As we March towards the 2024 American Physiology Summit, this month’s publication review covers recent advancements in the realm of muscle physiology, including the development of an improved resistance training method, the long-term musculoskeletal consequences of chemotherapy, and the characterization of crossbridge kinetics in cardiac trabeculae. Taken together, these studies reveal a promising trend of breakthroughs in muscle physiology.

Featured image (©Huot et al. (2024), licensed under CC BY-NC 4.0 DEED) providing an overview of the investigation into the lasting musculoskeletal consequences of Folfiri, a common chemotherapeutic, in young C57BL/6J male mice.

Stuart has got the PoWeR! Skeletal muscle adaptations to a novel heavy progressive weighted wheel running exercise model in C57BL/6 mice

Skeletal muscle plays a critical role in our day-to-day activities, enabling movement, stabilizing joints, and sustaining body posture and position. Despite the inevitable age-related loss of muscle mass, researchers have long known exercise is able to maintain skeletal muscle health throughout the lifespan. In fact, several resistance-type exercise models have been employed to investigate the mechanisms regulating muscle mass, including synergist ablation, electrical stimulation, weighted ladder climbing, and weighted pulling. More recently, Dungan et al. (2019) developed a progressive weighted wheel running model, termed PoWeR, to better replicate traditional resistance exercises in mice. However, the ability of the mice to run great distances despite the added resistance prompted Koopmans et al. (2024) to investigate whether a heavier loading protocol would bias PoWeR towards a more resistance-type exercise.

To test this, Koopmans et al. (2024) administered a 9-week protocol in 3-8 month-old C57BL/6 mice, with each mouse randomly assigned to three treatments: sedentary (SED), PoWeR, and heavy PoWeR (hPoWeR). At the end of the 9-week protocol, Aurora Scientific’s 1305A 3-in-1 muscle system was used to assess in-situ contractile function of the gastrocnemius-plantaris-soleus (GPS) muscle complex in the mice hindimbs. Interestingly, the normalized in-situ peak force of the GPS complex increased +9.5% and +17.0% in PoWeR and hPoWeR respectively. Moreover, fibre type transitions in the hPoWeR mice were found to be blunted and running distance was significantly lower than PoWeR mice in the final weeks of training. Altogether, the novel hPoWeR model developed by Koopmans et al. (2024) modestly biased PoWeR towards a more resistant-type exercise while maintaining a voluntary, high-throughput and low stress protocol for the animal. In this way, hPoWeR offers a more faithful recapitulation of typical resistance training adaptations, in line with decreased running volume and exposure to higher resistance.

Long-term musculoskeletal consequences of chemotherapy in pediatric mice

Advancements in cancer research have drastically improved survival outcomes in pediatric patients, with many surviving into adulthood. In particular, chemotherapeutics play a vital role in cancer treatments by disrupting cell division and eradicating cancerous cells. Unfortunately, the effects of these anti-cancer agents are often not isolated to cancer cells, and in turn, elicit toxic reactions on the body. In fact, studies have shown that chemotherapies can drive musculoskeletal alterations and oxidative stress, leading to immediate side effects on skeletal muscle and bone. Despite our increased understanding of these short-term effects, the long-term effects of chemotherapeutics administered during child development remain critically understudied. To address this, Huot et al. (2024) investigated the lasting musculoskeletal consequences of Folfiri, a common chemotherapeutic, in young (four-week-old) C57BL/6J male mice.

Specifically, Huot et al. (2024) administered either Folfiri or the vehicle intraperitoneally for up to 5 weeks. In a separate cohort of mice, Folfiri was administered in age-matched mice for 5 weeks and followed up 4 weeks after treatment cessation. Using Aurora Scientific’s 1300A 3-in-1 muscle system, paired with the 800B dual-bath apparatus, they performed both in-vivo and ex-vivo contractility experiments to assess muscle function. For the in-vivo assessments, plantarflexion torque was measured at baseline, 2 weeks, and 5 weeks into treatment, while ex-vivo assessments involved dissection of the EDL upon euthanasia, followed by maximum twitch and force-frequency analysis. The force data for both experiments were collected and analyzed with Aurora Scientific’s 615A: Dynamic Muscle Control and Analysis software.

Upon analysis, Huot et al. (2024) found that Folfiri-treated animals showed a marked reduction in skeletal muscle force generation at 2 weeks and 5 weeks compared to the controls. Significant reductions were also observed at 5, 7, and 9 weeks, indicating a lasting musculoskeletal defect after treatment cessation. Similarly, ex-vivo EDL contractions revealed significant force reductions in the Folfiri-treated mice at the time of sacrifice. In addition to muscle contractility defects, the authors also found that chemotherapy-treated mice had alterations in body weight, fat, and bone mass, along with muscle innervation deficits and abnormal mitochondrial homeostasis. As such, their findings reveal long-lasting musculoskeletal complications in actively growing pediatric mice treated with chemotherapy, and underscores the importance of further investigation into the long-term effects of cancer treatments in pediatric populations.

Strain rate of stretch affects crossbridge detachment during relaxation of intact cardiac trabeculae

Mechanical control of relaxation refers to how the rate of stretching, or strain rate, just prior to relaxation influences the relaxation of a muscle. Although changes in strain and strain rate are known to influence muscle contractility, their influence on muscle relaxation is currently not well understood. Tanner et al. (2024) therefore sought to investigate this by precisely controlling the strain rate and the time-to-stretch of intact cardiac trabeculae and assessing the crossbridge kinetics as the myocardium relaxed.

To evaluate intact cardiac trabeculae mechanics, trabeculae were loaded onto Aurora Scientific’s 802D-500: Permeabilized/Intact Fiber Apparatus and mounted between the 403A force transducer and 322C high-speed length controller.  After equilibration, a normal twitch was measured, followed by a reference load-clamp twitch, typically held at 50% of the peak “normal” twitch. The trabeculae were then either:

  • held at isometric length as the muscle relaxed (known as the reference trace)
  • ramp-stretched at 1% muscle length and four variable strain rates (25-1000 s-1), then held at isometric length as the muscle relaxed (known as the ramp-stretch trace).

The difference between the reference and ramp-stretch trace was then calculated to determine the stress response. Tanner et al. (2024) found that following a ramp-stretch, peak stress-response decreased as the time-to-stretch increased and as the strain rate decreased. In contrast, the minimum stress-response increased with time-to-stretch but was not dependent on strain rate. Additionally, the group observed rapid changes in the stress-response at higher strain rates, indicating enhanced crossbridge detachment during relaxation. Collectively, these findings demonstrate that peak stress is dependent on strain rate, but minimum stress and time-to-minimum stress values are not, therefore supporting the idea that crossbridge detachment is accelerated by strain rate.

Conclusions

To summit all up, these studies have climbed to the forefront of muscle physiology research, establishing new experimental physiology techniques, and advancing our understanding of muscle mechanics. As we venture towards new feats in the field, the key-takeaways from Koopmans et al. (2024), Huot et al. (2024), and Tanner et al. (2024) will allow us to delve deeper into physiological insights, and ultimately reach new heights.