the snow shovels used in [6] are presented in Figure 4. It should be
noted that out of all the previous studies examined, this was the only study which
included women in the sample size, although the results were not broken down by
gender. The study found the L5/S1 peak extension moment was 0.627 N·m/kg·m for the
bent-shaft shovel compared with 0.703 N·m/kg·m for the straight-shaft shovel.
Additionally, the peak upper body flexion for the bent-shaft shovel was 74.3°, reduced
from a value of 84.8° with the straight-shaft shovel. These results indicate that the
ergonomic shovel does reduce mechanical loading on the lower back. Graphs of the
L5/S1 extension moments and the upper body flexion are presented in Figure 5 and
Figure 6, respectively. In an attempt to account for the different body proportions, the
extension moment was divided by the total mass and height of each participant.
4
Figure 5: Mean L5/S1 extension moment-time curves (sagittal plane) [6]
Figure 6: Mean upper body flexion-time curves (sagittal plane) [6]
Although this study did include women in the study, its presentation of the final results
in the form of averages speaks only to the average individual. A female of the 5th
percentile height is fifteen inches shorter than a male of 95th percentile height [7]. A
95th percentile height male who is also in the 95th percentile in weight can weigh more
than three times more than a 5th percentile female in height and weight. These
discrepancies in height should greatly affect the flexion angle and the discrepancies in
weight should greatly affect the extension moment.
5
3. Methodology
This evaluation will expand upon previously completed work in [6]. Will ergonomic
shovels reduce mechanical loading of the lower back across the human adult population?
Figure 7 shows the weight range of North American and European adults within various
height ranges. While the subjects selected in Figure 7 are at the extremes for their
heights, the 5th percentile female is 4 feet, 11 inches tall and weighs on average 113 lbs
(150 cm, 51.26 kg) and the 95th percentile male is 6 feet, 2 inches tall and weighs on
average 246 lbs (188 cm, 111.58 kg) [7].
Figure 7: Heaviest and lightest weights for subjects of given heights [7]
To understand the ergonomic value of the bent-shaft shovel, it must be proven to reduce
the mechanical loading of the lower back in a great proportion of the adult population. A
small female will not need to bend over much to reach the snow. Additionally, her upper
body will weigh significantly less than that of a large male. The extension moment is
theorized to increase with both flexion angle and upper body weight; therefore,
ergonomic advantages gained by use of the bent-shaft shovel may diminish as height and
weight decrease.
6
To evaluate the effectiveness of the ergonomic value across the adult human population,
a kinematic analysis will be completed. This analysis will use Maple (a computer
algebra program) to compute the body joint positions throughout the shoveling motions.
Kinematic analysis will first be accomplished for an adult matching the average height
and weight of the subjects in [6] (1.77 m, 73.5 kg) to reproduce the results shown in
Figure 5 and Figure 6. When this kinematic model is complete, the variables defining the
height and weight can be modified to represent a 5th percentile female and a 95th
percentile male and the results will be compared.
3.1 Initial Phase
Two separate Maple worksheets were created, one each for the standard shovel and the
bent shovel. A global coordinate system is defined with the ground below the base of the
trunk as (0,0,0). The x-direction is the front-to-back direction, the y-direction is the
vertical direction, and the z-direction is the left-to-right direction. The height and weight
of the subject can be input in inches and pounds. Using these values, and the ratios
discussed in Appendix A, the body segment weights and lengths can be computed.
The subject is assumed to stand with their upper arm rotated 10 degrees from
vertical, and their lower arms parallel to the ground. This can be seen in Figure 8.
Knowing these angles permits the calculation of the base of the trunk, the top of the
trunk, both shoulders, both elbows, both hands, the shovel shaft end, and the center of
the shovel blade. The x-, y-, and z-coordinates of these points are calculated. For the
bent shovel, additional points are necessary to define the two vertices which comprise
the bent shaft.
7
Figure 8: Initial positions of upper arm (green) and lower arm (blue) for the Lewinson Average
The total moment on the base by summing the contributions of each item. For
simplicity, the center of mass for the trunk and head was considered to be the
supersternale height. The center of mass for the upper and lower arms was calculated to
be at the center of each arm. The center of mass for the shovel was located at the shaft
termination and the snow load was applied to the blade position. To determine the
contributions each of these items have towards the total moment, the moment arm was
determined using the distance from the base in the x- and z-directions as shown in Figure
9. Only the x-direction contribution is considered for the upper and lower arms as the zdirections are counteracted by the left and right symmetry.
8
Figure 9: Calculation of moment arm
3.2 Second Step
In the second step, the trunk rotates at a constant angular velocity until the peak trunk
flexion angle (84.8° for the straight shovel, 74.3° for the bent shovel) is reached. The
upper arm rotates another 20° with respect to the vertical and the lower arm rotates until
it is perpendicular to the ground. The positions of these arms is shown in Figure 10.
9
Figure 10: Position of back and arms at the end of the second step for the straight-shaft (left) and
the bent-shaft (right) shovels
3.3 Third Step
During the third step, the left arm lowers while the right arm raises to reduce the blade
height to zero. This is accomplished by estimating what angle the shovel needs to be
rotated to in order to reduce the blade height at the end of the second step to zero. Since
the lower arm remains perpendicular to the ground, this raising and lowering is
accomplished by increasing and decreasing the angle at the shoulder. As a result, the
right arm moves closer to the body and the left arm moves further away, causing a
rotation of the shovel in the x-z plane as well as the y-z plane.
The estimation used in determining these angles is a small source of error in the
overall calculation. Theoretically, the blade height (y-direction) should be zero at the end
of this step. The actual blade heights for the three body types are shown in Table 1.
Table 1: Errors associated with estimation in step three
Straight shovel
Bent shovel
5th percentile female
0.73333 in
-0.04672 in
Lewinson average
1.17676 in
-0.39272 in
95th percentile male
1.42104 in
-0.54481 in
10
The size of these errors was concerning, considering some of them exceeded one
inch. To minimize the effects of these errors, an offset was added or subtracted to the
angle between the shovel and the ground. Using trial and error, the offset correction in
degrees was determined to be 1+(height-59)*0.08 for the straight shovel, and 0.1+(height-59)*0.05 for the bent shovel. Table 2 shows the actual value of these angles
added or subtracted as well as the new resultant errors.
Table 2: Offset angles applied and resultant errors and percent reduction in step three
5th percentile
Lewinson
95th percentile
female
average
male
Straight Shovel
Angle Offset
+1.00 °
+1.86 °
+2.20 °
Error
+0.02864 in
-0.05048 in
+0.03635 in
Percent Error Reduced by
96.1%
95.7%
97.4%
Angle Offset
+0.10 °
+0.63 °
+0.85 °
Error
+0.02599 in
+0.05187 in
+0.04019 in
Percent Error Reduced by
44.4%
86.8%
92.6%
Bent Shovel
As shown in Table 2, these applied offsets greatly reduced the error created in
approximation. In nearly all cases the error was reduced by at least one order of
magnitude. Although this source of error was greatly reduced, it was determined to not
be a major effect the maximum moment for any of the individuals by any measurable
amount when the moment was considered rounded to the nearest integer.
3.4 Fourth Step
During the fourth step, the weight of the snow linearly ramps over the time period.
Based on [6], the weight of the snow is assumed to be 6 pounds.
11
3.5 Fifth, Sixth, and Seventh Step
During the remaining steps, the motions are completed in reverse. This is accomplished
by setting the position values equal to the appropriate position value in the
corresponding step.
IS A SPECIFIC LIST OF ASSUMPTIONS NEEDED?
12
4. Results
The computer programs (Maple worksheets) for the straight shovel and bent shovel are
separate, so in order to graph any of the results on the same plot, the results were
exported to Microsoft Excel.
A graph of the moment for the Lewinson average individual (69.69 inches tall and
162 lbs) is shown in Figure 11. This graph can be generally compared to the
experimental results shown in Figure 5.
Straight Shovel
Bent Shovel
70000
Moment (in*lbs)
60000
50000
40000
30000
20000
10000
0
0
25
50
75
Time (normalized to % cycle)
Figure 11: Graph of the moment for the Lewinson average individual with both shovels
Graphs for all five of the representative individuals are shown in Figure 12. The
annotations on the right correspond to the order they are presented in
13
100
Table 12, where 1 is the smallest individual (light 5th percentile female) and 5 is the
largest individual (heavy 95th percentile male).
120000
100000
Straight-1
Moment (in*lbs)
80000
Straight-2
Straight-3
Straight-4
60000
Straight-5
Bent-1
Bent-2
40000
Bent-3
Bent-4
Bent-5
20000
0
0
25
50
75
100
Time (normalized to % of cycle)
Figure 12: Graphs for all five subjects with straight shovel and bent shovel
Table 3 presents the peak moments observed for both shovels for each of the five
representative individuals. The percent reduction is also presented.
Table 3: Maximum Moments for the five subjects
Straight shovel
Bent shovel
% Reduction
5th percentile female (light)
32,548 in*lb
26,654 in*lb
18.1%
5th percentile female (average)
37,139 in*lb
30,987 in*lb
16.6%
Lewinson average
57,337 in*lb
50,237 in*lb
12.4%
95th percentile male (average)
86,689 in*lb
78,018 in*lb
10.0%
14
95th percentile male (heavy)
101,421 in*lb
91,922 in*lb
9.4%
In [6], results were presented in a normalized manner; they were all divided by the
height and weight of the individual. The results of Table 3 are presented again in Table
4, normalized by height and weight.
Table 4: Maximum Moments for the five subjects, normalized by height and weight
Straight shovel
Bent shovel
5th percentile female (light)
6.269 in*lb/ in*lb
5.134 in*lb/ in*lb
5th percentile female (average)
5.571 in*lb/ in*lb
4.648 in*lb/ in*lb
Lewinson average
5.079 in*lb/ in*lb
4.450 in*lb/ in*lb
95th percentile male (average)
4.762 in*lb/ in*lb
4.286 in*lb/ in*lb
95th percentile male (heavy)
4.615 in*lb/ in*lb
4.182 in*lb/ in*lb
The extension angular impulse (integral of moment over the time period) is presented in
Table 5.
Table 5: Extension angular impulse for all five subjects
Straight shovel
Bent shovel
% Reduction
5th percentile female (light)
115,467 in*lb*s
102,628 in*lb*s
11.1%
5th percentile female (average)
134,825 in*lb*s
120,496 in*lb*s
10.6%
Lewinson average
222,755 in*lb*s
200,416 in*lb*s
10.0%
95th percentile male (average)
348,340 in*lb*s
315,277 in*lb*s
9.5%
95th percentile male (heavy)
410,624 in*lb*s
372,635 in*lb*s
9.3%
15
5. Discussion
In comparing Figure 5 with Figure 11, it is clear that there are some differences in the
data. Namely, the finite element analysis used in this report evaluated a very robotic,
precise motion, that is unlikely to occur in practice. However, looking at the two figures,
there still are some similarities. It is clear that the results of this study agree with [6] in
that use of a bent shovel will reduce the peak extension moment.
To further compare the results, the extension moments of [6] were de-normalized
and converted to imperial units. Using these values, and the heights and weights of the
five subjects, the Lewinson results for the five subjects can be predicted and are
presented in Table 6.
Table 6: Predicted maximum moments based on [6]
Straight shovel
Bent shovel
% Reduction
5th percentile female (light)
372 in*lb
332 in*lb
10.8%
5th percentile female (average)
478 in*lb
426 in*lb
10.8%
Lewinson average
809 in*lb
722 in*lb
10.8%
95th percentile male (average)
1,305 in*lb
1,164 in*lb
10.8%
95th percentile male (heavy)
1,576 in*lb
1,405 in*lb
10.8%
The percent reductions in Table 6 are all the same since [6] only presented
normalized moments. Based on the results from this study, it is clear that there should be
differences based on the weight of the individuals. Assuming each subject picks up the
same weight of snow with each shoveling motion, the weight of that snow is more
significant to the 88 lb individual than to the individual who weighs 297 lbs. This is why
Table 3 shows that the greatest percent reduction occurs for the 5th percentile light
female.
One disadvantage of this study is it assumed all subjects had the same trunk flexion
angle. A constant trunk flexion angle was assumed because [6] only provided an average
angle and did not break any information down by height. In this study use of a constant
trunk flexion angle resulted in the 95th percentile male ending step two with his hands
16
approximately four inches higher than the 5th percentile female. To have the blade
height equal zero at the end of step three, this means the 95th percentile male must rotate
his arms further than the 5th percentile female, resulting in the shovel load being held
20% further away from the body than the female, and a 20% increase in the moment
from the snow load.
In theory, the 5th percentile female would have a lower trunk flexion angle and the
95th percentile male would have a larger trunk flexion angle, resulting in the two
subjects reaching a similar height at the end of step two. Values for these trunk flexion
angles cannot be inferred from [6]. If an angle of the average minus one standard
deviation is assumed to occur for a subject of average height minus one standard
deviation, and the results are extrapolated over the 5th percentile female and 95th
percentile male range, this would result in a 5th percentile female bending only 44.7°
while the 95th percentile male would bend 101°. These numbers seem unrealistic.
It should be noted that the trunk flexion angles presented in [6] are significantly
higher than those reported in [5]. These differences are presented in Table 7.
Table 7: Peak Flexion Angle reported by [5] & [6]
Reference [5]
Reference [6]
Straight Shovel
Bent Shovel
Mean (S.D.)
Mean (S.D.)
49.2°
41.4°
(12.7)
(14.4)
84.8°
74.3°
(13.3)
(11.5)
Difference (°)
Difference (%)
-7.8°
-15.9%
-10.1°
-11.9%
Trunk flexion angles that are based on [5] may be more accurate than those from
[6]; however, both data sets could be improved by showing the relationship between
trunk flexion and height.
In real-world scenarios, it is likely that larger individuals would lift more mass of
snow with each action; therefore, lowering the percent reduction in moment for the
smaller individuals, and slightly raising it for the larger ones. Together, with the
anticipated flexion angle differences discussed above, it is likely that the differences in
percent reduction presented in both Table 3 and Table 5 would average out slightly.
17
Additionally, the method used in [6] for normalization assumes each kilogram of
mass and each meter of height affect the moment equally. When converted to imperial
units, this means that every additional pound of weight is approximately equal to every
17.858 inches in height. Using the results from this study it is very easy to see that the
proportion is incorrect, as each inch of height affects the maximum moment more than
each additional pound.
To determine a more appropriate normalization technique, first the results of this
study will be normalized using the methods of [6]. The mean average for each shovel
was calculated, as well as the average height and weight of the five subjects evaluated.
From these values, the normalized moments are presented in Table 8.
Table 8: Normalization of study data using the [6] method for normalization
This study
Values from [6]
Height
67.138 in
69.69 in
Weight
181.2 lbs
162.0 lbs
Max Moment (Straight)
63,027 lb-in
809 lb-in
Max Moment (Bent)
55,564 lb-in
722 lb-in
Max
Moment
(Straight- 5.181 lb-in/lb-in
0.072 lb-in
(Bent- 4.567 lb-in/lb-in
0.064 lb-in
Normalized)
Max
Moment
Normalized)
Instead of normalizing by dividing by a value equal to the height multiplied by the
weight, as was done in [6], different normalization techniques were experimented with.
Table 9 shows the results of normalizing by these different values. In the table header, h
is representative of the height, while w is representative of the weight. The (h*w)
column are the results of multiplying these two values together, as was done in [6]. (h)
and (w) are the results of normalizing by the height or weight only, respectively. The
remaining three columns attempt different combinations of sums of these values. The
accuracy is comparing the maximum moment, as calculated using Maple, with the
estimated moment, as calculated by using a normalized moment and multiplying by the
factor in the header row.
18
Table 9: Accuracy of different normalization methods
(h*w)
(h)
(w)
(h+w)
(h+4w)
(h+2w)
Straight Shovels
59 in // 88 lbs
17%
70%
6%
15%
0%
6%
59 in // 113 lbs
7%
49%
6%
18%
10%
13%
69.69 in // 162 lbs
2%
14%
2%
3%
0%
1%
74 in // 246 lbs
9%
20%
1%
6%
3%
4%
74 in // 297 lbs
12%
32%
2%
7%
1%
3%
59 in // 88 lbs
1%
83%
1%
23%
8%
14%
59 in // 113 lbs
11%
58%
12%
24%
16%
19%
69.69 in // 162 lbs
16%
15%
1%
3%
0%
1%
74 in // 246 lbs
21%
22%
3%
8%
5%
6%
74 in // 297 lbs
24%
33%
1%
10%
4%
6%
Average Variance
12%
40%
3.5%
12%
4.7%
7.3%
Bent Shovel
Table 9 shows that the effect of height on the maximum moment is limited. The
most accurate normalization method (overall) above does not consider the height at all.
The normalization method (h+4w) is also fairly accurate and does consider the height.
This shows care must be taken when presenting normalized values. [6] used units of
meters and kilograms, so inherently these units considered the stronger influence of
mass/weight on the maximum moment. Use of centimeters and kilograms in the metric
system or inches and pounds in the imperial system would yield non-informative results
unless a system similar to h+4w was used.
The success of a weight-only of h+4w method is not surprising. A 10% increase in
mass could be expected to increase the moment proportionally. However, a 10%
increase in height will not necessarily increase the moment by the same percentage.
Someone who is 10% taller also has longer arms, so the shovel would not necessarily
begin 10% higher.
19
6. Conclusions
Snow shoveling is a strenuous physical activity that many Americans participate in
without formal training. Improper usage of a traditional snow shovel induces large
moments at the base of the spine leading to injury. Ergonomic shovels exist in the
market to improve posture during operation. These shovels claim to reduce stress in the
lower back, and prior research studies reached similar conclusions for a person of
average height and weight.
The adult human population varies greatly in height and weight. To determine if an
ergonomic shovel reduced stress across the population, a kinematic evaluation was
completed using Maple, a computer algebra program. Subjects were chosen to represent
the human population from a 5th percentile female to a 95th percentile male. Subject
motion was predicted based on trunk flexion angles recorded in previous research
studies.
This evaluation concludes that use of an ergonomic (bent-shaft) shovel will reduce
the moment induced in the lower back across the adult human population when
compared with a traditional (straight-shaft) shovel. Reduction of moment at the base of
the stain will reduce stress and muscle strain leading to reduced injuries.
This evaluation also determined the effect of height when finding the maximum
moment is limited when compared to the effect of weight.
Future evaluations in this area should consider inclusion of 5th percentile females
and 95th percentile males and include specific output and/or discussion, rather than
presentation of only averages. The discussion section of this report noted previous work
reported only a single trunk flexion angle which was used across all heights in this
evaluation. Future evaluations should experimentally determine how height impacts the
trunk flexion angle to improve the accuracy of the results.
20
7. References
[1]
National Oceanic and Atmospheric Administration. (2014, February). National
Overview - February 2014, Winter Snowfall Departure from Average. Retrieved
February 23, 2016, from
https://www.ncdc.noaa.gov/sotc/national/2014/2/supplemental/page-4/
[2]
Watson, D. S., Shields, B. J., & Smith, G. A. (2011). Snow shovel-related
injuries and medical emergencies treated in US EDs, 1990 to 2006. American
Journal of Emergency Medicine, 29(1), 11-17.
[3]
Huang, C., & Paquet, V. (2002). Kinematic evaluation of two snow-shovel
designs. International Journal of Industrial Ergonomics, 29(6), 319-330.
[4]
Degani, A., Asfour, S. S., Waly, S. M., & Koshy, J. H. (1993). A comparative
study of two shovel designs. Applied Ergonomics, 24(5), 306-312.
[5]
McGorry, R. W., Dempsey, P. G., & Leamon, T. B. (2003). The effect of
technique and shaft configuration in snow shoveling on physiologic, kinematic,
kinetic and productivity variables. Applied Ergonomics, 34(3), 225-231.
[6]
Lewinson, R. T., Rouhi, G., & Robertson, D. G. E. (2014). Influence of snow
shovel shaft configuration on lumbosacral biomecahnics during a load-lifting
task. Applied Ergonomics, 45(2), 234-238
[7]
Dowell, B., & Gscheidle, G. (2003). The Evolution of Anthropometrics and User
Control: The Science and Research Behind the Mirra 2 Chair. Retrieved
February 21, 2016, from http://hermanmiller.com/research/solution-essays/theevolution-of-anthropometrics-and-user-control.html
[8]
Clauser, C. E., McConville, J. T., & Young, J. W. (1969). Weight, Volume, and
Center of Mass of Segments of the Human Body (Tech. No. AMRL-TR-69-70).
Wright-Patterson AFB, Ohio: USAF Aerospace Medical Research Laboratory.
21
8. Appendices
8.1 Appendix A - Determination of Body Segment Weights
Weights, volumes, and center of masses for the human body and body segments are
presented in [8]. The body mass segments in this study will be used to determine the
mass of each segment as a percentage of the total body weight. Using this percentage,
the body mass segments of any body type can be extrapolated proportionally. Similar
extrapolation can be accomplished for body segment lengths. Table 10 presents body
lengths and heights while Table 11 presents body segment masses.
Table 10: Average body heights and lengths [8]
Total height
Head + trunk
Suprasternale height
Chest breadth
Trunk
Arm
Length (cm)
172.72
81.92
141.05
33.23
57.89
77.45
Percentage of Height
100%
47.43%
81.66%
19.24%
33.52%
44.84%
Table 11: Average body segment weights [8]
Total body
Head + trunk
Total arm
Trunk
Upper arm
Forearm and hand
Mass (kg)
65.606
38.061
3.216
33.312
1.730
1.483
The weights and heights presented in
22
Percentage of Weight
100%
58.01%
4.901%
50.78%
2.64%
2.26%
Table 12 will be used in this study.
23
Table 12: Average heights and weights for across the adult human population
Body Type
Height (in) Weight (lbs)
59.00
88
Light 5th percentile female [7]
113
Average 5th percentile female [7] 59.00
69.69
162
Lewinson Average [6]
74.00
246
Average 95th percentile male [7]
74.00
297
Heavy 95th percentile male [7]
To calculate the respective heights and lengths across the adult population, the height
from
24
Table 12 will be multiplied by the segment height of length from Table 10 and then
divided by the total height from Table 10. These results are shown in Table 13.
Table 13: Body segment lengths and heights across the human adult population
Total height
Head + trunk
Suprasternale
Chest breadth
Trunk
Arm
Light 5th Average
Lewinson
percentile 5th
Average
female
percentile
female
59.00 in
59.00 in
69.69 in
27.98 in
27.98 in
33.05 in
48.18 in
48.18 in
56.91 in
11.35 in
11.35 in
13.41 in
19.77 in
19.77 in
23.36 in
26.46 in
26.46 in
31.25 in
Average
95th
percentile
male
74.00 in
35.10 in
60.43 in
14.24 in
24.80 in
33.18 in
Heavy
95th
percentile
male
74.00 in
35.10 in
60.43 in
14.24 in
24.80 in
33.18 in
To calculate the respective body segment weights across the adult population, the height
from Table 13 will be multiplied by the segment height of length from Table 11 and then
divided by the total height from Table 11. These results are shown in Table 14.
Table 14: Body segment weights across the human adult population
Total body
Head + trunk
Total arm
Trunk
Upper arm
Forearm + hand
Light 5th Average
Lewinson
percentile 5th
Average
female
percentile
female
88.00 lbs 113.00 lbs 162.00 lbs
51.05 lbs
65.56 lbs
93.98 lbs
4.31 lbs
5.54 lbs
7.94 lbs
44.68 lbs
57.38 lbs
82.26 lbs
2.32 lbs
2.98 lbs
4.27 lbs
1.99 lbs
2.55 lbs
3.66 lbs
25
Average
95th
percentile
male
246.00 lbs
142.72 lbs
12.06 lbs
124.91 lbs
6.49 lbs
5.56 lbs
Heavy
95th
percentile
male
297.00 lbs
172.30 lbs
14.56 lbs
150.80 lbs
7.83 lbs
6.71 lbs
8.2 Appendix B - Maple Worksheet for the Traditional (straight-shaft)
shovel
In the real final report, the PDF print of the maple worksheet will follow this page. Until
then, the document is separate and can be found in this folder.
26
8.3 Appendix C - Maple Worksheet for the Ergonomic (bent-shaft)
shovel
As with Appendix B, in the real report the PDF will immediately follow this page. For
now the document can be found elsewhere in this folder.
27