Using a backpack is a common a task for many people,(students, hikers, military) carrying a backpack load changes the mechanicsinvolved in walking, one of the most notable effects is that it increasesforward lean of the trunk. Increasing the load also further increases theforward lean (Chow et al., 2005).
Below 50% ofbodyweight, stance phase duration remains the same, however swing phaseduration is decreased (Tilbury-Davis and Hooper, 1999). Increasing the load also raises braking force,lateral force, propulsive force and ground reaction force (Ghori and Luckwill, 1985).It can be concluded that the effects of load masswere not changed by walking distance. The results from Quesada et al, (2000) Supportthe idea that kinematics change under prolonged load carriage to try and absorbthe impact forces to try and absorb the impact forces and reduce injury. Large magnitudesof impact forces or large numbers of impact forces are known to be a riskfactor for the lower limb developing overuse injuries.
Stride length did notchange between 0 to 40% BW during a 8km walk and approximately 5000 impacts,this indicated that the number of impacts does not depend on load mass (Simpson, Munro and Steele, 2012).When loading is applied, anteroposterior andvertical GRF’s increase. An increase in load has also been associated with ahigher rate of contraction from the rectus abdominus, as the centre of gravityis shifted backwards. The biceps femoris and vastus lateralis muscles seemed tobe unaffected by various loads, which suggests that the majority of strain doesnot affect the lower extremity muscles, whittfield et al reported mostmusculoskeletal problems from wearing a schoolbag were in the upper back(36.7%) lower back 35% shoulders 57.9% and neck (44%) (Al-Khabbaz, Shimada and Hasegawa, 2008).
The biomechanical changes that occur caused by anincreased load can be the cause of many problems such as back pain, jointproblems and muscle discomfort in general. The forces put on themusculoskeletal system are higher at increased gait cadences as a largermagnitude of forces are generated at the heel strike. If muscles are unable tocope with the impact forces, then joint contact forces will also be increased.Vertical GRF forces have been found to be lower at a high gait cadence whilewalking a carrying a backpack, this would suggest that there are adaptations inthe gait pattern helping to minimise potential damage to the musculoskeletal system(Castro et al., 2015). Most research indicates that there is a linear relationshipbetween the applied load and anteroposterior and vertical GRF’s, with anincrease of 10N of force per 1KG and load added in 8 KG increments have aproportional relationship with GRF parameters. An increase in force can also bedescribed as the static effect that load has instead of changes in accelerationacting on the body. Overuse injuries aremuch more likely to occur under excessive amounts of impact forces, be itvolume or magnitude.
This can often result in injuries of the tibia,metatarsals and knee joint, most commonly these injuries are stress fractures,(Birrell, Hooper and Haslam, 2007). Predicting the amount of load that can beapplied before injury is also important for the prevention of injuries. Theaim of this study is to determine if there is a proportional increase inimpulse and GRF parameters when compared to the weight of a backpack. Method: Participants:The sample size was 15 BSc sport and exercisescience students which were split into 3 groups consisting of 5 people pergroup. In the smaller groups 2 people had to participate in the study. Eachsubjects mass was also noted pre-back pack load. EquipmentData was collected from a force platform with asampling frequency of 1000 HZ for a duration of 1.00 seconds with the walkingdirection and positive y direction going the same way.
Three pairs ofphotoelectric lights were set up at hip height so that walking speed was 1.25m/s ((±5%). The photoelectric timing lights wereset 2.5 metres apart with the middle light in line with the centre of the forceplatform. Walking speed was then recorded on approach to the force platform andwalking away from the force platform. As acceleration and deceleration affectsGRF, it was important that speed recorded was in the desired range.
ProcedureBeforethe subject can walk on the force platform, the platform must be set back tozero which corresponds to 0.100 seconds prior to foot impact when looking atthe force time trace. The participant can then proceed to walk on the platform,but it is only recorded if the right foot contacts the force plate without anychanges in gait. One trial must be recorded from each individual with andwithout the backpack. Using bioware software, the GRF data recorded is savedwhich includes the subjects name and load carriage condition. The data was thennormalised by total body weight for each condition (subjects body mass+ 20 kgbackpack).
Tomake sure that the participants were familiar with the correct walking speedand the conditions needed for a successful trial, an unlimited number ofpractise walks were allowed. These conditions measured were impact peak, thrustmaximum, braking force, vertical and mediolateral impulse and stance time. ResultsPairedsamples Test Mean Std.
Deviation T test P test Pair 1 Impactpeakzero-impactpeak20 .50018 .65497 2.645 .023 Pair 2 TZ1zero-TZ1twenty -3.
80328 7.06427 -1.865 0.89 Pair 3 FZ3zero-FZ3twenty -47991 .60607 2.743 0.
19 Pair 4 TZ3zero-TZ3twenty -2.27530 14.77899 -533 .604 Pair 5 FZ2zero-FZ3twenty -.
03916 .42307 -.321 .
754 Pair 6 TZ2zero-TZ2twenty -3.79832 12.84691 -1.024 .328 Pair 7 FY1zero-FY1twenty -.08122 .
44222 -.636 .538 Pair 8 TY1zero-TY1twenty -1.27329 8.07192 -.546 .596 Pair 9 FY3zero-FY3twenty .06048 .
19998 1.048 .317 Pair 10 TY3zero-TY3twenty -1.61132 8.78144 -636 .538 Pair 11 TY2zero-TY2twenty -2.89671 6.76594 -1.
483 .166 Pair 12 Verticalimpulsezero-Verticalimpulse20 -00101 .24701 -.014 .989 Pair 13 Breakingimpulsezero-Breakingimpulse20 -00726 .09588 -.262 .
798 Pair 14 Posteriorimpulsezero-Posteriorimpulsetwenty -00247 .03931 -217 .832 Pair 15 Netanteriorposteriorimpulsezero-Netanteriorposteriorimpulsetwenty -00972 .11476 -293 .775 Tosee if there were any significant differences between backpack loads, Pairedsamples T test was carried out in IBM spss version 24. The tests showed thatthere were significant differences (P<0.05) between the measured thrust maximum variable(FZ3) and inimpact peak (FZ1) shown by the table above.
TheVGRF data represented on the graph have two noticeable peaks. The first peakrepresents the time period instantly after the heel makes contact with theforce plate. The centre of gravity is going towards the ground, which in turnincreases the reaction force from the ground towards the vertical direction.Thesecond peak is when the front of the foot is pushing off the force plate, thedrop in between these phases happens when the centre of gravity is rising awayfrom the ground. Discussion Knudson,D.
(2007) demonstrated that increased load in 8kg increments resulted in aproportional increase in vertical and anterior posterior GRF parameters.Changes to the vertical and horizontal position can be attributed to shifts inthe body’s centre of mass. The change in COM is also linked to restrictions ofthe natural arm swing patterns (Birrell, Hooperand Haslam, 2007).Whensomeone is carrying a backpack for the centre of gravity (CG) to stay in itsstability limits as a person moves forward, gait pattern must adapt, momentummust increase when load is carried behind the CG in order to bring the CG overthe supporting foot.Shiftingthe load in front of the cg allows the load to increase momentum. If speed wasto increase, GRF also increases, this is because overall momentum of the body increases(Hsiang and Chang, 2002). In order tocompensate for the added weight stance time becomes shorter, if there is ashorter stance time then there is less time to produce an impulse, tocounteract this the peak forces should be higher, which our results did notsupport (Tongen and Wunderlich, 2018).
Our results show that there was a significant difference in theimpact peak variables, some evidence goes against this as Tilbury-Davis andHooper, (1999) indicated that impact parameters between 0 and 20kg are reducedbut a load greater than 40kg show a significant difference. This is evidencethat there is a threshold which when exceeded activates a compensatingmechanism. However, the threshold that activates this mechanism lies between 20and 40kg.
Up to 64% of body weight has little to no effect on sagittalplane motion, it is an important finding that increased load carriage in abackpack increases ground reaction forces proportionally to total mass. A 20kgload causes an activation of the forces needed for balance in all subjects.After the threshold is activated higher loads do not cause any furtherincreases in impact forces (Tilbury-Davis and Hooper,1999). In Birrell and Haslams (2008) study the load was infront of the body, the data showed a common trend for a decrease in thrustmaximum. Active momentum is thought to be the key factor in decreasing thrustmaximum, this momentum is being produced earlier in the gait cycle. Thevertical impulse did not change which is surprising as usually when there is asignificant increase in impact peak then there is usually a decrease in forceminimum, which we did not observe in our results.