Plyometrics, Performance, and Bone Health

One of the topics I am most interested in relates to plyometric training for endurance runners. An article by Ebben (2001) states that, due to the instability of the surface of cross-country running, an estimated 5-10% of energy in a three- to six-mile race comes from anaerobic sources. With that said, it is important to train for this anaerobic component, even in a primarily aerobic sport. This can be done via the use of explosive plyometric exercises.

Plyometrics and Performance

Ebben (2001) discusses the principle of specificity as it relates to training in a sport-specific manner. Force application to the ground is extremely important in cross-country running, as it directly generates power for covering the greatest horizontal distance possible with optimal efficiency. Plyometric training, especially in the single-leg modality, is highly specific to the single-leg force application that occurs in a runner’s stride. Plyometric exercises with a greater horizontal component are even more specific to running, such as multiple reactive single-leg hops moving forward.

A study by Ramirez-Campillo et al. (2014) examined the use of plyometric training in highly competitive middle- and long-distance runners for the purpose of developing explosive strength in performance. Plyometric training adapts the stretch-shortening cycle (SSC) and increases the rate of activation of a muscle’s motor units. They initiated this study because prior research lacked a sufficient number of participants, failed to evaluate the effects in elite runners, applied a very high volume of plyometric activity per study length, and/or failed to include a time trial to assess distance running performance change.

The experiment included a simultaneous application of plyometric and endurance training to test the effect on both time trial endurance performance and explosive strength adaptations (Ramirez-Campillo et al., 2014). The plyometric exercises included drop (depth) jumps from 20 centimeters and 40 centimeters to test for maximum jump height and minimum ground contact time, a countermovement jump (with arms) for slow SSC action, a 20-meter sprint test to assess horizontal explosive strength with fast SSC action, and a 2.4-kilometer endurance test on an outdoor track. Total plyometric training time was less than 60 minutes per week for the six-week study. The experimental group had an improvement in time trial performance that was three times greater than the control group; in all other explosive metrics, the experimental group improved significantly while the control group showed a reduction in performance.

I thought this study was well-done and covered a lot of the bases that were missing in previous research to-date. It’s important to see these adaptations in the elite population: Since they are already highly efficient individuals, small gains in performance are crucial and highly visible with new training strategies. In the general, untrained, or even moderately trained population, it’s very easy to manipulate variables to create positive results; this is much more difficult to achieve in elite populations.

Speed potential has been found to be 85-90% predetermined by the genetic makeup of an individual (Karalejic, Stojiljkovic, Stojanovic, Andjelkovic & Nikolic, 2014). The ability of an athlete to develop their ultimate speed potential requires the highest functionality and efficiency of the neuromuscular system. This motor development begins at a young age; often in elementary school when a child learns various games and sports that require running and jumping in some form or another. The greatest increase in development occurs in pre-adolescence, from ages 8-12 years old, and developmental ability is drastically lost after this point of growth and adaptation.

Karalejic et al. found that working on dynamic power improves the speed handling of the body. This can be achieved with the incorporation of plyometric training for athletes, since plyometric activity is known to effectively develop the elastic forces of the tendons, which directly return energy from the ground during a horizontal sprint. The general recommendations from this article were that plyometric sessions should not exceed 40 to 60 jumps for beginners, two to three times per week with 24 to 48 hours between sessions, and not in same-day combination with strength activity due to central nervous system fatigue. These exercises are also not recommended until 13 years of age, with depth jumping height between 40 and 120 centimeters and all landing on the forefoot, not allowing the heel to touch the ground.

I was in accordance with this article until I reached the recommendation section, which gave no reasoning behind the age limit, height limit, or “proper” landing. I definitely do not teach a forefoot landing on a depth jump, due to the high demand it places on the bones of the foot, shins, and knees. I find much better results from a full-footed landing, and an attenuated risk of injury too.

Plyometrics and Bone Health

Due to the high impact nature of these exercises, they inadvertently develop the eccentric strength of the muscles and aid in bone and tendon remodeling and growth, respectively (Ebben, 2001). The high-risk impact associated with plyometric training should be preempted with a strong base in strength training so the body is well-prepared for the landing forces involved. Coaches should limit the volume of these exercises to a minimum to supplement a strength training regimen, remaining mindful of the high-volume nature of a runner’s mileage throughout the season. Early prevention of bone weakening disorders such as osteoporosis has become a focus of research even as early as adolescence. Statistics show that approximately 60% of all cases of osteoporosis in adulthood link to low bone mineral content in childhood, since 50% of total bone mass develops during this time (Vlachopoulos, Barker, Williams, Knapp & Metcalf, 2015).

Vlachopoulos et al. (2015) has such a study in progress, on three groups of athletes (n=105) ages 12-14 years old, participating in the sports of football, cycling, and swimming. Football is an osteogenic (bone-developing) impact sport, whereas cycling and swimming are non-osteogenic, non-impact sports. The purpose of this study is to assess bone metabolism in each group over the course of 12 months, as well as the effect of a nine-month plyometric training program on bone health in these athletes.

The longitudinal study will track the following markers in each subject: body composition via dual energy X-ray absorptiometry (DEXA), nutrition, bone stiffness with bone ultrasonometry, pubertal maturation, physical fitness, bone turnover markers and vitamin D. Following 12 months of sport-specific training, the groups will be split into interventional (plyometric-enhanced) and non-interventional (sport-only) subgroups. In the interventional groups, a progressive plyometric regimen consisting of 10 minutes per day and 3-4 times per week will be implemented for nine months. Unfortunately, this particular study is longitudinal; therefore, the results will not be published for some time. I think the design is extremely well done and I will be looking forward to the results once they are published.

Another study, however, did measure the effects of plyometric activity on markers of bone turnover in boys and young men (Kish, Mezil, Ward, Klentrou, & Falk, 2015). Twelve boys (mean age 10.2) and 14 men (mean age 22) performed 144 jumps, and venous blood sample markers for bone resorption were compared from pre-, 5-min, 1-hr, and 24-hrs post-exercise. Bone alkaline phosphatase (ALP) and osteoprotegerin (OPG) were used to measure bone formation; N-telopeptides of type I collagen (NTx) and receptor activator of nuclear factor KB ligand (RANKL) were used to measure bone resorption.

The results of this study concluded that some age-related differences exist between the markers of bone formation and resorption at rest, but not post-exercise (Kish et al., 2015). Boys carried higher levels of ALP and NTx at rest but both boys and men experienced increases in ALP (peak at 24-hrs post-exercise) and OPG (immediately post-exercise) for bone formation. Boys demonstrated bone ALP and NTx increases greater than 20% post exercise, while men were less than 10%. These numbers were not statistically significant due to the sample size, so future research may explore this difference further.

It appears that even a single session of plyometric activity can accelerate bone formation. Share on X

It appears that even a single session of plyometric activity can accelerate bone formation, possibly more so in developing boys than fully grown men. Many conservative coaches still refuse to add this type of training to a distance runner’s program, without seeing the direct translation to force-velocity development as is obvious in a sprinter or jumper in track and field. Hopefully, this mindset will evolve as these coaches see the benefits that plyometric training can have on the speed, efficiency, and injury-prevention of a cross-country runner on unpredictable terrain.

References

• Ebben, W. P. (October 2001). “Maximum power training and plyometrics for cross-country running.” National Strength and Conditioning Association, 23(5): 47-50.
• Karalejic, S., Stojiljkovic, D., Stojanovic, J., Andjelkovic, I. & Nikolic, D. (2014). “Methodics of developing speed in young athletes.” Activities in Physical Education and Sport, 4(2): 159-161.
• Kish, K., Mezil, Y., Ward, W. E., Klentrou, P. & Falk, B. (2015). “Effects of plyometric exercise session on markers of bone turnover in boys and young men.” European Journal of Applied Physiology, 115, 2115-2124.
• Ramírez-Campillo, R., Alvarez, C., Henríquez-Olguín, C., Baez, E. B., Martínez, C., Andrade, D. C. & Izquierdo, M. (2014). “Effects of plyometric training on endurance and explosive strength performance in competitive middle- and long-distance runners.” Journal of Strength & Conditioning Research, 28(1): 97–104.