Dr. Jared Fletcher is currently an assistant professor in the Department of Health and Physical Education at Mount Royal University in Calgary, AB, Canada. There, he teaches courses in exercise physiology, biomechanics, and statistics. His current research interests lie in examining the energetic implications of muscle-tendon interaction during running and walking and physiological methods to optimize performance in athletes. Specifically, Dr. Fletcher is interested in how measures of heart rate variability and skeletal muscle fatigue can be used to best prescribe training intensity and duration in elite and recreational athletes.
Freelap USA: Potentiation is popular with coaches, but social media often tends to inflate and misconstrue what really happens physiologically. Can you identify mistakes you see in terms of coaches who assume they use PAP but do not actually perform such an event?
Dr. Jared Fletcher: Coaches and sports scientists have long thought that PAP (post activation potentiation) contributes to improved human performance. This probably began with a popular (and excellent) review by Digby Sale in 20021, highlighting what muscle physiologists had known for some time: Muscle twitch force, or low-frequency force, increases following a conditioning contraction like a maximal voluntary contraction, the mechanism of which is myosin light chain phosphorylation. Coaches and sport scientists then took this phenomenon to imply that if contractile force could be improved following these conditioning contractions, then so could sport performance. An athlete’s warm-up should, therefore, incorporate these conditioning contractions to try to elicit PAP to improve subsequent performance.
Many human studies have been designed to try to demonstrate this. For example, athletes perform two types of warm-up: one without a type of conditioning contraction and one with these contractions followed by a performance outcome. When the performance outcome is improved with the warm-up containing the conditioning contractions, the mechanism is said to be PAP.
These types of studies typically make several key assumptions or errors, which are well described in a paper by MacIntosh et al.2. [Full transparency: Brian MacIntosh was my graduate supervisor, but I was not involved in the writing of this paper.]
- PAP, at the level of the muscles, is often never measured. That is, the response to a single, electrically evoked twitch is not quantified, so there is no way to know whether muscle twitch force increases.
- Confounding factors to the conditions, such as muscle temperature, blinding of the subjects, and/or the study investigators, are often not accounted for.
- Performance is often measured at times when you would not expect PAP to be present. PAP dissipates quickly following a conditioning contraction, but performance is often measured 10 minutes or more following the contraction.
- PAP is thought to be the mechanism behind improved performance, but perhaps it is as simple as the conditioning contractions improving the warm-up, so performance is improved simply because the athlete had a better warm-up.
- Almost all sporting events involve multiple contractions—they themselves should evoke PAP (and also fatigue), but these repetitive “conditioning” contractions are never considered. For example, coaches spend every effort to potentiate the performance with conditioning contractions like maximal countermovement jumps, med ball throws, etc. However, they ignore the fact that the first contraction of an event will also potentiate the subsequent contraction, the second will potentiate the third, and so on…
This isn’t to say that these “conditioning contractions” often performed as part of an athlete’s warm-up can’t improve performance—clearly, there are many studies showing performance improves following this additional warm-up. However, it is difficult to determine specifically whether this improved performance is a result of PAP or something else.
Freelap USA: Swimming is seen as a sport from another world due to the properties of water. For coaches to take advantage of the medium, how should they treat pool training in their monitoring if they are a land-based team sport? It’s possible to get a high metabolic load with low eccentric stress, making it a tricky training session for some coaches who don’t know how to quantify the workout.
Dr. Jared Fletcher: When I first began working with swim teams and coaches as a graduate student, I was frankly (naively) amazed at the volumes these athletes could apparently handle. As a former middle-distance runner, I was amazed that a 400m freestyle swimmer could do so much more work than a 1500m runner, despite the duration of the events being similar.My message to coaches is often a simple one: Find some method you are confident in to quantify athlete load and stick with it, says @jfletcher14. Click To Tweet
Quantifying the workout/training week/month/year of any athlete is a contentious issue, both in the training literature and on social media, probably because it is so difficult to try to quantify “load” in any sport. We wonder whether intensity x duration is enough, how to define “intensity,” etc. All of these methods, from simple to extremely complex, have their pros and cons. Therefore, my message to coaches is often a simple one: Find some method you are confident in and stick with it. With regular monitoring of various physiological and performance measures, coaches and physiologists start to get a good sense of what is “too much” or “too little” (however that may be quantified).
Freelap USA: You have a lot of experience with shoes and athletes, and cycling seems to have more going on than people once believed. In the past, the focus was above the ankle, especially with quads and hip flexors; now, a lot of attention is how the foot works within the stroke. Can you share how this may matter for those involved in soccer who use bike routines to improve fitness or aerobic capacity?
Dr. Jared Fletcher: Thanks very much for that compliment, but I would consider myself very low on the footwear totem pole compared to many of my colleagues (within and outside of the footwear industry). That said, when I was a postdoc in Dr. Benno Nigg’s lab, we were tasked with quantifying the mechanical and physiological effects of different cycling shoes, and we eventually published.3
The key difference between the shoe conditions we tested was in the torsional (twisting) stiffness, such that one shoe type allowed slightly more “twisting” along the longitudinal axis of the shoe. This would be closely equivalent to allowing pedal “float” on a bike setup. The hypothesis was that allowing the ankle/shoe complex to move relative to the pedal would reduce knee moments, which would eventually reduce overuse injuries. We found some minor, subject-specific differences in knee moments, but no differences in gross efficiency, so we began to question what the real benefit of cycling shoes could be.
A follow-up study then used a third shoe condition (a lightweight running shoe) as an “extreme” torsional stiffness shoe, for which we also show no difference in gross efficiency and, again, some small subject-specific differences in knee and ankle joint moments. These results essentially confirm the results of Straw and Kram4, who also showed gross efficiency was no different between running and cycling shoes. We, and others, hypothesize that the benefits of cycling shoes may be seen over long-duration trials (perhaps several hours, as is seen in stage races) and/or during the sprint finish of stage races, but this remains to be tested.If you use cycling shoes, you probably want to ensure your shoe/pedal interface has at least some float to reduce knee and/or ankle joint loads, says @jfletcher14. Click To Tweet
So, for those athletes cycling as part of a cross-training program, my simple suggestion is that shoe-type probably doesn’t matter for most of us: we cycle at relatively low power outputs for relatively short periods of time. However, if you use cycling shoes (anecdotally, they “feel” better, I will admit), you probably want to ensure your shoe/pedal interface has at least some float to reduce knee and/or ankle joint loads.
Freelap USA: HR monitoring is useful but obviously limited. Can you share what is truly useful with heart rate? It seems some coaches have abandoned it, while others are overconfident with what they can do. Perhaps you could share some middle ground?
Dr. Jared Fletcher: The usefulness of HR is that it is a measure of exercise intensity: HR increases linearly with speed or power output. The issue with HR measurement is that HR takes some time to reach a steady state (2–3 minutes), so for intervals shorter than this, the measured HR will be lower than what the intensity would suggest. Further, above the anaerobic threshold, HR will not reach a steady state, and so the time at which HR is measured will affect the actual HR measurement. Lastly, over long-duration events, cardiovascular drift will also affect the interpretation of HR over time.
So, coaches need to account for all of these factors (and others, like caffeine use, over/under training, previous exercise, etc.) to properly interpret HR measurements during a workout session. Is it useful to monitor the intensity of an easy off-day run or swim to ensure the athlete isn’t going too hard? Probably. Is it beneficial to try to measure HR at the end of a 100m freestyle swim to determine if the athlete “gave it their all”? Probably not. We coaches and sport scientists need to manage the on and off kinetics of HR to properly interpret what the HR measurement actually implies.
Freelap USA: Isometric strength and Achilles tendon development is a hot topic. How should we look at the role and function of the Achilles and how do we prepare it for sport? Coaches are confused about how elastic energy is utilized, and a lot of return to play strategies are failing now. Is there something most coaches are missing?
Dr. Jared Fletcher: Of course, I’m biased, but what an exciting time to be conducting Achilles tendon research! Back in the early 2000s, several studies from Kubo et al.5–8 showed that isometric strength training increased Achilles tendon stiffness (how much force is required to stretch it).
One of my first graduate studies9 attempted to replicate these studies in a group of well-trained distance runners and then examine the impact of this training on changes in Achilles tendon stiffness and running economy. The idea was based on a previous study by Arampatzis et al.10, which showed that runners with good running economy (a low-energy cost to run a given distance) had a higher Achilles tendon stiffness. So, using a crossover, randomized design, we tested the hypothesis that running economy would improve following a period of isometric strength training of the ankle plantar flexors as a result of increases in Achilles tendon stiffness compared to another group of similarly trained runners who did not perform the isometric training.
While we found a relationship between how much the Achilles tendon stiffness increased/decreased and how much the energy cost decreased/increased, a larger question arose: How or why do increases in Achilles tendon stiffness result in a lower energy cost of running? Since then, we (and many, many others) have performed a range of studies, trying to examine how the muscle and tendon interact during various forms of exercise.
In short, our lower limbs consist of various muscle-tendon units and having a tendon that can stretch and recoil at various lengths and velocities allows the muscle length changes and velocities to remain low. If our lower limbs did not have tendons, this length change would have to be accomplished by the muscle itself and this lengthening/shortening costs energy and/or reduces the force or power capability of the muscle. So, tendons serve to reduce the energy cost of muscle contraction during long-duration events and may also act as power amplifiers during short-duration events (because the tendon can stretch and recoil faster than the muscle can).Tendons serve to reduce the energy cost of muscle contraction during long-duration events and may also act as power amplifiers during short-duration events, says @jfletcher14. Click To Tweet
When a tendon is stretched, it stores elastic strain energy, which can subsequently be released during shortening. This strain energy contributes to the mechanical work that the muscle would otherwise have to do. It is often thought that this strain energy contributes quite a substantial portion of the total energy required to run. We11 recently proposed the idea that this elastic strain energy does not come with zero metabolic cost, so we calculated the cost of storing/releasing this strain energy of the Achilles during running at various speeds and in different levels of runners (trained males and females, and some well-trained male runners). We proposed that the metabolic energy required to store and release energy from the tendon is quite high; even greater than the strain energy returned from the tendon.
From these data, we proposed a bit of a paradigm shift: Runners are able to run economically, not necessarily because their tendons store and return large amounts of strain energy, but because these economical runners’ tendons allow the muscles to operate at favorable lengths and velocities to keep their metabolic costs low. The idea that different runners have an “optimal tendon stiffness” has been proposed in the literature, depending on whether the tendon helps to reduce metabolic cost (like in distance running) or allow the whole muscle-tendon unit to lengthen and shorten faster to amplify power (like in sprinting).
In terms of return to play strategies, coaches and athletes should be aware: 1) of the basic physiology and mechanics of what the tendon might be doing during the activity, and 2) that the tendon is sufficiently strong to handle the stretch-shortening imposed on it during the activity. Certainly, there is some great work out of Dr. Keith Baar’s lab (and some excellent podcasts recently) showing various exercise (i.e., skipping rope) and nutritional (consuming gelatin) interventions to increase collagen synthesis and improve tendon health.12
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1. Sale, D.G. “Postactivation potentiation: role in human performance.” Exercise and Sport Sciences Reviews. 2002; 30(3): 138–143.
2. MacIntosh, B.R., Robillard, M.-E., and Tomaras, E.K. “The effect of torsional shoe sole stiffness on knee moment and gross efficiency in cycling.” Journal of Sports Sciences. 2019; 37(13): 1–7.
4. Straw, A.H. and Kram, R. “Effects of shoe type and shoe–pedal interface on the metabolic cost of bicycling.” Footwear Science. 2016; 8(1): 19–22.
5. Kubo, K., Kanehisa, H., and Fukunaga, T. “Effects of different duration isometric contractions on tendon elasticity in human quadriceps muscles.” The Journal of Physiology. 2001; 536(Pt 2): 649–655.
6. Kubo, K., Kanehisa, H., and Fukunaga, T. “Effects of resistance and stretching training programmes on the viscoelastic properties of human tendon structures in vivo.” The Journal of Physiology. 2002; 538(Pt 1): 219–226.
7. Kubo, K., Kanehisa, H., Ito, M., and Fukunaga, T. “Effects of isometric training on the elasticity of human tendon structures in vivo.” Journal of Applied Physiology (Bethesda, Md.: 1985). 2001; 91(1): 26–32.
8. Kubo, K., Kanehisa, H., Kawakami, Y., and Fukunaga, T. “Influences of repetitive muscle contractions with different modes on tendon elasticity in vivo.” Journal of Applied Physiology (Bethesda, Md.: 1985). 2001; 91(1): 277–282.
9. Fletcher, J.R., Esau, S.P., and MacIntosh, B.R. “Changes in tendon stiffness and running economy in highly trained distance runners.” European Journal of Applied Physiology. 2010; 110(5): 1037–1046.
10. Arampatzis, A., De Monte, G., Karamanidis, K., Morey-Klapsing, G., Stafilidis, S., and Bruggemann, G.P. “Influence of the muscle-tendon unit’s mechanical and morphological properties on running economy.” The Journal of Experimental Biology. 2006; 209(Pt 17): 3345–3357.
11. Fletcher, J.R. and MacIntosh, B.R. “Achilles tendon strain energy in distance running: consider the muscle energy cost.” Journal of Applied Physiology (Bethesda, Md.: 1985). 2015; 118(2): 193–199.
12. Shaw, G., Lee-Barthel, A., Ross, M.L.R., Wang, B., and Baar, K. “Vitamin C-enriched gelatin supplementation before intermittent activity augments collagen synthesis.” The American Journal of Clinical Nutrition. 2016; (C): 1–8.