Updated: Jan 8, 2021
The spring-mass model is a simple model for describing how the lower extremity functions during running. During stance phase, the leg acts like a ‘spring’; compressing under the force from gravity and the vertical ground reaction force. During this compression, as the leg ‘resists’ deformation, the leg stores elastic energy in the tendons; working much like a compressed pogo stick. After midstance the leg spring is released and the centre of mass rises, partly due to elastic energy release from the tendons. This gives humans the characteristic ‘bounce’ during running.
Lower extremity ‘stiffness’ refers to how much the limb is compressed during stance phase, with the combination of stiffness from muscles, ligament, tendon, bone and cartilage all adding to the stiffness properties of the spring. Higher lower extremity stiffness results in less joint excursions of the hip, knee and ankle as they resist deformation under loading. High compliance on the other hand, results in larger joint excursions, with more flexion occurring at the hip, knee and ankle during stance phase, resulting in a less elastic lower limb spring. Increased compliance of the lower extremity has been associated with longer ground contact times and is likely to result in more elastic energy loss as heat; as the elastic energy in the tendon is only available for a limited time. Therefore, the ability of the lower limb to store and release elastic energy predominantly determines one’s lower extremity stiffness.
The stretch-shortening cycle
During the stretch-shortening cycle, an eccentric contraction causes the storage of elastic energy in the tendon. During tendon recoil, the stored energy is released, increasing force production, and allowing more work to occur with less energy; maximising efficiency. Energy produced from muscular contraction requires the presence of oxygen, which is not the case for elastic energy from the tendon. Enhancing stretch-shortening cycle ability will increase the amount of elastic energy supplied from the tendon, which is essentially ‘free’ energy, occurring without metabolic processes or aerobic demand.
Long distance runners benefit greatly from elastic energy by allowing significant conservation of energy via efficient energy storage and release, as opposed to the elastic energy specifically increasing force per stride. The amount of muscle force generated during running is closely associated with the energetic cost of running, suggesting that maximising elastic energy output via the stretch-shortening cycle will result in improvements in the energetics of running. Elastic energy production is metabolically efficient, as unlike muscular contraction, occurs without the use of oxygen. This ability to passively store and release elastic energy therefore saves muscular work and reduces metabolic costs. Exercises to train stretch shortening cycle ability should therefore gravitate towards minimising ground contact time to minimise the amortization phase and train the ability to both store and release elastic energy via the stretch shortening cycle.
Plyometric training has the capacity to improve running economy in long distance runners, with the mechanisms thought to be an improved ability to utilise the stretch shortening cycle. It was demonstrated that 6 weeks of plyometric training lead to improvements in running performance in a group of 18 regular runners. However, the runners demonstrated no improvements in jump height that would have indicated improvements in the ability to store and release elastic energy. It’s possible that plyometric training improved running economy by increasing neuromuscular efficiency, as opposed to specific stretch shortening cycle ability. Either way, training aimed at improving lower extremity stiffness via plyometric training has the potential to improve running economy with the mechanisms still unclear.
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