What is the benefit of a springy gait?
The characteristic spring or bounce of running is a result of the lower limbs absorbing energy of the body as the center of mass falls and slows in the first half of stance. Work must then be performed to reaccelerate and lift the center of mass in the second half of stance (1).
Some of this work is done by muscular contractile elements and some of it is done by the storage and recovery of elastic energy in the tendon (6). The muscle uses metabolic energy to perform work, as well as to absorb it, but storage and recovery of elastic strain energy in tendons is independent of metabolic processes, and, at the level of the tendon, effectively ‘free’.
The benefits of a springy gait relate to the fact that any work performed by the tendon during running doesn't have to be performed by muscles. While muscle work is metabolically expensive, tendon elasticity and energy production isn't completely cost free (9).
Tendons operate in tandem with muscles, and are only able to perform as effective springs when the muscle generates force. This force generation from the muscle does require energy and is therefore a cost to utilising the tendon spring (9).
To help explain this further we have to look at two properties of skeletal muscle. These properties help to explain the metabolic and efficiency benefits of tendon elasticity in running (9).
Firstly; the 'Fenn effect' describes the fact that active muscles utilise more energy when performing work than when only generating force (2).
What is 'work' you might ask?
Basically, a muscle performs work when it is contracting and changing length - during a concentric contraction a muscle is performing 'positive work', during an eccentric contraction a muscle is performing 'negative work'. When the muscle works isometrically, it performs 'no work', even though it is still producing force (2). As a muscle not performing work consumes less energy,, utilising muscle contraction in this way is a good example of being efficient.
If the tendon has enough compliance to undergo the majority of the lengthening during running gait, then this allows the muscle to work in an ideal contraction state - producing force isometrically and lowering the energy consumption of each active muscle fibre (5). If there were no tendon compliance, the muscle would have to undergo the lengthening–shortening pattern to maintain the same joint movement. The stretch– shortening pattern of the tendon therefore allows the muscle fibers to generate force nearly isometrically (3).
Kawakami et al. 2002
Secondly; When a muscle contracts, the amount of force it can generate is related to it's shortening velocity.
When the velocity is zero (ie. a muscle is working isometrically), the amount of force that can be produced is maximal. As muscle shortening velocity increases, force production goes down. Compliance and lengthening of the tendon decreases the shortening velocity of the muscle, which effectively improves its force generating capacity. Had the muscle undergone a length change rapidly, it's ability to produce force would be diminished. Therefore, tendon compliance has a large effect on the shortening velocity and therefore force production capability of the muscle.
The benefits of a springy gait are easily understood when we consider that maximizing effective energy is the same as minimizing useless energy (4). Leaking energy makes the metabolic cost of performing at high intensities or over long durations expensive and performance limiting (4).
Elastic mechanisms in runners save energy not just because they reduce the work muscles must do, but because they allow muscles to operate at lengths and shortening velocities that are favourable for economic force production (7,8,9).
1. Blickhan, R. (1989). The spring-mass model for running and hopping. J. Biomech. 2, 1217-1227.
2. Fenn, W. O. (1924). The relation between the work performed and the energy liberated in muscular contraction. J. Physiol. 58, 373-395
3. Kawakami, Y, T Muraoka, T., Ito, S., Kanehisa, H., T Fukunaga, T. (2002). In vivo muscle fibre behaviour during counter-movement exercise in humans reveals a significant role for tendon elasticity.J Physiol. 540(Pt 2): 635–646.
4. Joyce, D., & Lewindon, D. (2014). High-performance training for sports. Champaign, IL: Human Kinetics.
5. Lichtwark, G. A., Bougoulias, K. and Wilson, A. M. (2007). Muscle fascicle and series elastic element length changes along the length of the human gastrocnemius during walking and running. J. Biomech. 40, 157-164.
6. Moritz, C. T. and Farley, C. T. (2005). Human hopping on very soft elastic surfaces: implications for muscle pre-stretch and elastic energy storage in locomotion. J. Exp. Biol. 208, 939
7. Roberts, T. J. (2002). The integrated function of muscles and tendons during locomotion. Comp. Biochem. Physiol. A Mol. Integr. Physiol. 133, 1087-1099.
8. McMahon, T. A. and Cheng, G. C. (1990). The mechanics of running: how does stiffness couple with speed? J. Biomech. 23, 65-78
9. Roberts, T. J. and Azizi, E. (2010). The series-elastic shock absorber: tendons attenuate muscle power during eccentric actions. J. Appl. Physiol. 109, 396-404