A mechanism to reduce shoulder strain of backpack wearers

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Introduction
Since ages humans have created various ways to reduce the weight they must personally carry, first with the help of domesticated animals and then by using machines.Even now a large amount of weight can be carried easily by people with the help of wheels on smooth surfaces such as a bus terminal, but this becomes ineffective in uneven terrain or in the wilderness, where the backpack remains the only possible option.But apart from its advantages it has got certain limitations like, when standing still, the static force acting on our body because of the backpack is equal to the weight of the backpack.However, while walking or running the forces exerted on our body by the backpack can increase dramatically as compared to that of the static force.It was found that walking with a backpack load of 15% body weight and 30% body weight resulted in increase in lumbosacral force of 26.7% and 64%, respectively when compared to walking without a backpack load [1].It was also found that deducting the weight of the body by stimulation of reduced gravity decreases the net metabolic rate [2].Again carrying load with compliant poles increases the oxygen consumption compared to no load condition in a motorized treadmill.However, the compliant poles minimize the peak shoulder forces and loading rates.The peak vertical ground reaction force is only slightly increased above unloaded levels [3].It was stated that a backpack load limit of 30% body weight may reduce the risk of lower limb injury for female hikers during prolonged walking [4].It was also mentioned in a critical review that the backpack loads contributed to an average of over 15% of the body weight of the school students, which increased the risk of musculoskeletal injury, fatigue, and discomfort [5].A survey was conducted to assess the impact of backpack load on a group of school students aged 8-12 years.The percentage of students suffering pain was much higher and it depends on the duration of use and the method of carrying [6].A group of 57 school children aged 7-9 years old was assessed and it was found that the heavier the weight of the backpack higher the pressure and force exerted under the foot [7].The changes in the gait and posture of the military were also examined due to the increasing load carriage in military load carriage systems [8].In another study, a backpack was designed to reduce the moment arm by locating its center of gravity near the spine.When tested with 18 adults for spinal alignment parameters, the radiological images showed that the designed backpack wearer's lumbar lordosis and spinal alignment were improved in comparison to a normal backpack wearer [9].
It is found that there is an increase in forces acting on the body of the wearer due to alternate deceleration and reacceleration of the backpack because of the movement of the hips on every step.This increase in force makes it difficult to move at a faster speed and also increase in peak forces puts high pressure on joints resulting in various orthopedic and muscular injuries.It also increases the net metabolic rate.To eliminate these forces, it is necessary to keep the load in a constant position in the vertical plane to reduce the dynamic forces acting on the body.So far such a problem has not been addressed yet.Motivated by such a problem the aim of the present work is to develop a mechanism to reduce shoulder strain of backpack wearers.The novelty of the proposed mechanism lies in its mechanical design, which can be easily assembled and disassembled with any existing backpack and maintains the backpack at a constant position by sliding it up and down while the wearer is moving.Its design will consist of two frames eliminating the repeated up and down motions of the backpack with the shoulder movements of the wearer maintaining a constant position relative to the ground by moving it relative to the wearer.This will reduce the strain and discomfort on the shoulders.In this paper, a detailed design, fabrication, and testing of the mechanism is done to check its effectiveness in significantly reducing the impact forces on the wearer's shoulders.This innovative solution offers a potential improvement in the design of backpacks to enhance user comfort and reduce the risk of shoulder injuries.
The paper is organized as follows, problem description is described in the "Introduction" section, followed by design of the proposed mechanism in the "Problem description" section, fabrication and testing in the "Methods" section, and results and discussion in the "Results and discussion" section.Finally, the paper is concluded in "Conclusions" section.

Problem description
While walking or running with a traditional backpack, the wearer's body moves up and down so, the load due to the backpack has to be decelerated and reaccelerated each time the wearer's foot hits the ground.Due to such repeated changes in the loading of the backpack, the peak vertical forces exerted on the shoulders of the wearer are more than the actual weight of the backpack leading to back pain or injury.To overcome such a problem of the excessive forces acting on the shoulders of the wearer a mechanism is proposed here.The mechanism is an additional system with two frames which is attached on the back side of a backpack.The first frame contains the harness to attach the backpack to the wearer like a normal backpack.The second frame contains the backpack or the load and it is attached to the first frame by long elastic bungee cords.
When the person wearing the backpack with the mechanism takes a step, the movement of the wearer causes the first frame to move up with respect to the ground.The suspended mechanism guides the second frame to move down relative to the first frame.Similarly, when the first frame moves down due to the movement of the wearer, the mechanism guides the second frame in the upward direction.Due to the movement of the second frame in the opposite direction, the load stays at a constant height relative to the ground.This eliminates the need to decelerate and reaccelerate the load with our body, thus reducing the impact forces on the shoulders.

Methods
The mechanism proposed here is designed and fabricated.The mechanism is composed of two frames.The components of the two frames and their construction are described below:

First frame
The first frame consists of two vertical aluminum pipes connected to two horizontal hollow rectangular frames on opposite ends.Two additional aluminum pipes are also attached on the outside of the rectangular frame to strengthen it and to attach the harness.The frame also consists of four pulleys two at the top and two at the bottom portion of the frame and two bungee cords.The pulleys are fixed on the rectangular frames.One end of each bungee cord is attached to the first frame by passing the cord through a hole drilled through the rectangular frame.A knot is then tied above the frame.The tension in the bungee cord is set by the knot.Each cord is then passed over two pulleys and then attached to the second frame.Two additional aluminum pipes are provided to increase the strength of the frame and to attach the harness.The first frame also contains a cover plate on which limits are attached to restrict the vertical relative motion of the frames up to a certain extent.It also avoids any contact between the person and the bungee cords.The components of the first frame are shown in Figs. 5, 6, 7, 8, 9, 10 and 11 (see Appendix).

Second frame
The second frame consists of a load plate on which the backpack is mounted.The second frame, i.e., the entire load of the bag will be suspended from the first frame using the bungee cords.Bungee cords are attached to the load plate by using hooks.Four bushings are provided on the load plate to facilitate the vertical motion of the pipes of the first frame and also to minimize any horizontal motion.Limits are provided to restrict the motion between the frames so that the bushings do not hit the rectangular hollow frames.Handles are also provided on the cover plate to attach the bungee cord to the first frame.The components of the second frame are shown in Figs. 12, 13, 14 and 15 (see Appendix).A backpack with the mechanism is shown in Fig. 1.The final assembled mechanism with the harness and cover plate is shown in Fig. 2.

Fabrication and testing
The proposed mechanism was fabricated in the Central Workshop of Tezpur University.The mechanism with the backpack is shown in Fig. 3.
The mechanism was then tested to assess the impact force felt on the shoulders of the user.The setup for analyzing the forces experienced by a wearer consisted of a load cell connected to an Arduino Nano through an HX711 analog-to-digital converter (ADC).The circuit diagram of the test setup is shown in Fig. 4.
The load cell was fitted between the shoulder of the subject and the harness of the backpack to detect the impact force acting on the shoulder.The Arduino was connected to a computer and the outputs were received on the monitor for both static and dynamic conditions for each of the normal backpack and the enhanced backpack.

Results and discussion
Using the testing procedure as discussed in the preceding section, initially, Arduino readings for loads 5 kg, 10 kg, 12.5 kg, and 15 kg were obtained for the normal backpack for both static and dynamic conditions.Then the equation of the curve obtained from the Arduino reading v/s Load graph was derived as shown in Eq. ( 1).
In Eq. ( 1) y is the Arduino reading and x is the load (kg).Following a similar procedure, a backpack with the mechanism was tested for loads 10 kg, 12.5 kg, and 15 kg.As the weight of the mechanism was 5.4 kg, so the total weight of the enhanced backpack became 15.4 kg, 17.9 kg, and 20.4 kg.Then by using Eq. ( 1), the expected Arduino readings for the normal backpack with loads 15.4 kg, 17.9 kg, and 20.4 kg were obtained as shown in Table 1.The corresponding Arduino readings for the enhanced backpack with added 10 kg, 12.5 kg, and 15 kg were also obtained.
The readings obtained for the backpack with mechanism were found to be less than the expected readings.This shows that there has been a reduction in the impact force acting on the shoulders of the wearer.The actual impact force acting on the wearer was then calculated using the Arduino readings in Eq. 1.
It is observed from Table 1 that in the enhanced backpack with the mechanism the impact force of 15.4 kg was acting equivalent to that of 12.45 kg whereas the impact force of 17.90 kg was acting equivalent to that of 13.61 kg and the impact force of 20.4 kg was acting equivalent to that of 13.71 kg on the wearer's shoulder.
Since the mechanism weighs 5.4 kg itself, thus it can be concluded that for an added load of 10 kg, 12.5 kg, and 15 kg on the backpack with the mechanism, the impact force acting on the shoulders of the wearer is equivalent to that of 12.45 kg, 13.61 kg, and 13.71 kg, respectively.Thus the mechanism is found to be effective for an added load of 15 kg.
The results obtained from the study were able to show that the proposed mechanism is effective in reducing the downward force applied on the shoulder of a person carrying a backpack.To measure the downward force exerted on a person wearing a backpack, the research employed an Arduino-based system integrated with a load cell sensor during the experimental setup.The findings indicated a substantial reduction in the downward pressure exerted on the shoulders by utilizing a special backpack relative to its normal counterpart without the mechanism.
(1) y = 63362x − 3123.4The increase in the effectiveness of the enhanced backpack with the mechanism when carrying loads of 15 kg or higher implies its potential usefulness, particularly for people carrying heavier weights.This indicates the effectiveness of the mechanism especially for those carrying heavier loads.
In general, the novel solution obtained from this study suggests that the proposed mechanism can effectively reduce the impact forces on the shoulders of backpack wearers by minimizing the shoulder strain and potential injuries of heavy backpack wearers.

Conclusions
The weight of the conventional backpack often causes severe strain on the shoulders of the wearer leading to injury.Such discomfort experienced by the wearer is mainly due to the impact force exerted by it on the wearer's shoulders.To address such a problem a mechanism is proposed in this paper to minimize the impact force on the shoulders of the wearer.The mechanism is designed in a way that the relative velocity of the backpack with respect to the ground remains the same which will contribute to reducing the force on the shoulders.
The components of the mechanism are fabricated and assembled.The final mechanism is then incorporated into a normal backpack and tested to check its effectiveness.Loads 10 kg, 12.5 kg, and 15 kg were added to the normal backpack and the enhanced backpack with the mechanism.Using an Arduino-based experimental setup, the Arduino readings for a typical backpack equipped with the mechanism under dynamic conditions for different loads were taken.The expected Arduino readings for the enhanced backpack were derived from an equation obtained on the basis of the graph of Arduino reading against load.Then using these readings together with this equation obtained, the actual impact forces acting on the wearer's shoulders can be calculated.
The results showed a decrease in shoulder impact forces while utilizing this backpack with a mechanism.Furthermore, it was observed that the measured values for the backpack fitted with the mechanism were less than what was expected thus indicating reduced impact forces.However, when the additional weight was at least 15 kg, we could observe certain effectiveness of it.
Future work may include testing the mechanism for its effectiveness with a greater number of loads and in various practical scenarios.As the weight of the mechanism was 5.4 kg, the mechanism can be designed with lesser weight which may give better results.

Fig. 1 Fig. 2
Fig. 1 Mechanism without a harness and cover plate

Fig. 3 aFig. 4
Fig. 3 a Back view and b side view of the final assembled mechanism after fabrication

Figures 5 ,
Figures 5,6,7,8,9, 10 and 11  shows the 2D CAD models of the components of the first frame of the mechanism.

Figures 12 ,
Figures 12, 13, 14 and 15 shows the 2D CAD models of the components of the second frame of the mechanism.

Table 1
Peak Arduino readings and equivalent load under dynamic conditions for the normal backpack without the mechanism and the Enhanced backpack with the mechanism