Long distance nature-based walking is a popular recreational physical activity [1]. In 2021, 1.14 million New Zealanders went on at least one hike [2], establishing itself as a popular outdoor activity. Hiking involves prolonged walking periods across varying terrain, exploring wilderness, and reaching remote destinations. Depending on the difficulty level, hiking expeditions can last several days with heavy loads of equipment carried on uphill and downhill slopes [3]. Carrying a heavy backpack for long walking periods may affect a hiker’s walking mechanics, and potentially lead to musculoskeletal strain. The weight of a typical backpack load reaches between 10–20% of a hiker’s body weight, therefore it is recommended that hikers use backpacks with supportive frame around the hip and sternum. Lumbar and cervical spine loading is influenced by a backpack’s design and weight distribution pattern [6]. Contemporary backpacks have indicated design considerations to improve the users’ postural biomechanics. Balance backpacks (BBP) are a uniquely designed carriage system that allows the user to balance loads posteriorly and anteriorly and have been reported to have less impact on a users' forward lean during gait [7]. Traditional backpacks (TBP) mainly have posterior loading during carriage, whereas a BBP concept aims to reduce the posterior loading impact and balances the load [8]. The loading differences between TBP and BBP suggests possible postural differences in the sagittal plane kinematics and kinetics. Efficient backpack carriage relies on strategic load placement [9]. The BBP may encourage lower energy expenditure, delaying fatigue onset [10]. Altering a TBP’s design to match the user’s anthropometry may reduce a user’s fatigue [11].
Studies investigating the relationship between gait and backpack carriage have identified certain influential biomechanical variables such as kinematics and kinetics [15–21], muscle activity [22–25], spatiotemporal parameters [26, 27] and comfort [28–30]. Studies have assessed the biomechanical gait changes during military backpack [32, 33] and school backpack carriage [34]. However, these findings may not apply to hikers because of the differences in backpack design, carrying loads, and participant characteristics.
Changes to posture due to fatigue has been reported to affect sports performance [35]. Prolonged backpack carriage also affects the user’s postural stability [36]. Heavy posterior loading causes a higher lumbar forward lean [8, 27, 37] with significant changes observed when backpack loads reach 10% body weight (BW) among female hikers [19]. A decrease in maximum lumbar extension (LE), or a greater forward trunk lean occurs to compensate for the heavy posterior loading to maintain balance. Sagittal plane kinematic adjustments align the trunk optimally over the hip joint and minimises hip flexion leading to improved gait efficiency [15]. This is a learned adaptation among regular hikers who often carry heavy loads in uneven terrain [38]. This trunk angle adaptation has been observed in the stance phase of army men who often carry heavy loads during training drills [32]. Posterior loading shifts the line of gravity before the base of support, leading to trunk flexion as the head tilts forward to restore stability [28]. As a consequence, to maintain a forward field of view the neck must be hyperextended to look ahead and not at the ground [8]. Increased LE to maintain balance can lead to higher muscle activity in the semispinalis, erector spinae, and trapezius, which may result in lumbar fatigue, discomfort, and potential pain [37]. Both load distribution and load magnitude impacts carriage performance efficiency [39]. Ground reaction forces (GRF) provide insights into an individual’s postural stability, and backpack designs influence a user’s GRF [40]. To maintain dynamic body stability, GRF has been reported to increase in as the backpack load increases [15, 19, 21]. Centre of pressure (COP) is the specific point under a person's foot where the GRF is concentrated, and COP adjustments and adaptations are made to maintain balance and prevent falling. Placement and magnitude of loads applied during gait have been linked to COP measures of postural stability [41]. Anteroposterior COP displacement changes when the backpack loading increases by 5–10% of BW, compared to no load [42]. Analysing COP displacement path helps differentiate a normal and abnormal gait [43]. During each step cycle, the centre of mass travels from behind to the front of the support base, reflected in the COP displacement, smoothly transitioning the kinetic energy, and requiring less mechanical work to move [44]. Mechanical stress information is therefore relatable to COP displacement and GRFs and further relatable to joint contact forces, which have the potential towards developing pathological conditions in the lower body [15]. Considering the biomechanical changes backpack design and loading may place on a user, prolonged abnormal walking gait could lead to an early fatigue onset for a hiker. Therefore, this study aimed to investigate the design of two backpacks (TBP and BBP), assessing the different loading system changes when walking on a flat, incline and decline gradients as compared to NBP loading condition. The focus was exclusively on LE and COP as they are crucial factors that influence the kinematics and kinetic carriage dynamics. Therefore, the primary aim was to compare the LE and COP changes of hikers walking on a treadmill (flat) under three loading conditions (NBP, TBP and BBP). The secondary aim was to compare the LE and COP changes of hikers walking on a treadmill (incline and decline) under three loading conditions (NBP, TBP and BBP).