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First published online March 28, 2008
Journal of Experimental Biology 211, 1281-1288 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.011932

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Changing the demand on specific muscle groups affects the walk–run transition speed

Jamie L. Bartlett^* and Rodger Kram

Locomotion Laboratory, Department of Integrative Physiology, University of Colorado, Boulder, CO 80309, USA

View larger version (6K): [in this window] [in a new window]   Fig. 1. Dorsiflexor assist (DFA) device used to reduce dorsiflexor muscle demand. The device secures around the subject's knee (A) with a Velcro-compatible surface. The subject dorsiflexed the foot allowing strap (B) to be pulled through the cam-style buckle. This allowed the rubber tubing (C) to be stretched applying a force and thus a torque around the ankle.

View larger version (10K): [in this window] [in a new window]   Fig. 2. Leg swing assist (LSA) device and aiding horizontal force (AHF) device. Schematic adapted from Gottschall and Kram (Gottschall and Kram, 2005). The system of rubber tubing and pulleys apply forces at either the feet or near the center of mass.

View larger version (9K): [in this window] [in a new window]   Fig. 3. Rectified electromyographic (EMG) signals (after 7 Hz high-pass filter) versus time [normal walking, walking with the dorsiflexor assist (DFA) device] for the TA muscle for a representative subject. Vertical broken lines indicate heel strikes and toe-off for the right leg. Black horizontal bars indicate the burst analyzed during swing and just post heel-strike (between 60–110%). The bar graph on the right depicts mean muscle activity (± s.e.m.) during the burst for all subjects while walking normally (white) and walking with the DFA device (grey). The asterisk denotes a significant difference compared with normal walking (P=0.003).

View larger version (9K): [in this window] [in a new window]   Fig. 4. Rectified electromyographic (EMG) signals (after 7 Hz high-pass filter) versus time [normal walking, walking with the leg swing assist (LSA) device] for the rectus femoris (RF) muscle for a representative subject. Vertical dashed lines indicate heel strikes and toe-off for the right leg. Black horizontal bars show the portion of the stride analyzed (between 50–85%). The bar graph on the right depicts mean muscle activity (± s.e.m.) for the burst during swing initiation for all subjects while walking normally (white) and walking with the LSA device (grey). The asterisk denotes a significant difference compared to normal walking (P=0.006).

View larger version (34K): [in this window] [in a new window]   Fig. 5. Rectified electromyographic (EMG) signals (after 7 Hz high-pass filter) versus time [normal walking, walking with the aiding horizontal force (AHF) and impeding horizontal force (IHF) devices, respectively] for the soleus (SOL), medial gastrocnemius (MGAS) and lateral gastrocnemius (LGAS) muscles for a representative subject. Vertical lines indicate heel strikes and toe-off for the right leg. Black horizontal bars indicate the burst analyzed during stance (between 0–60%). The bar graphs on the right depict mean muscle activity (± s.e.m.) during stance for all subjects while walking normally (white), walking with the AHF (grey), and walking with the IHF (black). The asterisk denotes a significant difference in mean muscle activity compared to normal walking (all P values <0.012).

View larger version (8K): [in this window] [in a new window]   Fig. 6. Mean electromyographic (EMG) activity normalized to walking at PTS for three muscles: (A) tibialis anterior (TA), (B) rectus femoris (RF), (C) medial gastrocnemius (MGAS) during normal walking (filled squares), normal running (filled circles) and walking with an assistive device: (A) dorsiflexor assist (DFA; open squares)), (B) leg swing assist (LSA; open squares), (C) aiding horizontal force (AHF; open squares) at preferred walking speed (PWS; 1.25 m s–1), preferred transition speed (PTS; 1.94±0.03 m s–1), at speeds less than PTS (–0.3, –0.2, –0.1 m s–1) and speeds greater than PTS (+0.1, +0.2 m s–1).

View larger version (12K): [in this window] [in a new window]   Fig. 7. Conceptual hypothetical framework illustrating the factors that influence the preferred walk–run transition speed (PTS). The horizontal axis represents increasing speed and the vertical axis represents the influence of each factor. A critical threshold for the PTS is represented by the broken horizontal lines. The thin lines 1,2,3 arbitrarily represent muscle groups. The thick line in each figure represents a proposed underlying factor that ultimately limits the walk–run transition speed. (A) PTS under normal conditions; (B) PTS at a slower speed as a result of increased demand in muscle group 1; (C) PTS at a faster speed as a result of decreased demand in muscle group 1; (D) PTS at the same speed as in C despite decreased muscle demand in all groups; (E) PTS at a slower speed as a result of another underlying factor that occurs during simulated reduced gravity.

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