Age-related greater Achilles tendon compliance is not associated with larger plantar flexor muscle fascicle strains in senior women

Abstract

The aim of the present study was to test the hypothesis that the age-associated decrease of tendon stiffness would necessitate greater muscle fascicle strains to produce similar levels of force during isometric contraction. Greater fascicle strains could force sarcomeres to operate in less advantageous regions of their force-length and force-velocity relationships, thus impairing the capacity to generate strong and explosive contractions. To test this hypothesis, sagittal-plane dynamic velocity-encoded phase-contrast magnetic resonance images of the gastrocnemius medialis (GM) muscle and Achilles tendon (AT) were acquired in six young (YW; 26.1 ± 2.3 yr) and six senior (SW; 76.7 ± 8.3 yr) women during submaximal isometric contraction (35% maximum voluntary isometric contraction) of the plantar flexor muscles. Multiple GM fascicle lengths were continuously determined by automatically tracking regions of interest coinciding with the end points of muscle fascicles evenly distributed along the muscle's proximo-distal length. AT stiffness and Young's modulus were measured as the slopes of the tendon's force-elongation and stress-strain curves, respectively. Despite significantly lower AT stiffness at older age (YW: 120.2 ± 52.3 N/mm vs. SW: 53.9 ± 44.4 N/mm, P = 0.040), contraction-induced changes in GM fascicle lengths were similar in both age groups at equal levels of absolute muscular force (4-5% fascicle shortening in both groups), and even significantly larger in YW (YW: 11-12% vs. SW: 6-8% fascicle shortening) at equal percentage of maximum voluntary contraction. These results suggest that factors other than AT stiffness, such as age-associated changes in muscle composition or fascicle slack, might serve as compensatory adaptations, limiting the degree of fascicle strains upon contraction.

Keywords: Achilles tendon; VE-PC; gastrocnemius muscle; strain; stress.

Figures

Fig. 1.

Typical examples of the high-resolution images obtained in the proximal (A) and distal (B) region of the gastrocnemius muscle-tendon unit. Lf, fascicle length; Aposup, superficial aponeurosis; Apodeep, deep aponeurosis; α, pennation angle; MA, Achilles tendon moment arm.

Fig. 2.

Achilles tendon force-elongation (A) and stress-strain (B) curves as obtained in six young (YW) and six senior women (SW). Values are means ± SD.

Fig. 3.

Dynamic changes in gastrocnemius medialis fascicle length (LF; A and B) and pennation angles (PA; C and D) during submaximal isometric contraction as measured in YW (n = 6) and SW (n = 6). Note, subplots A and C reflect the data measured in YW (n = 6), B and D represent the results obtained in the SW cohort (n = 6). Values are means ± SD.

Fig. 4.

Representative curves illustrating the typical relationship between fascicle shortening and rise of force in YW and SW. Note: the data presented were obtained in one YW and one SW, who were selected due to their similar muscular force and baseline LF. Solid and dashed lines represent the phase of contraction and relaxation, respectively. The arrows below the x-axis highlight the “toe” regions, typically seen at the onset of contraction. Δ, Change.