The Science of Running Economy

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Paula Radcliffe, the holder of the women's world record for the marathon (2:15:25) has gained much of her improvement through greater Running Economy.

Running Economy is how much energy it takes you to run. The better your economy, the faster and further you can run. Running economy is obviously determined to some extent by biomechanical efficiency. Changes in things like arm movement and the amount of "bounce" can have a direct impact on running economy. However there is some evidence that biochemical changes may have a significant impact on running economy. For instance slow twitch muscles require less oxygen for the same level of work as fast which muscles do. Running Economy can vary by as much as 30% between runners of a similar V̇O2max[1]. The two charts below show the V̇O2max and running economy of Paula Radcliffe over a 10 year period[2]. Over that time Paula Radcliffe's race performance dramatically improved even though her V̇O2max did not. This suggests that for elite athletes at least, improvements in running economy are critical.

Paula Radcliffe's V̇O2max
Paula Radcliffe's Running Economy (V̇O2 consumption at 16 Km/hr, 6:00 min/mile)
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1 Shoes

The research on how shoes effect running economy is rather confusing, with many apparently contradictory findings in some areas. Some of the problems include:

  • Many studies use a treadmill, which may have some level of cushioning built in. This has an obvious and profound effect on studies that are looking at how cushioned and un-cushioned footwear or barefoot running change running economy.
  • Few studies controlled for Cadence or Foot Strike, and barefoot has lower ground contact time[3][4][5], and a higher cadence[3][5] than shod.
  • The type of shoe, especially the raised heel may also influence outcome.

1.1 Shoe Weight

Studies have consistently shown that heavier shoes reduce running economy[6][7][8][9]. Each 100g/3.5oz added to the weight of each shoe reduces running economy by about 1%[10][9][11][12].

1.2 Shoe Cushioning

Shoe mass against running economy[13].Even on a cushioned treadmill, it appears that shoe cushioning can provide an improvement in running economy. The tests were performed at paces corresponding to various percentages of V̇O2max.

The results of studies that have compared barefoot and shod running (or running in un-cushioned and cushioned shoes) have provided conflicting information. Some studies show that running economy is worse when wearing shoes, with the bulk of the detrimental effect explained by the shoe weight. However, other studies have shown that the cushioning provided by shoes can compensate for the reduction in running economy due to shoe weight, and sometimes the shoes can actually be provide better economy than barefoot. I believe that the key to understanding this conflict lies in the type of running a surface that is used by the study. Not surprisingly, if a study uses a cushioned treadmill, the cushioning provided by the shoe does not confer any additional advantage over the barefoot condition. Of the 11 studies I found, 3 of the 4 studies using cushioned treadmills showed no improvement in running economy for shoe cushioning, while the 4 studies using un-cushioned treadmills and one using a cushioned treadmill showed an improvement for shoe cushioning. There were three studies that did not give sufficient information to determine the type of treadmill, and two showed no improvement while the remainder did. I conclude that a well cushioned running shoe can improve Running Economy by an estimated 2-3.5% compared with a weight matched un-cushioned shoe. In practice, most shoes will have a beneficial impact on Running Economy due to cushioning and a negative impact due to their weight.

  • Studies on cushioned treadmills
    • A thesis study showed that barefoot is more economic than shoed (New Balance M1500, 344-368g/shoe), but the same as shod when compensating for shoe mass[14]. The barefoot condition had a greater Cadence than shod. The treadmill (Marquette model 1900) was cushioned.
    • Habitually FFS runners are 2.4% more efficient in minimally shod (FiveFingers with) than cushioned (GEL-Cumulus 10) shoes at 9 min/mile pace and with weights to ensure equal mass[15]. This study used a Vision T9250 treadmill that is cushioned, with a reputable reviewer rating as 6.8/10 for cushioning[16].
    • A study compared barefoot (BFT), minimally shod (MS), and shod (SH) showed that BFT was 2.6-5.1% more efficient than SH[17]. The study also compared MS at the Cadence of BFT (165) and the cadence of SH (160), with an improvement only seen at the lower cadence. The participants had no prior barefoot or minimal running experience, and no adjustment was made for shoe mass. However, this study used a Woodway PPS treadmill with the manufacture claim that "Our patented Slat Belt running surface helps absorbs more of the impact than any other treadmill([18])".
    • A study comparing barefoot running with the participants using their normal running shoes showed that barefoot condition had a 4.4% improvement in running economy[13]. However, 80% of the change in running economy could be explained by shoe mass. In fact, the slope of the line between shoe mass and running economy predicts that a shoe weighing 9.2oz/260g would have the same running economy has barefoot. The shoes used weighed between 8.5-15oz (239-430g). Interestingly while running economy improved, V̇O2max did not. The treadmill used was a WoodWay Pro-XL, a highly cushioned "slatted" treadmill similar to the one noted above.
  • Studies on un-cushioned treadmills
    • Running in lightweight (150g/5.3oz, Nike Mayfly) shoes was 2% more efficient than barefoot[10]. The study used runners with a midfoot Foot Strike and an un-cushioned treadmill (Quinton 1860).
    • A thesis study showed that running in lightweight shoes (Nike Mayfly) was 3.4% more efficient than weight matched barefoot, and 2.1% more efficient than barefoot[9]. The study used runners with a midfoot Foot Strike and an un-cushioned treadmill (Quinton 1860). When the weight of the Nike Mayfly shoes was doubled using weights, the running economy was still lower than barefoot. The study also added EVA foam to the treadmill surface, but this did not change the running economy of barefoot running. However, the cushioning was added in slabs, and participants noted that the gaps felt "like running on a trail".
    • Running barefoot and in Nike 3.0 shoes had no difference in running economy on an un-cushioned treadmill (Quinton 1860)[19]. However, adding 10mm of cushioning to the treadmill improved barefoot running economy by 1.9%[19]. Using 20mm cushioning made no further improvement over 10mm.
    • A comparison of barefoot and shoes weighing 150g and 350g showed the same running economy in barefoot and 150g shoes, but 3.6% lower running economy in 350g shoes[20]. The treadmill used (ADAL 3D, HEF-Tecmachine) appears to be un-cushioned. This study also showed that compared with barefoot, shoes reduced the total energy required to run (work) without changing V̇O[20], suggesting that barefoot is using more elastic energy, further suggesting that the barefoot condition in this study uses forefoot strike[21].
  • Unknown Treadmill
    • Barefoot was 2.0% more economic than shod on a treadmill and 5.7% more economic on an indoor track[22]. However, the type of treadmill and indoor track was not described and may have been cushioned. No indication was given of the type or weight of the shoes used.
    • Barefoot was 1.3% more efficient than running in 340g shoes, while Vibram FiveFingers was 2.8% more efficient[5]. The Cadence was 91.2 barefoot, 88.3 in FiveFingers, and 86.0 in shoes. While it seems strange that the FiveFingers were more efficient than barefoot, I suspect this is likely to be a statistical anomaly due to the small sample size (8 runners). The runners were all experienced in barefoot running. It is unclear if the Zebris FDM-T treadmill is cushioned or not.
    • A study using Nike Air shoes showed an average 2.4% improvement in running economy compared with firmer shoes of a similar weight[23]. The improvement in running the economy varied by individual, between 0.5% and 6%, SD=1.8%. No details of the type of treadmill used were recorded.

1.3 Other Shoe Characteristics

There are a handful of studies that have looked at other shoe characteristics, there is insufficient information to reach any conclusions.

  • A study showed that a stiffer midsole results in improved running economy[24].
  • A study has suggested that shoe comfort affects running economy, with the most comfortable shoes having the greatest economy[25], but due to the many differences in the shoes used, the study seems too flawed to be of much use.
  • A comparison of identical shoes, one made with EVA foam and the other with Adidas' softer "Boost" foam indicated that the softer foam had a 1% better running economy[26].

2 Fatigue

A short (30 minute) high intensity run does not change economy[27], but economy deteriorates during a marathon run, possibly due to muscle damage and the need for greater neurological muscle activation to produce the force required to maintain pace[28]. However, a marathon run does not significantly change running biomechanics such as ground contact time[29].

3 Breathing

It takes energy to breathe, and this can have a significant impact on running economy.

4 Glycogen depletion & Fat Burning

Because burning fat requires more oxygen than carbohydrate, the switch to fat burning due to Glycogen Depletion means more oxygen is required. This does not directly change running economy as the energy required remains similar[28]. However, breathing rate becomes higher to supply the required oxygen, and the amount of oxygen extracted in each breath is lower, further driving the breathing rate.

5 Heat

TBD.

6 DOMS

Delayed Onset Muscle Soreness reduces running economy[30][31].

7 Flexibility

Greater Standing External Hip Rotation reduces running economy.

Greater flexibility reduces running economy. Runners with greater standing external hip rotation and dorsiflexion of the foot have poorer running economy[32]. (Dorsiflexion of the foot is bending the ankle so the toes move towards the shin, which is what happens when you do a calf stretch. Standing External Hip Rotation is shown above.) Another study showed that greater overall flexibility is associated with poorer economy, with the most flexible third of the studied group using 9% more energy than the least flexible third[33].

  • Stretching Programs. Most studies show that Stretching programs do not reduce running economy[34][35].
  • Stretching before running. One study[36] has shown that stretching directly before running reduces performance and running economy, but most studies indicate no impact[37][38], even though the stretching impairs other muscular functioning. (One study on subjects with limited hip range of motion showed that stretching before running improved running economy[39], but the subjects were only tested with 4 minutes of running, and a steady state requires 4-15 minutes[40]).

8 Body weight and fat percentage

  • Body Mass. Greater body mass is associated with better running economy[41][42]. This may be because larger individuals can store more elastic energy on each step to reduce their energy consumption[43].
  • Body Fat. Perhaps surprisingly, obese individuals have similar running economy as leaner individuals[43]. Remember that running economy is energy used divided by weight. So the heavier you are, the more energy required overall, and more body fat results in a lower power output. So being overweight reduces your fitness (aerobic capacity) but not your running economy.

9 Gender

Women may improve their running economy in response to training more effectively than men[41].

10 Muscle Fiber Types

Fast twitch Muscle fibers require more energy to provide contraction than slow twitch muscles[44][45][46], so a runner with a higher proportion of slow twitch fibers will have better running economy.

11 Running Dynamics

  • Cadence. A review of the scientific studies showed consistently that an increased Cadence reduces shock at the hip, knee, and ankle, vertical oscillation, and ground contact time[47].
  • Balance and step width. Maintaining side to side balance is estimated to cost 2% of the energy of running[48]. This may be due to step width, as increasing step width can reduce running economy by 11%[49].
The blue line is the center line of the body and the red line indicates the center of the foot placement. The distance between the two lines is the step width.
  • Arm Swing. While it obviously costs energy to swing the arms while running, this arm swing actually improves running economy[48][49], probably by improving balance.
  • Ground Contact Time. In a wide range of animals, from a 32g kangaroo rat to a 140Kg pony, the energy cost of running has been shown to be proportional to the time each foot spends in contact with the ground[50]. The energy taken to run is mostly taken with supporting the weight of the body.
    • Factors related to top running speed and economy
      • RE correlated to longer GCT
      • GCT measured at 5.8-6.6 m/s (4:37-4:04), then 3.3-5.56 m/s
      • RE measured at 3.9 m/s (6:50)
      • GCT differs FFS/RFS http://www.ncbi.nlm.nih.gov/pubmed/3891372
      • One study showed that longer GCT was correlated to better RE, but that study measured RW at 3.9 m/s (6:50 min/mile), and the correlation with longer GCT was only seen at speeds between 5.8-6.6 m/s (4:37-4:04 min/mile), rather radically faster.
    • Neuromuscular characteristics and muscle power as determinants of 5-km running performance
      • 3.67 m•s – 7:18
      • 4.17 m•s 6:26
      • CV = 4.5, 6:00
      • Furthermore, REtrack2 correlated with the mean CT of constant velocity laps during the 5K (r = 0.64, P < 0.001).
      • REtread was measured as a steady-state oxygen uptake (V•O2, U mL•kg-0.75•min-1) (38)
      • Longer GCT has greater VO2 cost (poorer RE)
    • Explosive-strength training improves 5-km running time by improving running economy and muscle power
      • Interesting – explosive training improves RE, decreases GCT, but control group has increased GCT
  • Foot strike
    • No difference in running economy, but higher work for FFS suggesting more elastic storage[21]
    • No difference between FFS/MFS and RFS except for habitual RFS doing FFS had a lower economy[51]

12 Magnesium

They is evidence that Magnesium deficiency increases the energy cost of exercise[52].

13 Framework for Factors Effecting Running Economy

A schematic of the factors effecting running economy[53].

14 Notes

Most studies use Oxygen use as a proxy for energy use, but this is not always accurate[54]. Measuring Running Economy is reasonably repeatable, with a CV of ~1-2%, though these variation are sufficient to cause issues in determining changes in economy due to different situations[55].

15 References

  1. ↑ JT. Daniels, A physiologist's view of running economy., Med Sci Sports Exerc, volume 17, issue 3, pages 332-8, Jun 1985, PMID 3894870
  2. ↑ Andrew M. Jones, The Physiology of the World Record Holder for the Women's Marathon, International journal of Sports Science and Coaching, volume 1, issue 2, 2006, pages 101–116, ISSN 1747-9541, doi 10.1260/174795406777641258
  3. ↑ 3.0 3.1 C. Divert, G. Mornieux, H. Baur, F. Mayer, A. Belli, Mechanical comparison of barefoot and shod running., Int J Sports Med, volume 26, issue 7, pages 593-8, Sep 2005, doi 10.1055/s-2004-821327, PMID 16195994
  4. ↑ EB. Lohman, KS. Balan Sackiriyas, RW. Swen, A comparison of the spatiotemporal parameters, kinematics, and biomechanics between shod, unshod, and minimally supported running as compared to walking., Phys Ther Sport, volume 12, issue 4, pages 151-63, Nov 2011, doi 10.1016/j.ptsp.2011.09.004, PMID 22085708
  5. ↑ 5.0 5.1 5.2 R. Squadrone, C. Gallozzi, Biomechanical and physiological comparison of barefoot and two shod conditions in experienced barefoot runners., J Sports Med Phys Fitness, volume 49, issue 1, pages 6-13, Mar 2009, PMID 19188889
  6. ↑ T. Lussiana, N. Fabre, K. Hébert-Losier, L. Mourot, Effect of slope and footwear on running economy and kinematics, Scandinavian Journal of Medicine & Science in Sports, volume 23, issue 4, 2013, pages e246–e253, ISSN 09057188, doi 10.1111/sms.12057
  7. ↑ LN. Burkett, WM. Kohrt, R. Buchbinder, Effects of shoes and foot orthotics on VO2 and selected frontal plane knee kinematics., Med Sci Sports Exerc, volume 17, issue 1, pages 158-63, Feb 1985, PMID 3982270
  8. ↑ S. Sobhani, S. Bredeweg, R. Dekker, B. Kluitenberg, E. van den Heuvel, J. Hijmans, K. Postema, Rocker shoe, minimalist shoe, and standard running shoe: a comparison of running economy., J Sci Med Sport, volume 17, issue 3, pages 312-6, May 2014, doi 10.1016/j.jsams.2013.04.015, PMID 23711621
  9. ↑ 9.0 9.1 9.2 The separate effects of shoe mass and cushioning on the energetic cost of barefoot vs. shod running. Wierzbinski, Corbyn. University of Colorado at Boulder. Departmental Honors Thesis. http://digitool.library.colostate.edu///exlibris/dtl/d3_1/apache_media/L2V4bGlicmlzL2R0bC9kM18xL2FwYWNoZV9tZWRpYS8xMTkyODM=.pdf
  10. ↑ 10.0 10.1 JR. Franz, CM. Wierzbinski, R. Kram, Metabolic cost of running barefoot versus shod: is lighter better?, Med Sci Sports Exerc, volume 44, issue 8, pages 1519-25, Aug 2012, doi 10.1249/MSS.0b013e3182514a88, PMID 22367745
  11. ↑ Frederick , E. C. The energy cost of load carriage on the feet during running. In: Winter, D.A., R. W. Norman, R. P. Wells, K. C. Hayes, and A. E. Patla (Editors), Biomechanics IX-B Human Kinetics Publ., Champaign, IL, pp.295-300, 1985
  12. ↑ Frederick, E. C., Physiological and ergonomics factors in running shoe design. Applied Ergonomics 15(4): 281-287, 1984
  13. ↑ 13.0 13.1 KA. Reeves, J. Corbett, MJ. Barwood, Barefoot running improves economy at high intensities and peak treadmill velocity., J Sports Med Phys Fitness, Jul 2014, PMID 24998616
  14. ↑ Flaherty R F. Running economy and kinematic differences among running with the foot shod, with the foot bare, and with the bare foot equated for weight [Thesis] . Springfield (MA): Springfi eld College 1994 ; 106
  15. ↑ DP. Perl, AI. Daoud, DE. Lieberman, Effects of footwear and strike type on running economy., Med Sci Sports Exerc, volume 44, issue 7, pages 1335-43, Jul 2012, doi 10.1249/MSS.0b013e318247989e, PMID 22217565
  16. ↑ Vision T9250 Deluxe Treadmill Review, http://www.treadmilldoctor.com/vision-t9250-deluxe-treadmill-review-09, Accessed on 19 October 2014
  17. ↑ Isabel Sarah Moore, Andrew Jones, Sharon Dixon, The pursuit of improved running performance: Can changes in cushioning and somatosensory feedback influence running economy and injury risk?, Footwear Science, volume 6, issue 1, 2014, pages 1–11, ISSN 1942-4280, doi 10.1080/19424280.2013.873487
  18. ↑ WOODWAY PPS Ultimate Medical Treadmill, http://medical.woodway.com/cardiac_rehab/cardiac_rehab_pps.html, Accessed on 19 October 2014
  19. ↑ 19.0 19.1 KD. Tung, JR. Franz, R. Kram, A test of the metabolic cost of cushioning hypothesis during unshod and shod running., Med Sci Sports Exerc, volume 46, issue 2, pages 324-9, Feb 2014, doi 10.1249/MSS.0b013e3182a63b81, PMID 24441213
  20. ↑ 20.0 20.1 C. Divert, G. Mornieux, P. Freychat, L. Baly, F. Mayer, A. Belli, Barefoot-shod running differences: shoe or mass effect?, Int J Sports Med, volume 29, issue 6, pages 512-8, Jun 2008, doi 10.1055/s-2007-989233, PMID 18027308
  21. ↑ 21.0 21.1 LP. Ardigò, C. Lafortuna, AE. Minetti, P. Mognoni, F. Saibene, Metabolic and mechanical aspects of foot landing type, forefoot and rearfoot strike, in human running., Acta Physiol Scand, volume 155, issue 1, pages 17-22, Sep 1995, doi 10.1111/j.1748-1716.1995.tb09943.x, PMID 8553873
  22. ↑ NJ. Hanson, K. Berg, P. Deka, JR. Meendering, C. Ryan, Oxygen cost of running barefoot vs. running shod., Int J Sports Med, volume 32, issue 6, pages 401-6, Jun 2011, doi 10.1055/s-0030-1265203, PMID 21472628
  23. ↑ Frederick, E.C., E.T. Howley, S.K. Powers. Lower oxygen cost while running in soft soled shoes. Research Quarterly 57: 174-177 , 1986
  24. ↑ JP. Roy, DJ. Stefanyshyn, Shoe midsole longitudinal bending stiffness and running economy, joint energy, and EMG., Med Sci Sports Exerc, volume 38, issue 3, pages 562-9, Mar 2006, doi 10.1249/01.mss.0000193562.22001.e8, PMID 16540846
  25. ↑ Geng Luo, Pro Stergiou, Jay Worobets, Benno Nigg, Darren Stefanyshyn, Improved footwear comfort reduces oxygen consumption during running, Footwear Science, volume 1, issue 1, 2009, pages 25–29, ISSN 1942-4280, doi 10.1080/19424280902993001
  26. ↑ Jay Worobets, John William Wannop, Elias Tomaras, Darren Stefanyshyn, Softer and more resilient running shoe cushioning properties enhance running economy, Footwear Science, volume 6, issue 3, 2014, pages 147–153, ISSN 1942-4280, doi 10.1080/19424280.2014.918184
  27. ↑ DW. Morgan, PE. Martin, FD. Baldini, GS. Krahenbuhl, Effects of a prolonged maximal run on running economy and running mechanics., Med Sci Sports Exerc, volume 22, issue 6, pages 834-40, Dec 1990, PMID 2287262
  28. ↑ 28.0 28.1 H. Kyröläinen, T. Pullinen, R. Candau, J. Avela, P. Huttunen, P. V. Komi, Effects of marathon running on running economy and kinematics, European Journal of Applied Physiology, volume 82, issue 4, 2000, pages 297–304, ISSN 1439-6319, doi 10.1007/s004210000219
  29. ↑ C. Nicol, P. V. Komi, P. Marconnet, Effects of marathon fatigue on running kinematics and economy, Scandinavian Journal of Medicine & Science in Sports, volume 1, issue 4, 2007, pages 195–204, ISSN 09057188, doi 10.1111/j.1600-0838.1991.tb00296.x
  30. ↑ William A. Braun, Darren J. Dutto, The effects of a single bout of downhill running and ensuing delayed onset of muscle soreness on running economy performed 48 h later, European Journal of Applied Physiology, volume 90, issue 1-2, 2003, pages 29–34, ISSN 1439-6319, doi 10.1007/s00421-003-0857-8
  31. ↑ Smith LL. Causes of delayed onset muscle soreness and the impact on athletic performance: a review. J Appl Sport Sci Res 1992; 6 (3): 135-41
  32. ↑ MW. Craib, VA. Mitchell, KB. Fields, TR. Cooper, R. Hopewell, DW. Morgan, The association between flexibility and running economy in sub-elite male distance runners., Med Sci Sports Exerc, volume 28, issue 6, pages 737-43, Jun 1996, PMID 8784761
  33. ↑ GW. Gleim, NS. Stachenfeld, JA. Nicholas, The influence of flexibility on the economy of walking and jogging., J Orthop Res, volume 8, issue 6, pages 814-23, Nov 1990, doi 10.1002/jor.1100080606, PMID 2213338
  34. ↑ AG. Nelson, J. Kokkonen, C. Eldredge, A. Cornwell, E. Glickman-Weiss, Chronic stretching and running economy., Scand J Med Sci Sports, volume 11, issue 5, pages 260-5, Oct 2001, PMID 11696209
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  37. ↑ Sarah J. Allison, David M. Bailey, Jonathan P. Folland, Prolonged static stretching does not influence running economy despite changes in neuromuscular function, Journal of Sports Sciences, volume 26, issue 14, 2008, pages 1489–1495, ISSN 0264-0414, doi 10.1080/02640410802392715
  38. ↑ PR. Hayes, A. Walker, Pre-exercise stretching does not impact upon running economy., J Strength Cond Res, volume 21, issue 4, pages 1227-32, Nov 2007, doi 10.1519/R-19545.1, PMID 18076223
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  46. ↑ EF. Coyle, LS. Sidossis, JF. Horowitz, JD. Beltz, Cycling efficiency is related to the percentage of type I muscle fibers., Med Sci Sports Exerc, volume 24, issue 7, pages 782-8, Jul 1992, PMID 1501563
  47. ↑ A. G. Schubert, J. Kempf, B. C. Heiderscheit, Influence of Stride Frequency and Length on Running Mechanics: A Systematic Review, Sports Health: A Multidisciplinary Approach, volume 6, issue 3, 2013, pages 210–217, ISSN 1941-7381, doi 10.1177/1941738113508544
  48. ↑ 48.0 48.1 C. J. Arellano, R. Kram, The energetic cost of maintaining lateral balance during human running, Journal of Applied Physiology, volume 112, issue 3, 2011, pages 427–434, ISSN 8750-7587, doi 10.1152/japplphysiol.00554.2011
  49. ↑ 49.0 49.1 CJ. Arellano, R. Kram, The effects of step width and arm swing on energetic cost and lateral balance during running., J Biomech, volume 44, issue 7, pages 1291-5, Apr 2011, doi 10.1016/j.jbiomech.2011.01.002, PMID 21316058
  50. ↑ Rodger Kram, C. Richard Taylor, Energetics of running: a new perspective, Nature, volume 346, issue 6281, 1990, pages 265–267, ISSN 0028-0836, doi 10.1038/346265a0
  51. ↑ A. H. Gruber, B. R. Umberger, B. Braun, J. Hamill, Economy and rate of carbohydrate oxidation during running with rearfoot and forefoot strike patterns, Journal of Applied Physiology, volume 115, issue 2, 2013, pages 194–201, ISSN 8750-7587, doi 10.1152/japplphysiol.01437.2012
  52. ↑ HC. Lukaski, FH. Nielsen, Dietary magnesium depletion affects metabolic responses during submaximal exercise in postmenopausal women., J Nutr, volume 132, issue 5, pages 930-5, May 2002, PMID 11983816
  53. ↑ PU. Saunders, DB. Pyne, RD. Telford, JA. Hawley, Factors affecting running economy in trained distance runners., Sports Med, volume 34, issue 7, pages 465-85, 2004, PMID 15233599
  54. ↑ J. R. Fletcher, S. P. Esau, B. R. MacIntosh, Economy of running: beyond the measurement of oxygen uptake, Journal of Applied Physiology, volume 107, issue 6, 2009, pages 1918–1922, ISSN 8750-7587, doi 10.1152/japplphysiol.00307.2009
  55. ↑ DW. Morgan, MW. Craib, GS. Krahenbuhl, K. Woodall, S. Jordan, K. Filarski, C. Burleson, T. Williams, Daily variability in running economy among well-trained male and female distance runners., Res Q Exerc Sport, volume 65, issue 1, pages 72-7, Mar 1994, doi 10.1080/02701367.1994.10762210, PMID 8184214
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