On your toes . . . straight as an arrow . . . light as a feather . . . shock absorber . . . recoil like a spring.

VGRFHow many times have we heard that we need to teach athletes and patients to land softly from a jump or when running? How many times have we taught it? Let’s look at three well-established and accepted points before answering the ultimate question . . . will teaching athletes to land quieter cause them to run slower and/or not jump as high? Specifically:

  1. a higher vertical ground reaction force (VGRF) contributes to injury in general* AND
  2. VGRF can decrease with training BUT
  3. a higher VGRF allows athletes to run faster

There is a missing step in the rehab and training processes, specifically adding activities involving higher VGRF’s


Higher Vertical Ground Reaction Force is Related to Injury Risk

It seems intuitive that decreasing the VGRF will decrease the risk of injury both when landing and when running.

  • In an award winning article, researchers from Ohio found a 20% higher VGRF during a landing task in those that went on to tear their anterior cruciate ligament (ACL) as compared to those who did not (Hewett et al., 2005).
  • Miranda et al., from Brown University published a study in 2013 that showed females with intact ACL landed and cut with a larger VGRF than males with intact ACL and subjects with reconstructed ACL’s had a significantly lower peak vertical GRF just after impact as compared with the intact ACL subjects. The Discussion section of their study was quite fascinating, including the following:
    • “. . . female subjects who have never had an ACL reconstruction appeared to perform the jump-cut maneuver with greater landing stiffness (smaller amount of knee flexion angle excursion combined with larger peak VGRF) than males with or without a history of ACL reconstruction and other females with a history of ACL reconstruction.”
    • “. . . the male and female ACL reconstructed subjects appear to perform the jump-cut maneuver with less energy than the intact ACL subjects, resulting in a lower peak VGRF even 5 yr or more after their reconstruction.”
    • “Interestingly, the time-to-peak vertical GRF appears to be smaller in intact ACL subjects as compared with ACL reconstructed subjects.”
  • This is not all about the ACL, however, Similar findings have been found with runners as well. In fact, Bryan Heiderscheit has published extensively on this, including the novel step rate manipulation to decrease GRF.
  • I do believe that performing exercises whose goal is to reduce VGRF helps improve the athlete’s ability to eccentrically control movement.

From Padua and DiStefano:

Ground reaction forces during athletic tasks may also influence the magnitude of anterior tibial shear by affecting knee flexion-extension moments that must be balanced by the quadriceps and hamstrings muscles. Yu et al (2006) demonstrated that increased posterior ground reaction force during a stop-jump task resulted in greater quadriceps muscle force and greater ACL loading. ACL loading during landing peaks at the time of maximum vertical ground reaction force (VGRF) immediately after initial contact. Research by Yu et al revealed that peak posterior ground reaction force was simultaneous with peak VGRF. More recently, Sell et al (2007) reported that posterior ground reaction force and knee flexion moment were significant predictors of anterior tibial shear during a stop-jump task. These findings suggest that knee flexion angle, knee flexion-extension moment, and ground reaction forces are all important factors that influence the magnitude of anterior tibial shear

Training Can Reduce VGRF

LESSWe know that training can reduce VGRF when landing; also from Padua and DiStefano:

There is strong evidence to indicate that VGRF can be reduced with proper instruction on jumping and landing technique and with direct supervision. Each study that demonstrated significant decreases in VGRF incorporated technique instruction and trained professional supervision and feedback (Hewett et al., 1996; Irmischer et al., 2004; Prapavessis et al., 2003). Those studies that did not incorporate technique instruction and training session supervision failed to significantly decrease VGRF (Chappell and Limpisvasti, 2008; Herman et al., 2008Lephart et al., 2005). ACL injury prevention programs should include proper technique instruction, and they should be performed under the supervision of trained professionals who provide feedback on movement quality during the exercise sessions.

Previous research demonstrated reductions in VGRF following a single session of instruction (augmented feedback), whether verbal, visual, or both (Cowling et al., 2003; McNair et al., 2000Onate et al., 2005Onate et al., 2001; Prapavessis et al., 1999). Cowling et al (2003) and Onate et al (2005) both demonstrated significant increases in knee flexion and increased hamstrings muscle activation following a single session of instruction, similar to those reporting decreased VGRF (Hewett et al., 1996; Irmischer et al., 2004; Prapavessis et al., 2003).  In fact, the increased knee flexion may have facilitated the decreased VGRF. An inverse relationship between VGRF and knee flexion angle/displacement during landing tasks has been identified (Devita et al., 1992Dufek and Bates, 1990Dufek et al., 1996).

Further, the aforementioned Heiderscheit found that variations in step rate while running decrease ground reaction forces as well.

Lastly, a study published earlier this year in the American Journal of Sports Medicine found that while VGRF can indeed be decreased when landing, performance (as measured by jump height and movement speed, among other things) significantly decreased (Dai et al., 2015). We’re missing something if this indeed is our goal.

Producing Higher VGRF Results in Faster Running and Higher Jumping

sprinting mechanicsWhat makes athletes and patients run faster and jump higher? Understanding things like arm movement do impact an athlete’s running speed, for the purposes of this post, let’s focus only on the lower body. Peter Weyand–current Director of the SMU Locomotor Performance Laboratory–published a fascinating article in 2000 during his time at Harvard. Specifically, he and his colleagues found that running speed is primarily due to applying greater force to the ground, not the speed of swinging lower body. In fact, a faster swing phase had almost no difference in helping athletes running faster. Similar findings were recently published by French researchers. Therefore, because the ground reaction force is equal to the force applied to the ground by the running athlete, for those faster running speeds, VGRF is indeed higher.

The same finding was published last year in the Journal of Applied Physiology. The authors highlight the figure at right in concluding:GRF

. . . the fastest sprinters apply substantially greater forces than non-sprinters during the early portion of the stance period. Consistent pattern asymmetry among the swiftest sprinters, and less pronounced pattern asymmetry among less-swift athletes lead us to conclude that: 1) the fastest athletes have converged on a common mechanical solution for speed, and 2) that less-swift athletes generally do not execute the pattern. On this basis, we suggest that the force-time pattern documented here for the most competitive sprinters in our sample constitutes a ground force application signature for maximizing human running speeds.

Take a look at that figure again, this time ignore the dark, Elite Sprinters curve. The difference between non-sprinters and sub-elite sprinters is significant. The sub-elite sprinters also produce significantly higher VGRF. Simply stated, the faster athletes produce higher VGRF.


So, back to this:

  • a higher VGRF contributes to injury in general* AND
  • VGRF can decrease with training BUT
  • a higher VGRF allows athletes to run faster

Given this need to produce higher VGRF to run faster and jump higher, “should we do this?”**

I’m not sure this is a question that deserves a Yes or No answer. Some thoughts in an attempt to answer the question:

  • cheetah-runningOne of the things other, faster species do is spend MORE time on the ground (Alexander, 1988) . . . could this pattern of landing lightly provide a similar benefit and make our athletes faster?
    • Probably not. Though other animals tend to have longer stance phases, they have other adaptations that allow them to develop and use this longer stance phase (e.g., different use of the spine, different joint movements–see Usherwood & Wilson, 2005).
    • Weyand et al. (2010) looked at that by comparing sprinting and single leg hopping. The latter had longer stance phases, but produced a significantly slower speed (20.6 v. 12.9 mph, respectively). Very cool article if you have a chance to read it.
  • stepping-stonesIs learning to land lightly a necessary step/milestone before learning more advanced methods of applying force to the ground?
    • Stated differently, should athletes learn to land in control and softly BEFORE learning to produce more force?
    • This seems quite likely. When children don’t reach certain physical milestones, we begin to worry that missing those points will not allow proper physical growth. I worked with a teenage soccer player a number of years ago who never learned to crawl. Her parents asked me to help her run faster when playing. Though she did get faster, she continued to move using uncoordinated movements that would never allow her to achieve the speed she and her parents so desired.
    • I would suggest, then, that learning to land quietly is a milestone that should be learned and mastered before moving to more advanced activities, like those producing high VGRF’s.
  • Different advice for elite athletes v. recreational athletes?
    • Perhaps this is true. But, given my experience with most recreational athletes, they tend to apply elite training paradigms and progressions to their own programs without having accomplished the aforementioned milestones.
    • Therefore, this is quite a bit more challenging than it would seem.
  • My answer:
    • Higher VGRF is, in fact, essential to running faster and jumping higher. We, as strength coaches and physical therapists, have done a poor job preparing our athletes to tolerate them. In general, most practitioners do not appropriately increase the loads for our athletes so they can in fact become more resilient to their use.
      • Cristiano RonaldoI would suggest that when athletes get injured when training, those injuries are due to poor training habits either self-selected or as prescribed by a sport or strength and conditioning coach. I have seen far too many athletes develop stress fractures, tendinopathies and other overuse injuries because they began doing more than their bodies were prepared to handle. I believe the body of an athlete can do and can tolerate amazing things, but not without adequate preparation. That begins with coaches and the proper introduction and progression of forces.
      • Lastly, I would suggest that there is a missing step in the rehab and training processes, specifically adding activities involving those higher VGRF’s (e.g., certain plyometric exercises). Whether physical therapists like it or not (I personally like it!), high forces are involved in sports. Athletes need to know how to tolerate and adapt when they happen. Sure, teach athletes to land quietly, but don’t end there. They need to know how to produce and adapt to movements that result in high VGRF to successfully return to their activities. If not, we are doing those athletes a great disservice and not allowing them to return with the least likelihood of re-injury possible.

Ultimately, a lot remains unknown, a lot that researchers like Terry Grindstaff, Greg Myer, Bryan HeiderscheitNick Stergiou, and others should look at to further help our athletes return to the activities they love.


References

Alexander RM. Why mammals gallop. Am Zool 28: 237245, 1988.

Chappell JD, Limpisvasti O. Effect of a neuromuscular training program on the kinetics and kinematics of jumping tasks. Am J Sports Med. 2008;36(6):1081-1086

Cowling EJ, Steele JR, McNair PJ. Effect of verbal instructions on muscle activity and risk of injury to the anterior cruciate ligament during landing. Br J Sports Med. 2003;37(2):126-130.

Dai, B, Garrett, WE, Gross, MT, Padua, DA, Queen, RM, Yu, B. The Effects of 2 Landing Techniques on Knee Kinematics, Kinetics, and Performance During Stop-Jump and Side-Cutting Tasks. Am J Sports Med. 2015;43(2):466-474.

Devita P, Skelly WA. Effect of landing stiffness on joint kinetics and energetics in the lower extremity. Med Sci Sports Exerc. 1992;24(1):108-115.

Dufek JS, Bates BT. The evaluation and prediction of impact forces during landings. Med Sci Sports Exerc. 1990;22(3):370-377.

Dufek JS, Zhang S. Landing models for volleyball players: a longitudinal evaluation. J Sports Med Phys Fitness. 1996;36(1):35-42.

Herman DC, Weinhold PS, Guskiewicz KM, Garrett WE, Yu B, Padua DA. The effects of strength training on the lower extremity biomechanics of female recreational athletes during a stop-jump task. Am J Sports Med. 2008;36(4):733-740.

Hewett TE, Myer GD, Ford KR, et al. Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study. Am J Sports Med. 2005;33:492–501.

Hewett TE, Stroupe AL, Nance TA, Noyes FR. Plyometric training in female athletes. Decreased impact forces and increased hamstring torques. Am J Sports Med. 1996;24(6):765-773.

Irmischer BS, Harris C, Pfeiffer RP, DeBeliso MA, Adams KJ, Shea KG. Effects of a knee ligament injury prevention exercise program on impact forces in women. J Strength Cond Res. 2004;18(4): 703-707.

Lephart SM, Abt JP, Ferris CM, et al. Neuromuscular and biomechanical characteristic changes in high school athletes: a plyometric versus basic resistance program. Br J Sports Med. 2005;39(12):932-938.

McNair PJ, Prapavessis H, Callender K. Decreasing landing forces: effect of instruction. Br J Sports Med. 2000;34(4):293-296.

Miranda, DL, Fadale, PD, Hulstyn, MJ, Shalvoy, RM, Machan, JT, Fleming, BC. Knee biomechanics during a jump-cut maneuver: Effects of sex and ACL surgery. Med Sci Sports Med. 2013;45(5):942-51.

Onate JA, Guskiewicz KM, Marshall SW, Giuliani C, Yu B, Garrett WE. Instruction of jump-landing technique using videotape feedback: altering lower extremity motion patterns. Am J Sports Med. 2005;33(6):831-842.

Onate JA, Guskiewicz KM, Sullivan RJ. Augmented feedback reduces jump landing forces. J Orthop Sports Phys Ther. 2001;31(9):511-517.

Padua, DA, DiStefano, LJ. Sagittal plane knee biomechanics and vertical ground reaction forces are modified following ACL injury prevention programs: A systematic review. Sports Health. 2009;1(2):165-173.

Prapavessis H, McNair PJ. Effects of instruction in jumping technique and experience jumping on ground reaction forces. J Orthop Sports Phys Ther. 1999;29(6):352-356.

Prapavessis H, McNair PJ, Anderson K, Hohepa M. Decreasing landing forces in children: the effect of instructions. J Orthop Sports Phys Ther. 2003;33(4):204-207.

Sell TC, Ferris CM, Abt JP, et al. Predictors of proximal tibia anterior shear force during a vertical stop-jump. J Orthop Res. 2007;25(12):1589-1597.

Usherwood, JR, Wilson, AM. No force limit on greyhound sprint speed. Nature 438: 753-754, 2005.

Weyand, PG, Sandell, RF Prime, DNL and Bundle, MW. The biological limits to running speed are imposed from the ground up. J Appl Physiol 108: 950 –961, 2010

Yu B, Lin CF, Garrett WE. Lower extremity biomechanics during the landing of a stop-jump task. Clin Biomech (Bristol, Avon). 2006;21(3):297-305.


Notes:

*I am not sure I truly believe this, though I do agree it is a factor; from my experience, especially specific to the ACL, it is more likely unwanted–or improperly prepared for–movement in the frontal and transverse planes.

**As an aside, I am not attempting to answer the question of which is better, VGRF or horizontal forces. I think both have their place.


 

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