Axial muscle activation provides stabilization against perturbations while running.

Incidence of traumatic brain injury is an important hazard in sports and recreation. Unexpected (blind-sided) impacts with other players, obstacles, and the ground can be particularly dangerous. We believe this is partially due to the lack of muscular activation which would have otherwise provided protective bracing. In this study participants were asked to run on the treadmill while undergoing perturbations applied at the waist which pulled participants in the fore-aft and lateral directions. To determine the effect of unexpected impacts, participants were given a directional audio-visual warning 0.5 s prior to the perturbation in half of the trials and were unwarned in the other half of the trials. Perturbations were given during the start of the stance phase and during the start of the flight phase to examine two distinct points within the locomotor cycle. Muscle activity was monitored in axial muscles before, during, and after the perturbations were given. We hypothesized that the presence of a warning would allow for voluntary axial muscle activity prior to and during perturbations that would provide bracing of the body, and decreased displacement and acceleration of the head compared to unwarned perturbations. Our results indicate that when a warning is given prior to perturbation, the body was displaced significantly less, and the linear acceleration of the head was also significantly lessened in response to some perturbations. The perturbations given in this study caused significant increases in axial muscle activity compared to activity present during control running. We found evidence that cervical and abdominal muscles increased activity in response to the warning and that typically the warned trials displayed a lower reflexive muscle activity response. Additionally, we found a stronger effect of the warnings on muscle activity within the perturbations given during flight phase than those given at stance phase. Results from this study support the hypothesis that knowledge regarding an impending perturbation is used by the neuromuscular system to activate relevant core musculature and provide bracing to the athlete.

The Human Neck is Part of the Musculoskeletal Core: Cervical Muscles Help Stabilize the Pelvis During Running and Jumping.

During locomotion, cervical muscles must be active to stabilize the head as the body accelerates and decelerates. We hypothesized that cervical muscles are also part of the linked chain of axial muscles that provide core stabilization against torques applied to the hip joint by the extrinsic muscles of the legs. To test whether specific cervical muscles play a role in postural stabilization of the head and/or core stabilization of the pelvic girdle, we used surface electromyography to measure changes in muscle activity in response to force manipulations during constant speed running and maximum effort counter-movement jumps. We found that doubling the mass of the head during both running and maximum effort jumping had little or no effect on (1) acceleration of the body and (2) cervical muscle activity. Application of horizontal forward and rearward directed forces at the pelvis during running tripled mean fore and aft accelerations, thereby increasing both the pitching moments on the head and flexion and extension torques applied to the hip. These manipulations primarily resulted in increases in cervical muscle activity that is appropriate for core stabilization of the pelvis. Additionally, when subjects jumped maximally with an applied downward directed force that reduced acceleration and therefore need for cervical muscles to stabilize the head, cervical muscle activity did not decrease. These results suggest that during locomotion, rather than acting to stabilize the head against the effects of inertia, the superficial muscles of the neck monitored in this study help to stabilize the pelvis against torques imposed by the extrinsic muscles of the legs at the hip joint. We suggest that a division of labor may exist between deep cervical muscles that presumably provide postural stabilization of the head versus superficial cervical muscles that provide core stabilization against torques applied to the pelvic and pectoral girdles by the extrinsic appendicular muscles.

Durante la locomoción, los músculos cervicales deben estar activos para estabilizar la cabeza a medida que el cuerpo acelera y desacelera. Presumimos que los músculos cervicales también son parte de la cadena unida de músculos axiales que brindan estabilización central contra las torsiones aplicadas a la articulación de la cadera por los músculos extrínsecos de las piernas. Para evaluar si los músculos cervicales específicos desempeñan un papel en la estabilización postural de la cabeza y/o la estabilización central de la cintura pélvica, utilizamos electromiografía de superficie para medir los cambios en la actividad muscular en respuesta a las manipulaciones de fuerza durante la carrera a velocidad constante y los saltos con contramovimiento de esfuerzo máximo. Descubrimos que duplicar la masa de la cabeza durante la carrera y el salto de máximo esfuerzo tuvo poco o ningún efecto sobre (1) la aceleración del cuerpo y (2) la actividad de los músculos cervicales. La aplicación de fuerzas horizontales dirigidas hacia adelante y hacia atrás en la pelvis durante la carrera triplicó las aceleraciones medias hacia adelante y hacia atrás, aumentando así tanto los momentos de cabeceo en la cabeza como los pares de flexión y extensión aplicados a la cadera. Estas manipulaciones dieron como resultado principalmente aumentos en la actividad de los músculos cervicales que son apropiados para la estabilización central de la pelvis. Además, cuando los sujetos saltaban al máximo aplicando una fuerza dirigida hacia abajo que reducía la aceleración y, por lo tanto, la necesidad de los músculos cervicales para estabilizar la cabeza, la actividad de los músculos cervicales no disminuía. Sugerimos que puede existir una división del trabajo entre los músculos cervicales profundos que presumiblemente brindan estabilización postural de la cabeza versus los músculos cervicales superficiales que brindan estabilización central contra los torques aplicados a las cinturas pélvica y pectoral por los músculos apendiculares extrínsecos.

Pendant la locomotion, les muscles cervicaux doivent être actifs pour stabiliser la tête lorsque le corps accélère et décélère. Nous avons émis l"hypothèse que les muscles cervicaux font également partie de la chaîne liée des muscles axiaux qui assurent la stabilisation du noyau contre les couples appliqués à l"articulation de la hanche par les muscles extrinsèques des jambes. Pour tester si des muscles cervicaux spécifiques jouent un rôle dans la stabilisation posturale de la tête et/ou la stabilisation centrale de la ceinture pelvienne, nous avons utilisé l"électromyographie de surface pour mesurer les changements dans l"activité musculaire en réponse aux manipulations de force pendant la course à vitesse constante et les sauts de contre-mouvement à effort maximal. Nous avons constaté que doubler la masse de la tête pendant la course et le saut à effort maximal avait peu ou pas d"effet sur (1) l"accélération du corps et (2) l"activité des muscles cervicaux. L"application de forces horizontales dirigées vers l"avant et vers l"arrière au niveau du bassin pendant la course a triplé les accélérations moyennes vers l"avant et vers l"arrière, augmentant ainsi à la fois les moments de tangage sur la tête et les couples de flexion et d"extension appliqués à la hanche. Ces manipulations ont principalement entraîné une augmentation de l"activité des muscles cervicaux, ce qui est approprié pour la stabilisation centrale du bassin. De plus, lorsque les sujets sautaient au maximum avec une force dirigée vers le bas qui réduisait l"accélération et donc le besoin de muscles cervicaux pour stabiliser la tête, l"activité des muscles cervicaux ne diminuait pas. Ces résultats suggèrent que lors de la locomotion, plutôt que d"agir pour stabiliser la tête contre les effets de l"inertie, les muscles superficiels du cou suivis dans cette étude aident à stabiliser le bassin contre les couples imposés par les muscles extrinsèques des jambes au niveau de l"articulation de la hanche. Nous suggérons qu"une division du travail peut exister entre les muscles cervicaux profonds qui assurent vraisemblablement la stabilisation posturale de la tête et les muscles cervicaux superficiels qui assurent la stabilisation centrale contre les couples appliqués aux ceintures pelvienne et pectorale par les muscles appendiculaires extrinsèques.

GDNF synthesis, signaling, and retrograde transport in motor neurons.

Glial cell line-derived neurotrophic factor (GDNF) is a 134 amino acid protein belonging in the GDNF family ligands (GFLs). GDNF was originally isolated from rat glial cell lines and identified as a neurotrophic factor with the ability to promote dopamine uptake within midbrain dopaminergic neurons. Since its discovery, the potential neuroprotective effects of GDNF have been researched extensively, and the effect of GDNF on motor neurons will be discussed herein. Similar to other members of the TGF-ß superfamily, GDNF is first synthesized as a precursor protein (pro-GDNF). After a series of protein cleavage and processing, the 211 amino acid pro-GDNF is finally converted into the active and mature form of GDNF. GDNF has the ability to trigger receptor tyrosine kinase RET phosphorylation, whose downstream effects have been found to promote neuronal health and survival. The binding of GDNF to its receptors triggers several intracellular signaling pathways which play roles in promoting the development, survival, and maintenance of neuron-neuron and neuron-target tissue interactions. The synthesis and regulation of GDNF have been shown to be altered in many diseases, aging, exercise, and addiction. The neuroprotective effects of GDNF may be used to develop treatments and therapies to ameliorate neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). In this review, we provide a detailed discussion of the general roles of GDNF and its production, delivery, secretion, and neuroprotective effects on motor neurons within the mammalian neuromuscular system.