Is it time to embrace the Spring Model in Physiotherapy?
A new way to assess and manage lower limb injuries
THE SPRING MODEL
Leg stiffness is defined as resistance of the limb (incorporating all joints) to a change in length . Therefore lower limb is compared to a Spring that repeatedly follows a Stretch-Shortening cycle (SSC) during our daily and sports activities. The SSC is a phenomenon associated with human locomotion, describing the muscle function in which a pre-activated musculotendinous unit lengthens then immediately shortens .
In other words, stiffness is the relationship between the deformation of an object in response to an applied force. From a biomechanical point of view, optimal running, jumping and hopping performance require an appropriate level of lower extremity stiffness to absorb ground reaction forces (GRFs), as well as to store and reuse elastic energy [1, 3].
It seems that the human body is equipped with a protective strategy that lowers the GRF and/or loading rate (LR) as muscles fatigue in order to protect the body from possible injuries . A sub-optimal variation of this mechanism, such as high bilateral difference in leg stiffness, has been shown to be related to the incidence of soft tissue injury in Australian football players .
Hence, it may appear quite intuitive that a low level of leg stiffness may lead to an augmented “deformation” of the lower limb that we see in the clinic as an excessive joint motion. In fact over the last decade, considerable focus has been given to motor control in the assessment and management of numerous musculoskeletal and sports related disorders. The theoretical basis assumed that the so called “abnormal movement patterns” or “excessive joint motion” could be causative and could predispose active people to musculoskeletal injuries.
A typical example of this theoretical model is reflected in the huge amount of research investigating lower limb kinematics. Hip internal rotation, knee valgus and excessive foot pronation became the culprits for a wide range of musculoskeletal injuries together with reduced hip strength. However, although these features may be present in some cluster of patients with MSK disorders  and in some injury mechanism , they may not mean causation!
We know that interventions targeting neuromuscular control may not alter the way patients move, they may just reduce the tissue sensitivity and increase the load tolerance to a specific task or movement. Also, they are commonly prescribed focusing on body parts (rather than on movement result), which is an internal focus, thus leading to poor motor learning [8, 9]. The fact that they are prescribed with low loads and high repetitions may not lead to movement pattern changes, but to graded exposure, associative learning  and also to secondary accidental strength gains .
In fact several prospective studies and systematic review have found a discord between lower limb kinematics, hip strength and musculoskeletal pain [12-15]. Recently, on his blog, Peter Malliaras cited an article  where increased hip abduction strength was linked with dynamic knee valgus. As most of the readers may know, Peter Malliaras weekly posts interesting reviews of articles regarding Tendinopathy with his own insights and the last week topic was hip strength data in mid-portion Achilles Tendinopathy .
It is indeed the new and exciting research in lower limb tendinopathies that is changing the way we look at lower limb injuries. In fact there is little evidence that biomechanics influences tendinopathies and as stated by Professor Jill Cook “it is more about capacities” . Although most researchers focused their attention on neuromuscular control and muscular strength, explosive strength, Rate of Force Development and power are “capacities” that cannot be neglected when assessing and rehabilitating patients with lower limb injuries [19-21]. Dynamometry may reflect strength and endurance, but cannot easily measure power . Furthermore, the so called “transfer” between strength gains and power output is not automatic [3, 22] and, as outlined by Rio et al. , in some symptomatic subjects full strength may be preserved.
Due to these premises, an increasingly number of studies has looked into the Stretch Shortening Cycle (SSC) and Ground Reaction Forces (GRFs). This underpins the concept that the lower limb is viewed as a spring, whose stiffness primarily depends on ankle stiffness during hopping and SSC activities . It is well known, although often underestimated, that the calf complex is vital in sporting activities involving hopping, running and change of direction [25, 26].
The Spring Model considers the lower limb as a Spring, whose stiffness is influenced by its local components (ankle plantar flexors, knee extensors and hip extensors muscles), by the Central Nervous System , presence of pathology  and fatigue [4, 29].
Therefore the intervention consists of reaching the optimal “capacity” values for each component through optimal application of resistance training regimens , according to tissues tolerance and the requirements of the patient population’s characteristics. Once reached optimal tissue capacities, specific strength and power training aimed to improve the whole leg stiffness should be implemented to transfer strength gains into fast SSC activities. In fact, this specific type of Rehabilitation is able to increase the Rate of Force Development, to improve jump performance, running economy and at the same time to increase leg stiffness [19, 30-32].
It has to remain clear that low load interventions, such as neuromuscular exercises, still have their own place in rehabilitation when combined to an external focus . For example, they may be useful in changing maladaptive landing kinematics . However, using this as stand-alone intervention is not sufficient as expressed in this article. Loads and exercises not meeting specific parameters cannot deliver the outcomes.
Could the adoption of the Spring Model help us and our patients into a more comprehensive rehabilitation plan able to reduce pain and at the same time to improve performance?
1. Butler, R.J., H.P. Crowell, 3rd, and I.M. Davis, Lower extremity stiffness: implications for performance and injury. Clin Biomech (Bristol, Avon), 2003. 18(6): p. 511-7.
2. Debenham, J., et al., Eccentric Fatigue Modulates Stretch-shortening Cycle Effectiveness–A Possible Role in Lower Limb Overuse Injuries. Int J Sports Med, 2016. 37(1): p. 50-5.
3. Brazier, J., et al., Lower Extremity Stiffness: Effects on Performance and Injury and Implications for Training. Strength & Conditioning Journal, 2014. 36(5): p. 103-112.
4. Zadpoor, A.A. and A.A. Nikooyan, The effects of lower extremity muscle fatigue on the vertical ground reaction force: a meta-analysis. Proc Inst Mech Eng H, 2012. 226(8): p. 579-88.
5. Pruyn, E.C., et al., Relationship between leg stiffness and lower body injuries in professional Australian football. J Sports Sci, 2012. 30(1): p. 71-8.
6. Witvrouw, E., et al., Patellofemoral pain: consensus statement from the 3rd International Patellofemoral Pain Research Retreat held in Vancouver, September 2013. Br J Sports Med, 2014. 48(6): p. 411-4.
7. Walden, M., et al., Three distinct mechanisms predominate in non-contact anterior cruciate ligament injuries in male professional football players: a systematic video analysis of 39 cases. 2015. 49(22): p. 1452-60.
8. Benjaminse, A., et al., Optimization of the anterior cruciate ligament injury prevention paradigm: novel feedback techniques to enhance motor learning and reduce injury risk. J Orthop Sports Phys Ther, 2015. 45(3): p. 170-82.
9. Wulf, G., C. Shea, and R. Lewthwaite, Motor skill learning and performance: a review of influential factors. Med Educ, 2010. 44(1): p. 75-84.
10. Zusman, M., Associative memory for movement-evoked chronic back pain and its extinction with musculoskeletal physiotherapy. Physical Therapy Reviews, 2008. 13(1): p. 57-68.
11. Mitchell, C.J., et al., Resistance exercise load does not determine training-mediated hypertrophic gains in young men. Journal of Applied Physiology, 2012. 113(1): p. 71-77.
12. Rathleff, M.S., et al., Is hip strength a risk factor for patellofemoral pain? A systematic review and meta-analysis. Br J Sports Med, 2014. 48(14): p. 1088.
13. Heiderscheit, B., LOWER EXTREMITY INJURIES: IS IT JUST ABOUT THE HIP STRENGTH? The Journal of orthopaedic and sports physical therapy, 2010. 40(2): p. 39-41.
14. Bolgla, L.A., et al., Comparison of hip and knee strength in males with and without patellofemoral pain. Phys Ther Sport, 2015. 16(3): p. 215-21.
15. Hespanhol Junior, L.C., et al., Lower limb alignment characteristics are not associated with running injuries in runners: Prospective cohort study. Eur J Sport Sci, 2016. 16(8): p. 1137-44.
16. Bandholm, T., et al., Increased external hip-rotation strength relates to reduced dynamic knee control in females: paradox or adaptation? Scand J Med Sci Sports, 2011. 21(6): p. e215-21.
17. Habets, B., et al., Hip muscle strength is decreased in middle-aged recreational male athletes with midportion Achilles tendinopathy: A cross-sectional study. Physical Therapy in Sport.
18. Cook, J.L. and S.I. Docking, “Rehabilitation will increase the ‘capacity’ of your …insert musculoskeletal tissue here….” Defining ’tissue capacity’: a core concept for clinicians. Br J Sports Med, 2015. 49(23): p. 1484-5.
19. Maffiuletti, N.A., et al., Rate of force development: physiological and methodological considerations. Eur J Appl Physiol, 2016. 116(6): p. 1091-116.
20. Angelozzi, M., et al., Rate of force development as an adjunctive outcome measure for return-to-sport decisions after anterior cruciate ligament reconstruction. J Orthop Sports Phys Ther, 2012. 42(9): p. 772-80.
21. Morin, J.B. and P. Samozino, Interpreting Power-Force-Velocity Profiles for Individualized and Specific Training. Int J Sports Physiol Perform, 2016. 11(2): p. 267-72.
22. Van Hooren, B. and F. Bosch, Influence of Muscle Slack on High-Intensity Sport Performance: A Review. Strength & Conditioning Journal, 2016. 38(5): p. 75-87.
23. Rio, E., et al., Tendon neuroplastic training: changing the way we think about tendon rehabilitation: a narrative review. 2016. 50(4): p. 209-15.
24. Farley, C.T. and D.C. Morgenroth, Leg stiffness primarily depends on ankle stiffness during human hopping. J Biomech, 1999. 32(3): p. 267-73.
25. O’Neill, S., P. Watson, and S. Barry, 75 Plantarflexor Muscle Power Deficits In Runners With Achilles Tendinopathy. British Journal of Sports Medicine, 2014. 48(Suppl 2): p. A49.
26. Marshall, B.M., et al., Biomechanical factors associated with time to complete a change of direction cutting maneuver. J Strength Cond Res, 2014. 28(10): p. 2845-51.
27. Ferris, D.P., K. Liang, and C.T. Farley, Runners adjust leg stiffness for their first step on a new running surface. J Biomech, 1999. 32(8): p. 787-94.
28. Debenham, J.R., et al., Achilles tendinopathy alters stretch shortening cycle behaviour during a sub-maximal hopping task. J Sci Med Sport, 2016. 19(1): p. 69-73.
29. Oliver, J.L., et al., Altered neuromuscular control of leg stiffness following soccer-specific exercise. Eur J Appl Physiol, 2014. 114(11): p. 2241-9.
30. Lum, D., et al., Effects of intermittent sprint and plyometric training on endurance running performance. Journal of Sport and Health Science.
31. Pellegrino, J., B.C. Ruby, and C.L. Dumke, Effect of Plyometrics on the Energy Cost of Running and MHC and Titin Isoforms. Med Sci Sports Exerc, 2016. 48(1): p. 49-56.
32. Denadai, B.S., et al., Explosive Training and Heavy Weight Training are Effective for Improving Running Economy in Endurance Athletes: A Systematic Review and Meta-Analysis. Sports Med, 2016.
33. Voskanian, N., ACL Injury prevention in female athletes: Review of the literature and practical considerations in implementing an ACL prevention program. Current Reviews in Musculoskeletal Medicine, 2013. 6(2): p. 158-163.