Using a backpack is a common a task for many people,
(students, hikers, military) carrying a backpack load changes the mechanics
involved in walking, one of the most notable effects is that it increases
forward lean of the trunk. Increasing the load also further increases the
forward lean (Chow et al., 2005). Below 50% of
bodyweight, stance phase duration remains the same, however swing phase
duration 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 mass
were not changed by walking distance. The results from Quesada et al, (2000) Support
the idea that kinematics change under prolonged load carriage to try and absorb
the impact forces to try and absorb the impact forces and reduce injury. Large magnitudes
of impact forces or large numbers of impact forces are known to be a risk
factor for the lower limb developing overuse injuries. Stride length did not
change 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 and
vertical GRF’s increase. An increase in load has also been associated with a
higher rate of contraction from the rectus abdominus, as the centre of gravity
is shifted backwards. The biceps femoris and vastus lateralis muscles seemed to
be unaffected by various loads, which suggests that the majority of strain does
not affect the lower extremity muscles, whittfield et al reported most
musculoskeletal 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 an
increased load can be the cause of many problems such as back pain, joint
problems and muscle discomfort in general. The forces put on the
musculoskeletal system are higher at increased gait cadences as a larger
magnitude of forces are generated at the heel strike. If muscles are unable to
cope 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 while
walking a carrying a backpack, this would suggest that there are adaptations in
the gait pattern helping to minimise potential damage to the musculoskeletal system
(Castro et al., 2015).

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Most research indicates that there is a linear relationship
between the applied load and anteroposterior and vertical GRF’s, with an
increase of 10N of force per 1KG and load added in 8 KG increments have a
proportional relationship with GRF parameters. An increase in force can also be
described as the static effect that load has instead of changes in acceleration
acting on the body.  Overuse injuries are
much more likely to occur under excessive amounts of impact forces, be it
volume 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 be
applied before injury is also important for the prevention of injuries.

The
aim of this study is to determine if there is a proportional increase in
impulse and GRF parameters when compared to the weight of a backpack.

 

 

 

 

Method:

 

Participants:

The sample size was 15 BSc sport and exercise
science students which were split into 3 groups consisting of 5 people per
group. In the smaller groups 2 people had to participate in the study. Each
subjects mass was also noted pre-back pack load.

 

Equipment

Data was collected from a force platform with a
sampling frequency of 1000 HZ for a duration of 1.00 seconds with the walking
direction and positive y direction going the same way. Three pairs of
photoelectric lights were set up at hip height so that walking speed was 1.25
m/s ((±5%). The photoelectric timing lights were
set 2.5 metres apart with the middle light in line with the centre of the force
platform. Walking speed was then recorded on approach to the force platform and
walking away from the force platform. As acceleration and deceleration affects
GRF, it was important that speed recorded was in the desired range.

 

Procedure

Before
the subject can walk on the force platform, the platform must be set back to
zero which corresponds to 0.100 seconds prior to foot impact when looking at
the 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 any
changes in gait. One trial must be recorded from each individual with and
without the backpack. Using bioware software, the GRF data recorded is saved
which includes the subjects name and load carriage condition. The data was then
normalised by total body weight for each condition (subjects body mass+ 20 kg
backpack).

To
make sure that the participants were familiar with the correct walking speed
and the conditions needed for a successful trial, an unlimited number of
practise walks were allowed. These conditions measured were impact peak, thrust
maximum, braking force, vertical and mediolateral impulse and stance time.

 

Results

Paired
samples 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

 

 

To
see if there were any significant differences between backpack loads, Paired
samples T test was carried out in IBM spss version 24. The tests showed that
there were significant differences (P<0.05) between the measured thrust maximum variable(FZ3) and in impact peak (FZ1) shown by the table above. The VGRF data represented on the graph have two noticeable peaks. The first peak represents the time period instantly after the heel makes contact with the force plate. The centre of gravity is going towards the ground, which in turn increases the reaction force from the ground towards the vertical direction. The second peak is when the front of the foot is pushing off the force plate, the drop in between these phases happens when the centre of gravity is rising away from the ground.                                     Discussion Knudson, D. (2007) demonstrated that increased load in 8kg increments resulted in a proportional increase in vertical and anterior posterior GRF parameters. Changes to the vertical and horizontal position can be attributed to shifts in the body's centre of mass. The change in COM is also linked to restrictions of the natural arm swing patterns (Birrell, Hooper and Haslam, 2007). When someone is carrying a backpack for the centre of gravity (CG) to stay in its stability limits as a person moves forward, gait pattern must adapt, momentum must increase when load is carried behind the CG in order to bring the CG over the supporting foot. Shifting the load in front of the cg allows the load to increase momentum. If speed was to increase, GRF also increases, this is because overall momentum of the body increases (Hsiang and Chang, 2002). In order to compensate for the added weight stance time becomes shorter, if there is a shorter stance time then there is less time to produce an impulse, to counteract this the peak forces should be higher, which our results did not support (Tongen and Wunderlich, 2018). Our results show that there was a significant difference in the impact peak variables, some evidence goes against this as Tilbury-Davis and Hooper, (1999) indicated that impact parameters between 0 and 20kg are reduced but a load greater than 40kg show a significant difference. This is evidence that there is a threshold which when exceeded activates a compensating mechanism. However, the threshold that activates this mechanism lies between 20 and 40kg. Up to 64% of body weight has little to no effect on sagittal plane motion, it is an important finding that increased load carriage in a backpack increases ground reaction forces proportionally to total mass. A 20kg load causes an activation of the forces needed for balance in all subjects. After the threshold is activated higher loads do not cause any further increases in impact forces (Tilbury-Davis and Hooper, 1999).   In Birrell and Haslams (2008) study the load was in front of the body, the data showed a common trend for a decrease in thrust maximum. Active momentum is thought to be the key factor in decreasing thrust maximum, this momentum is being produced earlier in the gait cycle. The vertical impulse did not change which is surprising as usually when there is a significant increase in impact peak then there is usually a decrease in force minimum, which we did not observe in our results. 

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