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What is Contact Strength and How is it Different from Finger Strength? | Contact Strength Pt. 1

Jason Hooper

Hooper’s Beta Ep. 97

INTRODUCTION

Contact strength is a largely confusing term in climbing. Unlike something like max hangs on a hangboard, which is a pretty clear-cut concept at this point, I’ve heard all kinds of differing opinions on contact strength. And that’s no good for us science nerds! We need consistency and repeatability in order to reach useful conclusions. So is there actually one, science-based explanation of contact strength or is it just another climbing slang term that no one can agree on? Today, we find out.

ANSWERS FROM THE RESEARCH

Wow, super informative, thanks Emile! Now let’s dive into the research to see if we can figure out the one true definition of contact strength. That’s right… SCIENCE B!&CH (breaking bad reference).

Alright so, Emile, can you pull up the research that mentions contact strength on screen real quick?

[Emile: Here you go!]

Cool, this first article is titled “Differences in Climbing-Specific Strength Between Boulder and Lead Rock Climbers,” and in it the authors say: “As pointed out by Watts et al. (23), the hand-hold ‘contact strength,’... is defined as the ability to quickly grasp a hold and grip to it.”

This sounds perfect! Let’s dive into their research. 

[Emile: it actually has nothing to do with contact strength and never mentions it again.]

Ooookay. No worries, let’s just pull up the research article by Watts et al. You know, where they pulled the information from.

[Emile: There ya go.]

[Only shows abstract]

That’s just the abstract. I need the full article! 

[Emile: Well it’s from 1996 and the full article is nowhere to be found online. And the abstract doesn’t mention contact strength… at all.]

So... you’re saying it’s a total dead end. 

[Emile: Yep!]

And are there any other research articles that mention contact strength?

[Emile: Nope!]

Omg *facepalm.* Okay, soooo in summary, crazy as it may seem, there isn’t a single research article that we could find that provides a solid definition of contact strength. There are a few articles that do mention it, but only as a passing reference or with a completely unhelpful description, like “hand and finger strength for rock climbing.”

Looks like we’re going to have to try a different approach. Let’s see if some of the prominent members in the climbing training community can lend us any insight.


ANSWERS FROM THE CLIMBING COMMUNITY

Steve Bechtel refers to contact strength as “...one’s ability to grasp a hold with maximum strength ‘on contact’.”

The Rock Climber's Training Manual calls it “...the amount of force you can generate during the period of initial contact with the hold.”

Eric Horst says it’s the “Initial grip strength upon touching a handhold; directly related to the rate of force development in the finger flexor muscles.”

And the Power Company says contact strength is “...the rate of force development in the finger flexors.”

Okay, it seems like there’s a general consensus that contact strength has something to do with the amount of force you can produce when you grab a hold. There were also a couple mentions of this term “rate of force production,” which they say is closely related to contact strength. So what happens if we search for more info on “rate of force production”?

Success! There are multiple research articles that mention rate of force production, or RFD. Let’s dive into RFD and try to understand what it really means in a climbing context.

RFD RESEARCH ARTICLE ONE

The first article on RFD we’ll look at is called Four Weeks of Finger Grip Training Increases the Rate of Force Development and the Maximal Force in Elite and Top World-Ranking Climbers.

In this experiment, the researchers looked at how a specific type of training would affect the RFD of their athletes’ fingers. They instructed the subjects to hold a force-measuring device “...as strongly as you can and as fast as possible.” Okay, that’s interesting! “Hold” the small hold as “strongly” and as “fast” as possible. Let’s keep those words in mind in case they can help us later.

The researchers felt this method was important, stating: “As the typical time needed during a bouldering event will always be shorter than the time needed to achieve the maximal force, increasing the RFD is, therefore, a crucial way to increase performance.” Basically, because we have such a short window of time to generate force when we grab a hold, it’s important we can generate force quickly. According to the research, they want to train their athletes to hold on as strongly and as fast as possible, because they believe with bouldering the way to improve performance is to improve RFD.

Let’s keep digging and see what else we can find. 

RFD RESEARCH ARTICLE TWO

This second article, titled “Rate of force development and maximal force: reliability and difference between non-climbers, skilled and international climbers”...

… has some really interesting info for us! They discuss the fact that “Climbers have very little time to grip strongly during dynamic movements. This ability to develop a high level of force in a short time, that is... the rate of force development (RFD).” They also state that “Explosive force is of great importance in climbing and is defined as ‘the capacity to increase contractile force from a low or resting level as quickly as possible.’ (Folland, Buckthorpe, & Hannah, 2014).” 

Again we’re seeing this concept of RFD pop up, which appears to be very similar to our rough definition of contact strength. The researchers believe RFD is a key component in climbing: that ability to generate force in your fingers in a short amount of time.

This all sounds great, but how is it different from regular old finger strength? Why are we using terms like contact strength and RFD when we appear to just be talking about generating force in the fingers?

RFD VS PLAIN OLD FINGER STRENGTH

The key to understanding RFD is to realize how short of a timeframe we’re talking about. Because the timeframe is so small, the way our bodies produce and modulate force is a bit more nuanced than you might think. 

Popping back to the first article real quick: they state that the first 200ms of developing force is due to “an increase in the motor unit discharge and contractile impulse,” while “the second part of the RFD, is linked closely to changes in the tendon-muscle coupling and to the contractile properties of the muscle.” Basically, in the first 200 ms of engagement, your strength is mostly limited by neural factors. After 200 ms, the limiting factor is the actual strength of the tendons and muscles.

To make things a bit clearer, let’s do a little experiment with a crane scale.

So according to the research article, that measurement was mostly limited by the contractile strength of our muscles. Now we’re going to try to generate as much force as we can in the shortest amount of time.

Okay, that time we got totally different values when we look at specific moments in time! That’s because, according to the article, the amount of force we can generate in the first fraction of a second has a lot more to do with neural factors than pure muscle strength.]

The rate at which you develop force is of course limited by the maximum strength of your fingers, BUT the rate is not necessarily determined by only that strength. In other words, you can’t produce 50 lbs of force in 2 seconds if your max strength is 40 lbs. On the other hand, just because your max strength is 40 lbs does not mean you’ll be able to generate all 40 lbs in 2 seconds. (Quick clarification on this example: the timeframe we chose (2 seconds) was totally arbitrary. It’s not a scientifically significant number in any way. The purpose was to illustrate the concept of RFD using simple numbers, but we realize it might be a bit confusing. In reality, it would never take you 2 seconds to exert your maximum force. Max force can generally be achieved in a fraction of a second.)

That was a lot of info, so here’s the takeaway: RFD appears to play a crucial role in climbing as we often only have a split second to generate force on holds. Our force output in that split second seems to be just as much a result of neural factors as it is muscle strength.

WHAT DOES IT ALL MEAN?

Okay, we’ve gone through the research articles and the expert opinions and we’ve collected some pretty useful information along the way. Now, let’s see if we can now answer our original question at the start of this video!

Answer? Yes, I believe so! While the term “contact strength” does not seem to have much or any scientific backing, the concept of contact strength certainly does. Through our search, we found contact strength essentially refers to the rate of force production in the fingers. Interestingly, this rate is determined not only by your absolute strength, but also by the efficiency of your neural connections, particularly in the first 200 ms of engagement with a hold. In other words, contact strength is a measure of tissue strength and neurological “speed.” Amazing!

At this point you might be thinking, “Okay, contact strength sounds pretty cool, but exactly how important is it for climbers? Like, should I be training regular strength, or focusing on contact strength to advance my climbing?”

That is an excellent question and it’s what we’ll be covering in Part 2 of this contact strength series, which will be coming out after this video. Finally, we’ll also have a Part 3 which will cover how to train contact strength. So many good videos in the works!

CONCLUSION

That’s gonna be it for this video. If you found it helpful, or at least entertaining, consider giving it a thumbs up and subscribing. If you did not find it helpful, try setting the playback speed to 75%.


Train all your friends to use the correct definition of contact strength. Climb your proj knowing that none of them care. Send them a hand-written note asking for forgiveness. Aaaaaand repeat. But this time, try citing more articles.

DISCLAIMER

As always, exercises are to be performed assuming your own risk and should not be done if you feel you are at risk for injury. See a medical professional if you have concerns before starting new exercises.

Written and Produced by Jason Hooper (PT, DPT, OCS, SCS, CAFS) and Emile Modesitt

IG: @hoopersbetaofficial

RESEARCH


Title

Upper body rate of force development and maximal strength discriminates performance levels in sport climbing. 

Citation

Stien N, Vereide VA, Saeterbakken AH, Hermans E, Shaw MP, Andersen V. Upper body rate of force development and maximal strength discriminates performance levels in sport climbing. PLoS One. 2021 Mar 26;16(3):e0249353. doi: 10.1371/journal.pone.0249353. PMID: 33770128; PMCID: PMC7997018.

Key Takeaways 

  • Moreover, in previous studies examining climbers, the strength and rate of force development (RFD) of the finger flexors has also discriminated between climbing performance levels [8] and disciplines

  • RFD is defined as the rate of the rise in force during isometric contractions, and has been used to quantify the ability to generate force rapidly

  • When climbing harder routes, the smaller holds and more difficult moves cause a need for more force to be exerted in a shorter time window to avoid falling off the route. RFD may, therefore, be a key factor for predicting climbing performance [4,5,8], and has discriminated between skilled and international performance levels when calculated using longer time periods

  • In one recent study [20], RFD was measured using a hand dynamometer, which have been shown to be less valid than specific tests (e.g., using climbing-specific holds and common climbing-positions

  • Conversely, Fanchini et al. [16] and Michailov et al. [19] used climbing-specific holds but isolated the finger flexors, excluding the arm- and back muscles from the testing. This might reduce the validity as, when climbing, the fingers are only responsible for maintaining contact with the holds whilst the vertical propulsive force of the climber is produced mainly by other prime movers (i.e., elbow flexors and shoulder extensors).

  • only two studies [18,22] have assessed the RFD of the entire pulling- apparatus (finger-, arm-, shoulder- and back-muscles) in one exercise (isometric pull-ups on a climbing-specific hold).

    • However, the authors compared climbers of different disciplines rather than performance levels.

  • absolute RFD (RFD100%; calculated from the onset of force to peak force

  • the shorter time periods (50–250 ms) could be associated with the explosive strength required for hard and dynamic climbing moves

  • Finally, it has been suggested that RFD data should be normalized (RFD relative to maximal force) to highlight whether or not differences in RFD are caused by a difference in maximal strength alone

  • The elite climbers produced higher RFD than the intermediate group at RFD100 (p 0.032) and RFD150 (p = 0.040), and higher RFD than the advanced group at RFD50 (p = 0.032) and RFD100

  • The elite group produced higher peak force output than the intermediate (ES = 1.77, p < 0.001) and advanced groups

  • In line with the primary hypothesis, the elite climbers produced higher RFD than the intermediate and advanced climbers. Conversely, no significant differences were found between the intermediate and advanced climbers.

  • Based on these findings, RFD may not be a crucial component for climbing performance before reaching the more demanding grades (> 24 IRCRA).

    • >24 = 

      • >    V9

      • =/> 7C

      • =/>  5.13c

      • =/> 8a+ 

  • The higher RFD produced by the elite climbers was accompanied by a notably higher peak force output than the other groups

  • Importantly, the RFD in the elite group was still greater than in the intermediate and advanced groups following normalization. Hence, the higher peak force alone did not cause the differences in RFD

  • However, it should be noted that the ES for the differences were reduced following normalization, suggesting that a meaningful portion of the differences in RFD is caused by the higher peak force output in the elite group.

  • Since advanced and intermediate climbers possess less climbing-specific strength of the finger flexors than the elites, performing a maximal-effort contraction using the shallow rung might limit the RFD substantially

  • One potential explanation could be that maximal strength accounts for less of the difference than neurological adaptations to years of attempting hard routes that require rapid force production [28]. In contrast to the absolute measures, the relative measures produced both lower CV values and more distinct between-groups differences, especially when examining the longer durations from the onset of force. As previously speculated [8], the maximal number of muscle fibers recruited while exerting maximal force is likely more reproducible than the time taken to recruit the fibers. As large variations between individuals’ times to reach peak force were observed in this (150 to 730 ms) and previous studies (~ 400 to 1000 ms) [8,18,22], using relative time periods should be the preferred method when examining the entire pulling-apparatus of climbers. For example, if an individual uses . 500 ms to reach peak force, the longest absolute time period (250 ms) would still represent the earlier phase of the force curve. Hence, relative time periods could be more functionally applicable than the traditional division of early and late phases [28] in tasks typically requiring longer than 250 ms to reach peak force

  • Examining the remaining relative (RFD50%—RFD100%) and absolute measures ((RFD100—RFD250), the intermediate and advanced climbers produced notably higher CV values (16.9– 30.1%) than the elite group (8.9–19.7%). These findings are in agreement with those of Levernier and Laffaye [8] who proposed that increasing skill level could be associated with an improved ability to reproduce similar force outputs across several attempts.

  • More climbing experience probably also produces a more efficient recruitment of the available motor units [34], thereby allowing for a more rapid force production across attempts.

  • Importantly, only male climbers were included in this study and the findings might not necessarily be generalizable to female climbers at the same level.

  • Furthermore, no familiarization session was performed as it was expected that experienced climbers would be able to perform the test adequately.

Title

Four Weeks of Finger Grip Training Increases the Rate of Force Development and the Maximal Force in Elite and Top World-Ranking Climbers

Citation

Levernier G, Laffaye G. Four Weeks of Finger Grip Training Increases the Rate of Force Development and the Maximal Force in Elite and Top World-Ranking Climbers. J Strength Cond Res. 2019 Sep;33(9):2471-2480. doi: 10.1519/JSC.0000000000002230. PMID: 28945641.


Key Takeaways 

  • The training program was designed in conjunction with the French national team’s coaches and was repeated 3 times a week for 4 weeks. The other 3 training sessions involved regular exercises. Finally, the protocol did not change the frequency of training for both groups

    • Each climber observed a 2-day resting period before each test to avoid the effect of fatigue caused by earlier climbing sessions.

    • Subjects stood with 1 hand on the dynamometer. The angle between the arm and the chest was 208 in the sagittal plane and the angle between the arm and the forearm was set to 908. During the test, the subjects were advised not to move. The other arm stayed still along the body. Three different holds were selected (i.e., the slope crimp, half crimp, and full F1 crimp, Figure 1). These brought into play the flexor digitorum profundus (FDP) muscle and the flexor digitorum superficialis (FDS) muscle as the main muscles

    • The climbers were in a standing position, hanging from a personalized small hold (slats ranging from 25 mm to 6 mm off the mark 1808) (rue des dolmens, 46,220 Prayssac) and HRT (Mladost 4, 1715 Sofia, Bulgaria) with 1 hand. They had to hold on as long as possible without AU8 making contact between the foot and the ground, before falling with a 1208 angle between the arm and the forearm

      • The size and the grip of the hold were chosen individually in such a way that athletes could not stay in 1 hold for more than 6 seconds.

        • To limit the risk of injury, climbers warmed up their upper limbs with suspensions that used a large degree of prehension

          • In addition, if pain occurred during the exercise, they immediately had to stop. 

    • This exercise was repeated for both hands in both conditions (slope and half crimps), with the training plane detailed in Table 1. 

    • The training session lasted about 45 minutes.

    • The instructions were to “hold the device as strongly as you can and as fast as possible”

  • When focusing RFD in regard with expertise, results show an increase of the values of ICC with skill level.

  • Concerning the averaged RFD,mean coefficients of variation decrease from 17.78% for averaged conditions in non-climbers to 12.40% in international climbers. This decreasing value of CV with skill level could be explained by the ability to reproduce a stable pattern of force over time is related to training status, including the ability to recruit motor units quickly with a high level of neural drive

  • Indeed, the ratio of strength-to-body weight has been shown to be a determining factor of climbing ability

  • To the best of our knowledge, only Amca’s study with experienced climbers compared the values between these 3 grip techniques, revealing that climbers develop more force (+11%) in the full crimp (546.2 6 40.9 N) compared with the half crimp (490.1 6 37.4 N) and the slope crimp (+21%); (435.7 6 41.6 N)

  • Moreover, a recent study (30) has shown that using the thumb during a hold produces an increase in the force of finger flexor of 12% (442 6 42.9 N without thumb vs. 494 6 68.8 N with thumb during the full crimp

  • Amca et al. (2) recorded during the full crimp a tension of 254.8 N on the A2 pulley, whereas the A4 pulley received 220.9 N, for an external force of 95.6

  • On the contrary, in the slope crimp the tension was much lower (57.4 N for the A4 and 8.1 N for the A2 for the same external force)

  • According to Schweizer (32) in the full crimp position, at 25% of maximum strength, the A2 pulley received 3 times as much force applied on the fingers.

  • These data suggest that for 505 N, which was developed by the climber of our study in the finger grip, the pressure of pulleys A2 and A4 received an overload that could damage this passive anatomic structure. Thus, it seems more reasonable to use the half crimp and the slope crimp in training rather than the full crimp to avoid injury.

  • Methodological studies have suggested that focusing only on the RFD peak is not a relevant method because it only takes into account a part of the curve, which is highly sensitive to variability and sudden changes

    • Rather, it is more accurate to investigate the evolution of force as a function of a given time

  • Fanchini and White (14) showed that boulderers are able to develop more strength at a faster rate than lead climbers.

  • Our results show a significant increase in RFD200 ms for the training group for the 3 conditions of crimps (half, slope, and full). A 32% gain for the slope crimp, a 27.5% gain for the half crimp, and a 28% for the full crimp was recorded, whereas no change was recorded for the control group, with changes of—3% for the half crimp and +6% for the full crimp.

  • According to the literature on RFD changes with training, a gain in the early part of the force-time curve is due to changes in the neural control of muscular contraction.

  • Indeed, the activation of the muscle during a rapid and explosive contraction is mainly determined by the discharge of motor units, i.e., the neural factor (1). This discharge occurs at the start of the contraction and is decisive in the first 100–200 ms (34). The time for the experimental group to reach the maximal force in our study before training was 2.62 6 0.36 seconds. As the typical time needed during a bouldering event will always be shorter than the time needed to achieve the maximal force, increasing the RFD is, therefore, a crucial way to increase performance.

  • The change of RFD200 ms is due to an increase in the motor unit discharge and the contractile impulse, as suggested by Aagaard et al. (1). A gain later in the force-time curve, i.e., in the second part of the RFD, is linked closely to changes in the tendon-muscle coupling and to the contractile properties of the muscle, which increase later in the RFD curve

  • specific training performed by climbers is primarily impacted by the neural factor and by a probable increase in the discharge of the motor units. Therefore, a 4-week training program is sufficient to increase the force and RFD for the finger flexor for both elite and top world-ranking boulderers.

  • On the other hand, the training did not have an effect on the absolute RFD95%. The literature of RFD gain with training highlights that a gain on absolute RFD of the force-time curve is a combination of changes in the neural factor in the early phase and changes in the musculo-tendinous structure.

    • The fact that there is no effect on the RFD95% tells us that a 4-week training had probably no impact on the structural factors (i.e., the muscle architecture, cross-sectional area, and type fibers II) (3), but had an important impact on the neural factor, more particularly on the increase of the discharges of the motor units, as suggested by the gain obtained during the first 200 ms

  • Our study suggests that it is not necessary to work specifically on the full crimp grip to increase the force in this position; rather, working with the half crimp or the slope crimp grip can result in an increase in finger flexor force and rate of force for all grips.



Title

Rate of force development and maximal force: reliability and difference between non-climbers, skilled and international climbers

Citation

Levernier G, Laffaye G. Rate of force development and maximal force: reliability and difference between non-climbers, skilled and international climbers. Sports Biomech. 2021 Jun;20(4):495-506. doi: 10.1080/14763141.2019.1584236. Epub 2019 Apr 30. PMID: 31038051.

Key Takeaways 

  • Fanchini et al. (2013) revealed that the peak of RFD during a task of isometric flexor of fingers is 36.73% higher in boulderers than in lead climbers

  • Another study reveals 16.70% higher performance obtained between elite and skilled climbers during an arm jump test, and 23.30% between skilled and novice climbers

  • Explosive force is of great importance in climbing and is defined as ‘the capacity to increase contractile force from a low or resting level as quickly as possible’

  • Elite climbers having 22.19% greater finger grip strength than skilled climbers who have 44.85% greater strength than novices

  • Climbers have very little time to grip strongly during these dynamic movements. This ability to develop a high level of force in a short time, that is, the rate of force development (RFD),

  • Fanchini et al. (2013) revealed that the peak of RFD during a task of isometric flexor of fingers is 36.73% higher in boulderers than in lead climbers

  • Thirty-one participants (12 international, 10 skilled and 9 non-climbers) were divided

    • (kind of weak still)

  • The aim was to analyse both RFD and maximal force (Fmax) and

  • Concerning the averaged RFD,mean coefficients of variation decrease from 17.78% for averaged conditions in non-climbers to 12.40% in international climbers. This decreasing value of CV with skill level could be explained by the ability to reproduce a stable pattern of force over time is related to training status, including the ability to recruit motor units quickly with a high level of neural drive

  • Moreover, this observation has a great impact especially when considering the functional importance of explosive force production in a wide variety of situations, such as to stabilise joints quickly to prevent falling or to maintain or adjust incorrect posture or balance.

    • Indeed, postural regulation in a short time is a key moment in climbing and involves neural processes such as sensory feedback and reflex, and is a highly adaptive strategy in preventing falls.

  • RFD100ms reveals a difference of 11.61% between international and skilled, 37.11% between skilled and non-climbers, and 44.41% between international and non-climbers. This part of the curve during the isometric task is influenced by neural drive

  • Furthermore, differences were observed in RFD200ms for all conditions: 24.51% between international and skilled, 34.04% between non-climbers and skilled, and 50.21% between international and non-climbers. Andersen & Aagaard

  • RFD between 150 and 250 ms is highly dependent on either the contractile properties of the muscle-tendon unit, such as cross-section area and neural drive, or a combination of both.

  • For RFD95%, a 45.18% difference between international and skilled, and 57.05% difference between international and non-climbers has been observed, meaning that international climbers would be able to reach 95% of Fmax more quickly than the others.

    • This two-times greater value found in international climbers compared to skilled climbers and non-climbers reveals their high level of adaptation in a wide variety of motions (fast and strong such as the ‘dyno’  movement or explosive movement, including postural or reflex adaptation and slower movements) compared to skilled climbers.

  • To conclude, RFD of the finger flexor in isometric contraction is a reliable and discriminating variable for climbing at 200 ms and at 95% of maximal force, and could be used in monitoring training.

  • Moreover, this parameter is able to discriminate skill level, as shown by the difference recorded between international and skilled, and skilled and non-climbers.

  • This implies for trainer that (i) designing workout for climbers based on holding as quick and as strong as possible is a good way to increase their finger rate of force development and consequently their climbing efficiency, especially in movement, such as the dyno or dynamic movements

  • recording the rate of force development at 200 ms is a good way to monitor a climber during training session, to assess the effect of a specific workout procedure on the way he produces the force or to compare values between different climbers.

  • The results reveal a high reliability for international climbers for the RFD, especially for the RFD200ms, suggesting that this variable could be used with a good accuracy for  intra or inter subject comparison for training purpose. Moreover, this parameter is able to discriminate skill level, as shown by the difference recorded between international and skilled, and skilled and non-climbers.


Title

Reliability and Validity of Finger Strength and Endurance Measurements in Rock Climbing

Citation

Michailov ML, Baláš J, Tanev SK, Andonov HS, Kodejška J, Brown L. Reliability and Validity of Finger Strength and Endurance Measurements in Rock Climbing. Res Q Exerc Sport. 2018 Jun;89(2):246-254. doi: 10.1080/02701367.2018.1441484. Epub 2018 Mar 26. PMID: 29578838.

Key Takeway

  • However, low-to moderate reliability was found for all parameters of rate of force development in the AF position.


Title

Comparison of climbing-specific strength and endurance between lead and boulder climbers

Citation

Stien N, Saeterbakken AH, Hermans E, Vereide VA, Olsen E, Andersen V. Comparison of climbing-specific strength and endurance between lead and boulder climbers. PLoS One. 2019 Sep 19;14(9):e0222529. doi: 10.1371/journal.pone.0222529. PMID: 31536569; PMCID: PMC6752829.

Key Takeway

  • Boulder climbers demonstrated a higher maximal and explosive strength in all strength and power measurements (26.2-52.9%, ES = 0.90-1.12, p = 0.006-0.023), whereas the finger flexor endurance test showed no significant difference between the groups (p = 0.088). Both groups were able to utilize 57-69% of peak force, average force and RFD in the ledge condition compared to the jug condition, but the relative utilization was not different between the groups (p = 0.290-0.996).

  • In conclusion, boulder climbers were stronger and more explosive compared to lead climbers, whereas no differences in finger flexor endurance were observed. Performing climbing-specific tests on a smaller hold appears to limit the force and power output equally between the two groups