Curvature of gastrocnemius muscle fascicles as function of muscle-tendon complex length and contraction in humans

Abstract

It has been shown that muscle fascicle curvature increases with increasing contraction level and decreasing muscle-tendon complex length. The analyses were done with limited examination windows concerning contraction level, muscle-tendon complex length, and/or intramuscular position of ultrasound imaging. With this study we aimed to investigate the correlation between fascicle arching and contraction, muscle-tendon complex length and their associated architectural parameters in gastrocnemius muscles to develop hypotheses concerning the fundamental mechanism of fascicle curving. Twelve participants were tested in five different positions (90°/105°*, 90°/90°*, 135°/90°*, 170°/90°*, and 170°/75°*; *knee/ankle angle). They performed isometric contractions at four different contraction levels (5%, 25%, 50%, and 75% of maximum voluntary contraction) in each position. Panoramic ultrasound images of gastrocnemius muscles were collected at rest and during constant contraction. Aponeuroses and fascicles were tracked in all ultrasound images and the parameters fascicle curvature, muscle-tendon complex strain, contraction level, pennation angle, fascicle length, fascicle strain, intramuscular position, sex and age group were analyzed by linear mixed effect models. Mean fascicle curvature of the medial gastrocnemius increased with contraction level (+5 m^-1 from 0% to 100%; p = 0.006). Muscle-tendon complex length had no significant impact on mean fascicle curvature. Mean pennation angle (2.2 m^-1 per 10°; p < 0.001), inverse mean fascicle length (20 m^-1 per cm^-1 ; p = 0.003), and mean fascicle strain (-0.07 m^-1 per +10%; p = 0.004) correlated with mean fascicle curvature. Evidence has also been found for intermuscular, intramuscular, and sex-specific intramuscular differences of fascicle curving. Pennation angle and the inverse fascicle length show the highest predictive capacities for fascicle curving. Due to the strong correlations between pennation angle and fascicle curvature and the intramuscular pattern of curving we suggest for future studies to examine correlations between fascicle curvature and intramuscular fluid pressure.

Keywords: biomechanics; connective tissue; physiology; ultrasound.

FIGURE 1

Study flow. Joint angles are given as geometrical knee/ankle angle.

FIGURE 2

Processed panoramic ultrasound image of medial gastrocnemius with manually labeled superficial and deep aponeuroses and nine fascicles.

FIGURE 3

Segment curvature $c_{\mathrm{s},2;3}$ calculation based on circle section fitting. The fitted circle section is defined by the segments' $s_2$ and $s_3$ midpoints as circle points and their distance $d_{2;3}$ as circle chord. $c_{\mathrm{s},2;3}$ is calculated as inverse circle radius $r_{2;3}$ using Equation (1).

FIGURE 4

Four competing models to explain mean fascicle curvature in the medial gastrocnemius ${\overline{c}}_{{\mathrm{f}}_{\mathrm{GM}}}$ with different independent variables: (a) with muscle–tendon complex strain ${\epsilon }_{\mathrm{MTC}}$ and contraction level $M_{\mathrm{PF}}$; (b) with mean deep pennation angle ${\overline{\alpha }}_{\mathrm{d}}$; (c) with mean fascicle length ${\overline{l}}_{\mathrm{f}}$; (d) with mean fascicle strain ${\overline{\epsilon }}_{\mathrm{f}}$. Model fit equations are given within the plot, visualized via a black plane for a and black lines for b–d. For simplicity, model estimates within the equations are displayed without units. mR², marginal R ²; cR², conditional R ². Significance markers: ^† p < 0.01; ^‡ p < 0.001.

FIGURE 5

Intermuscular differences (a) in mean fascicle curvature (${\overline{c}}_{\mathrm{f}}$), (b) in mean pennation angle (${\overline{\alpha }}_{\mathrm{d}}$), and (c) in model intercept and slope between medial and lateral gastrocnemius (GM and GL); mR², marginal R ²; cR², conditional R ². Significance markers: ^‡ p < 0.001.

FIGURE 6

Distribution of (a) pennation angle (${\alpha }_{\mathrm{d}}$) and (b) fascicle curvature ($c_{\mathrm{f}}$) from distal to proximal intramuscular position (${\mathrm{Pos}}_{\mathrm{DP}}$), in (c) combined into a linear mixed model with intramuscular position defined from central to peripheral (${\mathrm{Pos}}_{\mathrm{CP}}$) to create linearity. mR², marginal R ²; cR², conditional R ². Significance markers: ^‡ p < 0.001.