Fusimotor reflexes to antagonistic muscles simultaneously assessed by multi-afferent recordings from muscle spindle afferents

Anterior Cruciate Ligament Injury Prevention Exercises: Could a Neuromuscular Warm-Up Improve Muscle Pre-Activation before a Soccer Game? A Proof-of-Principle Study on Professional Football Players

Neuromuscular warm-up has been shown to decrease the risk of anterior cruciate ligament (ACL) injury improving muscular firing patterns. All preventive training programs described in the literature have a duration of several weeks. To date, no studies have explored the immediate effect of a neuromuscular warm-up exercise on pre-activation time of the knee stabilizer muscles. Thus, this proof-of-principle study aimed at evaluating the acute effects of a neuromuscular warm-up exercises on the electromyographic activation of knee stabilizer muscles’ activation pattern. We included 11 professional football players, mean aged 23.2 ± 4.5 years, from a Southern Italy football team. All of them underwent a standard warm-up exercise protocol at the first day of the evaluation. At 1 week, they underwent a structured neuromuscular warm-up exercise protocol. We assessed as outcome measure the pre-activation time (ms) of rectus femoris (RF), vastus medialis (VM), biceps femoris (BF), and medial hamstrings (MH) upon landing. Outcomes were assessed before and after the standard warm-up and neuromuscular warm-up. Pre-activation time of RF, VM, BF and MH significantly improved only after neuromuscular warm-up (p < 0.05); moreover, there was a significant (p < 0.05) between-group difference in pre-activation time of all muscles after the neuromuscular warm-up compared with the standard warm-up. These findings suggested that physical exercise consisting of a structured injury prevention neuromuscular warm-up might have an immediate effect in improving the activation time of the knee stabilizer muscles, thus potentially reducing the risk of ACL injury.

Gamma Loop Dysfunction as a Possible Neurophysiological Mechanism of Arthrogenic Muscle Inhibition: A Narrative Review of the Literature

Context: Quadriceps activation failure has been observed following various pathological conditions in a knee joint such as knee surgery, pain, effusion in knee, and osteoarthritis also could be aging matter. Those patients are unable to attain maximal quadriceps strength for a long period of time although their quadriceps itself is not damaged. This impairment is termed arthrogenic muscle inhibition (AMI). AMI has been of concern to clinicians because this weakness hinders the rehabilitation process considerably and delays recovery because strengthening protocols for the AMI could be largely ineffective. Clinically, it is important to understand neurophysiological mechanisms of the AMI to treat patients with the impairment. Objectives: This is a narrative review of the literature. The purpose of this review is to understand the following: (1) Why investigations of only peripheral spinal reflexive pathways are not enough for elucidation of the mechanisms of the AMI? (2) What we know about the role of the gamma spindle system in AMI so far? (3) Could a dysfunctional gamma spindle system contribute to AMI lead neural changes in upper central nervous system? and (4) Concerns that a clinician should take into consideration when deciding whether to apply therapeutic interventions for AMI. Data Sources: The databases PubMed, MEDLINE, SPORTDiscus, and CINAHL were searched with the terms arthrogenic muscle inhibition (AMI), reflex inhibition, joint mechanoreceptor, gamma loop, corticospinal pathway, spinal reflex, effusion, and joint injury. The remaining citations were collected from references of similar papers. Conclusions: AMI is a limiting factor in the rehabilitation of joint injury. Motor unit recruitment could be hindered in patients with AMI as a result of a dysfunctional gamma spindle system. Clinicians should understand the mechanism of AMI well in order to establish effective rehabilitation programs for AMI. Indeed, AMI is not caused by a single factor, but rather, multiple neural factors can change over time following the appearance of AMI. Therefore, multiple interventions targeting different neural pathways should be combined to achieve the ideal therapeutic goal for the treatment of AMI.

Genetically identified spinal interneurons integrating tactile afferents for motor control

Our movements are shaped by our perception of the world as communicated by our senses. Perception of sensory information has been largely attributed to cortical activity. However, a prior level of sensory processing occurs in the spinal cord. Indeed, sensory inputs directly project to many spinal circuits, some of which communicate with motor circuits within the spinal cord. Therefore, the processing of sensory information for the purpose of ensuring proper movements is distributed between spinal and supraspinal circuits. The mechanisms underlying the integration of sensory information for motor control at the level of the spinal cord have yet to be fully described. Recent research has led to the characterization of spinal neuron populations that share common molecular identities. Identification of molecular markers that define specific populations of spinal neurons is a prerequisite to the application of genetic techniques devised to both delineate the function of these spinal neurons and their connectivity. This strategy has been used in the study of spinal neurons that receive tactile inputs from sensory neurons innervating the skin. As a result, the circuits that include these spinal neurons have been revealed to play important roles in specific aspects of motor function. We describe these genetically identified spinal neurons that integrate tactile information and the contribution of these studies to our understanding of how tactile information shapes motor output. Furthermore, we describe future opportunities that these circuits present for shedding light on the neural mechanisms of tactile processing.

Selfishness examined: Cooperation in the absence of egoistic incentives

Social dilemmas occur when the pursuit of self-interest by individuals in a group leads to less than optimal collective outcomes for everyone in the group. A critical assumption in the human sciences is that people's choices in such dilemmas are individualistic, selfish, and rational. Hence, cooperation in the support of group welfare will only occur if there are selfish incentives that convert the social dilemma into a nondilemma. In recent years, inclusive fitness theories have lent weight to such traditional views of rational selfishness on Darwinian grounds. To show that cooperation is based on selfish incentives, however, one must provide evidence that people do not cooperate without such incentives. In a series of experimental social dilemmas, subjects were instructed to make single, anonymous choices about whether or not to contribute money for a shared “bonus” that would be provided only if enough other people in the group also contributed their money. Noncontributors cited selfish reasons for their choices; contributors did not. If people are allowed to engage in discussion, they will contribute resources at high rates, frequently on irrational grounds, to promote group welfare. These findings are consistent with previous research on ingroup biasing effects that cannot be explained by “economic man” or “selfish gene” theories. An alternative explanation is that sociality was a primary factor shaping the evolution of Homo sapiens. The cognitive and affective mechanisms underlying such choices evolved under selection pressures on small groups for developing and maintaining group membership and for predicting and controlling the behavior of other group members. This sociality hypothesis organizes previously inexplicable and disparate phenomena in a Darwinian framework and makes novel predictions about human choice.

On the function of muscle and reflex partitioning

Studies have shown that in the mammalian neuromuscular system stretch reflexes are localized within individual muscles. Neuromuscular compartmentalization, the partitioning of sensory output from muscles, and the partitioning of segmental pathways to motor nuclei have also been demonstrated. This evidence indicates that individual motor nuclei and the muscles they innervate are not homogeneous functional units. An analysis of the functional significance of reflex localization and partitioning suggests that segmental control mechanisms are based on subdivisions of motor nuclei–muscle complexes. A partitioned organization of segmental control mechanisms could utilize (1) the potential functional diversity of muscle fiber types, (2) the variety of mechanical actions of individual muscles arising from their distributed origins and insertions, and (3) diverse architectural features such as intramuscular variations in pinnation and complex in-series and in-parallel arrangements of muscle fibers. The differentiated activity observed in some muscles during natural movements also calls for localized segmental control mechanisms. Partitioning may also play a role in mechanical interactions between contracting motor units and in increasing the stability of neuromuscular systems. The functional advantages of reflex localization and partitioning suggest they are probably common features of segmental systems, whose organization reflects the structure and function of their associated neuromuscular systems.