Mechanical model of the arm with dimensions and reference points.

(A) Schematic depiction of upper arm and forearm with elbow joint together with biceps and triceps brachii muscle models. Mechanical support structures representing the bones are plotted as bold lines. Three externally palpable landmarks (acromion, ac as well as medial and lateral epicondyles, ec_m and ec_l) are illustrated in green. The hand/wrist marks the lateral end of the forearm. The musculotendon complexes (MTC) for both muscles were depicted schematically as spring-mass systems with separate tendon elasticities. The MTCs were spanned between the respective insertion points (Abic|tric, Bbic|tric). The angle of the elbow joint θ was defined as 0 for the fully extended arm and positive for counter-clockwise flexion movements. (B) shows a reduced model version of (A) in which most insertion points were placed on the support structures and the tendons were neglected. The graphic also shows the definition of the length specifications. The MTC of the triceps was guided over a pulley wheel with constant radius. (C) Posterior view of upper arm with two heads of biceps brachii in shades of blue and the three externally palpable landmarks in green. Landmarks were used to estimate the depicted length measures (for details see text).