Airway smooth muscle adapting in dynamic conditions is refractory to the bronchodilator effect of a deep inspiration

Airway smooth muscle (ASM) is continuously strained during breathing at tidal volume. Whether this tidal strain influences the magnitude of the bronchodilator response to a deep inspiration (DI) is not clearly defined. The present in vitro study examines the effect of tidal strain on the bronchodilator effect of DIs. ASM strips from sheep tracheas were mounted in organ baths and then subjected to stretches (30% strain), simulating DIs at varying time intervals. In between simulated DIs, the strips were either held at a fixed length (isometric) or oscillated continuously by 6% (length oscillations) to simulate tidal strain. The contractile state of the strips was also controlled by adding either methacholine or isoproterenol to activate or relax ASM, respectively. Although the time-dependent gain in force caused by methacholine was attenuated by length oscillations, part of the acquired force in the oscillating condition was preserved postsimulated DIs, which was not the case in the isometric condition. Consequently, the bronchodilator effect of simulated DIs (i.e., the decline in force postsimulated versus presimulated DIs) was attenuated in oscillating versus isometric conditions. These findings suggest that an ASM operating in a dynamic environment acquired adaptations that make it refractory to the decline in contractility inflicted by a larger strain simulating a DI.

Spinal excitability is increased in the torque-depressed isometric steady state following active muscle shortening

Torque depression (TD) is the reduction in steady-state isometric torque following active muscle shortening when compared with a purely isometric contraction at the same muscle length and level of activation. The purpose of the present study was to assess spinal and supraspinal excitability in the TD state during submaximal contractions of the dorsiflexors. Eleven young (24 ± 2 yrs) males performed 16 contractions at a constant level of electromyographic activity (40% of maximum). Half of the contractions were purely isometric (8 s at an ankle angle of 100°), whereas the other half induced TD (2 s isometric at 140°, a 1 s shortening phase at 40° s −1 and 5 s at 100°). Motor evoked potentials (MEPs), cervicomedullary motor evoked potentials (CMEPs) and compound muscle action potentials (M-waves) were recorded from tibialis anterior during the TD steady-state and purely isometric contractions. When compared with values in the purely isometric condition, following active shortening, there was a 13% decrease in torque ( p < 0.05), with a 10% increase in normalized CMEP amplitude (CMEP/Mmax) ( p < 0.05) and no change in normalized MEP amplitude (MEP/CMEP) in the TD state ( p > 0.05). These findings indicate that during voluntary contractions in the TD state, the history-dependent properties of muscle can increase spinal excitability and influence voluntary control of submaximal torque production.

Potential of M-Wave Elicited by Double Pulse for Muscle Fatigue Evaluation in Intermittent Muscle Activation by Functional Electrical Stimulation for Motor Rehabilitation

Clinical studies on application of functional electrical stimulation (FES) to motor rehabilitation have been increasing. However, muscle fatigue appears early in the course of repetitive movement production training by FES. Although M-wave variables were suggested to be reliable indices of muscle fatigue in long lasting constant electrical stimulation under the isometric condition, the ability of M-wave needs more studies under intermittent stimulation condition, because the intervals between electrical stimulations help recovery of muscle activation level. In this paper, M-waves elicited by double pulses were examined in muscle fatigue evaluation during repetitive movements considering rehabilitation training with surface electrical stimulation. M-waves were measured under the two conditions of repetitive stimulation: knee extension force production under the isometric condition and the dynamic movement condition by knee joint angle control. Amplitude of M-wave elicited by the 2nd pulse of a double pulse decreased during muscle fatigue in both measurement conditions, while the change in M-waves elicited by single pulses in a stimulation burst was not relevant to muscle fatigue in repeated activation with stimulation interval of 1 s. Fatigue index obtained from M-waves elicited by 2nd pulses was suggested to provide good estimation of muscle fatigue during repetitive movements with FES.

Visual Feedback of Bilateral Bite Force to Assess Motor Control of the Mandible in Isometric Condition

The assessment of the individual ability of modulating and coordinating the right and left bite force is poorly investigated. The present study describes a methodology for the assessment of the bilateral control of the biting force and evaluates the test-retest reliability in a sample of 13 healthy subjects. By modulating the intensity and the left/right balance of the biting force, the subject was able to drive a cursor on the screen to “reach and hold” targets, randomly generated within the physiological “range of force” of the subject. The average motor performance was evaluated by the mean cursor-target distance = 13 ± 5%, the Offset Error = 9 ± 5% and the standard deviation of the force vector = 17.7 ± 6.1% (expressed as % of the target). Mean distance and standard deviation indices had acceptable reliability. This technique improves the characterization of the mandibular motor function and it may have a relevant role for the assessment and rehabilitation of the neuromusculoskeletal disorders affecting the orofacial system.

Higher Neuromuscular Manifestations of Fatigue in Dynamic than Isometric Pull-Up Tasks in Rock Climbers

Neuromuscular assessment of rock climbers has been mainly focused on forearm muscles in the literature. We aimed to extend the body of knowledge investigating on two other upper limb muscles during sport-specific activities in nine male rock climbers. We assessed neuromuscular manifestations of fatigue recording surface electromyographic signals from brachioradialis and teres major muscles, using multi-channel electrode arrays. Participants performed two tasks until volitional exhaustion: a sequence of dynamic pull-ups and an isometric contraction sustaining the body at half-way of a pull-up (with the elbows flexed at 90°). The tasks were performed in randomized order with 10 minutes of rest in between. The normalized rate of change of muscle fiber conduction velocity was calculated as the index of fatigue. The time-to-task failure was significantly shorter in the dynamic (31 ±10 s) than isometric contraction (59 ±19 s). The rate of decrease of muscle fiber conduction velocity was found steeper in the dynamic than isometric task both in brachioradialis (isometric: −0.2 ±0.1%/s; dynamic: −1.2 ±0.6%/s) and teres major muscles (isometric: −0.4±0.3%/s; dynamic: −1.8±0.7%/s). The main finding was that a sequence of dynamic pull-ups lead to higher fatigue than sustaining the body weight in an isometric condition at half-way of a pull-up. Furthermore, we confirmed the possibility to properly record physiological CV estimates from two muscles, which had never been studied before in rock climbing, in highly dynamic contractions.

Contractive Spectral Triples for Crossed Products

Connes showed that spectral triples encode (noncommutative) metric information. Further, Connes and Moscovici in their metric bundle construction showed that, as with the Takesaki duality theorem, forming a crossed product spectral triple can substantially simplify the structure. In a recent paper, Bellissard, Marcolli and Reihani (among other things) studied in depth metric notions for spectral triples and crossed product spectral triples for $Z$-actions, with applications in number theory and coding theory. In the work of Connes and Moscovici, crossed products involving groups of diffeomorphisms and even of étale groupoids are required. With this motivation, the present paper develops part of the Bellissard-Marcolli-Reihani theory for a general discrete group action, and in particular, introduces coaction spectral triples and their associated metric notions. The isometric condition is replaced by the contractive condition.

Morphometric Studies on Subadult Liza subviridis and Sillago sihamafrom Sonmiani Bay (Miani Hor), Balochistan, Pakistan

Length-weight relationship of Liza subviridis and Sillago sihama (Family: Mugilidae; Class: Pisces and Family: Sillaginidae; Class: Pisces) in subadults sampled from Somiani Bay Balochistan, Pakistan during 2002, 2003 and 2006 was analyzed. Liza subviridis showed positive allometry (3.23) in 2002, and nHgative allometry during 2003 (2.95) and (1.95). Sub adults of Sillago sihama showed positive allometry during 2002 (3.10) and 2003 (3.13). An isometric condition (3.02) was observed in the samples of Sillago sihama collected from Bhaira in 2006.

Cooperativity of myosin molecules through strain–dependent chemistry

There is mounting evidence that the myosin head domain contains a lever arm which amplifies small structural changes that occur at the nucleotide–binding site. The mechanical work associated with movement of the lever affects the rates at which the products of ATP hydrolysis are released. During muscle contraction, this strain–dependent chemistry leads to cooperativity of the myosin molecules within a thick filament. Two aspects of cooperative action are discussed, in the context of a simple stochastic model. (i) A modest motion of the lever arm on ADP release can serve to regulate the fraction of myosin bound to the thin filament, in order to recruit more heads at higher loads. (ii) If the lever swings through a large angle when phosphate is released, the chemical cycles of the myosin molecules can be synchronized at high loads. This leads to stepwise sliding of the filaments and suggests that the isometric condition is not a steady state.