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. 2023 Oct;108(10):1308-1324.
doi: 10.1113/EP091334. Epub 2023 Aug 23.

Ultrasonographic measurements of fascicle length overestimate adaptations in serial sarcomere number

Avery Hinks  1 , Martino V Franchi  2   3 , Geoffrey A Power  1
Affiliations

Ultrasonographic measurements of fascicle length overestimate adaptations in serial sarcomere number

Avery Hinks et al. Exp Physiol. 2023 Oct.

Abstract

Ultrasound-derived measurements of muscle fascicle length (FL) are often used to infer increases (chronic stretch or training) or decreases (muscle disuse or aging) in serial sarcomere number (SSN). Whether FL adaptations measured via ultrasound can truly approximate SSN adaptations has not been investigated. We casted the right hindlimb of 15 male Sprague-Dawley rats in a dorsiflexed position (i.e., stretched the plantar flexors) for 2 weeks, with the left hindlimb serving as a control. Ultrasound images of the soleus, lateral gastrocnemius (LG), and medial gastrocnemius (MG) were obtained with the ankle at 90° and full dorsiflexion for both hindlimbs pre and post-cast. Following post-cast ultrasound measurements, legs were fixed in formalin with the ankle at 90°, then muscles were dissected and fascicles were teased out for measurement of sarcomere lengths via laser diffraction and calculation of SSN. Ultrasound detected an 11% increase in soleus FL, a 12% decrease in LG FL, and an 8-11% increase in MG FL for proximal fascicles and at full dorsiflexion. These adaptations were partly reflected by SSN adaptations, with a 6% greater soleus SSN in the casted leg than the un-casted leg, but no SSN differences for the gastrocnemii. Weak relationships were observed between ultrasonographic measurements of FL and measurements of FL and SSN from dissected fascicles. Our results showed that ultrasound-derived FL measurements can overestimate an increase in SSN by ∼5%. Future studies should be cautious when concluding a large magnitude of sarcomerogenesis from ultrasound-derived FL measurements, and may consider applying a correction factor. NEW FINDINGS: What is the central question of this study? Measurements of muscle fascicle length via ultrasound are often used to infer changes in serial sarcomere number, such as increases following chronic stretch or resistance training, and decreases with ageing: does ultrasound-derived fascicle length accurately depict adaptations in serial sarcomere number? What is the main finding and its importance? Ultrasound detected an ∼11% increase in soleus fascicle length, but measurements on dissected fascicles showed the actual serial sarcomere number increase was only ∼6%; therefore, measurements of ultrasound-derived fascicle length can overestimate serial sarcomere number adaptations by as much as 5%.

Keywords: casting; fascicle; immobilization; pennation angle; sarcomere; sarcomerogenesis; ultrasound.

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Conflict of interest statement

No conflicts of interest, financial or otherwise, are declared by the authors.

Figures

FIGURE 1
FIGURE 1
(a) Example images of applying the splint and brace for the dorsiflexion cast. (b–d) Set‐up and example of ultrasound images obtained from the left lateral gastrocnemius (b), soleus (c) and medial gastrocnemius (d), with the ankle fixed at 90° using tape. White arrows indicate the muscle of interest in each image. (e) The area highlighted by the white box in (c), showing representative tracings of fascicle length (orange) and pennation angle (green).
FIGURE 2
FIGURE 2
Fascicle length of the un‐casted and casted lateral gastrocnemius (LG) measured via ultrasound from n = 15 rats. For the un‐casted LG, there was an effect of region (a) and an interaction between joint position and time (b). For the casted LG, there were effects of region (c), joint position (d) and time (e). Data are presented as means ± standard deviation. *Significant difference between indicated means (P < 0.05).
FIGURE 3
FIGURE 3
Fascicle length of the un‐casted and casted soleus measured via ultrasound from n = 15 rats. For the un‐casted soleus, there was an interaction between region and position (a) and no effect of time (b). For the casted soleus, there was an effect of position (c) and an interaction between region and time (d). Data are presented as means ± standard deviation. *Significant difference between indicated means (P < 0.05).
FIGURE 4
FIGURE 4
Fascicle length of the un‐casted and casted medial gastrocnemius (MG) measured via ultrasound from n = 15 rats. For the un‐casted MG, there was an interaction between region and time (a) and an effect of position (b). For the casted MG, there were interactions between region and time (c) and position and time (d). Data are presented as means ± standard deviation. *Significant difference between indicated means (P < 0.05).
FIGURE 5
FIGURE 5
Changes in pennation angle of the un‐casted (a) and casted (b) lateral gastrocnemius (LG) from pre to post‐cast in n = 15 rats. Data are presented as means ± standard deviation. *Significant difference between indicated means (P < 0.05).
FIGURE 6
FIGURE 6
Changes in pennation angle of the un‐casted (a) and casted (b,c) soleus from pre to post‐cast in n = 15 rats. Data are presented as means ± standard deviation. *Significant difference between indicated means (P < 0.05).
FIGURE 7
FIGURE 7
Changes in pennation angle of the un‐casted (a) and casted (b) medial gastrocnemius (MG) from pre to post‐cast in n = 15 rats. Data are presented as mean ± standard deviation. *Significant difference between indicated means (P < 0.05).
FIGURE 8
FIGURE 8
Comparison of muscle wet weight between the casted and un‐casted leg for the lateral gastrocnemius (LG), soleus, and medial gastrocnemius (MG) from n = 15 rats. Data are presented as means ± standard deviation. *Significant difference between indicated means (P < 0.05).
FIGURE 9
FIGURE 9
Effects of time (a–c) and effects of region (d–f) on serial sarcomere number (a,d), average sarcomere length (b,e) and fascicle length (c,f) of the lateral gastrocnemius from dissected fascicles in n = 15 rats. Data are presented as means ± standard deviation. *Significant difference between indicated means (P < 0.05).
FIGURE 10
FIGURE 10
Effects of time on serial sarcomere number (a), average sarcomere length (b) and fascicle length (c) of the soleus from dissected fascicles in n = 15 rats. Data are presented as mean ± standard deviation. *Significant difference between indicated means (P < 0.05).
FIGURE 11
FIGURE 11
Effects of time (a–c) and effects of region (d–f) on serial sarcomere number (a,d), average sarcomere length (b,e), and fascicle length (c,f) of the medial gastrocnemius from dissected fascicles in n = 15 rats. Data are presented as means ± standard deviation. *Significant difference between indicated means (P < 0.05).
FIGURE 12
FIGURE 12
Relationships between ultrasound‐derived fascicle length at 90° and fascicle length of dissected fascicles (a–c), ultrasound‐derived fascicle length at 90° and SSN (d–f), and ultrasound‐derived FL at full dorsiflexion and SSN (g–i) for the soleus, lateral gastrocnemius (LG) and medial gastrocnemius (MG). Each graph displays data from a total of n = 30 muscles (n = 15 right leg, n = 15 left leg) from n = 15 rats. *Significant relationship (P < 0.05).
FIGURE 13
FIGURE 13
For all muscles combined, relationships between the percentage change in ultrasound‐derived fascicle length (FL) from pre to post‐cast as measured with the ankle at 90° (a) and full dorsiflexion (b) and the percentage change in serial sarcomere number (SSN) from the un‐casted to casted leg determined from dissected fascicles. Each graph displays data from a total of n = 45 muscles (n = 15 of each of the lateral gastrocnemius (LG), soleus and medial gastrocnemius (MG)) from n = 15 rats. *Significant relationship (P < 0.05).
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