Doctor of Philosophy
School of Health Sciences
Simpson, Katrina, Females lugging loads: What is an appropriate backpack load for female recreational hikers?, Doctor of Philosophy thesis, School of Health Sciences, University of Wollongong, 2010. http://ro.uow.edu.au/theses/3284
Recreational hiking is an increasingly popular outdoor recreational pursuit for females. However, the positive health benefits of the activity can be reduced by the occurrence of injury, pain or discomfort, with females incurring more injuries than their male counterparts. Despite this association between hiking injury and female gender, there is a lack of research on the physiological, biomechanical and subjective responses of women to prolonged load carriage upon which to develop load mass guidelines for recreational hiking. Various biomechanical and neuromuscular factors such as muscle activation patterns, kinematics and kinetics are altered after fatiguing exercise and epidemiologic research suggests a relationship between fatigue and musculoskeletal injury rates. In addition, pre-existing gender differences in various biomechanical and neuromuscular factors may also be amplified by fatigue caused by prolonged load carriage and may help to explain the mechanisms that may lead to load carriage injuries in females. Therefore, the aim of this thesis was to determine the effect of variations in load mass during prolonged load carriage on the performance of female recreational hikers and develop load mass guidelines for this cohort or recreational hikers. To achieve this aim, the thesis was divided into two sections, in which three experimental studies were conducted.
In Section A the backpack loading methodology for Section B was established by determining the appropriate control condition for load carriage studies (Chapter 2) and whether the vertical position of the load impacted load carriage (Chapter 3). In Chapter 2 it was found that either an unloaded backpack or no backpack could serve as a control condition in load carriage studies. Chapter 3 then established that vertical load position in a backpack did not elicit substantial gait modifications or significantly alter selfreported responses, indicating that vertical load position was not a critical factor affecting load carriage performance when carrying a backpack. Therefore, the backpack loading methodology selected for Section B included no backpack as the control condition to reduce any irritation and discomfort caused by an empty backpack and the load mass was distributed evenly along the vertical axis of the backpack and close to the backpack harness.
Section B investigated the effect of load mass on biomechanical, physiological and subjective parameters displayed by female recreational hikers during prolonged load carriage. Fifteen experienced female recreational hikers carrying four different backpack loads (0%, 20%, 30% and 40% body weight (BW)) walked 8 km over-ground on a load carriage course at a self-selected speed at their normal hiking pace. Sagittal plane lower limb kinematics, kinetics and electromyographic (EMG) patterns, together with ratings of perceived exertion (RPE), heart rate and discomfort measures, were collected throughout each trial at the 0, 2, 4 and 8 km distances As distance increased, subjects reported significantly increased shoulder, neck and upper back discomfort and displayed increased forward trunk flexion. The participants also displayed lower integrated EMG of the vastus lateralis (VL), semitendinosus (ST) and gastrocnemius medialis (GM) muscles at 2 km compared to 0 km but no further decrease was seen as distance increased. At 8 km, a shorter VL activation and local muscle fatigue of tibialis anterior suggested that the functional roles of these muscles may have been compromised, reducing the ability of the lower limb to absorb the repetitive loads associated with prolonged load carriage. Altered muscle activity may have also resulted in the changes in knee flexion and ankle plantar flexion evident at initial foot-ground contact with increased load carriage distance.
With increased load mass, the vertical and anteroposterior ground reaction forces (GRF) generated during walking were proportional to the increase in load, albeit up to 30% BW, where no further increases in GRF were found as load increased from 30% BW to 40% BW. Although changes in double support time, velocity, cadence and knee flexion at initial foot-ground contact and early stance were seen with load carriage, they did not change as load mass increased. Single-limb stance time was the only variable to increase when loads heavier than 30% BW were carried. Activity of VL, ST and GM also increased in order to compensate for the additional load. However, changes in VL and biceps femoris co-activation were only seen when participants carried the 40% BW load. The changes in kinematic, GRF and muscle activation patterns suggest that as load mass increases to 30% BW the walking patterns adopted by female recreational hikers are aimed at providing greater shock absorption and stability. Posture and subjective data were all significantly affected by load mass, such that the self-reported RPE and shoulder discomfort results suggest that carrying 30% BW was perceived as reasonable by participants, whereas carrying 40% BW was considered excessive.
It was concluded that backpack load mass and hiking distance affect self-reported, physiological and biomechanical responses to load carriage. The findings of this thesis support a maximum load mass recommendation of 30% BW for experienced female recreational hikers. Further research should address whether this recommendation needs to be modified for less experienced or conditioned female hikers, when hiking over longer distances or over multiple days or when hiking in more challenging environmental conditions and whether adherence to this recommendation assist female hikers to reduce their risk of injury when participating in this popular recreational activity.