Despite full voluntary effort, neuromuscular activation of the quadriceps femoris muscle appears inhibited during slow concentric and eccentric contractions. Our aim was to compare neuromuscular activation during maximal voluntary concentric and eccentric quadriceps contractions, hypothesizing that inhibition of neuromuscular activation diminishes with resistance training. In 15 men, pretraining electromyographic activity of the quadriceps muscles [vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF)] was 17-36% lower during slow and fast (30 and 240 degrees/s) eccentric and slow concentric contractions compared with fast concentric contractions. After 14 wk of heavy resistance training, neuromuscular inhibition was reduced for VL and VM and was completely removed for RF. Concurrently, electromyographic activity increased 21-52, 22-29, and 16-32% for VL, VM, and RF, respectively. In addition, median power frequency decreased for VL and RF. Eccentric quadriceps strength increased 15-17%, whereas slow and fast concentric strength increased 15 and 8%, respectively. Pre- and posttraining median power frequency did not differ between eccentric and concentric contractions. In conclusion, quadriceps motoneuron activation was lower during maximal voluntary eccentric and slow concentric contractions compared with during fast concentric contraction in untrained subjects, and, after heavy resistance training, this inhibition in neuromuscular activation was reduced.

This short review discusses changes in the fibre type distribution, myosin heavy chain isoform composition and histological appearance of the very elderly human skeletal muscle. Point of origin of the discussion comes from data that we have obtained from muscle biopsies from the vastus lateralis muscle of a group of frail very elderly subjects (age: 88 +/- 3 years, range 85-97). Myosin heavy chain composition of muscle homogenates and single fibres, fibre type distribution, fibre size and capillary density were examined and compared with muscle biopsies from the young vastus lateralis muscle. Histological preparations of the muscle biopsies from our elderly subjects showed extended "grouping" (Nygaard & Sanchez, Anat Rec 1992: 202: 451-459) of the fibre types as well as significant changes in the appearance and size of the individual muscle fibres. On average, the fibre type composition of our very elderly subjects do not seem to be different to what is observed in a corresponding young group when examined with ATPase histochemistry. Likewise, the MHC composition of the muscle homogenates is comparable to what is observed in young subjects. Nevertheless, a detailed examination of the MHC composition of single fibres from the old subjects revealed that the most prominent phenotype was fibres co-expressing MHC I and MHC IIA. This is very different from what is observed in the young muscle. Detailed investigation of longitudinally cut fibres indicated that some fibres in the very old muscle, in contrast to the young muscle, switch fibre type along the length of the fibre or contain areas or nuclear domains in which the MHC expression is different from the remaining part of the fibre.

Spinal cord injured (SCI) individuals most often contract their injury at a young age and are deemed to a life of more or less physical inactivity. In addition to the primary implications of the SCI, severe SCI individuals are stigmatized by conditions related to their physically inactive lifestyle. It is unknown if these inactivity related conditions are potentially reversible and the aim of the present study was, therefore, to examine the e ect of exercise on SCI individuals. Ten such individuals (six with tetraplegia and four with paraplegia; age 27 ± 45 years; time since injury 3 ± 23 years) were exercise trained for 1 year using an electrically induced computerized feedback controlled cycle ergometer. They trained for up to three times a week (mean 2.3 times), 30 min on each occasion. The gluteal, hamstring and quadriceps muscles were stimulated via electrodes placed on the skin over their motor points. During the ®rst training bouts, a substantial variation in performance was seen between the subjects. A majority of them were capable of performing 30 min of exercise in the ®rst bout; however, two individuals were only able to perform a few minutes of exercise. After training for 1 year all of the subjects were able to perform 30 min of continuous training and the work output had increased from 4+1 (mean+SE) to 17+2 Kilo Joules per training bout (P50.05). The maximal oxygen uptake during electrically induced exercise increased from 1.20+0.08 litres per minute measured after a few weeks habituation to the exercise to 1.43+0.09 litres per minute after training for 1 year (P50.05).Magnetic resonance cross sectional images of the thigh were performed to estimate muscle mass and an increase of 12% (mean, P50.05) was seen in response to 1 year of training. In biopsies taken before exercise various degrees of atrophy were observed in the individual muscle ®bres, a phenomenon that was partially normalized in all subjects after training.The ®bre type distribution in skeletal muscles is known to shift towards type IIB ®bres (fast twitch, fast fatiguable, glycolytic ®bres) within the ®rst 2 years after the spinal cord injury. The muscle in the present investigation contained of 63% myosin heavy chain (MHC) isoform IIB, 33% MHC isoform IIA (fast twitch, fatigue resistant) and less than 5% MHC isoform I (slow twitch) before training. A shift towards more fatigue resistant contractile proteins was found after 1 year of training. The percentage of MHC isoform IIA increased to 61% of all contractile protein and a corresponding decrease to 32% was seen in the fast fatiguable MHC isoform IIB, whereas MHC isoform I only comprised 7% of the total amount of MHC. This shift was accompanied by a doubling of the enzymatic activity of citrate synthase, as an indicator of mitochondrial oxidative capacity.It is concluded that inactivity-associated changes in exercise performance capacity and skeletal muscle occurring in SCI individuals after injury are reversible, even up to over 20 years after the injury. It follows that electricall...

Very little attention has been given to the combined effects of aging and disuse as separate factors causing deterioration in muscle mechanical function. Thus the purpose of this study was to investigate the effects of 2 wk of immobilization followed by 4 wk of retraining on knee extensor muscle mechanical function (e.g., maximal strength and rapid force capacity) and muscle fiber morphology in 9 old (OM: 67.3 ± 1.3 yr) and 11 young healthy men (YM: 24.4 ± 0.5 yr) with comparable levels of physical activity. Following immobilization, OM demonstrated markedly larger decreases in rapid force capacity (i.e., rate of force development, impulse) than YM (∼ 20-37 vs. ∼ 13-16%; P < 0.05). In contrast, muscle fiber area decreased in YM for type I, IIA, and IIx fibers (∼ 15-30%; P < 0.05), whereas only type IIa area decreased in OM (13.2%; P < 0.05). Subsequent retraining fully restored muscle mechanical function and muscle fiber area in YM, whereas OM showed an attenuated recovery in muscle fiber area and rapid force capacity (tendency). Changes in maximal isometric and dynamic muscle strength were similar between OM and YM. In conclusion, the present data reveal that OM may be more susceptible to the deleterious effects of short-term muscle disuse on muscle fiber size and rapid force capacity than YM. Furthermore, OM seems to require longer time to recover and regain rapid muscle force capacity, which may lead to a larger risk of falling in aged individuals after periods of short-term disuse.