5 ways endurance sport and triathlon affects your musculoskeletal system

Endurance sports and triathlons affects the musculoskeletal system in a number of ways, and understanding why and how it adapts can help prevent future problems occurring, says physiotherapist Hayley Lord

Endurance sports and triathlons inflict change upon numerous systems of the human body; our resting heart rate reduces (cardiovascular) and we get an increase in the production of the body’s ‘feel good’ hormones, endorphins (central nervous). But how much do we really know about the influence of such sports on our musculoskeletal system?

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1.  Maintains and increases bone mineral density (BMD)

Like muscles, bones are a living tissue that can be strengthened. When adequate stress is applied to bone, the process of bone remodelling occurs: bone clearing cells (osteoclasts) break down mature tissue and bone building cells (osteoblasts) replace these with new bone. If the activity between these two cell types is equal, bone mineral density (BMD) is maintained or increased. BMD is a direct indicator of bone strength, health and an individual’s resistance to bone stress. Resistance training, particularly weight bearing, is recognised as being most beneficial for increasing BMD.

Of triathlon sports, running has the greatest influence on increasing BMD because of its repeated weight-bearing foot-to-floor nature. Some literature suggests that running 20-30Km per week increases BMD – particularly in the lower limb and spinal bones. Due to the limited and non-weight-bearing nature of cycling and swimming respectively, their influence on increasing BMD is negligible. However, they undoubtedly help to maintain BMD as the repeated muscle contractions cause pulling tendon forces on the bone which will still stress and stimulate bone remodelling, albeit to a lesser degree than in running.

A note of warning… repeated over-training can have detrimental effects on BMD as it causes a chronic increase in our stress hormone, cortisol, which causes the osteoclasts to remove more bone than the osteoblasts cells can replace. This reduces BMD, increasing susceptibility to stress. In the least severe form, bone stress may present as medial tibial stress syndrome (aka. ‘shin splints’) – something that physios commonly see when runners rapidly ramp up their mileage and training frequency. In more severe cases, bone stress can result in stress fractures. Having reduced BMD for a prolonged period of time can lead to long-term bone health complications such as osteoporosis.

There are many variable factors and much debate on the definition of over-training, optimal training and the role of rest days. Some literature states repeatedly running upwards of 80Km per week is detrimental to BMD. There is little cited on this for cycling and swimming.

So BMD…. it’s important to adequately stress – not over-stress! – bones.

2. Increases resistance to delayed onset muscle soreness (DOMS)

DOMS is that dull, achy feeling that makes our muscles tender and stiff 24-72 hours post-exercise. It’s very common for beginner triathletes to experience DOMS, but experienced, well-conditioned triathletes that have trained consistently for years can also get it post-race or vigorous activity.

Delayed onset muscle soreness in the calves – simple exercises to fix it

The primary cause of DOMS is repeated high force eccentric contractions, where the muscle lengthens under tension whilst trying to contract resulting in the muscle is being pulled from either end. The opposite of this is a concentric contraction whereby the muscle shortens as it contracts.

For triathletes, running is the main culprit for DOMS because both the quadriceps and calf muscles must work eccentrically to help them to catch and control their body weight each time their foot strikes the ground. This is exacerbated when running at higher intensities or downhill. As swimming and road cycling are non-impact activities, you may still experience post-exercise stiffness, but the actual muscle soreness will be negligible.

Through endurance training, you build muscular resistance to fatigue and DOMS. This mostly correlates to improved aerobic capabilities (discussed later) and ‘muscle memory’. Alongside this, triathletes commonly use prophylactic and preventative measures such as nutrition, low intensity activity, compression garments and massage to manage DOMS.

3. Increases the number and size of mitochondria in active muscles:

Energy production (respiration) can be aerobic or anaerobic. Aerobic respiration is where oxygen and nutrients from food are combined to produce energy. It produces large amounts of sustainable energy and hence is imperative within endurance sports. Conversely, anaerobic respiration occurs when the body enters a more effortful state where it must rely on limited energy stores within muscles to produce very short, powerful bursts of energy in the absence of oxygen. Anaerobic respiration also produces lactic acid, the by-product traditionally believed to increase fatigue and DOMS. Anaerobic respiration is thus an unsustainable source of energy, used only in power feats such as the sprint finish of an endurance event.

Mitochondria are the powerhouse part of our cells that partake in aerobic respiration. Endurance training increases both the number and size of mitochondria in active muscles which therefore increases our ability to uptake oxygen and continue producing large quantities of sustainable energy via aerobic respiration; enhancing our efficiency in endurance sports. 

Increasing mitochondria improves performance

Mitochondria structure changes in elite athletes


4. Increases the number of enzymes in active muscles: 

Specific enzymes catalyse the process of aerobic respiration. During endurance training, the number of these enzymes increases; accelerating the rate of reaction and the body’s subsequent ability to produce energy.

Myoglobin, an enzyme in muscles that binds to and stores oxygen, is believed to increase by up to 80% with endurance training. When oxygen becomes limited during endurance exercise and the body begins to reach its ‘aerobic respiration threshold’- the point at which the muscle will switch to anaerobic respiration, myoglobin releases stored oxygen to mitochondria. This allows them to continue to fuel aerobic respiration, enhancing the body’s ability to tolerate performing at the same level for longer; increasing endurance performance.

5) Increase in muscle fibre conversion from Type II to Type I 

We are born with an approximate 50/50 split of type I and type II muscle fibres.

Type I (slow-twitch) fibres can sustain force for prolonged periods of time but cannot generate a significant amount of force. They contain an abundance of mitochondria and are best suited to aerobic respiration and endurance feats such as marathon running.

Type II (fast-twitch) fibres can elicit greater forces than type I fibres but fatigue quickly. They contain far fewer mitochondria than type I fibres and will thus preferentially undergo anaerobic respiration. They are best suited to powerful activities such as sprinting.

During endurance training and sports, we can covert up to 10% of type II to type I muscle fibres. This increases our muscular endurance, resistance to fatigue and endurance performance.

Additionally, you may be genetically ‘gifted’. Individuals with a natural talent for certain sports (power versus endurance) are believed to be born with an inexact split of muscle fibres. For example, one would imagine that Usain Bolt was likely born with over 50% of type II fibres, which he has further enhanced through specific sprint training. 

In summary…..

Whilst we are often aware of the role of the musculoskeletal system, we do not always appreciate precisely how it changes in response to endurance-based sports; which is ironic considering that 85% of injuries suffered by triathletes are to the musculoskeletal system.

The human body is a highly adaptive machine. Generally, if you undertake training for any endurance event in a progressive manner over a period of time, factoring in adequate recovery, your body should safely adapt to both enhance and prolong your aerobic respiration capabilities. In addition to this, repeated practice will aid skill refinement. Combined, these two will make you a more efficient endurance athlete.

Having a greater awareness of some of the effects on the musculoskeletal system should increase your appreciation of appropriate training, preventative conditioning and recovery.

Hayley Lord is a physiotherapist with, which has nine clinics in London and specialise in: 

•    Sports injury physio with emphasis on rehabilitation

•    Assessments & injury prevention advice

•    Sports massages: an integral part of any training plan


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