The free Achilles tendon (AT) consists of distinct fascicles arising from each of the triceps surae muscles that may give rise to non-uniform behavior during functional tasks such as walking. Here, we estimated in vivo deformations of the human AT during walking using simultaneous ultrasound and motion capture measurements. Ten subjects walked at three speeds (0.75, 1.00, and 1.25 m/s) on a force-measuring treadmill. A custom orthotic secured a linear array transducer in two locations: 1) the distal lateral gastrocnemius muscle-tendon junction and 2) the free AT, on average centered 6 cm superior to calcaneal insertion. We used motion capture to record lower extremity kinematics and the position and orientation of the ultrasound transducer. A 2D ultrasound elastography algorithm tracked superficial and deep tissue displacements within the free AT. We estimated AT elongation (i.e., change in length) relative to the calcaneal insertion by transforming the orthotic, transducer, and calcaneus kinematics into a common reference frame. Superficial and deep regions of the free AT underwent significantly different longitudinal displacements and elongations during walking. For example, we found that the superficial AT exhibited 16-29% greater peak elongation than the deep AT during the stance phase of walking (p<0.01). Moreover, superficial-deep AT tissue deformations became less uniform with faster walking speed (p<0.01). Non-uniform deformations of the free AT, which could reflect inter-fascicle sliding, may enable the gastrocnemius and soleus muscles to transmit their forces independently while allowing unique kinematic behavior at the muscle fiber level.

The Achilles tendon (AT) moment arm is an important determinant of ankle moment and power generation during locomotion. Load and depth-dependent variations in the AT moment arm are generally not considered, but may be relevant given the complex triceps surae architecture. We coupled motion analysis and ultrasound imaging to characterize AT moment arms during walking in 10 subjects. Muscle loading during push-off amplified the AT moment arm by 10% relative to heel strike. AT moment arms also varied by 14% over the tendon thickness. In walking, AT moment arms are not strictly dependent on kinematics, but exhibit important load and spatial dependencies.

The Achilles tendon (AT) moment arm transforms triceps surae muscle forces into a moment about the ankle which is critical for functional activities like walking. Moreover, the AT moment arm changes continuously during walking, as it depends on both ankle joint rotation and triceps surae muscle loading (presumably due to bulging of the muscle belly). Here, we posit that aging negatively effects the architecturally complex AT moment arm during walking, which thereby contributes to well-documented reductions in ankle moment generation during push-off. We used motion capture-guided ultrasound imaging to quantify instantaneous variations in the AT moment arms of young (23.9 ± 4.3 years) and older (69.9 ± 2.6 years) adults during walking, their dependence on triceps surae muscle loading, and their association with ankle moment generation during push-off. Older adults walked with 11% smaller AT moment arms and 11% smaller peak ankle moments during push-off than young adults. Moreover, as hypothesized, these unfavourable changes were significantly and positively correlated (r = 0.38, p < 0.01). More surprisingly, aging attenuated load-dependent increases in the AT moment arm (i.e., those between heel-strike and push-off at the same ankle angle); only young adults exhibited a significant increase in their AT moment arm due to triceps surae muscle-loading. Age-associated reductions in triceps surae volume or activation, and thus muscle bulging during force generation, may compromise the mechanical advantage of the AT during the critical push-off phase of walking in older adults. Thus, strategies to restore and/or improve locomotor performance in our aging population should consider these functionally important changes in musculoskeletal behavior.

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