Tag Archives: moment arm

Tendinopathy, musculoskeletal characteristics, systemic strategies, and running.

I came across a very interesting research article titled Running Biomechanics: Shorter Heels, Better Economy. Evidence is presented that running economy is determined by supposedly immutable factors in the athlete’s musculoskeletal structure, such as the moment arm of the achilles tendon, which refers to the distance between the achilles tendon and the ankle joint, which serves as the fulcrum of rotation. The evidence presented suggests that greater running economy—the amount of energy stored in the tendons, to be used in the next step—correlates with a shorter moment arm far more strongly than with other factors such as  lower leg volume or VO2 (a given rate of oxygen consumption). This has serious implications for the advice given to runners on how to improve their running economy.


The authors conclude that 56% of the variation in running economy between runners could be predicted by the moment arm of the achilles tendon. This is interesting, considering that other studies suggest that there are 20-30% differences in running economy even among elite athletes. The study, which selected highly-trained, competitive male runners as participants, corroborates these findings.

This body of data suggests that, by and large, training does not affect running economy, when running economy is a function of the body’s skeletal configuration. What does this mean? That when it is up to the physical characteristics of someone’s bone structure, changes to running economy cannot be easily made. Because the achilles tendon moment arm (which corresponds to the distance between the ankle joint and the heel bone) is fixed in adults, the abovementioned 56% in variation is also fixed.

However, factors that aren’t skeletal could affect running economy—factors such as poor muscle coordination and imbalance. For example, one of the most common problems in amateur runners is stiffness of the soleus and gastrocnemius (calf) muscles. Often, this contributes to excessive plantarflexion (pointing of the foot) and premature heel rise during the late stage of the stance phase of gait. (Heel rise should occur during pushoff phase).

dosiflexion_plantar_flexion (1)

By raising the heel, the achilles tendon moment arm increases, allowing the gastrocnemius to exert more force against the ground. However, as the above-referenced article would suggest, this means that comparatively less energy would be stored in the achilles tendon. Other research on achilles tendinopathy corroborates this, with findings that those who suffer from the condition often have a reduced activation of the tibialis anterior muscle. By and large, those who suffer from achilles tendinopathy will point the foot to decrease loading of the tendon.

It is likely that pointing the foot as a result of achilles tendinopathy is a two-pronged strategy: both the reduction in tendon loading and the increase in achilles moment arm contribute to maintaining a functioning system. In light of the abovementioned research, this increase in moment arm means that force exerted into the ground is achieved through active muscle contractions of the soleus and gastrocnemius, rather than passive energy storage in the achilles tendon. By offsetting the production of power from the tendon to the muscle, the limb can remain useful in a suboptimal state.

This means that there is no single way to improve running performance. In fact, unless you have severely impaired biomechanics—which, granted, is more than commonplace in modern runners—there is nothing much you can do about your running economy. But unless you already use your body perfectly, there is no point in worrying about a large achilles moment arm. And if you already do use your body perfectly, there is no point in worrying about it either: you’ll simply end up developing other faculties, such as the aerobic engine, as your body seeks to achieve greater speed and endurance.

In Running Science, Owen Anderson compares Steve Prefontaine and Frank Shorter, writing that even though both athletes had very similar times in the 10,000 meter race, Prefontaine had a markedly higher VO2max (maximum volume of oxygen consumption per minute) than Shorter.   Anderson’s analysis is that Shorter had superior biomechanics, while Prefontaine had to develop greater aerobic capacity. However, in light of the presented evidence (and a cursory glance over both athletes’ physiology and body type), it is likely that Shorter had musculoskeletal advantages over Prefontaine, such as a reduced achilles tendon moment arm.

Steve Prefontaine
                   Steve Prefontaine

Concretely, this means that faulty biomechanics aside, certain runners will benefit more from particular kinds of training than others. For example, a runner with a huge achilles tendon moment arm may benefit more from weightlifting and muscle power exercises, particularly those that develop the tibialis anterior, allowing for ankle stabilization during the landing phase at greater ankle dorsiflexion than runners with a smaller achilles tendon moment arm: as mentioned above, dorsiflexing the foot reduces the achilles moment arm and increases loading (which is why those with achilles tendinopathy avoid it).

Runners who have to increase dorsiflexion to a greater extent for a given running economy will still be relying on more muscle power than those who don’t, at least in some fashion: the moment arm of the tibialis anterior (which dorsiflexes the foot) increases throughout dorsiflexion. In other words, this will offset the need for muscle power from the rear muscles to the front muscles, at least in the calf region.

It’s likely that the same biomechanic advice—advice on how to develop running economy—won’t be equally useful for two different runners. Although running economy will be largely a function of achilles tendon moment arm, running speed, endurance, and overall performance is not. Runners should study their bodies (or get studied by an expert) to see what kinds of training will help them develop their race performance.

Keep in mind that running economy is not the same thing as running performance. Prefontaine and Shorter’s comparison should tell you that. However, certain people have attributes that favor specific skillsets. Some people have great muscle power, others have great economy. Lately, running trends have been focusing too much on the energy-return properties of the body—so much so that runners are either alienated or forget that the body has other properties. The body is always more complex than the latest trend says so. And even if the latest trend does not validate the attributes that we should develop to make us better runners, it doesn’t mean those attributes aren’t there, or are somehow less important. The human body is an extremely complex machine, capable of achieving great performance through many different avenues. With a bit of study, we can figure out what those are.