Effects of prolonged load carriage on ground reaction forces, lower limb kinematics and spatio-temporal parameters in female recreational hikers.

The effect of load carriage on female recreational hikers has received little attention. This study collected lower limb sagittal plane kinematic, spatio-temporal and ground reaction force (GRF) data from 15 female hikers carrying four loads (0%, 20%, 30% and 40% body weight (BW)) over 8 km. Increasing load resulted in a proportional increase in GRF up to 30% BW, increased stance time, and greater mediolateral impulse with 30% and 40% BW. Also seen were decreased velocity and cadence and increased double support and knee flexion when carrying load compared to no load. Increased distance resulted in increased knee flexion and ankle plantar flexion at initial foot-ground contact. It was concluded that, as load mass and distance increased, female hikers modified their gait to attenuate the lower limb impact forces. When carrying 30% and 40% BW loads, however, the changes aimed at attenuating the higher GRF may result in a less stable gait. PRACTITIONER SUMMARY: Limited research has investigated the biomechanical responses of female recreational hikers to prolonged load carriage. This study provides a better understanding of the effects of increasing load on lower limb kinematics, spatio-temporal parameters and the GRF generated by female hikers during prolonged load carriage. The results have implications for the development of load carriage guidelines to minimise the risk of injury to females who carry backpacks and to improve performance for this population.

Does load position affect gait and subjective responses of females during load carriage?

Recreational hikers carry heavy loads while often walking long distances over uneven terrain. Previous studies have suggested that not only the load mass but also the position of the load may influence load carriage. The purpose of this study was to determine the effect of vertical load position on gait and subjective responses of female recreational hikers. Fifteen experienced female hikers walked for 2 km over a simulated hiking trail carrying 30% BW in three vertical load positions (high, medium and low). Lower limb and trunk kinematic, electromyography (EMG) and ground reaction force (GRF) data were collected together with heart rate (HR), ratings of perceived exertion (RPE) and discomfort measures. Although HR, RPE and discomfort measures were not able to discern statistical differences between load positions, the high load position was the most preferred by participants. The high load position also resulted in a more upright posture (p < 0.001), decreased gastrocnemius integrated EMG compared to the medium (p = 0.005) and low load positions (p = 0.02) and a higher first peak deceleration vertical GRF compared to the low load position (p = 0.011). However, the absolute differences were small and unlikely to be functionally relevant in load carriage studies. Based on the findings of this study, a high, medium or low load position cannot be preferentially recommended for healthy, experienced, female hikers carrying 30% BW.

Backpack load affects lower limb muscle activity patterns of female hikers during prolonged load carriage.

This study investigated the effect of prolonged load carriage on lower limb muscle activity displayed by female recreational hikers. Electromyography (EMG) signals from vastus lateralis (VL), biceps femoris (BF), semitendinosus (ST), tibialis anterior (TA) and gastrocnemius (GM) were recorded for fifteen female hikers carrying four loads (0%, 20%, 30% and 40% body weight (BW)) over 8 km. Muscle burst duration, muscle burst onset relative to initial contact and integrated EMG signals (iEMG) were calculated to evaluate muscle activity, whereas the shift in mean power frequency (MPF) was used to evaluate muscle fatigue. Increased walking distance significantly decreased the MPF of TA; decreased the iEMG for VL, ST and GM; and shortened VL muscle burst duration. Furthermore, carrying 20-40% BW loads significantly increased VL and GM iEMG and increased BF muscle burst duration, whereas a 40% BW load caused a later VL muscle burst onset. The differences observed in muscle activity with increased load mass seem to be adjustments aimed at maintaining balance and attenuating the increased loads placed on the lower limbs during gait. Based on the changes in muscle activity, a backpack load limit of 30% BW may reduce the risk of lower limb injury for female hikers during prolonged walking.

Effect of load mass on posture, heart rate and subjective responses of recreational female hikers to prolonged load carriage.

Load carriage has been associated with a risk of upper and lower limb musculoskeletal disorders with women suffering significantly higher injury rates than their male counterparts. Despite this injury risk, there are limited evidence-based guidelines for recreational hikers, particularly female recreational hikers, regarding safe backpack loads. The purpose of the present study was to determine how variations in load mass affected the heart rate, posture and subjective responses of women during prolonged walking to provide evidence for a load mass limit for female recreational hikers. Heart rate (HR), posture and ratings of perceived exertion (RPE) and discomfort were collected for 15 female experienced recreational hikers (22.3 ± 3.9 years) while they hiked for 8 km at a self-selected pace under four different load conditions (0%, 20%, 30% and 40% of body weight (BW)). Although HR was not significantly affected by load mass or walking distance, increasing load mass and distance significantly affected posture, RPE and discomfort of the upper body. Carrying a 20% BW load induced significant changes in trunk posture, RPE and reported shoulder discomfort compared to the unloaded condition. The 20% BW load also resulted in a mean RPE rating of 'fairly light', which increased to 'hard' when carrying a 40% BW load. As load carriage distance increased participants reported significantly increased shoulder, neck and upper back discomfort. Based on the changes to posture, self-reported exertion and discomfort when carrying loads of 20%, 30% and 40% BW over 8 km, it was concluded that a backpack load limit of 30% BW should be recommended for female recreational hikers during prolonged walking.