We thank Dr. Wolf for his thoughtful and enthusiastic commentary on our work and especially his recognition that resistance training in neurologic populations is an effective approach for ‘brain’ training. Indeed, this is the point intended in our study design and we were rewarded to find the results confirmed our hypothesis. While the concept of strengthening is hardly novel to physical therapy practice, the most common intent of strengthening is to address muscular and mechanical factors: hypertrophy and joint stability. However, strength and strengthening involve both neural and muscular factors. The neural aspects of strength are often underappreciated as they are less well understood, especially in persons with neurologic impairment. Interestingly, weakness is among the most prominent characteristics of post-stroke hemiparesis, yet to date there is only equivocal evidence of significant muscle atrophy in persons with hemiparesis.1 Clearly, it is not about the muscle! But more to the point is the rationale that underlies resistance training. While the outward manifestations may look similar between orthopaedic and neurorehabilitative practice, resistance training constitutes a potent form of neuromotor training—with emphasis on the neuro—for persons with neurologic impairments. Data presented in our case indicate that resistance training is an effective approach to improve central motor activation. The specific mechanisms that mediate such neural adaptations remain to be elucidated, especially in persons with post-stroke hemiparesis, but are foremost among the topics of our ongoing neurophysiological research. Neural mechanisms of force production are both sophisticated and task-specific. The target movement and contraction mode are highly relevant to the nervous system and constitute critical signals for inducing training-related adaptation. This concept has been demonstrated through studies comparing: intended vs. actual movement speed (ie, ballistic vs. isometric contractions),2 and executed concentric or eccentric contractions when the intended movements were opposite (ie, eccentric and concentric, respectively). 3 EMG activation patterns precede executed contractions/movements and are consistent with the intended movement rather than the actual muscle contraction. Such findings clearly demonstrate that the primary mechanisms for force production and control are located supraspinally. As an additional piece of this puzzle, equivalent strength gains have been observed in response to training with either executed maximal voluntary contractions or imagined (ie, mental practice) maximal voluntary contractions.4 Importantly, the neural adaptations observed in each of these paradigms occurred in the absence of either actual muscle contraction or the peripheral feedback associated with muscle contraction. Taken together, these findings demonstrate that the intended movement or contraction mode drives supraspinal activity and from this central activity task-specific motor activation ultimately emerges. We assert the goal of resistance training in persons post-stroke is to optimize this central motor activation. This concept of intended movement or contraction mode bears further consideration. Intensity may indeed represent a critical element of the intervention described in this case. However, another critical parameter may be the dynamic nature of the training, that the subject was challenged to move at speeds greater than her existing capacity. Therapeutically-induced adaptations observed in force/torque, movement speed, area-under-the-curve, and EMG followed the course of the intended rather than the executed movements. We expect these mechanistic observations translate to changes at the level of movement (or ‘function’) and, as Dr. Wolf suggested, kinematic analysis documenting such outcomes will strengthen our arguments for the importance of resistance training for neural effects. Rather than its appropriateness in neurorehabilitation, the current focus has shifted to why resistance training is effective and how to improve its generality across a broad spectrum of hemiparetic individuals. Treatment parameters to optimize neural adaptations to resistance training likely differ, especially in neurologically impaired persons. A better understanding of these parameters is among the clinical deliverables anticipated from our research. Finally, Dr. Wolf underscored several mitigating factors to consider if this intervention approach is to gain acceptance in clinical neurorehabilitation practice. Among these is how the severity of motor impairment modifies the potential for positive functional outcome. Recently conducted studies5,6 examined elements of upper-extremity strengthening for persons post-stroke. Subgroup analysis revealed that higher-functioning individuals benefited more from strengthening than lower-functioning individuals. The subject presented in this case was classified by our convention as lower-functioning, yet produced a remarkable, even unexpected, positive functional outcome. Data from our larger clinical trial will afford the opportunity to determine the uniformity of strength gains and clinical/functional improvements across impairment level. We anticipate, however, that the chronicity of hemiparesis and the effects of intervention in acute rehabilitation are equally important covariates in determining the effectiveness of our experimental approach for promoting recovery of UE function.
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Jul 2, 2024
Eldon Graham + 2
Open Access