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First published online March 8, 2005
Journal of Experimental Biology 208, 799-808 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01435

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Ankle plantar flexor force production is an important determinant of the preferred walk-to-run transition speed

Richard R. Neptune^* and Kotaro Sasaki

Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX 78712, USA

View larger version (42K): [in a new window]   Fig. 1. Bipedal musculoskeletal model including the nine muscle groups per leg used to simulate walking and running.

View larger version (9K): [in a new window]   Fig. 2. Schematic of the musculotendon actuator used in the model. The properties of the musculotendon force generation (FT) were represented by an active contractile element (CE) in parallel with a passive elastic element (PEE). The muscle fiber was placed in series with tendon (SEE), which was represented by a non-linear elastic element. The pennation angle () denotes the angle between the muscle fibers and the tendon. All musculotendon parameters, including the origin and insertion of each muscle, pennation angle, tendon slack length, resting fiber length and maximum isometric force were based on Delp et al. (1990).

View larger version (18K): [in a new window]   Fig. 3. Intrinsic muscle properties: (A) normalized force–length and (B) force–velocity relationships (Delp and Loan, 1995; Zajac, 1989). With deviations from the muscle fiber's optimal length and increasing rates of shortening, the ability of a muscle to produce force decreases. Note, negative velocity values indicate muscle shortening.

View larger version (21K): [in a new window]   Fig. 4. Normalized experimental soleus (SOL), medial gastrocnemius (GAS) and tibialis anterior (TA) muscle EMG as walking speed increased from 40 to 120% PTS. The EMG magnitude of all muscles continued to increase beyond the PTS.

View larger version (24K): [in a new window]   Fig. 5. Experimental anterior/posterior and vertical ground reaction forces as walking speed increased from 40 to 120% PTS. The braking and propulsion phases are indicated with grey horizontal bars during 100% PTS walking.

View larger version (32K): [in a new window]   Fig. 6. Simulation tracking over the entire gait cycle of the hip, knee and ankle joint angles during walking and running at 100% PTS. The vertical lines indicate toe-off.

View larger version (32K): [in a new window]   Fig. 7. Simulation results of walking as walking speed increases from 60 to 120% PTS for the soleus (SOL), medial gastrocnemius (GAS) and tibialis anterior (TA). (A) Muscle activation as a percent of maximum (Activation) and musculotendon force (MT force), and (B) normalized muscle fiber length and velocity. The fiber length was normalized to its optimal length and the velocity was normalized to its maximum contraction velocity, which was estimated as ten times the muscle fiber resting length per second (Zajac, 1989). Negative velocity values indicate muscle shortening. The gray filled regions indicate general area of muscle activity.

View larger version (18K): [in a new window]   Fig. 8. Simulation muscle activation and force comparison between walking and running at 100% PTS.

View larger version (11K): [in a new window]   Fig. 9. Combined simulation plantar flexor muscle impulse from the soleus and medial gastrocnemius during the propulsion phase as walking speed increased from 60 to 120% PTS.

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