Stiffness… Like many things in training, and in life for that matter, too much is probably not good, and neither is too little. For example, bodybuilders present extremely high tendon stiffness which comes at the expense of movement ability, as is also the case in individuals with the very rare genetic condition called myostatin-deficiency (i.e., myostatin-related hypertrophy). On the other hand, extremely low passive tendon stiffness and hyperlaxity have been commonly associated with decreased force production and increased joint instability. Both extremes have profound implications on physical performance and injury risk.
Tendon stiffness has become an increasingly popular topic in athletic preparation. This is due to the influence that different stiffness levels may have on explosive-strength type performances, like jumping and sprinting, as well as on movement economy in endurance running (1-3). Interestingly though, stiffness used to be perceived as an undesirable property, probably because severe joint stiffness is associated with movement restrictions, decreased ranges of motion and pain in musculoskeletal conditions such as osteoarthritis and ligament sprains. This really feeds Arbesman’s idea that some cause-effect models in practice may work better compared to others at a given time, until new models evolve and falsify them, which advances our understanding (4). Hence the importance of not marrying to any one model…
Before diving into my perspective on tendon stiffness, plyometrics and sports performance, I think it is important to establish some definitions. Stiffness is the extent to which an object resists deformation in response to an applied force. In tendons, stiffness may be desirable to explosive-strength performance because force transmission from the muscle to the bone, which results in the movement observed, is optimised. In other words, stiffer tendons may mean greater and faster mechano-elastic energy transmission, less energy loss and therefore greater power performance. The opposite concept is compliance: i.e., the more compliant an object is, the less stiff it is and the easier it deforms in response to force. A visual example is a rope versus an elastic band: if you are pulling a car you would use a rope (stiff) not an elastic band (compliant). Plyometric training is a commonly employed, jump-based training means to improve explosive performance, primarily through increasing tendon stiffness (5). Plyometric exercises are high-velocity maximum-force movements (typically 0.1 to 0.2s contact times), which tend to have an eccentric (force-absorption) phase, an amortization (force transmission) phase, and a concentric (force production/ propulsion) phase; e.g., depth jumps and drop jumps. The term plyometrics has been bastardized to mean any hopping, sub-max jumping and landing skills, however this was not the original definition – when I speak about plyometrics I’m referring to the Russians’ Shock Training Method popularized by Yuri Verskoshansky.
What I don’t like of the car example is that the idea we get is that the stiffer the better; we must remember that pulling a car is a slow uni-directional strength movement, whereas in sports we may encounter situations where tendon compliance may be desirable (e.g., directional changes) and chronic tendon stiffness is probably not good. Equally, when looking at the physical demands of sports such as basketball, soccer, AFL, netball, volleyball and the rugby codes it is hard not to ask yourself the question: “how much good will more stiffness/ plyometric training do?”. Or in the words of Mike Boyle: “Is this bucket not filled already?”. Further, what level of risk is there in continuing to push for stiffness and provoking tissues which are regularly subjected to sports that are plyometric in nature? (see quotes/ extracts below).
“… Athletes change activity between 500 and 3000 times over the course of a competition, or once every 2–4 s. Studies of soccer reported the most frequent cutting (up to 800 per game), … Basketball (42–56 per game), handball (up to 90 per game), and volleyball (up to 35 per game) were found to require the most jumping.” Extracted from publication (6): Taylor, J. B., Wright, A. A., Dischiavi, S. L., Townsend, M. A., & Marmon, A. R. (2017). Activity demands during multi-directional team sports: a systematic review. Sports Medicine, 47(12), 2533-2551.
“Isn’t sprinting plyometric?” – Quote from expert track and field and strength coach Chris Korfist at the 2018 CVASPS conference.
I had two really interesting conversations on precisely this. One was a roundtable discussion with two basketball physios (Craig Barden and Simon Hall) working in professional basketball in England (BBL) and in Australia (NBL), respectively. And the second with Coach Jake Tuura out of YSU in the US. Both are available in the podcast.
Link 1 (Pro Basketball roundtable): https://open.spotify.com/episode/6WoCVao2ocXFdpFxmvqFaA?si=SrtMMuz9TZ6KP8IkYzhOuQ
Link 2 (Conversation w/ Jake Tuura): https://open.spotify.com/episode/6I9rK5B8UiwQqujKqwaucX?si=rmacZ0T1TJKoM5RSV0wGtQ
Nevertheless, these are hard questions because of the many factors surrounding whichever approach an S&C coach decides to take. Making the right call is made even harder by the fact that tendons don’t respond to loading until 24 to 48 hours after (see Dr. Ebonie Rio’s talk on tendons and isometrics available at: https://www.youtube.com/watch?v=SQoCkqdeVyI ) – that is that if a plyometric training exercise/ session was provocative, pain won’t be evident immediately (even in individuals with tendinopathy). And also, by the fact that competitive schedules across sports tend not to provide the most desirable recovery times (e.g. NBA, NCAA).
For long it has been known that to reap the benefits of plyometric training and tendon stiffness an athlete must have a significant baseline level of strength. With reference to the car and rope example, a 1-year old baby is not going to move the car even if he has the stiffest rope in the world. I think this is why the NSCA used to promote the 1.5 times BW squat guideline before getting athletes to perform plyometric training, which although I don’t necessarily agree with it and they have clearly revoked it, it serves as evidence that this idea that a baseline level of strength is necessary has been present in the S&C practice for long. Now, 1.5 times BW squat seems a bit of a stretch for athletes in some team sports, even at the elite level, but it is difficult to justify complex plyometrics for athletes without high strength levels, something which is made even more complicated when we look at temporal changes in strength parameters over a season in sports like football and basketball. Additionally, before a substantial strength is achieved, the benefits of strength on power performance and tendon stiffness are greatly evident whereas the reverse (i.e., with plyometric training on strength) is not as clear. Given that strength has substantial implications for injury risk reduction and that plyometric training has been in past associated with some degree of injury pre-disposition (7) – again not something I entirely agree with provided that sound technical coaching is present – it appears to me a valid argument to minimize, or remove, plyometric training at least from an in-season s&c programme in sub-elite/ college team sports for the sake of injury risk minimization and appropriate tissue recovery. Furthermore, when you look at the athletes that have most benefitted directly from a heavy emphasis on plyometric training and jumps in their preparation, you tend to see very competent movers; primarily track and field athletes (8). These guys can probably afford this as they have the technical mastery to get it right, however we know this is not the case in team sports and poor plyometric exercise performance over time does not seem like a great idea. We also know that track and field athletes are not exposed to such complex movement perturbations (e.g. multi-directional jumps, lateral accelerations, contacts/ collisions) in competition as are team sport athletes. These are considerations worth having.
Going back to the idea of baseline strength prior to implementing plyometric training for tendon stiffness, we must also consider further baseline physical characteristics of the athlete prior to implementing plyometric work. This is an opinion of mine that has changed with time. I used to promote the idea that every athlete should jump in every gym session, whereas now I think the picture is more complex. An important baseline physical characteristic worth considering is fat mass. I say this not only because of the influence that carrying extra non-functional mass may have on joint loading, force generation and movement efficiency, but also because improving power, speed and even tendon stiffness for that matter, in athletes whom have more fat mass than what they should have can be a lot easier than going straight into programming complex plyometrics. I’ll expand. I had an experience with an athlete who fitted this description. Over a few months, jumps (loaded and unloaded), Olympic lifts, strength training, sprints and throws alongside their sports training really had trivial effects on their speed and power performance. In fact, complaints of knee pain were not unusual. A few months later this athlete lost a lot of weight, a lot, with an improved nutrition, continuing basic strength training and performing regular aerobic work (continuous running and cycling). In a short time, they got faster, were able to jump higher, gym lifts began to progress at a faster rate and they clearly improved their aerobic fitness; all at the same adherence level. We were able to address more problems with a simpler training strategy that was also safer for the load-bearing joints.* I think it is important to identify these physical characteristics before assuming that because plyometric training is popular in the field and has been shown to improve tendon stiffness and power performance in world-class athletes, that all athletes have to do a load of jumping outside of their sports, which can aggravate other areas. This particular athlete is probably now in a better condition, structurally and physiologically, to further their explosive qualities simply by having reduced their fat mass. Where would maintaining the previous approach have led? I don’t know, but it did not seem like it led to the desired outcome.
*Note: To a similar end, we’ve seen many of Jake Tuura’s case-studies on social media where athletes improve their jump heights and tendon/ joint health by simply micro-dosing isometric exercises into their regular routines. This is not to say athletes shouldn’t jump, of course you must jump (and maximally) to improve your jumps; but maybe sports training and competition provide sufficient amounts of this stimulus and our priorities as S&C coaches lie elsewhere.
All in all, it really is a complex picture. It comes down to acknowledging that introducing a greater emphasis (i.e., bias) on an aspect of the athlete’s preparation will introduce variance in another; and picking what these are, well I guess that is the art of coaching… Plyometric training for the sake of explosive-strength development and stiffness can be a double-edged sword (i.e., high-risk, high-reward) and therefore its implementation requires a critical evaluation – more than what is currently being carried out. There are many routes to achieving tendon stiffness, and identifying the low-hanging fruit, be it strength deficits, extra fat mass, hypermobility, amongst others, will likely lead to a lower risk avenue than implementing plyometric training as the fixing tool. Individual athletes should be evaluated and observed as a n=1 study, and rarely should this result in a coach employing the same strategy for all athletes. This is where an integrative view of the athlete as a complex system comes into play.
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References & wider reading
1. Kubo, K., Ikebukuro, T., Yata, H., Tomita, M., & Okada, M. (2011). Morphological and mechanical properties of muscle and tendon in highly trained sprinters. Journal of Applied Biomechanics, 27(4), 336-344.
2. Kuitunen, S., Komi, P. V., & Kyröläinen, H. (2002). Knee and ankle joint stiffness in sprint running. Medicine and science in sports and exercise, 34(1), 166.
3. Kubo, K., Morimoto, M., Komuro, T., Tsunoda, N., Kanehisa, H., & Fukunaga, T. (2007). Influences of tendon stiffness, joint stiffness, and electromyographic activity on jump performances using single joint. European journal of applied physiology, 99(3), 235-243. Chicago
4. Arbesman, S. (2013). The half-life of facts: Why everything we know has an expiration date. Penguin.
5. Fouré, A., Nordez, A., & Cornu, C. (2010). Plyometric training effects on Achilles tendon stiffness and dissipative properties. Journal of applied physiology, 109(3), 849-854.
6. Taylor, J. B., Wright, A. A., Dischiavi, S. L., Townsend, M. A., & Marmon, A. R. (2017). Activity demands during multi-directional team sports: a systematic review. Sports Medicine, 47(12), 2533-2551.
7. Ignjatovic ́ A, Radovanovic ́ D. Physiological basis of force and strength training. Jagodina: Pedagogical Faculty; 2013.
8. Verkhoshansky, Y., & Verkhoshansky, N. (2011). Special strength training: manual for coaches (p. 274). Roma: Verkhoshansky Sstm.