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First published online May 30, 2008
Journal of Experimental Biology 211, 1882-1892 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.014407
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Motor unit recruitment patterns 1: responses to changes in locomotor velocity and incline

Emma F. Hodson-Tole1,* and James M. Wakeling2

1 The Structure and Motion Laboratory, The Royal Veterinary College, Hawkshead Lane, North Mymms, Hatfield, Herts, AL9 7TA, UK
2 School of Kinesiology, Simon Fraser University, Burnaby, BC, V5A 1S6, Canada


Figure 1
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  		Fig. 1. Mean myoelectric intensity spectra for the plantaris (A), soleus (B) and medial gastrocnemius (C) muscle during three locomotor conditions (N=6 for each muscle). Data are shown as mean (black line) and s.e.m. (grey lines), with the division between stance and swing phase denoted by the broken, grey vertical line; 0% stride duration represents initial ground contact. Partitioning of the stride is represented in D, including data from the plantaris muscle (10° 40 cm s–1). The division between stance and swing phase is shown by the broken, thick, vertical grey line.

 

Figure 2
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  		Fig. 2. Results of the principal component analysis. (A) The first two principal components (PCI, solid black line; PCII, broken black line) defined from the partitioned myoelectric spectra of all muscles under all conditions (N=245 740). (B) Vector representation of the spectra reconstructed from the PC weightings shown in A, with PCI –0.76 PCII (red) and PCI +0.54 PCII (blue), indicating the angle {theta}.

 

Figure 3
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  		Fig. 3. PCI–PCII loading score vector plots representing the changes in {theta} that would be expected to occur if: (A) orderly recruitment of motor units occurred with slower motor units activated before and deactivated after faster motor units; (B) slower motor units were activated and deactivated in advance of activation and deactivation of faster motor units.

 

Figure 4
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  		Fig. 4. (A) Temporal stride characteristics for each of the locomotor conditions and (B) incline conditions at a velocity of 20 cm s–1. The whole bar represents total stride duration; the lower section (hatched) represents stance duration (mean ± s.e.m., N=12); the upper section (white) represents swing duration (mean ± s.e.m., N=12; incline in degrees; velocity in cm s–1).

 

Figure 5
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  		Fig. 5. Stride frequency recorded for (A) each locomotor condition and (B) all conditions at a velocity of 20 cm s–1 (mean ± s.e.m., N=12; incline in degrees; velocity in cm s–1).

 

Figure 6
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  		Fig. 6. Myoelectric intensities recorded from the plantaris (hatched), soleus (horizontal stripe) and medial gastrocnemius (zig-zag) in (A) each locomotor condition and (B) incline conditions at a velocity of 20 cm s–1 (incline in degrees; velocity in cm s–1). Values are mean ± s.e.m. (N=6 for each muscle).

 

Figure 7
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  		Fig. 7. Mean myoelectric frequencies recorded from the plantaris (hatched), soleus (horizontal stripe) and medial gastrocnemius (zig-zag) in (A) each locomotor condition and (B) incline conditions at a velocity of 20 cm s–1 (incline in degrees; velocity in cm s–1). Values are mean ± s.e.m. (N=6 for each muscle).

 

Figure 8
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  		Fig. 8. Principal component loading scores for PCI and PCII from the plantaris (green), soleus (red) and medial gastrocnemius (blue) muscles during each locomotor condition. Each point shows the mean ± s.e.m. pooled from each muscle from six subjects. There are 20 points on each graph, which represent the 20 time windows within each stride. *Time points corresponding to 0% stride duration; ^time points corresponding to 100% stride duration. Points for the soleus and plantaris always track in a clockwise direction, whereas points for the medial gastrocnemius follow an anti-clockwise loop, except during the incline conditions 20° and 25° where a figure-of-eight configuration occurs. PCI loading score values are indicators of the level of muscle activity, with higher values indicating a greater level of activity. A scale bar is shown on the right.

 

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© The Company of Biologists Ltd 2008