Endurance training as an effective strategy to counteract impairments in muscle mass and function after 10 days of bed rest in older adults

Periods of muscle disuse are increasingly common in modern society, due to sedentary lifestyle, illnesses or hospitalization. Disuse can cause substantial losses in muscle mass, strength and power, particularly in older adults. Resistance training (RT) is the most commonly prescribed strategy to counteract such impairments in muscle mass and function. A recent study from our lab (1) has confirmed the effectiveness of RT in restoring muscle structure and function after 10 days of disuse. However, transcriptomic profile analyses from that same study suggest that exposure to short-term muscle disuse could prompt the muscle to primarily restore its metabolic function. Since endurance training is well known for enhancing oxidative metabolism, this study explores whether it can also effectively reverse disuse-induced impairments to skeletal muscle. Ten healthy male older adults (69 ± 3 years) underwent 10 days of strict horizontal bed rest (BR) without countermeasures, followed by 21 days (9 sessions) of moderate intensity endurance exercising (AR). Training sessions consisted of 15min of warm-up, followed by five 15s high intensity cycling sprints, and 30min of cycling at 80-100% of gas exchange threshold. Neuromuscular function assessments and muscle biopsies were performed both before (BR0) and after (BR10) bed rest, and after AR. An isokinetic dynamometer was used to assess maximum voluntary isometric contraction (MVIC) and maximum muscle power (P M ) of the knee flexors. The quadriceps femoris (QF) was manually outlined on magnetic resonance imaging (MRI) images obtained every 12mm, from the most distal MRI slice where QF can be seen, to the most proximal one. QF volume (VOL) was calculated using the truncated cone formula (2). Muscle thickness (MT), fascicle length (FL) and pennation angle (PA) were measured on ultrasound images obtained from the vastus lateralis at 50% of femur length. Transcriptional (RNAseq) analyses on muscle biopsies are ongoing. One-way repeated measures ANOVAs were performed to detect differences over timepoints. A statistical level of p< 0.05 was accepted. From BR0 to BR10 there were significant decreases in QF VOL (-6.2%, p<0.001), MT (-7.4%, p=0.02), PA (-7.7%, p=0.04) and MVIC (-13.3%, p<0.001). Changes in FL (-3.6%, p=0.06) and P M (-1.4%, p=0.84) were not significant. From BR10 to AR there were significant increases in QF VOL (11.2%, p<0.001), MT (13.7%, p=0.003), MVIC (8.9%, p=0.007) and P M (25.9%, p<0.001). Changes in FL (5.3%, p=0.09) and PA (9.4%, p=0.12) were not significant. Ten days of bed rest led to significant muscle atrophy and strength loss. Unlike the typically observed responses to endurance training, muscle volume was not only restored, but its change unexpectedly surpassed the baseline values after retraining, along with marked improvements in muscle strength and power. Ongoing transcriptional analyses will provide insights into the molecular mechanisms driving these unexpected hypertrophic and functional adaptations to endurance training. This study was cofinanced by the Slovenian Research and Innovation Agency (project J5-4593 and programme P2-0041), the Italian Space Agency (n.2024-5-E.0 - CUP n.I53D24000060005), and by NextGenerationEU/Italian Ministry of University and Research (MUR) PRIN 2022 PNRR, (ReActiveAge, Project n. P2022FNCPR). REFERENCES:1. Franchi et al. (2025) . J Physiol. 2. Tracy et al. (2003). MSSE. Slovenian Research and Innovation Agency; Italian Space Agency; NextGenerationEU/Italian MUR PRIN 2022 PNRR (ReActiveAge, Project n. P2022FNCPR). This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.