Influence of weighted downhill running training on serial sarcomere number and work loop performance in the rat soleus

Abstract

Increased serial sarcomere number (SSN) has been observed in rats following downhill running training due to the emphasis on active lengthening contractions; however, little is known about the influence on dynamic contractile function. Therefore, we employed 4 weeks of weighted downhill running training in rats, then assessed soleus SSN and work loop performance. We hypothesised trained rats would produce greater net work output during work loops due to a greater SSN. Thirty-one Sprague-Dawley rats were assigned to a training or sedentary control group. Weight was added during downhill running via a custom-made vest, progressing from 5-15% body mass. Following sacrifice, the soleus was dissected, and a force-length relationship was constructed. Work loops (cyclic muscle length changes) were then performed about optimal muscle length (LO) at 1.5-3-Hz cycle frequencies and 1-7-mm length changes. Muscles were then fixed in formalin at LO. Fascicle lengths and sarcomere lengths were measured to calculate SSN. Intramuscular collagen content and crosslinking were quantified via a hydroxyproline content and pepsin-solubility assay. Trained rats had longer fascicle lengths (+13%), greater SSN (+8%), and a less steep passive force-length curve than controls (P<0.05). There were no differences in collagen parameters (P>0.05). Net work output was greater (+78-209%) in trained than control rats for the 1.5-Hz work loops at 1 and 3-mm length changes (P<0.05), however, net work output was more related to maximum specific force (R2=0.17-0.48, P<0.05) than SSN (R2=0.03-0.07, P=0.17-0.86). Therefore, contrary to our hypothesis, training-induced sarcomerogenesis likely contributed little to the improvements in work loop performance. This article has an associated First Person interview with the first author of the paper.

Keywords: Collagen; Dynamic performance; Eccentric training; Force-length relationship; Muscle architecture; Sarcomerogenesis.

Fig. 1.

Rat weighted vest design (left) and a rat wearing a vest containing 15% of its body mass at week 4 of training (right).

Fig. 2.

Comparison of muscle wet weight (A), physiological cross-sectional area (PCSA) (B), total collagen concentration (C), and the percentage represented by pepsin-insoluble collagen (D) in control versus trained rats. Data are reported as mean±s.e. (A,B: n=18 control, n=13 training; C,D: n=11 control, n=12 training). No significant differences (P>0.05) were found between control and training groups for any of these variables.

Fig. 3.

Comparison of fascicle length (A), sarcomere length (B), and serial sarcomere number (C) in control versus trained rats. Data are reported as mean±s.e. (n=18 control, n=13 training). *Significant difference (P<0.05) between control and training.

Fig. 4.

Violin plot comparisons of average sarcomere length (A) and serial sarcomere number (B) in control versus trained rats with the sample size adjusted to treat each fascicle independently. Red dots represent the mean. *Significant difference (P<0.05) between control and training.

Fig. 5.

Comparison of active force-length relationships in absolute terms (A,B), normalised to physiological cross-sectional area (C,D), and normalised to maximum force (E,F), and passive force-length relationships (G,H) in control (solid lines) versus trained rats (dashed lines), with the collected data on the left, and the corresponding fitted asymmetric Gaussian (B,D,F) and exponential curves (H) on the right. The grey lines in F indicate the width of the plateau region. Data are reported as mean±s.e. (n=16 control, n=12 training). There were no between-group differences in optimal muscle length, maximum absolute or specific force, or the width of the plateau region. *The coefficient b indicated a less steep curve in trained compared to control rats (P<0.05).

Fig. 6.

Work of shortening (A), work of lengthening (B), and net work output (C) of the passive (i.e. no stimulation) work loops in trained and control rats. Data are reported as mean±s.e. (n=16 control, n=12 training).

Fig. 7.

Representative active (i.e. stimulation during shortening) work loop traces from 1 control and 1 trained rat. S indicates the start of the cycle. Orange lines indicate the stimulation period. Arrows indicate the direction of the cycle, with clockwise segments containing negative work and counterclockwise segments containing positive work.

Fig. 8.

Work of shortening (A), work of lengthening (B), and net work output (C) of the active (i.e. stimulation during shortening) work loops in trained (triangles) and control rats (circles). Data are reported as mean±s.e (n=16 control, n=12 training). *Significant difference (P<0.05) between control and training at the colour-coded cycle frequency. #Significant difference across cycle frequencies. †Significant difference across length changes.

Fig. 9.

Plots of the relationships between net work output and serial sarcomere number in the 1.5-Hz work loops at length changes of 1 and 3 mm. There were no significant relationships between serial sarcomere number and net work output in these work loops (P>0.05).