Effect of surface stiffness on the neural control of stretch‐shortening cycle movements

Abstract

Aim: It is accepted that leg stiffness (K leg ) increases when surface stiffness decreases, and vice versa. However, little is known how the central nervous system fulfils this task. To understand the effect of surface stiffness on the neural control of stretch-shortening cycle movements, this study aimed to compare modulation of spinal and corticospinal excitability at distinct phases after ground contact during two-legged hopping when changing from solid to elastic ground. Methods: Motor-evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS) and H-reflexes were elicited at the time of the short (SLR)-, medium (MLR)-and long (LLR)-latency responses of the soleus muscle (SOL) during two-legged hopping on different stiffness surfaces, elastic and stiff. Results: Soleus H-reflexes during two-legged hopping on the elastic surface were lower at SLR and larger at LLR than on the stiff surface (P < 0.05 for both comparisons). SOL MEP size was higher at the time of SLR during hopping on the elastic surface than on the stiff surface (P < 0.05) although the background EMG was similar. Conclusion: It is argued that this phase-specific adaptation in spinal reflex excitability is functionally relevant to adjust leg stiffness to optimally exploit the properties of the elastic surface. Thus, the increased corticospinal excitability on the elastic surface may reflect a more supraspinal control of the ankle muscles to compensate the decrease in reflexive stiffness at the beginning of touchdown and/or counteract the higher postural challenges associated with the elastic surface. Keywords H-reflex, leg stiffness, stretch-shortening cycle, transcranial magnetic stimulation.The neural control of stretch-shortening cycle (SSC) movements such as drop jumps, two-legged hopping, or running is highly complex incorporating feedforward (pre-programmed) and feedback (reflex) control (Taube et al. 2012a). Thus, multiple hierarchical levels of the central nervous system (CNS) have to closely interact to adjust neural control task (Leukel et al. 2008a) and context specifically (Leukel et al. 2008a(Leukel et al. , 2012 taking into account the biomechanical constraints of the human body. The CNS is therefore challenged to optimally exploit the capacity of the tendomuscular system to store kinetic energy during 1 Published in which should be cited to refer to this work.http://doc.rero.ch the eccentric phase (touchdown), which can subsequently be released in the concentric phase (push-off) without exposing the body to the risk of being overloaded. It was previously demonstrated that the CNS can adjust the recoil properties of the tendomuscular system in various ways at different times of the SSC movement; for instance Arampatzis et al. (2001) showed that the level of pre-activation (muscular activity before touchdown) is related to the stiffness of the lower extremity. Furthermore, the reflex-induced activity shortly after touchdown can be modulated. Several studies observed reduced H-reflex activity with increasing d...

Eccentric Fatigue Modulates Stretch-shortening Cycle Effectiveness – A Possible Role in Lower Limb Overuse Injuries

“…The most meaningful measure of SSC effectiveness is stiffness of the lower limb [8]. Stiffness increases or decreases, regulated in accordance with task and environmental changes by the modulation of muscle activity via feedforward and feedback neural activity [16,32]. Furthermore, stiffness is modified in response to acute and chronic loading histories [39,40], as well as in accordance with pathologic conditions such as AT [14], where as a rule, increased stiffness is positive, and decreased stiffness is negative to the individual's performance.…”

Effects of fatigue and surface instability on neuromuscular performance during jumping

“…There is a controversy in the literature regarding the effects of surface condition on leg muscle activity. In fact, whereas some research groups found significantly higher leg muscle activity during lower limb exercises (e.g., squats) on unstable compared with stable surfaces (Anderson & Behm, ), others demonstrated similar (Li et al., ; Márquez et al., ) or significantly lower activation levels (McBride et al., ; Prieske et al., ). In terms of maximal jumping, Prieske et al.…”

“…There is a controversy in the literature regarding the effects of surface condition on leg muscle activity. In fact, whereas some research groups found significantly higher leg muscle activity during lower limb exercises (e.g., squats) on unstable compared with stable surfaces (Anderson & Behm, 2005), others demonstrated similar (Li et al, 2013;Márquez et al, 2014) or significantly lower activation levels (McBride et al, 2006;Prieske et al, 2013). In terms of maximal jumping, Prieske et al (2013) reported significantly lower leg muscle activity during the preactivation (i.e., 21-24% in TA and GM), braking (i.e., 15% in VM), and push-off phase (i.e., 11-21% in VM and GM) in healthy, physically active subjects (23 ± 3 years) when performing DJs on unstable (i.e., AIREX® balance pad) compared with stable surfaces.…”

“…Task and surface specific effects were also found for lower limb muscle activities during jumping and landing on stable as compared with unstable surfaces. For instance, Márquez et al (2014) reported similar leg muscle activities during submaximal hoppings on unstable (i.e., gymnastic spring floor) compared with stable surface, whereas Prieske et al (2013) observed reduced lower leg muscle activities during the performance of DJs on unstable surfaces (i.e., balance pad).…”

“…For instance, Márquez et al. () reported similar leg muscle activities during submaximal hoppings on unstable (i.e., gymnastic spring floor) compared with stable surface, whereas Prieske et al. () observed reduced lower leg muscle activities during the performance of DJs on unstable surfaces (i.e., balance pad).…”