The Stretch Shortening Cycle (SSC) and Distance Running

A muscle can contract more rapidly when the shortening (concentric phase) is preceded by the muscle lengthening (eccentric phase).  An easy way to visualize this concept is during a vertical jump.  When jumping for maximal height athletes will use a countermovement prior to jumping.  This utilizes the SSC and increases jump height.  Try it yourself (or watch the NFL Combine)….Jumping with a countermovement allows for increased jump height when compared to a static jump (which doesn’t utilize the SSC).  Rarely in sport (or life for that matter) do we ever have purely eccentric or concentric muscle contractions.  Typically movement is characterized by a SSC which allows for increases in concentric performance through 1) The Return of Stored Elastic [Strain] Energy, 2) Utilization of the Stretch Reflex, and 3) Cross Bridge Potentiation.  This model is very applicable to long distance running with each stride utilizing a small SSC.  Plyometric Training (which exaggerates the SSC) is a great way to improve distance running performance.

The spring-mass model has been used to explain variations in Running Economy (RE) and Mechanical Efficiency (ME), and describes each running stride being counteracted by a spring behavior in the support leg.  During the eccentric phase of ground contact, mechanical energy is stored in the muscles, tendons, and ligaments of the support leg.  Stiffness of the muscle-tendon unit (MTU) can help to ensure that a maximal amount of that energy is returned into the ground during the subsequent stride.  It is possible that this stored energy can reduce muscle activation and spare energy while running (allowing us to run farther/faster).  Gajdosik and Riggin [19] found that trained distance runners had greater passive resistance of their calf MTU when compared to untrained subjects.  This lack of flexibility, they hypothesize, contributes to positive gains in running economy through a more efficient transfer of energy.  Other studies have found that during the eccentric phase of the SSC the calf MTU stores active stiffness which increases concentric torque during plantar flexion [20, 21].  Kryolainen et al. [22] also determined that increased stiffness of the calf MTU contributed to improvements in RE and ME.  Resistance training has been found to improve stiffness of the MTU and may lead to performance improvements in distance runners [6].

          
During SSC exercise the muscle preactivates and shortens prior to ground contact in order to prepare for footstrike [23].  The muscle is at its shortest length during the eccentric phase of the movement and gradually lengthens as it moves through the amortization and concentric phases.  In contrast, the tendon lengthens as it moves through the eccentric phase and begins to shorten as the concentric phase begins [24]  Stored elastic energy that is utilized in SSC movements is stored mostly in tendons during the eccentric phase and returned during the concentric phase [23-26]  This characteristic of the SSC allows for preservation of ATP through usage of stored elastic energy (which allows for faster sustained running)…

         
Designing training programs that enhance the SSC should be of interest to coaches, scientists, and athletes.  If a runner can conserve metabolic energy by utilizing mechanical energy through the SSC he/she should be able to run faster (which is the ultimate goal, isn’t it?)…. I think sometimes as distance running coaches we overemphasize capacity and undervalue efficiency.  Simple jumping drills as a part of a dynamic warm-up are an easy way to address this need...

6.         Paavolainen, L., et al., Explosive-strength training improves 5-km running time by improving running economy and muscle power. J Appl Physiol, 1999. 86(5): p. 1527-1533.

19.       Gajdosik, R.L. and T.J. Riggin, Passive elastic properties of the calf muscle-tendon unit of distance runners. Isokinetics & Exercise Science, 2005. 13(3): p. 207-216.

20.       Kurokawa, S., et al., Interaction between fascicles and tendinous structures during counter movement jumping investigated in vivo. J Appl Physiol, 2003. 95(6): p. 2306-2314.

21.       Svantesson, U., et al., Use of a Kin-Com dynamometer to study the stretch-shortening cycle during plantar flexion. Eur J Appl Physiol Occup Physiol, 1991. 62(6): p. 415-9.

22.       Kyrolainen, H., et al., Interrelationships between Muscle Structure, Muscle Strength, and Running Economy. Medicine & Science in Sports & Exercise, 2003. 35(1): p. 45-49.

23.       Kubo, K., Y. Kawakami, and T. Fukunaga, Influence of elastic properties of tendon structures on jump performance in humans. J Appl Physiol, 1999. 87(6): p. 2090-6.

24.       Kubo, K., et al., Measurement of viscoelastic properties of tendon structures in vivo. Scand J Med Sci Sports, 2002. 12(1): p. 3-8.

25.       Ito, M., et al., Nonisometric behavior of fascicles during isometric contractions of a human muscle. J Appl Physiol, 1998. 85(4): p. 1230-5.

26.       Muraoka, T., et al., Muscle fiber and tendon length changes in the human vastus lateralis during slow pedaling. J Appl Physiol, 2001. 91(5): p. 2035-40.