Chadwell Brad A, Hartwell Hadley J, Peters Susan E
Department of Biology, The University of North Carolina at Charlotte, 9201 University City Blvd., Charlotte, NC 28223, USA.
J Morphol. 2002 Mar;251(3):309-22. doi: 10.1002/jmor.1091.
The leopard frog (Rana pipiens) is an excellent jumper that can reach high take-off velocities and accelerations. It is diurnal, using long, explosive jumps to capture prey and escape predators. The marine toad (Bufo marinus) is a cryptic, nocturnal toad, typically using short, slow hops, or sometimes walking, to patrol its feeding area. Typical of frogs with these different locomotor styles, Rana has relatively long hindlimbs and large (by mass) hindlimb extensor muscles compared to Bufo. We studied the isometric contractile properties of their extensor muscles and found differences that correlate with their different hopping performances. At the hip (semimembranosus, SM), knee (peroneus, Per) and ankle (plantaris longus, PL), we found that Rana's muscles tended to produce greater maximum isometric force relative to body mass, although the difference was significant only for PL. This suggests that differences in force capability at the ankle may be more important than at other joints to produce divergent hopping performances. Maximum isometric force scaled with body mass so that the smaller Rana has relatively larger muscles and force differences between species may reflect size differences only. In addition, Rana's muscles exhibited greater passive resistance to elongation, implying more elastic tissue is present, which may amplify force at take-off due to elastic recoil. Rana's muscles also achieved a higher percentage of maximum force at lower stimulus inputs (frequencies and durations) than in Bufo, perhaps amplifying the differences in force available for limb extension during natural stimulation. Twitch contraction and relaxation times tended to be faster in Rana, although variation was great, so that differences were significant only for Per. Fatigability also tended to be greater in Rana muscles, although, again, values reached significance in only one muscle (PL). Thus, in addition to biomechanical effects, differences in hopping performance may also be determined by diverse physiological properties of the muscles.
豹蛙（北美豹蛙）是出色的跳跃者，能够达到很高的起跳速度和加速度。它是昼行性动物，通过长距离的爆发式跳跃来捕食猎物和躲避捕食者。海蟾蜍（美洲巨蟾蜍）是一种善于隐藏的夜行性蟾蜍，通常以短距离、缓慢的跳跃，或者有时以行走的方式在其觅食区域巡逻。与具有不同运动方式的蛙类一样，相较于海蟾蜍，豹蛙的后肢相对较长，且（按质量计算）后肢伸肌较大。我们研究了它们伸肌的等长收缩特性，发现这些差异与它们不同的跳跃表现相关。在髋部（半膜肌，SM）、膝部（腓骨肌，Per）和踝部（长跖肌，PL），我们发现，相对于体重而言，豹蛙的肌肉往往能产生更大的最大等长力，不过这种差异仅在PL肌肉上显著。这表明，在产生不同的跳跃表现方面，踝关节处的力量能力差异可能比其他关节处的差异更为重要。最大等长力与体重呈比例关系，因此体型较小的豹蛙具有相对较大的肌肉，物种之间的力量差异可能仅反映了体型差异。此外，豹蛙的肌肉对伸长表现出更大的被动阻力，这意味着存在更多的弹性组织，这可能会由于弹性回缩而在起跳时放大力量。在较低的刺激输入（频率和持续时间）下，豹蛙的肌肉也比海蟾蜍的肌肉能达到更高的最大力量百分比，这可能会放大自然刺激期间肢体伸展可用力量的差异。豹蛙的肌肉的抽搐收缩和舒张时间往往更快，尽管变化很大，所以差异仅在Per肌肉上显著。豹蛙肌肉的疲劳性也往往更大，不过同样，只有一块肌肉（PL）的值达到了显著水平。因此，除了生物力学效应外，跳跃表现的差异也可能由肌肉的多种生理特性决定。
