Resting Tendon Cross-Sectional Area Underestimates Biceps Brachii Tendon Stress: Importance of Measuring During a Contraction

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.

Keywords: aging; elbow flexion; in vivo; sex differences; tendon mechanics; ultrasound.

Figure 1

The ultrasound measurements of biceps brachii (BB) tendon cross-sectional area (CSA) for a young male at rest (A) and during a 20% MVC contraction (B), an old male at rest (C) and during a 20% MVC contraction (D), and a young female at rest (E) and during a 25% MVC contraction (F). The inherent measurement platform of the ultrasound provides the area in square centimeters (cm²) and this was converted to square millimeter (mm²) prior to the calculations of tendon stress using the resting and the instantaneous tendon CSA measures. MVC, maximal voluntary contraction.

Figure 2

The ultrasound measurements of tendon elongation in a young female during a 25% MVC contraction. (A) Measurement at rest of distance from muscle-tendon junction to edge of ultrasound field of view (Length 1), and measurement of distance from hyperechoic marker to the edge of ultrasound field of view (L2). (B) Measurement during the contracted state. The difference between the measurements during the contracted and resting states was used as the measure of tendon elongation. Images have been cropped for reproduction, but analysis captured the full length of the tendon. MVC, maximal voluntary contraction.

Figure 3

Johnson–Neyman regions of significance for the difference in stress calculated using the resting and instantaneous CSA for young males (A), old males (B), and young females (C). Values to the right of the shaded areas indicate the statistically significant differences between stress values calculated using the resting and instantaneous tendon CSA (p < 0.05). MVC, maximal voluntary contraction; MPa, megapascals.

Figure 4

(A) Tendon force across the submaximal force levels for young and old males and young females. Tendon force increased with the contraction intensity and was greater in young males compared with the old males and young females. (B) Tendon CSA at rest and across the submaximal force levels. CSA, cross-sectional area; MVC, Maximal voluntary contraction; N, Newtons. *differs across force levels; ^#differs between all the three groups at 5% and 10% MVC, and between young and old males at 20, 40, 60, and 80% MVC (p < 0.05).

Figure 5

Stress values calculated using the resting and instantaneous tendon CSA. CSA, cross-sectional area YM, young males; OM, old males; YF, young females. MPa, megapascals; MVC, maximal voluntary contraction.