Dick, Taylor J. M.Biewener, AndrewWakeling, James M.2018-02-212017Dick, Taylor J. M., Andrew A. Biewener, and James M. Wakeling. 2017. “Comparison of Human Gastrocnemius Forces Predicted by Hill-Type Muscle Models and Estimated from Ultrasound Images.” The Journal of Experimental Biology 220 (9) (February 15): 1643–1653. doi:10.1242/jeb.154807.0022-0949http://nrs.harvard.edu/urn-3:HUL.InstRepos:34858091Hill-type models are ubiquitous in the field of biomechanics, providing estimates of a muscle's force as a function of its activation state and its assumed force–length and force–velocity properties. However, despite their routine use, the accuracy with which Hill-type models predict the forces generated by muscles during submaximal, dynamic tasks remains largely unknown. This study compared human gastrocnemius forces predicted by Hill-type models with the forces estimated from ultrasound-based measures of tendon length changes and stiffness during cycling, over a range of loads and cadences. We tested both a traditional model, with one contractile element, and a differential model, with two contractile elements that accounted for independent contributions of slow and fast muscle fibres. Both models were driven by subject-specific, ultrasound-based measures of fascicle lengths, velocities and pennation angles and by activation patterns of slow and fast muscle fibres derived from surface electromyographic recordings. The models predicted, on average, 54% of the time-varying gastrocnemius forces estimated from the ultrasound-based methods. However, differences between predicted and estimated forces were smaller under low speed–high activation conditions, with models able to predict nearly 80% of the gastrocnemius force over a complete pedal cycle. Additionally, the predictions from the Hill-type muscle models tested here showed that a similar pattern of force production could be achieved for most conditions with and without accounting for the independent contributions of different muscle fibre types.en-USJournal Article2018-02-1320172018-02-2110.1242/jeb.154807