 This study investigates the relationship between muscle architecture and force production in Vivo. Two ultrasound-based approaches are used to assess the relationship between muscle architecture and force production in 21 healthy subjects. The results show that muscle architecture can be used to predict force production, but only when measured at rest close to optimal length. This suggests that muscle architecture may not be as important as previously thought in determining muscle performance. This article was authored by Amelie Workhausen, Oivind Glersen, Antoine Nordes, and others.