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.

Influence of 4 weeks of downhill running on calcium sensitivity of rat single muscle fibers

“…Accordingly, eccentric training has been shown to produce preferential increases in the size of Type II fibers (Douglas et al, 2017 ); however, in the present study, in the training group there was an increased CSA for both Type I and II fibers (Figure 2 ). Recently, Hinks et al ( 2022 ) reported no differences in muscle size or isometric force production for the soleus muscle tested in vitro following 4 weeks of downhill running between the control and training groups. Of note, this is the same sample of rats used for single fiber analysis in the present study.…”

“…The training protocol consisted of 4 weeks of downhill running training 3 days/week, with custom‐made weighted vests (Hinks et al, 2022 ) to add load during running. Rats ran on an EXER 3/6 animal treadmill (Columbus Instruments) at a 15° decline.…”

“…The vastus intermedius was selected as the hindlimb muscles of rats undergo active lengthening (i.e., eccentric contractions) during similar downhill running protocols as the current study (Butterfield et al, 1985 ; Lynn & Morgan, 1994 ). Importantly, the vastus intermedius has repeatedly been shown to have strong adaptations to downhill eccentric training in the form of increased serial sarcomere number (Butterfield et al, 1985 ; Hinks et al, 2022 ; Lynn & Morgan, 1994 ). While in the dissecting solution, several muscle fiber bundles of approximately 0.5–1 mm in width and 3 mm in length were dissected and transferred to a tube containing 2.5 ml of chilled skinning solution on ice for 30 min to be chemically permeabilized (Mazara et al, 2021 ).…”

“…With respect to eccentric training in particular, despite the lack of Ca 2+ sensitivity data in previous work, it is known that Ca 2+ sensitivity increases as a function of sarcomere length (Joumaa & Herzog, 2014 ; Martyn & Gordon, 2001 ). Data from our lab showed an increase in the number of serial sarcomeres and resting sarcomere length following eccentric training (i.e., weighted downhill running) in the rat soleus (Hinks et al, 2022 ). Therefore, it is possible that following chronic eccentric training, sarcomeres may be operating at a longer length, resulting in a corresponding increase in Ca 2+ sensitivity.…”

First person – Avery Hinks

“… ABSTRACT First Person is a series of interviews with the first authors of a selection of papers published in Biology Open, helping early-career researchers promote themselves alongside their papers. Avery Hinks is first author on ‘ Influence of weighted downhill running training on serial sarcomere number and work loop performance in the rat soleus ’, published in BiO. Avery is an MSc student (soon to be PhD student) in the lab of Dr Geoffrey Power at the University of Canada, investigating how muscle structure influences muscle function, particularly regarding adaptations following an intervention.…”