Roberts Thomas J, Higginson Brian K, Nelson Frank E, Gabaldón Annette M
Brown University, Ecology and Evolutionary Biology Department, Box GB205, Providence, RI 02912, USA.
J Exp Biol. 2007 Jul;210(Pt 14):2510-7. doi: 10.1242/jeb.003913.
We examined the length changes and electromyographic (EMG) activity of two hindlimb muscles in wild turkeys, to determine how these muscles modulate mechanical function with changes in running speed and slope. The muscles studied were the iliotibialis lateralis pars postacetabularis (ILPO), a biarticular knee and hip extensor, and the femorotibialis lateralis (FT), a knee extensor. Muscle length changes were recorded using sonomicrometry, and EMG activity was recorded from indwelling bipolar electrodes as the animals walked and ran at a range of speeds (1-3.5 m s(-1)). Treadmill slope was also varied, from a 12 degrees uphill slope to a downhill slope of -12 degrees. To test the hypothesis that the strain pattern in active muscles reflects the demand for mechanical work, we compared strain in the ILPO and FT across the range of slopes. Both muscles underwent active lengthen-shorten cycles during stance. We analyzed the lengthening and shortening part of the strain pattern separately to determine the response of muscle strain to surface slope. In both muscles stance phase shortening strain increased over the range of slopes studied, from 7.8+/-3.5% (ILPO) and 1.9+/-2.2% (FT) during downhill running at -12 degrees, to 30.3+/-3.9% (ILPO) and 8.2+/-5.6% (FT) during uphill running at 12 degrees. Stance-phase lengthening strain was also modulated with slope, from -15.6+/-3.2% (ILPO) and -22.1+/-9.6% (FT) during downhill running at -12 degrees, to -4.2+/-2.5% (ILPO) and -9.0+/-5.6% (FT) during uphill running at 12 degrees. The results suggest that for the ILPO and FT a change in net mechanical work output with running slope is likely mediated by a change in both the lengthening, energy absorbing portion of the contraction and the shortening, energy producing part of the contraction. We also found changes in the timing of EMG activity, and the relative portion of the stance period spent lengthening, which were consistent with a shift in muscle function from energy absorption during downhill running, to net energy production during uphill running. Generally, muscle strain was less affected by speed than by slope. Shortening strains were not significantly correlated with running speed. Only FT lengthening strain changed significantly with speed, ranging from -6.8+/-4.3% at 1 m s(-1) to -15.3+/-4.7% at 3.5 m s(-1). The consistent patterns of strain changes with running slope are evidence that strain pattern is modulated to meet the changes in demand for net mechanical work. The relatively poor relationship between strain and running speed may reflect the fact that changes in running speed during level running are not associated with a change in demand for net mechanical work. Taken together, the speed and slope results suggest that the demand for mechanical work is an important determinant of muscle length patterns in running and walking.
我们研究了野生火鸡两条后肢肌肉的长度变化和肌电图（EMG）活动，以确定这些肌肉如何随着奔跑速度和坡度的变化来调节机械功能。所研究的肌肉是髋臼后部分的外侧髂胫肌（ILPO），它是一个双关节的膝关节和髋关节伸肌，以及外侧股胫肌（FT），它是膝关节伸肌。使用超声测量法记录肌肉长度变化，并在动物以一系列速度（1-3.5米/秒）行走和奔跑时，通过植入的双极电极记录EMG活动。跑步机的坡度也有所变化，从12度上坡到-12度下坡。为了检验活动肌肉中的应变模式反映机械功需求这一假设，我们比较了ILPO和FT在不同坡度范围内的应变。两条肌肉在站立期都经历了主动的拉长-缩短循环。我们分别分析了应变模式的拉长和缩短部分，以确定肌肉应变对表面坡度的反应。在所研究的坡度范围内，两条肌肉的站立期缩短应变均增加，从-12度下坡奔跑时的7.8±3.5%（ILPO）和1.9±2.2%（FT），增加到12度上坡奔跑时的30.3±3.9%（ILPO）和8.2±5.6%（FT）。站立期拉长应变也随坡度而调节，从-12度下坡奔跑时的-15.6±3.2%（ILPO）和-22.1±9.6%（FT），变化到12度上坡奔跑时的-4.2±2.5%（ILPO）和-9.0±5.6%（FT）。结果表明，对于ILPO和FT，随着奔跑坡度的变化，净机械功输出的改变可能是由收缩过程中拉长的、吸收能量的部分以及缩短的、产生能量的部分的变化共同介导的。我们还发现了EMG活动时间的变化，以及站立期用于拉长的相对比例的变化，这与肌肉功能从下坡奔跑时的能量吸收，向上坡奔跑时的净能量产生的转变相一致。一般来说，肌肉应变受速度的影响小于受坡度的影响。缩短应变与奔跑速度没有显著相关性。只有FT的拉长应变随速度有显著变化，范围从1米/秒时的-6.8±4.3%到3.5米/秒时的-15.3±4.7%。随着奔跑坡度变化的应变变化的一致模式证明，应变模式被调节以满足净机械功需求的变化。应变与奔跑速度之间相对较弱的关系可能反映了这样一个事实，即水平奔跑时奔跑速度的变化与净机械功需求的变化无关。综合起来，速度和坡度的结果表明，机械功需求是跑步和行走中肌肉长度模式的一个重要决定因素。
