Muscle Stiffness & Efficiency during Movement

Part I: The Ankle Joint

When walking or running, the hip rotates forwards and the knee straightens. The power for this originates at the ankle, allowing us to push forwards through the front of the foot, with muscles around the thigh and hips stabilising the joints, adding control, and contributing to propulsion. The faster we walk or run, or the older we happen to be, the greater the input from the muscles at the front of the hip (hip flexors and rectus femoris). Propulsion from the ankle, when walking, is mostly passive, which reduces energy usage and promotes efficiency.

Passive stiffness refers to the resistance offered to a joint from inactive muscles, and is influenced by the amount of muscle, its contractile portions (at rest), and the proteins that link them (titin). In addition, there is resistance from other tissues, mostly the tendons but ligaments too, to some extent, and all other tissue types that might affect the joint (skin, vessels, the capsule itself). The active contributions to movement follow muscle contraction, so depends on the amount of contractile portions that are actively contracting. Because muscle contractions use energy, the greater the contribution from passive elements, the greater the overall movement efficiency.

The ankle is an unusual joint, in that the Achilles tendon contributes so much to movement, relative to the contributions to joints from other tendons in the body. As the heel strikes the ground, the shank (shin bone, tibia) moves forward, stretching out the Achilles tendon. The tendon stretches, absorbing then storing mechanical energy, which it then releases, like a spring, as we push forwards through the front of our foot. During this time, the muscles the Achilles joins onto (calf muscles, gastrocnemius and soleus), maintain an isometric contraction, which is a more energy-efficient type of contraction than one where they are changing length (lengthening or shortening).

Importantly, the muscle contraction only needs to contribute to the passive resistance the muscles are already offering against the Achilles, meaning that fewer fibres contract if someone has 'tight' calf muscles than if they are very compliant. In addition to the passive element, the proteins that link the contractile portions of muscle (titin), contribute even more resistance, especially when muscles are contracting and being stretched. This is known as passive force enhancement. Thus, a natural activity, such as walking at our preferred speed, will require the minimum amount of energy input, and fewer calories to be expended than a less efficient activity, such as speed walking, running, hopping, bounding, etc. When we change from walking to running, so efficiency is reduced as muscles are required to contract actively to propel the body faster, but the same mechanisms and interactions occur, albeit in a less efficient manner, using up more energy to permit increased speed.

A key point within this is the contribution of the passive elements, and how their efficiency is maintained. It was mentioned that if the calf muscles are very tight, less energy will be required than if they are very compliant. This is true up to a point, but when the muscles become 'too tight', opposing muscles might have to contract to permit normal movement, or else the body must compensate in some other way to promote the most efficient movement pattern it can (perhaps rotating outwards from the hips, increasing pronation at the ankle, for example). Part of the difficulty we face is that we do not know what a 'normal' amount of stiffness is at the ankle joint (I should add, it has been investigated by this author and two of his Master's students, as well as various other researchers, so information is in the pipeline).

If someone places greater stress on their ankle joint, this will lead to an adaptive increase in muscle stiffness, so as to promote efficiency. If someone has long leg bones, is tall, is heavy, has a lot of lower leg muscle mass, these will all increase the stiffness of the muscles relative to someone with a more petit stature. This increased stiffness is adaptive and specific to the individual, and a larger person needs to have stiffer calf muscles and Achilles tendon to maintain efficiency in walking.

Consider that some therapists will test range of motion of the ankle joint by pushing the front of the foot back, stretching the back of the ankle (dorsiflexing). How much harder would they have to push if assessing a bigger individual, compared with doing the same test on a much smaller person? How hard should they push, and what is a normal amount of flexibility required during this sort of test, or even during movement? Until we know what 'normal' is, relative to an individual's size and composition, it is not possible to say.

Muscle stiffness increases with age. At some point, this will potentially take the level of passive stiffness beyond some upper 'normal' level for the individual, and begin to require compensations from other muscles, using up energy and promoting inefficiency. As the ankle joint becomes 'too stiff', so the hip flexors compensate to make up for deficiencies in propulsion from the ankle. It is possible that a decreased preferred walking speed exacerbates this further.

Currently, research is underway to help us better understand passive stiffness, passive force enhancement, and the interactions between active and passive elements. There is also interest in trying to define what sort of exercise-based interventions might influence short- or long-term stiffness, and whether these would help or hinder efficiency (considering stiffness is likely to be adaptive to the activities we do most often). This certainly raises questions regarding the appropriateness of some approaches to cross-training.

Injury risk is another important area, as muscles that are 'too stiff' will increase the likelihood of muscle-tendon tears, whereas muscles that are not stiff enough will lead to increased potential for ligament injuries at a joint. Again, because stiffness will be specific to an individual's size and composition, and because it might be influenced by activities they do the most, it is a highly individual matter, and an area where the research is still in its infancy.



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