Activation of protein synthesis, regeneration, and MAPK signaling pathways following repeated bouts of eccentric cycling

In vivo intracellular Ca2+ profiles after eccentric rat muscle contractions: addressing the mechanistic bases for repeated bout protection

Eccentric contractions (ECC) are accompanied by the accumulation of intracellular calcium ions ([Ca2+]i) and induce skeletal muscle damage. Suppressed muscle damage in repeated bouts of ECC is well characterized; however, whether it is mediated by altered Ca2+ profiles remains unknown. We tested the hypothesis that repeated ECC suppresses Ca2+ accumulation via adaptations in Ca2+ regulation. Male Wistar rats were divided into two groups: ECC single bout (ECC-SB) and repeated bout (ECC-RB). Tibialis anterior (TA) muscles were subjected to ECC (40 times, 5 sets) once (ECC-SB) or twice 14 days apart (ECC-RB). Under anesthesia, the TA muscle was loaded with Ca2+ indicator Fura 2-AM, and the 340/380 nm ratio was evaluated as [Ca2+]i. Ca2+ handling proteins were measured by Western blots. ECC induced [Ca2+]i increase in both groups, but ECC-RB evinced a markedly suppressed [Ca2+]i (Time: P < 0.01, Group: P = 0.0357). Five hours post-ECC, in contrast to the localized [Ca2+]i accumulation in ECC-SB, ECC-RB exhibited lower and more uniform [Ca2+]i (P < 0.01). In ECC-RB, mitochondria Ca2+ uniporter complex (MCU) components MCU and MICU2 were significantly increased pre-second ECC bout (P < 0.01), and both SERCA1 and MICU1 were better preserved after contractions (P < 0.01). Fourteen days after novel ECC, skeletal muscle mitochondrial Ca2+ regulating proteins were elevated. Following subsequent ECC, [Ca2+]i accumulation and muscle damage were suppressed and SERCA1 and MICU1 preserved. These findings suggest that tolerance to a subsequent ECC bout is driven, at least in part, by enhanced mitochondrial and sarcoplasmic reticulum Ca2+ regulation.NEW & NOTEWORTHY We demonstrated a reduced [Ca2+]i profile with suppressed muscle damage after a repeated bout of ECC in vivo: the ECC-induced immediate [Ca2+]i increase was suppressed and the persistence of increased [Ca2+]i with localized accumulation was diminished after repeated ECC. This effect occurred consonant with the upregulation of the mitochondrial Ca2+ uniporter complex and better preservation of SERCA1 and MICU1. These findings suggest that the mechanistic bases for repeated bout protection involve adaptation of Ca2+ regulation.

Exercise, exerkines and exercise mimetic drugs: Molecular mechanisms and therapeutics

Chronic diseases linked with sedentary lifestyles and poor dietary habits are increasingly common in modern society. Exercise is widely acknowledged to have a plethora of health benefits, including its role in primary prevention of various chronic conditions like type 2 diabetes mellitus, obesity, cardiovascular disease, and several musculoskeletal as well as degenerative disorders. Regular physical activity induces numerous physiological adaptations that contribute to these positive effects, primarily observed in skeletal muscle but also impacting other tissues. There is a growing interest among researchers in developing pharmaceutical interventions that mimic the beneficial effects of exercise for therapeutic applications. Exercise mimetic medications have the potential to be helpful aids in enhancing functional outcomes for patients with metabolic dysfunction, neuromuscular and musculoskeletal disorders. Some of the potential targets for exercise mimetics include pathways involved in metabolism, mitochondrial function, inflammation, and tissue regeneration. The present review aims to provide an exhaustive overview of the current understanding of exercise physiology, the role of exerkines and biomolecular pathways, and the potential applications of exercise mimetic drugs for the treatment of several diseases.

Plasma α-Actin as an Early Marker of Muscle Damage After Repeated Bouts of Eccentric Cycling

Purpose: This study aimed to examine the changes in skeletal muscle (SM) α-actin, myoglobin (Mb) and hydroxyproline (HP) in plasma and other indirect markers of muscle damage after repeated bouts of eccentric cycling. Methods: Ten healthy men (23.3 ± 2.8 years) performed two 30-min eccentric cycling bouts at 100% of maximal concentric power output (230.7 ± 36.9 W) separated by 2 weeks (ECC1 and ECC2). Maximal voluntary isometric contraction (MVIC) peak force of the knee extensor muscles, muscle soreness (SOR), pain pressure threshold (PPT) and plasma levels of SM α-actin, Mb, and HP were measured before, 0.5, 3, 24-168 h after each cycling bout. Results: MVIC peak force decreased on average 10.7 ± 13.1% more after ECC1 than ECC2. SOR was 80% greater and PPT was 12-14% lower after ECC1 than ECC2. Plasma SM α-actin levels increased at 0.5, 3, and 24-72 h after ECC1 (26.1-47.9%), and SM α-actin levels at 24 h after ECC1 were associated with muscle strength loss (r = -0.56, P = .04) and SOR (r = 0.88, P = .001). Mb levels increased at 0.5, 3, and 24 h after ECC1 (200-502%). However, Mb levels at 24 h after ECC1were not associated with muscle strength loss and SOR. HP levels remained unchanged after ECC1. ECC2 did not increase SM α-actin, Mb and HP levels. Conclusion: Our results indicate that α-actin could be used as a potential marker for the early identification of SM damage due to its early appearance in plasma and its association with other indirect markers of muscle damage.

Unlocking the Therapeutic Potential of Irisin: Harnessing Its Function in Degenerative Disorders and Tissue Regeneration

Physical activity is well-established as an important protective factor against degenerative conditions and a promoter of tissue growth and renewal. The discovery of Fibronectin domain-containing protein 5 (FNDC5) as the precursor of Irisin in 2012 sparked significant interest in its potential as a diagnostic biomarker and a therapeutic agent for various diseases. Clinical studies have examined the correlation between plasma Irisin levels and pathological conditions using a range of assays, but the lack of reliable measurements for endogenous Irisin has led to uncertainty about its prognostic/diagnostic potential as an exercise surrogate. Animal and tissue-engineering models have shown the protective effects of Irisin treatment in reversing functional impairment and potentially permanent damage, but dosage ambiguities remain unresolved. This review provides a comprehensive examination of the clinical and basic studies of Irisin in the context of degenerative conditions and explores its potential as a therapeutic approach in the physiological processes involved in tissue repair/regeneration.

Molecular mechanisms of exercise contributing to tissue regeneration

Physical activity has been known as an essential element to promote human health for centuries. Thus, exercise intervention is encouraged to battle against sedentary lifestyle. Recent rapid advances in molecular biotechnology have demonstrated that both endurance and resistance exercise training, two traditional types of exercise, trigger a series of physiological responses, unraveling the mechanisms of exercise regulating on the human body. Therefore, exercise has been expected as a candidate approach of alleviating a wide range of diseases, such as metabolic diseases, neurodegenerative disorders, tumors, and cardiovascular diseases. In particular, the capacity of exercise to promote tissue regeneration has attracted the attention of many researchers in recent decades. Since most adult human organs have a weak regenerative capacity, it is currently a key challenge in regenerative medicine to improve the efficiency of tissue regeneration. As research progresses, exercise-induced tissue regeneration seems to provide a novel approach for fighting against injury or senescence, establishing strong theoretical basis for more and more "exercise mimetics." These drugs are acting as the pharmaceutical alternatives of those individuals who cannot experience the benefits of exercise. Here, we comprehensively provide a description of the benefits of exercise on tissue regeneration in diverse organs, mainly focusing on musculoskeletal system, cardiovascular system, and nervous system. We also discuss the underlying molecular mechanisms associated with the regenerative effects of exercise and emerging therapeutic exercise mimetics for regeneration, as well as the associated opportunities and challenges. We aim to describe an integrated perspective on the current advances of distinct physiological mechanisms associated with exercise-induced tissue regeneration on various organs and facilitate the development of drugs that mimics the benefits of exercise.

Effects of eccentric, concentric and eccentric/concentric training on muscle function and mass, functional performance, cardiometabolic health, quality of life and molecular adaptations of skeletal muscle in COPD patients: a multicentre randomised trial

Background

Chronic obstructive pulmonary disease (COPD) is the third cause of death worldwide. COPD is characterised by dyspnoea, limited exercise tolerance, and muscle dysfunction. Muscle dysfunction has been linked to dysregulation between muscle protein synthesis, myogenesis and degradation mechanisms. Conventional concentric cycling has been shown to improve several clinical outcomes and reduce muscle wasting in COPD patients. Eccentric cycling is a less explored exercise modality that allows higher training workloads imposing lower cardio-metabolic demand during exercise, which has shown to induce greater muscle mass and strength gains after training. Interestingly, the combination of eccentric and concentric cycling training has scarcely been explored. The molecular adaptations of skeletal muscle after exercise interventions in COPD have shown equivocal results. The mechanisms of muscle wasting in COPD and whether it can be reversed by exercise training are unclear. Therefore, this study aims two-fold: (1) to compare the effects of 12 weeks of eccentric (ECC), concentric (CONC), and combined eccentric/concentric (ECC/CONC) cycling training on muscle mass and function, cardiometabolic health, physical activity levels and quality of life in severe COPD patients; and (2) to examine the molecular adaptations regulating muscle growth after training, and whether they occur similarly in specific muscle fibres (i.e., I, IIa and IIx).

Methods

Study 1 will compare the effects of 12 weeks of CONC, ECC, versus ECC/CONC training on muscle mass and function, cardiometabolic health, levels of physical activity and quality of life of severe COPD patients using a multicentre randomised trial. Study 2 will investigate the effects of these training modalities on the molecular adaptations regulating muscle protein synthesis, myogenesis and muscle degradation in a subgroup of patients from Study 1. Changes in muscle fibres morphology, protein content, genes, and microRNA expression involved in skeletal muscle growth will be analysed in specific fibre-type pools.

Discussion

We aim to demonstrate that a combination of eccentric and concentric exercise could maximise the improvements in clinical outcomes and may be ideal for COPD patients. We also expect to unravel the molecular mechanisms underpinning muscle mass regulation after training in severe COPD patients.

Trial Registry: Deutshches Register Klinischer Studien; Trial registration: DRKS00027331; Date of registration: 12 January 2022. https://www.drks.de/drks_web/navigate.do?navigationId=trial.HTML&TRIAL_ID=DRKS00027331.

Effects of eccentric vs concentric cycling training on patients with moderate COPD

PURPOSE

The present study compared the effects of eccentric cycling (ECC) and conventional concentric cycling (CONC) training on muscle function, body composition, functional performance, and quality of life (QOL) of patients with moderate chronic obstructive pulmonary disease (COPD).

METHODS

Twenty patients (age: 69.6 ± 10.1 years, forced expiratory volume in 1-s: 73.2 ± 11.4% of predicted) were randomly allocated to ECC (n = 10) or CONC (n = 10) group. They performed 12 weeks of ECC or CONC training at similar perceived exertion. The workload, heart rate (HR), blood oxygen saturation (SpO₂), and dyspnea were monitored during cycling. Outcomes measures included maximal voluntary isometric contraction (MVC) strength of the knee extensors, rate of force development (RFD), lower limb fat-free (LLFFM) and fat (LLFM) mass, 6-min walking test (6MWT), timed up-and-go test (TUG), stairs ascending (SAWT) and descending walking time (SDWT), and QOL assessed by the Saint George's respiratory questionnaire.

RESULTS

ECC produced on average threefold greater (P < 0.001) workload (211.8 ± 106.0 kJ) than CONC (78.1 ± 62.6 kJ) over 34 training sessions. ECC showed 1.5 ± 2.1% greater SpO₂, 24.7 ± 4.1% lower HR, and 64.4 ± 29.6% lower dyspnea in average than CONC (P < 0.001). ECC increased LLFFM (4.5 ± 6.2%; P = 0.03), while CONC decreased LLFM (3.3 ± 6.4%; P = 0.04) after training. Both ECC and CONC reduced (P < 0.05) SAWT (- 16.1 ± 9.3% vs - 10.1 ± 14.4%) and SDWT (- 12.2 ± 12.6% vs - 14.4 ± 14.7%), and improved (P < 0.05) QOL (33.4 ± 38.8 vs 26.1 ± 36.6%) similarly, but only ECC improved (P < 0.05) RFD (69-199%), TUG (13.6 ± 13.6%), and 6MWT (25.3 ± 27.7%).

CONCLUSION

These results suggest that ECC training with less cardio-pulmonary demands was more effective in increasing functional performance and muscle mass for COPD patients than CONC training.

Aerobic Metabolic Adaptations in Endurance Eccentric Exercise and Training: From Whole Body to Mitochondria

A characteristic feature of eccentric as compared with concentric exercise is the ability to generate greater mechanical loads for lower cardiopulmonary demands. Current evidence concurs to show that eccentric training translates into considerable gains in muscle mass and strength. Less is known, however, regarding its impact on oxygen transport and on factors to be considered for optimizing its prescription and monitoring. This article reviews the existing evidence for endurance eccentric exercise effects on the components of the oxygen transport system from systemic to mitochondria in both humans and animals. In the studies reviewed, specially designed cycle-ergometers or downhill treadmill running were used to generate eccentric contractions. Observations to date indicate that overall, the aerobic demand associated with the eccentric training load was too low to significantly increase peak maximal oxygen consumption. By extension, it can be inferred that the very high eccentric power output that would have been required to solicit a metabolic demand sufficient to enhance peak aerobic power could not be tolerated or sustained by participants. The impact of endurance eccentric training on peripheral flow distribution remains largely undocumented. Given the high damage susceptibility of eccentric exercise, the extent to which skeletal muscle oxygen utilization adaptations would be seen depends on the balance of adverse and positive signals on mitochondrial integrity. The article examines the protection provided by repeated bouts of acute eccentric exercise and reports on the impact of eccentric cycling and downhill running training programs on markers of mitochondrial function and of mitochondrial biogenesis using mostly from animal studies. The summary of findings does not reveal an impact of training on skeletal muscle mitochondrial respiration nor on selected mitochondrial messenger RNA transcripts. The implications of observations to date are discussed within future perspectives for advancing research on endurance eccentric exercise physiological impacts and using a combined eccentric and concentric exercise approach to optimize functional capacity.

Submaximal Eccentric Cycling in People With COPD: Acute Whole-Body Cardiopulmonary and Muscle Metabolic Responses

BACKGROUND

Eccentric cycling (ECC) may be an attractive exercise method in COPD because of both low cardiorespiratory demand and perception of effort compared with conventional concentric cycling (CON) at matched mechanical loads. However, it is unknown whether ECC can be performed by individuals with COPD at an intensity able to cause sufficient metabolic stress to improve aerobic capacity.

RESEARCH QUESTION

What are the cardiopulmonary and metabolic responses to ECC in people with COPD and healthy volunteers when compared with CON at matched mechanical loads?

STUDY DESIGN AND METHODS

Thirteen people with COPD (mean ± SD age, 64 ± 9 years; FEV₁, 45 ± 19% predicted; BMI, 24 ± 4 kg/m²; oxygen uptake at peak exercise [V̇O_2peak], 15 ± 3 mL/kg/min) and 9 age-matched control participants (FEV₁, 102 ± 13% predicted; BMI, 28 ± 5 kg/m²; V̇O_2peak, 23 ± 5 mL/kg/min), performed up to six 4-min bouts of ECC and CON at matched mechanical loads of increasing intensity. In addition, 12 individuals with COPD underwent quadriceps muscle biopsies before and after 20 min of ECC and CON at 65% peak power.

RESULTS

At matched mechanical loads, oxygen uptake, minute ventilation, heart rate, systolic BP, respiratory exchange ratio (all P < .001), capillary lactate, perceived breathlessness, and leg fatigue (P < .05) were lower in both groups during ECC than CON. Muscle lactate content increased (P = .008) and muscle phosphocreatine decreased (P = .012) during CON in COPD, which was not evident during ECC.

INTERPRETATION

Cardiopulmonary and blood lactate responses during submaximal ECC were less compared with during CON at equivalent mechanical workloads in healthy participants and COPD patients, and this was confirmed at a muscle level in COPD patients. Submaximal ECC was well tolerated and allowed greater mechanical work at lower ventilatory cost. However, in people with COPD, a training intervention based on ECC is unlikely to stimulate cardiovascular and metabolic adaptation to the same extent as CON.

Relative lipid oxidation associates directly with mitochondrial fusion phenotype and mitochondria-sarcoplasmic reticulum interactions in human skeletal muscle

Disturbances in skeletal muscle lipid oxidation might induce ectopic fat deposition and lipotoxicity. Nevertheless, the cellular mechanisms that regulate skeletal muscle lipid oxidation have not been fully determined. We aimed to determine whether there was an association between relative whole body lipid oxidation and mitochondrial size or mitochondria-sarcoplasmic reticulum interactions in the skeletal muscle. Twelve healthy men were included [mean (standard deviation), 24.7 (1.5) yr old, 24.4 (2.6) kg/m²]. The respiratory quotient (RQ) was used to estimate relative lipid oxidation at rest and during exercise (50% maximal oxygen consumption, 600 kcal expended). A skeletal muscle biopsy was obtained from the vastus lateralis at rest. Transmission electron microscopy was used to determine mitochondrial size and mitochondria-sarcoplasmic reticulum interactions (≤50 nm of distance between organelles). Protein levels of fusion/fission regulators were measured in skeletal muscle by Western blot. Resting RQ and exercise RQ associated inversely with intermyofibrillar mitochondrial size (r = -0.66 and r = -0.60, respectively, P < 0.05). Resting RQ also associated inversely with the percentage of intermyofibrillar mitochondria-sarcoplasmic reticulum interactions (r = -0.62, P = 0.03). Finally, intermyofibrillar mitochondrial size associated inversely with lipid droplet density (r = -0.66, P = 0.01) but directly with mitochondria fusion-to-fission ratio (r = 0.61, P = 0.03). Our results show that whole body lipid oxidation is associated with skeletal muscle intermyofibrillar mitochondrial size, fusion phenotype, and mitochondria-sarcoplasmic-reticulum interactions in nondiabetic humans.

The Acute Physiological Responses of Eccentric Cycling During the Recovery Periods of a High Intensity Concentric Cycling Interval Session

Eccentric and concentric exercise is associated with disparate acute and chronic responses. We uniquely interspersed workload equivalent eccentric cycling during each recovery period of a high intensity interval training (HIIT) cycling trial to determine acute cardiopulmonary, thermal and psycho-physiological responses. Twelve males [age 28 years (SD 6), peak oxygen consumption 48 mL ⋅ kg^–1 ⋅ min^–1 (SD 6)] completed two high intensity interval cycling trials [4 × 5 min, 60% peak power output (PPO)] separated by 7–10 days. The CON_R trial required participants to cycle concentrically during each recovery period (5 min, 30% PPO). The ECC_R trial modified the recovery to be eccentric cycling (5 min, 60% PPO). High intensity workload (CON_R: 187 ± 17; ECC_R: 187 ± 21 W), oxygen consumption (CON_R: 2.55 ± 0.17; ECC_R: 2.68 ± 0.20 L ⋅ min^–1), heart rate (CON_R: 165 ± 7; ECC_R: 171 ± 10 beats ⋅ min^–1) and RPE legs (CON_R: 15 ± 3; ECC_R: 15 ± 3) were equivalent between trials. Eccentric cycling recovery significantly increased external workload (CON_R: 93 ± 18; ECC_R: 196 ± 24 W, P < 0.01) yet lowered oxygen consumption (CON_R: 1.51 ± 0.18; ECC_R: 1.20 ± 0.20 L ⋅ min^–1, P < 0.05) while heart rate (CON_R: 132 ± 13; ECC_R: 137 ± 12 beats ⋅ min^–1) and RPE of the legs (CON_R: 11 ± 7; ECC_R: 12 ± 7) remained equivalent. There was no significant difference in the aural temperature between the trials (ECC_R: 37.3 ± 0.1°C; CON_R: 37.4 ± 0.1°C, P > 0.05), yet during recovery periods mean skin temperature was significantly elevated in the ECC_R (ECC_R: 33.9 ± 0.2°C; CON_R: 33.3 ± 0.2°C, P < 0.05). Participants preferred ECC_R (10/12) and rated the ECC_R as more achievable (82.8 ± 11.4 mm) than CON_R (79.4 ± 15.9 mm, P < 0.01). In conclusion, eccentric cycling during the recovery period of a HIIT training session, offers a novel approach to concurrent training methodology. The unique cardiopulmonary and skeletal muscle responses facilitate the achievement of both training stimuli within a single exercise bout.