Bones are by far my favorite connective tissue. I used to think of them as a very static tissue - one that underwent lots of growth during youth/ adolescence, but didn’t change much after reaching skeletal maturity. Since becoming a Physical Therapist, my view of bones has changed completely. They are an incredibly dynamic tissue that undergoes constant remodeling throughout our lives and are greatly impacted by lifestyle choices- including exercise and nutrition. Although the majority of our bone density is accrued during adolescence,1 the choices we make as adults can have large influences upon both bone strength and injury risk. We rely heavily on our skeletal system for structural support, movement, and mineral homeostasis. Optimizing bone health is one of the most important things we can do as athletes from both a performance and general health perspective- so I figured it would be a great opportunity to explain the best ways to do this from an exercise/ training perspective (letting Sky tackle the nutrition side of this!).
Bones constantly undergo a process known as remodeling, which helps create new bone and strengthen our skeletons. There are two types of bone cells involved: osteoclasts (bone removing cells), and osteoblasts (bone building cells). When we stress our bones with movement (i.e. throwing, running, jumping), there is microdamage that occurs, which is a normal part of the bone lifecycle. Osteoclasts remove these damaged bone cells, which serves as a stimulus for osteoblasts to come in and replace these areas with new bone tissue.2 This process of remodeling follows Wolff’s law, or the phenomenon that bones will actually adapt and become stronger due to the loads placed upon them.3 So in an ideal (healthy) situation, the more we stress our bones, the stronger they get. When there are imbalances in the bone removal and building process, however, issues start to occur.
The repetitive workload caused by activities like running creates normal microdamage, as previously discussed. When the stress from training outweighs the bone’s ability to resist/repair, however, the bone becomes more susceptible to fatigue due to mechanical changes in the bone structure. This is typically the result of an imbalance between training load and energy availability, which is the amount of energy our body requires to sustain our normal physiology (i.e. bone growth) minus the metabolic cost of exercise. This can result in bone stress injuries (BSI), which frequently occur in conjunction with increased training loads. This is particularly true of an increase in intensity (i.e. running velocity), but also occurs in the case of increased mileage/duration.4 These injuries typically occur about 4-6 weeks after training has increased, as there is a lag between osteoclast and osteoblast activity- meaning that the warning signs of pain will lag behind tissue damage.5 It also means that you probably didn’t injure yourself because of “that one long run last weekend”, but because of cumulative load over the course of a few weeks.
Rehabilitation depends upon the location of the BSI, as certain bones receive less blood flow than others. Lower-risk bones, including the 2nd metatarsal shaft and tibia, can take 3+ months to heal. High-risk bones such as the base of the 5th metatarsal and femoral neck have lower healing rates due to poor blood supply, and typically require periods of non-weight bearing during rehabilitation- with even greater healing times (up to 12 months).5 Our bones are composed of two different types of bones: trabecular (spongy bone) and cortical (compact bone); our skeleton is 80% cortical bone and 20% trabecular bone. Cortical bone is
stronger than trabecular; however, trabecular is more metabolically active.2 Because of this, bones comprised of relatively higher percent of trabecular bone than cortical tend to be more susceptible to changes in energy availability, meaning that they respond quicker to an imbalance between training load and caloric intake. Thus, they tend to be more susceptible to BSIs caused from low energy availability, or RED-S (relative energy deficiency in sport).6 These bones include the pelvis, sacrum, and femoral neck, and because of their high turnover rate injuries can occur much quicker than I realized (think within weeks to months of a change in training/intake).
Because of the intricate balance between load, recovery, and energy availability (see Sky’s blog!), BSIs are some of the hardest injuries to manage and often require input from a physician, physical therapist, and registered dietician. There is a high risk of recurrence, and without proper management, patients can often “yo-yo” back and forth without fully recovering. Needless to say, these can be some of the most frustrating injuries to deal with, and a summer race schedule can be completely derailed because of a BSI.
This highlights the importance of progressive loading when creating a training plan; it is often so easy to gain momentum and progress too quickly- it's a mistake I’ve watched teammates make, and have made countless times myself. However, volume of running alone is not the only factor impacting bone health and susceptibility to injury. The type of loading is incredibly important- it plays a huge role in determining our bone density, which is a direct indication of bone strength. This is where I will hit you with the most important piece of information (seriously, if you take one thing from this article its this): Running is NOT a bone building activity. 4
Our bone cells actually desensitize to repetitive, unidirectional loads; they respond to about the first 20 loading cycles, after which they essentially “turn off” and stop adapting to continued load.4 This means that your bones are really only responding to the first few minutes of your long run. Obviously, there are other great benefits to running for more than a few minutes, but increasing your bone density is NOT one of them.
Rather, bones are stimulated by quick movements and heavy, weight-bearing loads. They thrive on multi-directional activity and jumping/sprinting in order to stay stimulated and metabolically active.4 This highlights the importance of participating in sports like soccer and gymnastics during adolescence, a time when we accrue the majority of our bone mineral density.1
As a PT, learning this information has been crucial for me in terms of appropriately programming training plans for running. While I always knew that activities like cycling and swimming weren’t great for building bone mass, the concept of a weight-bearing activity like running offering little to no benefit to our bones has been pretty revolutionary. I now incorporate explosive plyometrics and heavy load, low repetition exercises into strength plans for runners. This dosage is key, as it provides maximal stimulus for bone building.
When a muscle contracts, it puts tension on the tendon, pulling on and transmitting force in order to move the bone to which it attaches. The muscle’s internal load actually exceeds the ground reaction force (the force equal and opposite to that which your body exerts on the ground while running), serving as a better stimulus for osteoblastic bone-building activity than the impact of running itself.4 As counter-intuitive as it seems, this means that running uphill is a greater stress to your tibia (due to the pull of your plantarflexor muscles) than the impact of running downhill. The more weight you are able to push during a calf raise or squat, the stronger your calf/ quad must contract and pull on your tibia- providing a greater stimulus for osteoblasts. Additionally, focusing on a slow lowering phase (i.e. during a squat) followed by a quick, explosive phase to move the weight will maximize bone stress and adaptation. Likewise, an emphasis upon explosive take-off during plyometrics is key, as this maximizes muscle activation and subsequent pull on the bone, providing more load than the landing itself. Finally, multi-directional movements should be incorporated, as osteoblasts respond well to changes in direction as opposed to repetitive movements. 4
The idea of “high load, low repetition” strength training contrasts the traditional “runner’s” strength program, which often consists of band work and low weight, high-repetition exercises. Perhaps this for fear of “bulking-up” with more traditional hypertrophy-inducing strength routines (i.e. 3 sets of 8-12 reps). However, research has shown that when performed in conjunction with aerobic training, there is a negative effect upon muscle hypertrophy when compared to strength training alone. 7 Studies have shown no change in body composition in runners after extensive periods of low repetition strength training, despite significant improvements in maximal strength.8 Thus, there is a shift away from high-rep strengthening routines- instead opting for dosages that improve strength and power- preparing our tissues to withstand the high loads we place on them mile after mile.
This is the basis of a concept that is foundational to ETS- the idea that strength training is just as important for injury prevention as it is for performance. Even if power and speed aren’t your emphasis, strength training and plyometrics are still necessary to keep your bones strong and allow for high running volumes and subsequent stress on your tissues.
Another key concept to take into consideration is utilizing periodization in your training. Throughout a training block, your bone’s mechanosensitivity is decreased. By creating fluctuation in your training load, it allows your bone cells to reset and improve mechanosensitivity, facilitating proper osteoblast activity and healthy bone remodeling after a period of rest.4 This could look like taking one easy week during which you significantly decrease both volume and intensity per every three weeks of training. It is also a good idea to periodize throughout your week by taking one complete rest day from running each week.
Ok, so that was a lot! Here is a list of some of the key takeaways and tips for keeping your bones healthy, avoiding BSIs, and actually being able to participate in your training plan!
SUMMARY: TIPS FOR AVOIDING BSI:
Keep your hard days hard, easy days easy (i.e. intervals and strength training on the same day, easy distance on another day) to allow adequate bone recovery.
Take 1 rest day off running/week.
Progressively increase training load (a good rule of thumb is to avoid increases in total running volume over 10%/week).
Increase duration before intensity, or make sure to decrease total volume if increasing velocity/ number of interval sessions/week.
Periodize your training by taking a rest or “de-load” week (significant reduction in volume/intensity) per every 3 weeks of training or 2 weeks per every 3 months.
Add strength training 2-3x/week. Try to progress towards 2-4 sets of 3-5 reps (starting in the 8-12 rep range is a smarter starting place if you haven’t been resistance training!). This includes exercises that stress the spine, pelvis, upper and lower legs- think squats, deadlifts, split squats, calf raises).
Add multi-directional plyometrics 3-4x/week (i.e. 3 x 10-20 reps of pogo jumps, box jumps, lateral jump, etc.) Do this immediately pre-run or allow 4-8 hours between jumping and your run to allow your bones a reset in mechanosensitivity. To provide maximal bone stimulus, progress towards weighted plyos (think squat jumps with weight- just remember to get off the ground quick!).
Focus on being explosive and quick during both strength training (concentric phase) and plyometrics (take-off).
Increase your cadence, which has been shown to decrease tibial forces!4 This doesn’t mean running faster- try using a metronome to calculate your steps/min, then increase by 5-10%.
Ensure adequate nutritional (especially carbohydrate, as your bones rely heavily on carbs for energy) intake. Read Sky’s blog for more on this!
Happy Training!
References:
1. Baxter‐Jones AD, Faulkner RA, Forwood MR, Mirwald RL, Bailey DA. Bone mineral accrual from 8 to 30 years of age: An estimation of peak bone mass. Journal of Bone and Mineral Research. 2011;26(8):1729-1739. doi:10.1002/jbmr.412
2. Clarke B. Normal Bone Anatomy and Physiology. Clinical Journal of the American Society of Nephrology. 2008;3(Supplement_3):S131. doi:10.2215/CJN.04151206
3. Willie BM, Zimmermann EA, Vitienes I, Main RP, Komarova SV. Bone adaptation: Safety factors and load predictability in shaping skeletal form. Bone. 2020;131:115114. doi:10.1016/j.bone.2019.115114
4. Warden SJ, Edwards WB, Willy RW. Preventing bone stress injuries in runners with optimal workload. Curr Osteoporos Rep. 2021;19(3):298-307. doi:10.1007/s11914-021-00666-y
5. Warden SJ, Edwards WB, Willy RW. Optimal Load for Managing Low-Risk Tibial and Metatarsal Bone Stress Injuries in Runners: The Science Behind the Clinical Reasoning. J Orthop Sports Phys Ther. 2021;51(7):322-330. doi:10.2519/jospt.2021.9982
6. Lower Trabecular Bone Score and Spine Bone Mineral Density Are Associated With Bone Stress Injuries and Triad Risk Factors in Collegiate Athletes - Tenforde - 2021 - PM&R - Wiley Online Library. Accessed May 18, 2024. https://onlinelibrary.wiley.com/doi/10.1002/pmrj.12510
7. Lundberg TR, Feuerbacher JF, Sünkeler M, Schumann M. The Effects of Concurrent Aerobic and Strength Training on Muscle Fiber Hypertrophy: A Systematic Review and Meta-Analysis. Sports Med. 2022;52(10):2391-2403. doi:10.1007/s40279-022-01688-x
8. Beattie K, Carson BP, Lyons M, Rossiter A, Kenny IC. The Effect of Strength Training on Performance Indicators in Distance Runners. The Journal of Strength & Conditioning Research. 2017;31(1):9. doi:10.1519/JSC.0000000000001464
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