Leg Spring Stiffness - Response to Plyometrics in Distance Runners?
By Steve Palladino
I have previously reported observations on Leg Spring Stiffness (LSS) relative to Running Effectiveness (surrogate for running economy) and Speed(1), LSS relative to running surface(2), LSS relative to hills(3), and LSS and static stretching in distance runners(4)
There is documentation that LSS correlates with Running Economy.(5,6) Furthermore, Spurrs et al(7) demonstrated that plyometric training over a six week experimental period in male distance runners resulted in improved 3K run time, Running Economy, and lower leg musculotendinous stiffness*. The improvements in 3K time and Running Economy occurred in the absence of improvement in either VO2max or lactate threshold. It appears that improvements in LSS impact Running Economy, which, in turn, translates to improved running pace.
*LSS may be related to physical properties of tendons (such as the Achilles tendon), ligaments and fascia structures (such as the plantar fascia), and myofascial elements. It has been proposed that on loading, during contact through midstance phase of running, these structures store elastic energy. Following midstance, through propulsion, these structures provide some elastic recoil. Hence, these structures may account for some or all of the “spring” effect captured by the metric LSS.
I have had a few recent opportunities to look into the response of LSS to plyometric training:
1) I have several male high school runners that I coach that did a plyometric program with me 3 times per week from November through February 5. On February 6, they started official practices with their school track team. The school program did not include plyometrics, so they lapsed from the regular plyometrics program. By races on March 4 and 11, it appeared that their LSS had regressed. They were advised by me at that point to resume plyometric training 3 times per week.
Example athlete - average weekly LSS from 2/6/2017 through 3/26/2017
2) I also coach a female post-collegiate runner that started with me at the beginning of the year. She had not been on plyometrics prior to working with me. I introduced plyometrics only after she had settled into the regular run training program. She has been on an introductory training load of plyometrics 2 times per week for about 3-4 weeks at this point.
The following is the “LSS” program that I advise for my runners 2-3 times per week:
- 20m x 2-4 repetitions of each
- emphasis on bounce/springiness and quickness of ground contact time
- starting with 6 repetitions on double legged plyos and 3 on single legged plyos
- weekly rep progression
Now, let’s take a look at the three male high school runners. The following are WKO4 charts of daily average LSS (green line). (Training Stress Balance, TSB - yellow line, is included to reflect any potential relationship that fatigue may have on LSS. More negative TSB = lower degree of freshness.) For each runner, the first chart is for the period of 2/6/2017 through 3/11/2017, when their LSS program lapsed, and the second chart is for the period of 3/12/2017 through 3/26/2017, when they resumed plyometrics.
Runner 1 - 2/6/2017 through 3/11/2017
Runner 1 - 3/12/2017 through 3/26/2017
Runner 2 - 2/6/2017 through 3/11/2017
Runner 2 - 3/12/2017 through 3/26/2017
Runner 3 - 2/6/2017 through 3/11/2017
Runner 3 - 3/12/2017 through 3/26/2017
All three runners showed flat to declining LSS during the 2/6/2017 to 3/11/2017 period, when their plyometrics/LSS program lapsed. After 3/12/2016, when they re-started the plyometrics/LSS program, they showed improving LSS.
Now let’s turn to the female post-collegiate runner who started plyometric about 3-4 weeks ago. Again, the following are WKO4 charts of daily average LSS (green line). The first chart is for the period of 2/1/2017 through 2/28/2017, before she started plyometrics, and the second chart is for the period of 3/5/2017 through 3/26/2017, during which she was engaged with an introductory level of plyometrics (to allow for gradual adaptation and lower injury risk).
2/1/2017 through 2/28/2017
3/5/2017 through 3/26/2017
She is now slowly improving her LSS. Her apparent slower rate of improvement (than that seen with the high school males) may be related to a number of potential variables, including, but not limited to; a) the lower volume and intensity of plyometric work that she has done versus the guys (in other words, a dose/response relationship), and/or b) there could be inter-individual adaptation response relationships in play, including the testosterone difference between her and the 14-15yo males.
Regardless, her small gains have righted the LSS ship, and appear to be translating into small gains in Running Effectiveness (a surrogate for Running Economy):
The next chart displays her weekly Running Effectiveness (RE) for the past two weeks, demonstrating improvements in 1 minute RE, 3 minute RE, 5 minute RE, 10 minute RE, and 30 minute RE. The chart demonstrates greater improvements RE at the shorter durations, but all have improved to varying degrees.
Consistent with previously published reports, these data show that LSS appears to be responsive to a LSS development program that includes plyometrics. In turn, gains in LSS may translate into improvements in Running Effectiveness (a surrogate for Running Economy). This latter observation is in alignment with the findings of Spurrs et al(7).
One must temper this possible relationship between plyometrics and LSS, and in turn, LSS and Running Economy with an appreciation that many factors, many known, perhaps some unknown, can contribute to Running Economy.(6) It is possible that the beneficial effects of improvements in LSS on Running Economy can be diluted by the myriad of other factors in play. As Barnes et al(5) wrote:
“In summary, despite some substantial correlations between some lower-body measures and running economy, it seems that no single lower-body measure can completely explain differences in running economy within and between genders. Other factors such as body lengths, mass distribution, fiber type, vertical oscillation, footstrike patterns, and other kinetic and kinematics are also likely to affect running economy. Running economy is therefore likely determined from the sum of influences from multiple lower-body attributes.”
Nevertheless, it appears that LSS is an actionable metric. Because LSS is essentially a measure of elastic recoil, it represents “speed” with no metabolic cost - no oxygen is utilized in the elastic recoil process. Building LSS is “free speed”, which translates into improved Running Economy (greater speed without greater oxygen utilization). Therefore, given that LSS is actionable (improves with plyometrics), it should be considered another training objective, on top of the traditional development of metabolic parameters.
Additional references on LSS