Date Published: June 7, 2019
Publisher: Public Library of Science
Author(s): Tong Li, Qingguo Li, Tao Liu, Virgilio Mattoli.
The carrying pole has existed as a load carrying tool for thousands of years and is still popular in many parts of Asia. Previous studies attempted to determine whether the elasticity of the carrying pole is energetically beneficial compared with other load carrying methods. However, conflicting results indicate that the effects of the carrying pole stiffness on the carrier are still unclear. The carrying pole exhibits more complex characteristics beyond stiffness, which invites further investigation. As the first step towards the goal, this paper explores the underlying mechanics of the carrying pole, including the structural and dynamic properties, to determine its impact on the carrier. The structure of the carrying pole is modeled and characterized by pole length, pole stiffness, and length of suspension rope. We argue that maintaining the pole’s balance should be a major prerequisite during load carriage and that active feedback control from the carrier is required. Simulations reveal that the structural parameters of the pole have significant influences on the pole’s balance and the interaction between the pole and the carrier. This work suggests mechanical characteristics of the carrying pole can potentially have an extensive impact on gait mechanics and energetics of the carrier.
Transportation plays an important role in the development of human society as it enables trade between people at different locations. Advancement in vehicles has largely benefited the transportation system by increasing its speed and efficiency. However, manual load carriage is still an essential way of moving things around in our society. In some cases, manual load carriage may be the only available option for transporting goods, for example in mountain areas and congested markets where machines and vehicles cannot reach. Humans carry load using their head , hands , feet , etc. or utilize tools including vests , poles , packs , etc. Among different manual load carrying methods, the carrying pole is very commonly used in East Asian countries such as China and Vietnam from ancient times until today . Archaeological evidence shows that carrying pole usage dates back to Ancient Egypt times (2300BC) [8, 9] and the Han Dynasty in China (206BC-220AD) . Its prevalence starts to draw attention from academia in recent years aiming to discover the advantages and disadvantages of carrying poles in contrast to other load carrying methods [11, 12].
Carrying loads with a pole forms a human-pole-load system as depicted in Fig 1. The pole is usually used to carry heavy (in mass) and bulky (in dimension) loads such as water, crops, and materials for construction. From the carrier’s perspective, interaction with the pole will influence their walking pattern, joint loading, balance, and energetic cost. These considerations most likely lead to requirements for pole design to achieve better performance during load carriage in aspects such as comfort, convenience, safety, and efficiency.
In this work, we take the “frontal pole”  as a typical pole-carrying style for analysis (Fig 2). Some other carrying styles such as transverse pole are also used in some cases and can be explored in further work. Since human walking is usually studied in the sagittal plane, this configuration helps to simplify the overall model as the pitch motion of the pole is also in the sagittal plane. The motion of carried loads in the medial-lateral direction is thus neglected in this work. In the human-pole-load system, two loads of similar mass (m1,m2) are suspended at the two ends of the pole via ropes at the distances of Lp1 and Lp2 to the shoulder. The distances from the attachment point of the rope to the load’s center of mass can be regarded as the effective pendulum length (Lr1,Lr2). Due to the weight of the loads, the pole will deform like a beam resulting in a vertical displacement (h1,h2) for each end (horizontal displacement is negligible). According to the beam theory , the static deflection at each end (vertical displacement) due to the load can be calculated as:
assuming the pole is an ideal cantilever beam where E is Young’s modulus and I is the second moment of the area . In this case, we can derive the stiffness of the pole at each side:
In our simulation, we assume the two loads have identical point mass (m1 = m2 = 10Kg and I1 = I2 = 0). The pole is initially supported at the middle point which is the equilibrium point of the pole (Lp1=Lp2=L2, where L is the pole length). Variables m0 and I0 is assumed to be small but should not be zero in order to avoid singularity when solving angular acceleration. We use m0 = 1Kg and I0=m0(Lp1+Lp2)212. We also consider the pole stiffness and rope length at each end to be the same (k1 = k2,Lr1 = Lr2). The arm to control the pole’s balance is assumed to be massless. In reality, it can be compensated by the difference in load mass or a shift of the shoulder contact point. The model is driven by the two dimensional motion of the shoulder. If the body only has vertical motion (xp = 0,yp ≠ 0), the pole will not roll over since the supporting position will always be at the initial equilibrium point (middle point of the pole). This indicates the imbalanced condition will come from the horizontal motion of the body which will lead to a deviation of the shoulder supporting point. We first study the response of the system under a step input of the horizontal body motion (xo) as in Fig 5B in the following three sections to study the balance dynamics of the system.
The carrying pole’s long history of application has been documented in many different places around the world. It was used for a relatively short period of time in many places but continues to be popular in some underdeveloped areas in Asia. We believe the prevalence of the carrying pole in East Asia is a blended consequence of many aspects including history, economics, engineering, and environment. However, it is not the focus of this paper to explain the prevalence of the carrying pole, as it requires knowledge across multiple disciplines. Our goal is to explore the mechanics of the carrying pole to reveal the operational principles of the carrying pole and the influence of its structure and parameters on pole balance and human-pole interaction. Full understanding of its mechanism may also help to explain why people would prefer this tool over some other load carriage tools given certain conditions, which accounts as an essential step to explain the popularity of the carrying pole.
In this paper, we investigated the mechanics of the carrying pole, a popular load-carrying tool in many Asian countries. We did not directly analyze the influence of pole carriage on gait energetics, as it requires a full understanding of the mechanics of the carrying pole, gait mechanics and human motor control. Instead, as the first step, we focused on understanding the mechanics of the carrying pole that could potentially affect load carriage performance. Through simulation, we showed that this seemingly simple tool produces rather complex behavior. Different from other load-carrying tools like backpacks, the carrying pole requires the carrier to keep the pole’s balance which is essential during walking. We analyzed multiple factors including pole length, pole stiffness, and rope length. We found that these factors have large influences on pole balance and the human-pole interaction. The current study is still limited by the little knowledge of the pole-shoulder contact and human motor control strategy. Even so, it is believed that these structural and dynamic analyses can serve as a foundation for further development of human-pole interaction models. We hope this study can inspire more studies of this load carrying tool and point to potential directions for further investigation.
The EOM of the system is obtained via the Newton-Euler Method.