Shortening velocity and power output of muscles that produce ballistic movements

生物学的新见解往往来自对表现出某种极端行为的动物的研究。在这些动物中，所有动物共有的功能原理被夸大了，因此更容易观察和研究。蟾蜍的弹道式舌头投射就是这样一个极端。在弹道舌头投射过程中，蟾蜍舌头可以以2500 m/s2的加速度离开口腔。这些研究人员的实验室中以前和正在进行的研究集中在阐明蟾蜍张口肌肉产生这种极其快速运动所需的力量的机制。在这个项目中的工作将集中在理解力-速度关系和动力输出的两个肌肉参与提供动力的弹道运动：1）m。下颌降肌，其在舌伸出期间单独负责张口;和2）m.半膜肌沿着其它肌肉在跳跃时伸展后肢。拟议的研究将使用经典的负荷钳或快速释放技术来量化这些肌肉在从等长强直到等张缩短过渡期间的双相缩短行为。在负载钳技术中，激活的肌肉受到刺激而产生张力，同时对抗负载收缩，防止其缩短。当负荷减少时，肌肉首先迅速缩短，然后减速到更慢的缩短速度。与大多数只测量慢相缩短速度的最新研究相反，这些研究人员也测量了初始快相缩短的速度。已经发现了肌肉缩短行为的四个重要的新方面。在快速阶段：1）缩短速度可以很高（高达Vmax的近100倍），并且其在颌和后肢肌肉之间不同; 2）所进行的外部功有助于总功率输出，尽管在颌和后肢肌肉之间贡献的大小不同; 3）缩短速度随着等长预刺激的持续时间而增加;在慢相4）缩短速度也随着等长预刺激的持续时间而增加，至少在某些肌肉中是这样。这些结果是一致的模型，其中一系列的弹性组件内的肌节被拉伸的收缩元素在等长强直，和拉伸的程度增加的持续时间的等长预刺激。初步数据表明，肌肉在缩短的快阶段和慢阶段之间的动能分配方式自适应地不同。一种机制，如磷酸化的粗和/或细丝，控制之间的快速和缓慢阶段的动能分布可能是重要的，在控制肌肉力量输出弹道运动。拟议的研究将扩大这些初步研究。具体而言，研究人员将：1）比较无尾两栖类动物下颌和后肢肌肉的双相缩短，这些无尾两栖类动物在进食和跳跃性能方面存在差异; 2）量化等长预刺激持续时间和肌肉长度对双相缩短的影响; 3）检查自由行为动物运动速度和肌肉预激活持续时间之间的关系。拟议的研究可能会证明：1）在等长预刺激期间储存的大量弹性势能在释放到等张缩短的瞬间转化为动能;以及2）肌肉在缩短的快速和缓慢阶段之间分配这种动能的策略不同，从而在较短收缩与较长收缩期间优化其功率输出。通过这些方式，拟议的研究有可能改变我们对肌肉如何缩短的概念，特别是在弹道运动期间发生的高速运动中。

New insights in biology have often come from studies of animals that exhibit some extreme of performance. In such animals, principles of function common to all animals are exaggerated, and are therefore observed and studied more readily. Ballistic tongue projection in toads represents one such extreme. During ballistic tongue projection, toad tongues can leave the mouth at accelerations of 2500 m/s2. Previous and ongoing research in the laboratories of these investigators has focused on elucidating mechanisms by which mouth opening muscles of toads produce the power required for this extremely rapid movement. The work in this project will focus on understanding the force-velocity relationships and power output of two muscles that are involved in powering ballistic movements: 1) the m. depressor mandibulae, which is solely responsible for mouth opening during tongue projection ; and 2) the m. semimembranosus, which along with other muscles, extends the hind limb during jumping. The proposed studies will use the classic load clamp or quick release technique to quantify the biphasic shortening behavior of these muscles during the transition from isometric tetanus to isotonic shortening. In the load clamp technique, an activated muscle is stimulated to develop tension while contracting against a load that prevents it from shortening. When the load is reduced, the muscle first shortens rapidly and then decelerates to a slower shortening velocity. In contrast to most recent studies which measure only the velocity of shortening during the slow phase, these investigators measured velocity of shortening during the initial fast phase as well. Four important new aspects of the shortening behavior of muscle have been discovered. During the fast phase: 1) shortening velocity can be high (up to nearly 100 times Vmax), and it differs between jaw and hind limb muscles; 2) external work performed contributes to total power output, although the magnitude of the contribution varies between the jaw and hind limb muscles; 3) shortening velocity increases with the duration of isometric pre-stimulation; and during the slow phase 4) shortening velocity also increases with the duration of isometric pre-stimulation, at least in some muscles. These results are consistent with a model in which a series elastic component within the sarcomere is stretched by the contractile elements during isometric tetanus, and the degree of stretch increases with the duration of isometric pre-stimulation. Preliminary data suggest that muscles differ adaptively in how kinetic energy is distributed between the fast and slow phases of shortening. A mechanism, such as phosphorylation of the thick and/or thin filaments, that controls the distribution of kinetic energy between the fast and slow phases would likely be important in controlling muscle power output during ballistic movements. The proposed studies will extend these preliminary studies. Specifically, the investigators will: 1) compare biphasic shortening of anuran jaw and hind limb muscles in species of anurans that differ in feeding and jumping performance; 2) quantify the effects of duration of isometric pre-stimulation and muscle length on biphasic shortening; and 3) examine the relationship between movement velocity and duration of muscle pre-activation in freely behaving animals. The proposed studies are likely to demonstrate: 1) that a significant amount of elastic potential energy, stored during isometric pre-stimulation, is converted to kinetic energy at the instant of release to isotonic shortening; and 2) that muscles differ in their strategies for distributing this kinetic energy between the fast and slow phases of shortening, thereby optimizing their power output during shorter vs. longer contractions. In these ways, the proposed studies have the potential to change our concept of how muscles shorten, especially at the high velocities that occur during ballistic movements.