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First published online June 27, 2008
Journal of Experimental Biology 211, 2303-2316 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.016139
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Integration within and between muscles during terrestrial locomotion: effects of incline and speed

Timothy E. Higham* and Andrew A. Biewener

Department of Organismic and Evolutionary Biology, Concord Field Station, Harvard University, 100 Old Causeway Road, Bedford, MA 01730, USA


Figure 1
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  		Fig. 1. A diagram showing the location of the muscles relative to the animal's body (A), and the location of the sonomicrometry crystals implanted into the medial gastrocnemius (blue) and the lateral gastrocnemius (red) (B). The center of rotation at the knee (above) and ankle (below) are also shown. Adapted from Higham et al. (Higham et al., 2008Go).

 

Figure 2
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  		Fig. 2. Two consecutive representative strides at 0.5 m s–1 on a level (solid lines) and inclined (broken lines) treadmill showing the changes in fascicle length (A) for the LG (red), pMG (blue) and dMG (blue). A scale bar for fascicle strain is to the right of each panel. The corresponding EMG signals (for level only) are also shown in black with a scale bar to the right. The shaded regions indicate the stance phases of the strides. (B) Muscle stress corresponding with the strides in A for the LG (red) and MG (blue).

 

Figure 3
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  		Fig. 3. Two consecutive representative strides at 2.0 m s–1 on a level (solid lines) and inclined (broken lines) treadmill showing the changes in fascicle length (A) for the LG (red), pMG (blue) and dMG (blue). A scale bar for fascicle strain is to the right of each panel. The corresponding EMG signals (for level only) are also shown in black with a scale bar to the right. The shaded regions indicate the stance phases of the strides. (B) Muscle stress corresponding with the strides in A for the LG (red) and MG (blue).

 

Figure 4
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  		Fig. 4. (A) Mean EMG spike amplitude (MSA) for the LG (top), pMG (middle) and dMG (bottom). The black and grey bars represent strides at 0.5 and 2.0 m s–1, respectively. Strides on the level surface are on the left of each panel and strides on the inclined surface are on the right. Asterisks indicate a significant effect of locomotor speed from an ANOVA (P<0.05). Values are means ± s.e.m. (N=4). (B) Regressions of MSA versus maximum muscle force for the LG (top), pMG (middle) and dMG (bottom). Force and MSA were significantly, and positively, correlated for the LG (r2=0.52, P<0.001), pMG (r2=0.31, P<0.001) and dMG (r2=0.58, P<0.001). LG, y=152.9x+10.8; pMG, y=110.7x+19.1; dMG, y=98.3x+26.3.

 

Figure 5
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  		Fig. 5. The relative onset, offset and duration of the EMG bursts relative to force generation in the LG (red bars), pMG (blue bars) and dMG (black bars). Note that the onset of force is almost coincident with footfall for both the LG and MG. The upper and lower panels represent speeds of 0.5 and 2.0 m s–1, respectively. Values are means ± s.e.m. (N=4) of the EMG burst duration. Note that for all three muscle regions, the onset of EMG activity precedes the onset of force generation to a greater extent when running at 2.0 m s–1 compared with walking at 0.5 m s–1. There were no effects of incline (not shown).

 

Figure 6
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  		Fig. 6. Mean fascicle strain (A) and fascicle shortening velocity (B) for the LG, pMG and dMG walking (black; 0.5 m s–1) and running (grey; 2.0 m s–1) on a level (left pair of bars) and inclined (right pair of bars) treadmill. Note that strain increased for the LG and pMG with an increase in running speed on the level treadmill, but only for the LG on an inclined treadmill. Values are means ± s.e.m. (N=4).

 

Figure 7
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  		Fig. 7. Mean peak muscle stress for the LG and MG walking (black; 0.5 m s–1) and running (grey; 2.0 m s–1) on a level (left pair of bars) and inclined (right pair of bars) treadmill. Note that muscle force and stress increased with running speed on the level and inclined treadmill for both muscles, but force and stress were not affected by incline. Values are means ± s.e.m. (N=4).

 

Figure 8
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  		Fig. 8. Peak LG force versus peak MG force (A) and peak LG stress versus peak MG stress (B) during walking (black circles) and running (red triangles). The broken line represents the null hypothesis that both muscles share force and stress equally. Note that, while the MG generates more force relative to the LG, the LG consistently generates more stress. As speed increases, the LG exhibits a greater increase in force and stress compared with the MG. Also shown is LG force plotted against MG force (C) for a representative sequence of a bird running at 2 m s–1 on an inclined treadmill. The arrows indicate the direction of the loop starting at the onset of force in the LG. Note that force generation is weighted towards the LG during the early stages of stance and then towards the MG during the later part of stance. This pattern is extremely consistent among and between animals.

 

Figure 9
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  		Fig. 9. Apparent dynamic stiffness (ADS) for the LG, pMG and dMG during walking (black; 0.5 m s–1) and running (grey; 2.0 m s–1) on a level (left pair of bars) and inclined (right pair of bars) treadmill. Note that the dMG exhibited a much higher ADS compared with the pMG and LG. See Materials and methods regarding the calculation of ADS. Values are means ± s.e.m. (N=4).

 

Figure 10
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  		Fig. 10. Representative in vivo work loops for the LG (red; upper panels), pMG (blue; middle panels) and dMG (blue; bottom panels) walking at 0.5 m s–1 on a level treadmill (left column) and running at 2.0 m s–1 on an inclined treadmill (right column). The area within each loop represents the net work done by each muscle and muscle region. Arrows indicate the direction of the work loop, such that a counter-clockwise loop indicates net positive work and a clockwise loop indicates net negative work (energy absorption). Note that the amount of positive work increased substantially with an increase in speed and incline for the LG.

 

Figure 11
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  		Fig. 11. Mean positive and negative work per stride performed by the LG, pMG and dMG walking (black; 0.5 m s–1) and running (grey; 2.0 m s–1) on a level (left pair of bars) and inclined (right pair of bars) treadmill. Note that the positive work performed by the LG consistently exceeded that of both the pMG and dMG. Asterisks indicate a significant effect of speed from an ANOVA (P<0.05). Values are means ± s.e.m. (N=4).

 

Figure 12
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  		Fig. 12. Results (factor scores) from principal components analyses (PCA) using 13 variables related to muscle function for the LG (A,B) and pMG (C,D) during walking (black circles) and running (red triangles). The top panels (A,C) show PC1 versus PC2, and the bottom panels (B,D) show PC1 versus PC3. Note that PC1 separated the two speeds for the LG, and PC3 separated the two speeds for the pMG. See Tables 2 and 3 for component loadings. For the LG, variables related to stress and force loaded strongly on PC1 whereas variables related to force and EMG duration loaded strongly on PC3 for the pMG.

 

Figure 13
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  		Fig. 13. Results (factor scores) from principal components analyses (PCA) using eight variables related to muscle function for the pMG during walking (black circles) and running (grey triangles), and for the dMG during walking (red circles) and running (pink triangles). (A) PC1 versus PC2. (B) PC1 versus PC3. Note that PC1 separated the two regions of the muscles, and PC3 separated the two speeds. See Table 4 for component loadings.

 

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© The Company of Biologists Ltd 2008