Influence of biceps brachii tendon mechanical properties on elbow flexor force steadiness in young and old males.

Elbow flexor force steadiness (FS) depends on strength and decreases with age. Achilles tendon mechanics effect standing balance and isometric plantarflexion FS. This study investigated the influence of distal biceps brachii (BB) tendon mechanics and elbow flexor strength on age-related decline in FS. Nine young (23 ± 2 years) and nine old (77 ± 5 years) males performed submaximal isometric elbow flexion tasks at low (2.5%, 5%, 10% maximal voluntary contraction (MVC)) and high (20%, 40%, 60%, 80%MVC) forces in a neutral forearm position. Distal BB tendon elongation and cross-sectional area (CSA) were recorded on ultrasound to calculate mechanics of strain, stress, and stiffness. Coefficient of variation (CV) of force was used to assess relationship of FS to tendon mechanics and strength. Young were 22% stronger and 41% steadier than old (P < .05). Tendon stiffness (170.1 ± 132.9 N/mm; 113.0 ± 55.1 N/mm) did not differ with age (P > .05). Young had 40% less strain compared to old at 5% MVC, but 42% greater strain at 60% and 80% MVC (P ≤ .05). Stress was ~18% greater in young at 10%, 20%, and 80% MVC (P ≤ .05). At low forces, CV of force was predicted by stress (r2  = 0.56) in young, and stress and MVC (r2  = 0.641) in old. At high forces for both age groups, CV of force was predicted by MVC and stress (r2  = 0.39-0.43). Stress and strain is greater in young compared with old males. Because strength influences tendon mechanics and is also associated with FS, absolute strength is a large and modifiable contributor to age-related decline in FS.

Duration of fascicle shortening is affected by muscle architecture and sex.

PURPOSE: The purpose of this study was to examine muscle fascicle properties of the gastrocnemius medialis (GM) during contraction and stretch between males and females. During contraction muscle fascicles shorten and pennation angles increase to generate force. Due to the elastic nature of the attached tendon, the fascicles continue to shorten when maximal force is achieved in order to sustain isometric force and this duration of fascicle shortening (DFS) can be observed with ultrasonography. Linear and curved muscle fascicles both display these kinetics; however, it is currently unknown if static stretch prior to a maximal voluntary contraction (MVC) alters the DFS and whether the effect differs between males and females. METHODS: Subjects performed an isometric MVC of the plantar flexors before and after a 2-min maximal dorsi-flexion stretch. Plantar flexor force was measured and ultrasound videography used to record GM and Achilles tendon architecture. RESULTS: Males were stronger than females (p = 0.004). The DFS was longer for females compared to males (p = 0.001) and the addition of a static stretch increased the DFS for curved (p = 0.002), but not linear, fascicles. Curved fascicles were longer (p = 0.05) with larger pennation angles (p = 0.04) for both males and females when compared to linear fascicles. Tendon excursion was greater (p = 0.05) post-stretch during contraction when compared to pre-stretch. CONCLUSIONS: This study provides evidence that regardless of sex, curved muscle fascicles behave differently than linear fascicles and should be considered separately when muscle architecture is examined.