Effects of acute fatigue on the volitional and magnetically-evoked electromechanical delay of the knee flexors in males and females

Neuromuscular performance capabilities, including those measured by evoked responses, may be adversely affected by fatigue; however, the capability of the neuromuscular system to initiate muscle force rapidly under these circumstances is yet to be established. Sex-differences in the acute responses of neuromuscular performance to exercise stress may be linked to evidence that females are much more vulnerable to ACL injury than males. Optimal functioning of the knee flexors is paramount to the dynamic stabilisation of the knee joint, therefore the aim of this investigation was to examine the effects of acute maximal intensity fatiguing exercise on the voluntary and magnetically-evoked electromechanical delay in the knee flexors of males and females. Knee flexor volitional and magnetically-evoked neuromuscular performance was assessed in seven male and nine females prior to and immediately after: (i) an intervention condition comprising a fatigue trial of 30-seconds maximal static exercise of the knee flexors, (ii) a control condition consisting of no exercise. The results showed that the fatigue intervention was associated with a substantive reduction in volitional peak force (PFV) that was greater in males compared to females (15.0%, 10.2%, respectively, p < 0.01) and impairment to volitional electromechanical delay (EMDV) in females exclusively (19.3%, p < 0.05). Similar improvements in magnetically-evoked electromechanical delay in males and females following fatigue (21%, p < 0.001), however, may suggest a vital facilitatory mechanism to overcome the effects of impaired voluntary capabilities, and a faster neuromuscular response that can be deployed during critical times to protect the joint system

Interactions between Connected Half-Sarcomeres Produce Emergent Mechanical Behavior in a Mathematical Model of Muscle

Most reductionist theories of muscle attribute a fiber's mechanical properties to the scaled behavior of a single half-sarcomere. Mathematical models of this type can explain many of the known mechanical properties of muscle but have to incorporate a passive mechanical component that becomes ∼300% stiffer in activating conditions to reproduce the force response elicited by stretching a fast mammalian muscle fiber. The available experimental data suggests that titin filaments, which are the mostly likely source of the passive component, become at most ∼30% stiffer in saturating Ca2+ solutions. The work described in this manuscript used computer modeling to test an alternative systems theory that attributes the stretch response of a mammalian fiber to the composite behavior of a collection of half-sarcomeres. The principal finding was that the stretch response of a chemically permeabilized rabbit psoas fiber could be reproduced with a framework consisting of 300 half-sarcomeres arranged in 6 parallel myofibrils without requiring titin filaments to stiffen in activating solutions. Ablation of inter-myofibrillar links in the computer simulations lowered isometric force values and lowered energy absorption during a stretch. This computed behavior mimics effects previously observed in experiments using muscles from desmin-deficient mice in which the connections between Z-disks in adjacent myofibrils are presumably compromised. The current simulations suggest that muscle fibers exhibit emergent properties that reflect interactions between half-sarcomeres and are not properties of a single half-sarcomere in isolation. It is therefore likely that full quantitative understanding of a fiber's mechanical properties requires detailed analysis of a complete fiber system and cannot be achieved by focusing solely on the properties of a single half-sarcomere

Variable, but not free-weight, resistance back squat exercise potentiates jump performance following a comprehensive task-specific warm-up

Studies examining acute, high-speed movement performance enhancement following intense muscular contractions (frequently called "post-activation potentiation"; PAP) often impose a limited warm-up, compromizing external validity. In the present study, the effects on countermovement vertical jump (CMJ) performance of back squat exercises performed with or without elastic bands during warm-up were compared. After familiarization, fifteen active men visited the laboratory on two occasions under randomized, counterbalanced experimental squat warm-up conditions: (a) free-weight resistance (FWR) and (b) variable resistance (VR). After completing a comprehensive task-specific warm-up, three maximal CMJs were performed followed by three back squat repetitions completed at 85% of 1-RM using either FWR or VR Three CMJs were then performed 30 seconds, 4 minutes, 8 minutes, and 12 minutes later. During CMJ trials, hip, knee, and ankle joint kinematics, ground reaction force data and vastus medialis, vastus lateralis, and gluteus maximus electromyograms (EMG) were recorded simultaneously using 3D motion analysis, force platform, and EMG techniques, respectively. No change in any variable occurred after FWR (P > 0.05). Significant increases (P < 0.05) were detected at all time points following VR in CMJ height (5.3%-6.5%), peak power (4.4%-5.9%), rate of force development (12.9%-19.1%), peak concentric knee angular velocity (3.1%-4.1%), and mean concentric vastus lateralis EMG activity (27.5%-33.4%). The lack of effect of the free-weight conditioning contractions suggests that the comprehensive task-specific warm-up routine mitigated any further performance augmentation. However, the improved CMJ performance following the use of elastic bands is indicative that specific alterations in force-time properties of warm-up exercises may further improve performance

Dorsiflexor and plantarflexor torque-angle and torque-velocity relationships of classical ballet dancers and volleyball players

Resumo: O objetivo desse estudo foi comparar as relações torque-ângulo e torque-velocidade e a ativação dos músculos flexores plantares e dorsiflexores entre bailarinas clássicas (n=14) e atletas de voleibol (n=22). O pico de torque dos flexores plantares e dorsiflexores foi avaliado durante contrações voluntárias máximas isométricas nos ângulos de -10°, 0°, 10°, 20°, 30°, 40° e 50°, e durante contrações concêntricas nas velocidades angulares de 0°/s, 60°/s, 120°/s, 180°/s, 240°/s, 300°/s, 360°/s e 420°/s. Sinais eletromiográficos (EMG) de superfície foram obtidos dos músculos gastrocnêmio medial, sóleo e tibial anterior. A amplitude de movimento de dorsiflexão foi semelhante entre os grupos, enquanto bailarinas apresentaram maior amplitude de flexão plantar do que atletas de voleibol. Enquanto nos músculos flexores plantares a relação torque-ângulo das bailarinas deslocou-se para a esquerda quando comparada à das atletas de voleibol, nos flexores dorsais ela se deslocou para a direita nos menores comprimentos musculares. Os torques normalizados em todas as velocidades de flexão plantar e dorsiflexão foram mais elevados nas bailarinas do que nas atletas de voleibol. Os sinais EMG do gastrocnêmio medial e do sóleo permaneceram aproximadamente constantes entre os diferentes ângulos articulares nas bailarinas, mas diminuíram com a redução no comprimento muscular no caso das atletas de voleibol. Os sinais EMG do tibial anterior aumentaram com a redução dos ângulos do tornozelo em ambos os grupos. Os sinais EMG dos dorsiflexores nas diferentes velocidades angulares foram semelhantes nos grupos, enquanto os sinais EMG do sóleo e do gastrocnêmio foram mais elevados nas bailarinas comparados aos das atletas de voleibol. As adaptações dos flexores plantares podem ser explicadas por alterações musculares intrínsecas e alterações na ativação voluntária máxima, enquanto para os músculos dorsiflexores somente mudanças nas propriedades intrínsecas parecem explicar os resultados observados. Os torques relativos mais elevados das bailarinas comparados aos das atletas de voleibol são provavelmente resultantes da ativação aumentada dos flexores plantares e de um maior comprimento de fibra dos dorsiflexores. Palavras-chave: propriedades mecânicas musculares, eletromiografia, ballet clássico, voleibol. Abstract: The purpose of this study was to compare the torque-angle and torque-velocity relationships, and the electromyographic (EMG) activity of the plantar-and dorsiflexor muscle groups of classical ballet dancers (n=14) and volleyball players (n=22). Peak torques of the ankle plantar-and dorsiflexor muscles were evaluated for maximal voluntary isometric contractions performed at seven different ankle angles (-10°, 0°, 10°, 20°, 30°, 40°, 50°) and for maximal effort, concentric, voluntary contractions performed at angular velocities of 0°/s, 60°/s, 120°/s, 180°/s, 240°/s, 300°/s, 360°/s and 420°/s. Bipolar surface EMG signals were obtained from gastrocnemius medialis, soleus and tibialis anterior muscles, and ankle range of motion was measured with a goniometer. The range of motion for dorsiflexion was the same for both groups, while ballet dancers had a greater range for plantarflexion than volleyball players. For the plantarflexor muscles, the torque-angle relationship was shifted to the left for the ballet dancers compared to the volleyball players, while for the dorsiflexor muscles it was shifted to the right for short dorsiflexor lengths. The normalized torques at all speeds of plantar-and dorsiflexion were greater for the ballet dancers than the volleyball players. The gastrocnemius medialis and soleus EMGs remained nearly constant across all angles for the ballet dancers, but decreased with decreasing muscle length in the volleyball players. The tibialis anterior EMGs increased with decreasing ankle angles in both groups. The normalized dorsiflexor EMGs were the same for both groups across all speeds, while the EMGs for soleus and gastrocnemius were significantly greater for the ballet dancers than the volleyball players. These results support the idea that systematic physical activity changes the in vivo torque-angle and torque-velocity relationships in accordance with functional demands. The greater relative torques for the ballet dancers than volleyball players are likely caused by the increased activation of the plantarflexors and an increased fiber length for the dorsiflexors

Residual force enhancement in skeletal muscle

Residual force enhancement has been observed consistently in skeletal muscles following active stretching. However, its underlying mechanism(s) remain elusive, and it cannot be explained readily within the framework of the cross-bridge theory. Traditionally, residual force enhancement has been attributed to the development of sarcomere length non-uniformities. However, recent evidence suggests that this might not be the case. Rather, it appears that residual force enhancement has an active and a passive component. The active component is tentatively associated with changes in the cross-bridge kinetics that might be reflected in decreased detachment rates following active muscle stretching, while the passive component possibly originates from a structural protein, such as titin, whose stiffness might be regulated by calcium