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The function of the skeletal muscle declines throughout aging ; Aerobic exercise interventions are typically prescribed as a viable tool to promote oxygen delivery to contracting muscles However, previous findings on exercise interventions show that aging individuals predominantly rely on nonoxidative metabolism for energy production during exercise on- transients, which could lead to premature fatigue and exercise cessation ; In conclusion, There were no significant differences between the two groups in the time- course changes of VL oxidative profile ( τ ) during the on-transient exercise ; The data presented here suggest that factors other than aging or muscle oxidative capacity govern the cycling performance in GET in physically active women

The aim of the study was to examine whether an improvement in jumping performance after 8 weeks of plyometric training (PT) runs in parallel with changes in lower-limb skeletal muscle contractile properties. Using noninvasive tensiomyography (TMG), we assessed contraction time (Tc) and the maximal amplitude of radial displacement (Dm) in 20 subjects (50% men ; age 22.4 6 4.7 years of age), randomly divided in PT group (N = 10 ; PLYO) and a control group (N = 10 ; CTRL). The PLYO performed 8 weeks of PT. Tensiomyography was measured in 5 leg skeletal muscles: vastus lateralis (VL), biceps femoris (BF), tibialis anterior (TA), gastrocnemius medialis (GM), and gastrocnemius lateralis (GL). Additionally, we evaluated countermovement jump (CMJ) height improvement on a ground force plate. Assessments were repeated before and after PT. After 8 weeks of PT, CMJ height increased by 12.2% in PLYO (p = 0.015), but not in CRTL. Contraction time, which is related to myosin heavy-chain type 1 (MHC-1) proportion, decreased in VL (28.7% ; p , 0.001), BF (226.7% ; p = 0.032), TA (232.9% ; p = 0.004), and GL (225.8% ; p = 0.044), but not in GM (28.1% ; p = 0.158). The estimated VL MHC-1 proportion decreased by 28.2% (p = 0.041). The maximal amplitude of radial displacement, inversely related to muscle tone, decreased in BF (226.5% ; p = 0.032), GM (214.9% ; p = 0.017), GL (231.5% ; p = 0.017), but not in TA (216.8% ; p = 0.113) and VL (26.0% ; p = 0.654). After PT, jumping performance increased, which was paralleled by decreased Tc and decreased muscle tone. Additionally, adaptations to contractile properties were muscle specific, which is important for future studies. It seems that adjustments were dose dependent, being higher in muscles with lower habitual load.

KEY POINTS Few days of unloading are sufficient to induce a decline of skeletal muscle mass and function; notably, contractile force is lost at a faster rate than muscle mass. The reasons behind this disproportionate loss of muscle force are still poorly understood. We provide strong evidence of two mechanisms only hypothesized until now for the rapid muscle force loss in only 10 days of bed rest. Our results show that an initial neuromuscular junction instability, accompanied by alterations in the innervation status and impairment of single fibre sarcoplasmic reticulum function contribute to the loss of contractile force in front of a preserved myofibrillar function and central activation capacity. Early onset of neuromuscular junction instability and impairment in calcium dynamics involved in excitation-contraction coupling are proposed as eligible determinants to the greater decline in muscle force than in muscle size during unloading. ABSTRACT Unloading induces rapid skeletal muscle atrophy and functional decline. Importantly, force is lost at a much higher rate than muscle mass. We aimed to investigate the early determinants of the disproportionate loss of force compared to that of muscle mass in response to unloading. Ten young participants underwent 10 days of bed rest (BR). At baseline (BR0) and at 10 days (BR10), quadriceps femoris (QF) volume (VOL) and isometric maximum voluntary contraction (MVC) were assessed. At BR0 and BR10 blood samples and biopsies of vastus lateralis (VL) muscle were collected. Neuromuscular junction (NMJ) stability and myofibre innervation status were assessed, together with single fibre mechanical properties and sarcoplasmic reticulum (SR) calcium handling. From BR0 to BR10, QFVOL and MVC decreased by 5.2% (P = 0.003) and 14.3% (P