The Effect of Added Weight on Ground Reaction Force and Joint Angles of the Knee during Drop Jump Landing
Introduction
For a good successful drop landing, athletes will require, good muscle strength, stability and further capabilities of the major joints, these are all vital factors which will impact the defense against injury to the joint facets15, 16). Furthermore, jumping and landing, which happens at multiple different times and scenarios in sports/activity, can be soft or rigid, this is dependent on the loss of biomechanical energy 17, 18), this ultimately can be a factor which causes injury along with instability19, 20). Force of impact of ground reaction force (GRF) is at a higher level during rigid landings as opposed to soft landings. From this we can potentially see causes of decreases in major joints and muscles of the lower extremity 21). Numerous injuries (e.g. ACL Tears and ankle sprains) are seen to be attributed to the task of landing from a jump (Beynnon et al., 2005; Ferretti, 1986; Ferretti et al., 1990, 1992; Frank & Jackson, 1997; Griffin et al., 2000; Miyasaka et al., 1991).

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Previously, researchers have observed lower extremity kinematics during drop landing, for example, degrees of flexion in the knee, hip and ankle (Blackburn et al., 2009; Cortes et al., 2007). Investigations have also been made into kinetic variables during landing such as peak vertical ground reaction force (Ricard et al., 1994). Frobell et al (2008) stated that amount of ground reaction force creates a high risk on the lower extremity during landing from jumps. It has been suggested by multiple researchers that from a larger height there will be a higher peak in ground reaction force and flexion.
Jump landings require the body to exploit several movement patterns to absorb the body’s energy when conducting a landing. There are 2 popular strategies that are normally used when landing from a jump. These are, Toe landing first and Heel landing first. Athletes will normally have their own landing techniques which are often unique to them, these techniques will normally be based on their own preference and/or activity/sporting demands. Any of these strategies will have a particular kinematic pattern which can affect the lower extremity angular kinematic intersegmental values. Different techniques can make you prone to different injuries, Butler et al. (2003) reported that knee stiffness can be closely related to landing on the toes first. Instability of the knee joints leads to restricted movement within the range of motion (ROM), which can then further result in a ongoing cycle of repeated instability in joints3, 4).
The purpose of this study was to assess the effect of an added weight on drop jump landing, while looking at ground reaction force and kinematic differences. It is hypothesized that when weight is added there will be a significant difference in the peaks of ground reaction force, flexion and abduction.
Methods
Participants
This study was conducted with 6 participant’s whom study Sport and Exercise Science at Aberystwyth University. This was made up by 3 Males and 3 Females with a mean age of 21 ± 2. None of the participant’s had previous experience with drop jumps other than what they would have gained from general exercise/activity. Participants were all free of injury and had no problems which may affect performance. All participant’s gave consent before testing and the study was ethically approved by the Aberystwyth University Ethics Committee.
Protocol
Before the study was started participants needed to have reflective markers placed on their body. Markers were placed on the Thigh, Shank, Foot, Ankle, Heel, Medial Epicondyle of Ankle and Knee, Knee, Sacral Back, ASIS left and Right, Iliac Spine. The markers were placed using the Helen Hayes marker set. Once the participant’s had the markers placed onto them they were to complete 5 drop jumps from a 50cm high chair with both feet landing on a force plate (Force plate (9287BA, Kistler Instrumente AG Winterthur, Poland) with no weight. Then after that they were to pick a suitable/comfortable weight and complete 5 more drop jumps. While conducting the trials with the weight, participant’s were to hold the weight across their chest while stepping off. The best trial of each participant was taken to be analysed and processed and only data from the right side of the body was analysed. The study was randomly counterbalanced. Data of the joint movements were captured by an 8 Camera 3D motion analysis system (Eagle Digital Real Time Camera System, Motion Analysis, Santa Rosa, USA), this system was calibrated before testing was started.
Statistical Analysis
Data from the trials were collected into Cortex at a sampling rate of 1000Hz. The data in cortex was analysed and processed. Ensuring there were no gaps in the data and that all data was filtered and smoothed. Gap Filling and Marker ID was carried out in Cortex. The data was filtered using the Butterworth filter a 15Hz. Full flexion is 180 full extension is 0. Once data was completely processed in Cortex it was then exported into Excel (Microsoft, Redmond WA) where the maximum values of Ground Reaction Force and Peak Flexion and Peak Abduction were calculated. This data was then exported into SPSS (IBM, Sommers, NY) where a Paired Samples T-Test was performed.
Results
Table 1 displays the means and standard deviation of Vertical Ground Reaction Force (VGRF) of both trials. The results found that there was no significant difference between the trial with added weight (M=2648.33, SD=281.14) and the trial with no weight (M=2493.14, SD=372.40), t(5) = 0.71, p = 0.507. These results indicate that there was no effect on VGRF with the added weight to the participant.
Table 2 displays the means and standard deviation of Peak Flexion of the Knee Joint of both trials. The results found no significant difference between the trial with added weight (M=89.76, SD=14.72) and the trial with no weight (M=81.86, SD=16.98); t(5) = 2.17, p = 0.082. These results show that there was no effect on Peak Flexion at the Knee when weight was added to the participant.
Table 3 displays the means and standard deviation of the Peak Adduction of the Knee Joint for both trials. Among these results there was no significant difference found once again between the trial with added weight (M=10.36, SD=4.08) and the trial with no weight (M=9.59, SD=4.62); t(5) = 0.78, p = 0.471. These results imply that the knee joint showed no greater peak adduction with weight against no weight.
Table 1 – Means and Standard Deviation for the Peak VGRF in both trials.

Trial

Mean

Standard Deviation

Weight

2648.33

281.14

No Weight

2493.14

372.40

Table 2 – Means and Standard Deviation for the Peak Values of Flexion in the Knee Joint for both trials.

Trial

Mean

Standard Deviation

Weight

89.76

14.72

No Weight

81.86

16.98

Table 3 – Mean and Standard Deviation for the Peak Abduction at the Knee Joint for both trials.

Trial

Mean

Standard Deviation

Weight

10.36

4.08

No Weight

9.59

4.62

Discussion
The results found that there was no significant difference between the two separate trials in any of Vertical ground reaction force, peak flexion or peak abduction. This shows there was no significant difference when weight was added during drop jump landing. There are very limited studies which look at the effect of added weight on drop jump landing, so it is hard to compare the results of this study directly to findings. The technique of drop jump landings is seen to be the biggest factor among papers when looking at ground reaction forces, peak flexion and peak abduction.

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High impact loads could be reduced if athletes are taught and trained suitably in their technique when landing, this can help minimise risks of injuries arising from repetitive high vertical impact forces on the lower extremity.20 this has been seen to be especially important in gymnastics. It has been implied that individuals can drop GRF values by performing landings which use toe-heel contact patterns20,25 or also controlled landings deprived of heel contact by increasing joint flexion,18 . It has been found in research that greater ground reaction forces are found more often within rigid or upright landing postures.11,16
Research on various biomechanical factors that could be connected with a heightened risk of lower extremity injury (e.g. ACL tears and damage) can be found in previous literature (Cowling & Steele, 2001; Fagenbaum & Darling, 2003; Hewett et al., 2005). As mentioned previously Incorrect landing technique has been closely related to ACL injury during vigorous activities (Cowling & Steele, 2001; Hewett et al., 2005). It has been suggested in studies that the risk of collapse on the lower extremity can be reduced if the most appropriate and correct landing technique is used to help minimise the forces created (Hewett et al., 2005). 70% of ACL injuries occur with non-contact situations (Griffin, 2000), these situations are normally in turning motions or landing. Therefore, it is important to assess the techniques of landing so injury risk is reduced.
There were several potential limitations to this study which may have had an effect on some results. These were things such as, participants wearing loose clothing, the participants picking different weights, only picking the ‘best trial’ of each participant. If participants wore loose clothing to the test day, it could affect the marker placement and create untrue marker position results for joints. Ensuring participants wear tight fitted clothing will ensure that all markers placed stay true to their original placement and mirror the joints as precisely as possible. When using the weight, participants were to select a comfortable weight for themselves, but also for it to be heavy enough (weight was picked using the RPE Scale, around 14 on the scale). If all participants used the same weight, then results could potentially differ. The last limitation found was that only the ‘best trial’ was picked for each participant. Although this was decided for this study, when evaluating the results, it could have been better to average the results from all 5 trials and get a mean of all 5 for each participant.
In conclusion to this study, there was no significant difference between the two trials and the factors analysed. Future research could look at certain populations (e.g. Gender or Sporting level) and analyse specific landing techniques. Looking at the different techniques and the effects of these techniques on VGRF could show interesting results.
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