Abstract
Force produced by the muscle during contraction is applied to the tendon and distributed through the cross-sectional area (CSA) of the tendon. This ratio of force to the tendon CSA is quantified as the tendon mechanical property of stress. Stress is traditionally calculated using the resting tendon CSA; however, this does not take into account the reductions in the CSA resulting from tendon elongation during the contraction. It is unknown if calculating the tendon stress using instantaneous CSA during a contraction significantly increases the values of in vivo distal biceps brachii (BB) tendon stress in humans compared to stress calculated with the resting CSA. Nine young (22 ± 1 years) and nine old (76 ± 4 years) males, and eight young females (21 ± 1 years) performed submaximal isometric elbow flexion tracking tasks at force levels ranging from 2.5 to 80% maximal voluntary contraction (MVC). The distal BB tendon CSA was recorded on ultrasound at rest and during the submaximal tracking tasks (instantaneous). Tendon stress was calculated as the ratio of tendon force during contraction to CSA using the resting and instantaneous measures of CSA, and statistically evaluated with multi-level modeling (MLM) and Johnson–Neyman regions of significance tests to determine the specific force levels above which the differences between calculation methods and groups became statistically significant. The tendon CSA was greatest at rest and decreased as the force level increased (p < 0.001), and was largest in young males (23.0 ± 2.90 mm2) followed by old males (20.87 ± 2.0 mm2) and young females (17.08 ± 1.54 mm2) (p < 0.001) at rest and across the submaximal force levels. Tendon stress was greater in the instantaneous compared with the resting CSA condition, and young males had the greatest difference in the values of tendon stress between the two conditions (20 ± 4%), followed by old males (19 ± 5%), and young females (17 ± 5%). The specific force at which the difference between the instantaneous and resting CSA stress values became statistically significant was 2.6, 6.6, and 10% MVC for old males, young females, and young males, respectively. The influence of using the instantaneous compared to resting CSA for tendon stress is sex-specific in young adults, and age-specific in the context of males. The instantaneous CSA should be used to provide a more accurate measure of in vivo tendon stress in humans.
Highlights
Producing movement and torque around a joint requires the transfer of force from the muscle to the bone via the tendon
Prior studies of the Achilles tendon showed greater stress in young compared with old (Stenroth et al, 2012) and males compared with females (Stenroth et al, 2012; Lepley et al, 2018) when calculated using the resting tendon cross-sectional area (CSA), and we showed that distal biceps brachii (BB) tendon stress is greater in young males as compared with old males when calculated using the instantaneous CSA (Smart et al, 2018a)
The present study showed significant variation in the rate of increase in distal BB tendon stress values among young males, old males, and young females as well as between the resting and instantaneous CSA conditions, and that the tendon stress values were significantly greater in the instantaneous compared with the resting CSA condition
Summary
Producing movement and torque around a joint requires the transfer of force from the muscle to the bone via the tendon. Human in vivo studies evaluating the tendon stress traditionally rely on the measure of engineering stress which assumes that CSA is constant from rest to the maximal forces, and uses the resting tendon CSA for all the calculations of stress (Stenroth et al, 2012; Eriksen et al, 2018; Lepley et al, 2018). This experimental approach is appealing, as resting CSA measures are relatively easy to acquire (Stenroth et al, 2012; Eriksen et al, 2018). Applying resting CSA in the calculation of tendon stress fails to account for the dynamic nature of the tendon and likely misrepresents the true amount of stress experienced by the tendon across various force levels
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