Walking and running rely on complex coordination of the neurological, muscular, and skeletal systems. The role of muscles is to produce force, a task that is dramatically affected by the dynamics of muscle fibers and tendons. In walking and running, we do not know how these dynamics affect force generation because experimental tools are ill suited to these measurements.

Computer models can be powerful tools for estimating muscle–tendon dynamics that cannot be measured experimentally. Previous lower limb models are based on severely limited data describing limb muscle architecture (i.e., muscle fiber lengths, pennation angles, and physiological cross-sectional areas). I created a model, based on state-of-the-art muscle architecture data from 21 cadavers, that estimates fiber lengths and velocities during movement. The model includes geometric representations of the bones, kinematic descriptions of the joints, and Hill-type models of 44 muscle-tendon compartments. The model allows calculation of muscle-tendon lengths and moment arms over a wide range of body positions. The model also allows detailed examination of the force and moment generation capacities of muscles about the ankle, knee, and hip and is freely available at www.simtk.org.

I used this musculoskeletal model and experimental measurements of joint angles and muscle activation patterns during walking to produce a simulation of muscle–tendon dynamics and calculate fiber operating lengths (i.e., the length of muscle fibers relative to their optimal fiber length) for 17 lower limb muscles. These results indicated that when muscle–tendon compliance is low the muscle fiber operating length is determined predominantly by the joint angles and muscle moment arms. If muscle–tendon compliance is high muscle fiber operating length is more dependent on activation level and force–length-velocity effects. I found that muscles operate on multiple limbs of the force–length curve (i.e., ascending, plateau, and descending limbs) during the gait cycle, but are active within a smaller portion of their total operating range.

Finally, I used this model to create simulations of muscle fiber lengths and velocities for five subjects walking and running at multiple speeds. The lengths and velocities of muscle fibers have a dramatic effect on muscle force generation. It is unknown, however, whether the lengths and velocities of lower limb muscle fibers substantially affect the ability of muscles to generate force during walking and running. I examined this issue by developing simulations of muscle–tendon dynamics that calculate the lengths and velocities of muscle fibers from electromyography recordings of eleven lower limb muscles and kinematic measurements of the hip, knee, and ankle made as five subjects walked at speeds of 1.0-1.75 m/s and ran at speeds of 2.0-5.0 m/s.

The simulations revealed that force generation ability (i.e., the force generated per unit of activation) of eight of the eleven muscles was significantly affected by walking or running speed. For example, gastrocnemius lateralis and soleus (two major ankle plantarflexors) force generation ability decreased with increasing walking speed, but the transition from walking to running increased the force generation ability of these muscles by reducing their fiber velocities. The results demonstrate the influence of plantarflexor muscle architecture on the walk-to-run transition and the effects of muscle–tendon compliance on the plantarflexors’ ability to generate ankle moment and power. These results allow us to study lower limb muscles in unprecedented detail by relating muscle fiber dynamics and force generation to the mechanical demands of walking and running.

These findings support the hypothesis that the walk-to-run transition in human gait is related to the force generation ability of the plantarflexors, offer insights into dynamic properties of muscles that have not yet been measured during walking and running, and permit comparisons among muscles with diverse architecture. The model and simulations can be employed to guide experimental studies and explore the limitations of current modeling assumptions. The rich data set provided by these efforts permits the nature of muscle force development in walking and running to be studied in unprecedented detail, allowing the patterns of human movement to be related to the patterns of muscle structure.