Neurodynamic contributions of eccentric muscle contractions
Mechanisms explaining the benefits from eccentric exercises (EEx) for managing tendinopathy can be found on the (molecular) level of the muscle-tendon or tendon-to-bone complex and generally involve remodeling of tissue morphology. Mechanisms can also be found in the central nervous system as emerging evidence demonstrates the positive effects EEx have on restoring cortico-spinal excitability, i.e. enhancing neuromuscular control. The neurodynamic effects following EEx may mechanistically explain their important role in preventing and treating musculoskeletal injuries.
Changes in motor control after injury, e.g. after tendinopathy but also after anterior cruciate ligament (ACL) reconstruction 1, in chronic ankle instability 2 and following intense rehabilitation after ACL reconstruction 3 may be ‘adversely affecting control of the skeletal muscle system’ 4.
Neuromuscular alterations (e.g. cortico-spinal excitability) following injury and/or operation may be the result of ‘ongoing monitoring’, i.e. an ongoing protection strategy by the central nervous system, even after pain reduction and/or return to sports 5. Prolonged corticospinal hyper-inhibitory and excitability with concomitant changes in motor control, motor planning and sensory processing can increase the risk for recurrence of symptoms or secondary injury after e.g. return to sports 4. In their clinical review Lepley and colleagues provide interesting thoughts to treat and prevent these ‘neuromuscular deficits’ as a result of musculoskeletal injuries with eccentric exercise (EEx). The authors reason from a neural point of view (not from the traditional ‘ability of eccentric exercise to remodel muscle morphology’) and point out the unique aspects of eccentric contractions in successfully restoring neuromuscular control whereas ‘the concentric exercise approach does not’. Compared with concentric and isometric contractions, eccentric contractions induce higher levels of motor cortex excitability which can overrule spinal inhibition present after joint injuries. Spinal inhibition may ‘initiate long-term deficits in neuromuscular control’ and may be a key factor why restoring motor control fails with ‘traditional, concentrically driven rehabilitation programs’, Lepley et al writes. Furthermore, eccentric and concentric contractions induce distinct patterns of brain activity indicating different strategy of neuro-motor processing 6. Eccentric contractions target specific motor control pathways in the brain which could attenuate deficits in neuromuscular control. In addition, Lepley et al demonstrate preliminary data from functional magnetic resonance imaging (fMRI) showing differences in cerebellar and cortical activation during concentric and eccentric contractions in patients with ACL reconstruction compared to healthy controls. Concentric contractions decrease the cerebellar activity and increase cortical activity whereas eccentric contractions increase the cerebellar activity and decrease cortical activity. Lepley reasons that the different neural activation patterns concomitant with a typical concentric-focused rehabilitation induces a suboptimal cortical-cerebellar state associated with poor neuromuscular control. They found that EEx reverse this suboptimal cortical-cerebellar state and promotes neural activation patterns comparable with healthy controls.
Furthermore, eccentric contractions inhibit spinal modulation of muscle force, i.e. reduce spinal-induced reflexive contractions during muscle lengthening. According to Lepley et al this may ‘increase the need for heighted feed-forward control and accurate sensory predictions of the cerebellum to regulate motor output’ and thus ‘reduce motor coordination error’ associated with primary and secondary injury risk.
However, current tendinopathy protocols are based on EEx and recurrence of symptoms is still high in e.g. tendinopathy of the Achilles tendon 7. Rio et al write: ‘current rehabilitation fails to restore corticospinal control of the muscle-tendon complex(s).’5 Furthermore, the viewpoints from Lepley and colleagues contrast with the results from Rio and colleagues in which ‘isotonic’ – a better term would be ‘isoweight’, because ‘a muscle can only produce constant tension regardless of its length in isolated conditions 8 – and isometric resistance training (RT) was able to restore motor control. However, Rio’s isotonic exercise approach which restored neuromuscular function was a novel external pacing-based resistance training (EP-RT) protocol and not traditional self-paced RT which, most likely, Lepley and colleagues refer to. It is therefore important that future research compares self-paced RT and EP-RT protocols with different mode of contractions. This could help elucidate questions such as if eccentric contractions with or without EP is superior for restoring neuromuscular control and influencing symptomatic tendon pain.
- On a spinal level, EEx potentially restore cortico-spinal excitability and putatively attenuate deficits in neuromuscular control.
- In the brain, EEx promotes neural activation patterns comparable to a healthy state whereas concentric exercises do not.
- EEx modulate neuromuscular control by inducing high levels of motor cortex excitability overruling spinal inhibition present after joint injuries.
- External paced eccentric contractions may be superior in influencing symptomatic persisting tendon pain compared to other modes of contractions delivered in both self-paced and external-paced tendinopathy rehabilitation protocols.
Author and Affiliations:
By Michiel R.M. Twiss, @physiotwiss
Michiel R.M. Twiss is a Dutch physiotherapist in Buchs SG, Switzerland. He has a keen interest in systematic reviews and meta-analyses in gerontology research and specifically strength and conditioning training in old age. He holds a private practice in Buchs, Sankt Gallen, Switzerland.
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