[This is a guest blog from Jenna Burnett. Jenna has a Bachelors degree in physics and mathematics from Purdue University and is working on her Masters in kinesiology at Iowa State University. She is currently working as a Sports Science Research Intern at Athletic Lab.]
[Note from Mike Young: This post is the final segment in a 3 part series on the Exxentric kBox3. The kBox3 is a critical training tool for us at Athletic Lab and, as with any new sport technology we integrate, we test the equipment to not only ensure the data it produces is accurate, valid and reliable but also to determine best practices for its subsequent use. We regularly conduct in-house testing both for the companies and for ourselves and have published some of this data on this and other blogs. In this series, Jenna looked at various loading protocols for the kBox on the most commonly used exercise: the Kbox Squat. If you are interested in learning more about the kMeter there is an excellent writeup on SimpliFaster]
This installment will conclude the three-part series looking at power generating capabilities in different loading situations and in two different harness systems for the Exxentric kBox3. The first part examined the kBox3 and included an introduction to the study. The second part talked through the methods and included the results. This final installment will discuss what the results mean and conclude the series.
Concentric v Eccentric
Concentric and eccentric peak powers are innately a distinct quality within an individual. Most athletes are trained traditionally with concentric overloading and use eccentric loading more in a controlled fashion against gravity. Very rarely, in a traditional training situation, do individuals receive eccentric overloading, thereby naturally pre-disposing them toward having higher power capabilities in concentric motions over eccentric motions. However, when put in an eccentric overloading situation, some individuals can adjust their power capabilities and increase their peak power in eccentric movements due to the increased loading and resistance. The difference between the two peak powers for every protocol was computed and then plotted in Figure 5 and Figure 6 for both harness systems. Figure 5 displays the vest harness differences, while Figure 6 displays the hip belt differences.
Vest Harness Concentric and Eccentric Differences
To determine whether the participants could generate more power in the concentric or eccentric direction, the difference between the concentric and eccentric peak power was computed and then plotted in Figure 5 for each protocol. As stated in the results, the vest harness displayed approximately even distributions within each protocol toward positive and negative differentials. The even or approximately even distributions in all the protocols implies that the difference is a result of the individual’s biomechanics as well as the protocols potentially. However, it is worth noting that there was only one individual who was completely negative for every protocol, and three individuals who were consistently positive for every protocol. This lack of consistency in sign in most individuals implies that there may have been a slight influence by the protocol on the power in one direction over the other. In terms of what protocol was the most common to switch sign on, the lag and pinch protocol both had three participants where it was the odd protocol out, implying that individuals would experience either higher concentric or eccentric on only one protocol, perhaps the one that they were biomechanically challenged the most by.
Hip Belt Concentric and Eccentric Differences
Similar difference computations and plotting was done for the hip belt; Figure 6 displays the results. While the concentric peak power differentials in the vest harness were approximately evenly distributed, the peak power differentials in the hip belt displayed a slightly off center distribution. This slight weighting toward the eccentric direction may have been the result of the hip belt allowing more natural biomechanics in the squat, helping the individuals control the descent a little bit better than in the vest harness. However, further research using kinematic data would need to be done to determine whether this is true. Despite the slight weighting toward eccentric, the distribution indicates that the difference between the concentric and eccentric powers was more the result of the individual’s biomechanics rather than a strong trend created by the protocols. Similarly to the vest differentials, five participants were consistently positive or negative for all the protocols, with two all positive and three all negative. The other seven participants had one protocol that was the odd one out, with three participants changing signs in the lag protocol, and two participants each changing in the familiarization and pinch protocols. This may display which protocol challenged the participants the most eccentrically or concentrically, depending on the direction in which the peak powers fell or rose.
Hip Belt vs Vest Harness
The two different belts offered with the kBox3 create extremely different biomechanics in the squat. The vest harness goes over the shoulders and buckles across the chest and is worn almost like a backpack, allowing individuals to use their entire upper body to drive the concentric motion of the squat. They can generate power from pulling their chest and shoulders up, as well as from driving up with their legs and from popping their hips. The hip belt, on the other hand, attaches around the hip like a belt. This means that the power would really only come from popping their hips and driving up with their lower body. As a result, we expected the hip belt to have decreased peak powers in both the concentric and eccentric directions.
Concentric Peak Power Differentials
To determine the difference between the absolute peak powers for hip belt and the vest harness, the two values for each protocol were subtracted and then plotted in Figure 7. From looking at the plots, it is obvious that most participants decreased in power capabilities when using the hip belt for the concentric motion. This is seen by the consistently positive power difference values, as the difference was computed by subtracting the hip power from the vest power. It is interesting to note that one participant had negative concentric peak power differentials for all three protocols, but all the other negative values were from a mix of participants. The distribution toward higher peak power in the vest harness may be the result of the vest harness measuring more drive and power from the raising of the shoulders and upper body, while the hip belt could only measure the power created from the movement of the hips and lower body. Four participants displayed the largest power differential in the familiar protocol and another four participants had the highest power differential in the pinch protocol. The last four had the highest differential in the lag protocol. This even distribution implies that the largest differential was related to the individual’s specific biomechanics in the harness system.
Eccentric Peak Power Differentials
The eccentric peak power differentials, displayed in Figure 8, showed similar results to the concentric peak power differentials, with the majority of participants showing positive differentials between the hip belt and vest harness. There was again only one participant that displayed negative peak power differentials for every protocol, while the other negative differentials were the result of a mix of seven participants. It is interesting to note that the participant that was consistently negative for both concentric and eccentric peak power differentials was the same individual. This may be the result of the individual being able to create biomechanically more accurate squat form and control in the hip belt over the vest harness. However, this is something that would need to be tested using a force plate and is not something that fell within our testing capabilities. Five participants had the largest power differential in the pinch protocol for eccentric power, while four participants had the largest power differential in the lag protocol. The other three participants had the largest differential during the familiar protocol. Similar to the concentric differentials, this implied that the relatively even distribution of largest differentials was the result of the specific individual and the harness system rather than not being familiar with the kBox3 mechanisms, or the protocol.
The largest difference between the pinch protocol and the familiarization protocol was the addition of the isometric hold. This hold allowed the participants to “preload” their muscles for the concentric portion of the first full depth squat. This initial preload may have increased the amount of force they exerted on their first drive upward, and thus preloaded their following squats. If the individuals could not control the additional force, it would eventually dissipate as they could not put as much upward drive in on the following squats and would resist less on the way down in the eccentric movement. This would imply that their peak power for both directions would decrease. However, if they could fully control the additional force, this meant that their peak power should increase from their familiarization or baseline peak power. The lag protocol, on the other hand, was expected to do almost the opposite. At the top of the squat, the participants had to wait for the additional cord to unwind and then rewind around the flywheel. The waiting and change in timing may have influenced their ability to resist the downward pull during the eccentric movement of the squat, as the abrupt boomerang effect of the lagging cord would have hit the participant with the full force from the upward drive at an unexpected, slightly delayed time. The additional wait time at the top and extra cord may have allowed a dissipation effect on the flywheel. Due to friction, some of the velocity of the flywheel will be lost during the wait and this would have decreased the amount of force the participants would have experienced on the descent. However, that does not mean that the participants would control the sudden, abrupt force any better. Like the pinch protocol, if they could fully control the descent and abrupt downward pull, then their eccentric and concentric peak powers should increase above that of their familiarization peak powers.
Vest Harness Protocols Comparison
As expected, the highest peak power for most participants fell either in the pinch or the lag protocol for both concentric and eccentric movements wearing the vest harness, as seen in Figure 1 and Figure 3 respectively. This was expected due to the additional loading created from either the isometric hold or the “boomerang” effect of the extra cord at the top of the squat.
The concentric peak power was largely distributed in the lag protocol, indicating that the increased loading on the muscles, perhaps created by the extra cord and better eccentric control, could create increased power capabilities. As expected, the increased loading increased the power capabilities of the majority of the participants.
The highest eccentric peak power distribution was similar to the concentric peak powers. Most individuals experienced their highest peak power in either the pinch or lag test. This again was expected due to the increased loading from either the hold or extra cord, again indicating that increased loading led to increased power outputs.
The eccentric power capabilities are influenced by the concentric drive from the previous squat, indicating that it was important to analyze whether participants maintained their absolute peak powers within a protocol for both eccentric and concentric motions. If so, then this would imply that the high power was the result of the protocol rather than biomechanics. Seven participants maintained their highest peak power protocol for both the concentric and eccentric movements, while two participants switched from the pinch or lag protocol to the other, and three participants went from the pinch protocol to the familiarization protocol or vice versa. This maintenance in over half the participants indicates that the absolute peak powers were influenced by the protocol rather than biomechanics or experience. It is also worth noting that the pinch and lag protocols were the third and fourth sets respectively, with experience perhaps contributing to the highest peak powers being created in those sets. The learning curve may still have influenced the second familiarization set, leading to decreased peak powers in that set and artificially setting the participants up to achieve their highest peak powers in the last two sets. Full research with more experienced participants would need to be done to determine if experience influenced the increased peak power in the third and fourth set.
Hip Belt Protocols Comparison
While the distribution of highest peak power was almost completely in the pinch or lag protocols for the vest harness, the hip belt had a wider distribution of highest concentric and eccentric peak powers, as seen in Figure 2 and Figure 4 respectively. Similarly to the vest harness, over half the participants experienced their highest peak power in the lag protocol. However, what was surprising were the number of participants that created the highest peak power in the familiarization protocol. This was unexpected, as both the pinch and lag protocol had additional loading which leant themselves toward creating more force and power in the concentric motion. Perhaps the individuals did not control the velocity of the squat as effectively wearing the hip belt, as less mass could directly influence the drive and resistance of inertia, leading to the perceived increase in power in the baseline protocol.
Different results, shown in Figure 4, were seen in the eccentric motion of the squat using the hip belt. The familiarization protocol contained the majority of the participants in terms of peak power. This may have been the result of not having the additional loading on the body in that protocol and being able to control the force and velocity of the squat to a greater extent, creating increased peak powers. The lowered mass may also have influenced the pinch and lag protocol as well, leading to decreased power capabilities due to not controlling the increased load as effectively.
In the hip belt protocols, only four participants maintained their highest peak power in the same protocol for the concentric and eccentric motions of the squat. The other eight participants were a mix of protocols, with one participant created their highest peak power in the familiarization protocol concentrically and the lag protocol eccentrically, three participants had the opposite situation, two participants went from a high concentric peak power in the lag to a high eccentric peak power in the pinch protocol, and one participant did the opposite. The final participant created their peak concentric power in the familiarization protocol and their peak eccentric power in the pinch protocol. The lack of consistency may speak more to the use of the hip belt than to the actual protocol. As mentioned previously, the mass which can influence the power capabilities in the hip belt are decreased, leading to the potential for decreased ability to control eccentric loading as effectively and leading to random peak power eccentrically and peak powers in the individual’s biomechanically strongest protocol. A second factor that may have influenced which set created the peak power is that the familiarization set was first, when the participants were the least fatigued. As a result of creating an extremely high peak power during the first set, the other two sets may have been decreased due to fatigue, despite allowing them several minutes of rest between sets. Experience may also have played a role, as this was the second time the participants went through the protocols. They also had at least four previous squat sets on the kBox3 completed, and this may have influenced their power capabilities as a result.
As a result of this data, not only were we able to look at power capabilities in different loading situations with the kBox3, but also could compare the concentric and eccentric peak power capabilities and the influence of two different harness systems. For the peak powers in the vest belt, the consistently high concentric power in the lag protocol indicates that it was the most influential in creating peak power. However, the eccentric peak powers displayed an even distribution between the pinch and lag protocol for the majority of the participants, indicating that the eccentric power may be influenced by individual biomechanics as well as the protocol. Because these two protocols displayed the absolute peak powers, it implies that increased loading can lead to increased power capabilities for most individuals, perhaps leading to better training adaptations sooner in the training. In the hip belt, the majority of participants created their absolute concentric peak power in the lag protocol as expected; however, the participants that had their absolute peak power in the familiarization protocol consisted of the majority in the eccentric motion. This difference in the eccentric motion indicates that individuals did not control their eccentric motion as effectively with the additional loading while wearing the hip belt, perhaps a result of the decreased influential mass while in the hip belt.
For the concentric and eccentric differences in the vest and hip harness, differentials were computed for each protocol and the distribution between positive and negative values was analyzed. The vest harness displayed almost completely even splits for each protocol, indicating that the individual’s biomechanics strongly influenced the difference between their peak powers, as well as the protocol loading. While the split was not exactly even in every protocol for the hip belt, with slightly off center weighting toward the eccentric peak power, the hip belt still indicates that the difference between the concentric and eccentric powers is specific to the individual rather than as direction result of the protocol changing the loading.
The hip belt and vest harness peak powers for each protocol were also directly compared as a differential in the concentric and eccentric motions of the squat. The concentric peak powers were generally decreased in the hip belt, perhaps a result of the decreased mass which can directly influence the creation of power. The eccentric peak powers also displayed a decreased capability in the hip belt, again indicating that the decreased mass may also have influenced the controlling of the squat in the descent.
As a result of this data, we can say that, in general, the vest harness will allow athletes to create the highest power, while individual biomechanics and additional loading influenced their peak power capabilities in the different protocols. Because of this, peak power generation is not something that can necessarily generalized in terms of loading; to get the most out of a session, you should determine what either challenges your power generation the most or allows you to generate the most power and train in that situation.
- O’Sullivan, K., McAuliffe, S., & Deburca, N. (2012). The effects of eccentric training on lower limb flexibility: a systematic review. British Journal of Sports Medicine, 46(12), 838–45. https://doi.org/10.1136/bjsports-2011-090835