Achilles Tendon Morphology Is Related to Triceps Surae Muscle Size and Peak Plantarflexion Torques During Walking in Young but Not Older Adults

Abstract

The interaction of the triceps surae muscles and the Achilles tendon is critical in producing the ankle plantarflexion torque required for human walking. Deficits in plantarflexor output are a hallmark of reduced mobility in older adults and are likely associated with changes in the triceps surae muscles that occur with age. Structural differences between young and older adults have been observed in the Achilles tendon and in the triceps surae muscles. However, less is known about how age-related differences in muscle and tendon morphology correspond with each other and, furthermore, how those morphology differences correlate with age-related deficits in function. The goal of this work was to investigate whether there is a correlation between age-related differences in triceps surae muscle size and Achilles tendon cross-sectional area (CSA) and whether either is predictive of ankle plantarflexion torque during walking. We used magnetic resonance imaging (MRI) to measure triceps surae muscle volumes and tendon CSAs in young (n = 14, age: 26 ± 4 years) and older (n = 7, age: 66 ± 5 years) adults, and we determined peak plantarflexion torques during treadmill walking. We found that individual muscle volumes as a percentage of the total triceps surae volume did not differ between young and older adults, though muscle volumes per body size (normalized by the product of height and mass) were smaller in older adults. Achilles tendon CSA was correlated with body size and muscle volumes in young adults but not in older adults. The ratio of tendon CSA to total triceps surae muscle volume was significantly greater in older adults. Peak ankle plantarflexion torque during walking correlated with body size and triceps surae volume in young and older adults but was correlated with tendon CSA only in the young adults. Structure-function relationships that seem to exist between the Achilles tendon and the triceps surae muscles in young adults are no longer evident in all older adults. Understanding mechanisms that determine altered Achilles tendon CSA in older adults may provide insight into age-related changes in function.

Age-related changes to triceps surae muscle-subtendon interaction dynamics during walking

“…However, contrary to several prior studies, we did not observe an effect of age on peak ankle moment or positive ankle push-off work during walking, likely as a result of our relatively slow range of walking speeds 33 . Yet, similar to our findings, Knaus et al revealed that older adults who walked with similar ankle moment generation to younger adults presented with deleterious changes in AT structure–function relations that are not measurable using conventional biomechanical analyses alone 34 .…”

Using muscle-tendon load limits to assess unphysiological musculoskeletal model deformation and Hill-type muscle parameter choice

“…For the calculated F max,ens of 5500.3 N, a CSA ens value of 124.93 mm 2 would be required to achieve the desired max(F ens ) to F max ratio of 1.5. This is more than double the Achilles tendon CSAs measured by Kruse et al (60.9 ± 8.2 mm 2 ) [71] and Knaus et al (64.8 ± 16.34 mm 2 ) [83], which is beyond the magnitude of any measurement error or standard deviation. On the other hand, assuming a correctly set CSA of 60.9 mm 2 and calculating the theoretical F max based on the same target of max(F ens ) = 1.5 F max would indicate that the maximum force acting on the Achilles tendon could not exceed 2681.25 N, less than the F max of just the connected musculus soleus (3585.9 N [74]).…”

“…This retroactive scaling of experimental data was deemed necessary as no publication listing all required parameters for the transfer of a stress-strain curve to a normalised force-strain curve is known to the authors. A study by Knaus et al [83] has shown that Achilles tendon CSA grows with the volume of the connected muscles and thus their maximum force production capability. Thus, a possible source of uncertainty could be that the CSA and F max values we derived from different literature sources.…”

3D characterization of the triple‐bundle Achilles tendon from in vivo high‐field MRI

“…The soleus has a larger pCSA than the medial and lateral gastrocnemius muscles, and thus might be presumed to have a larger subtendon 16 . Additionally, modeling studies suggest that Achilles tendon twist may reduce along‐fiber strain 17–21 . These and other tissue properties could serve to reduce local tendon loading, mitigating injury risk.…”

“…16 Additionally, modeling studies suggest that Achilles tendon twist may reduce along-fiber strain. [17][18][19][20][21] These and other tissue properties could serve to reduce local tendon loading, mitigating injury risk.…”