The purpose of this study was to compare the steadiness and discharge rate of motor units during submaximal contractions performed by young and old adults. Subjects performed isometric and slow shortening and lengthening contractions with the first dorsal interosseous muscle. The steadiness of the isometric and slow anisometric contractions was less for the old subjects compared with young subjects, especially at the lower target forces and with the lightest loads. Furthermore, the steadiness of the lengthening contractions was less compared with the shortening contractions for the old subjects. Although the mean discharge rates of motor units were not different for the two groups of subjects, the variability of the discharge rates was greater for the old subjects during the isometric and anisometric contractions. We conclude that a more variable discharge by single motor units probably contributes to the reduced ability of old adults to perform steady muscle contractions.

The purpose of the study was to determine the association between steadiness and activation of the agonist and antagonist muscles during isometric and anisometric contractions. Young (n = 14) and old (n = 15) adults used the first dorsal interosseus muscle to perform constant-force and constant-load tasks (2.5, 5, 20, 50, and 75% maximum) with the left index finger. Steadiness was quantified as the coefficient of variation of force and the SD of acceleration normalized to the load lifted. The old adults were less steady at most target forces with isometric contractions (2.5, 5, and 50%) and with most loads during the anisometric contractions (2.5, 5, and 20%). Furthermore, the old adults were less steady when performing lengthening contractions (up to 50%) compared with shortening contractions, whereas there was no difference for young adults. The reduced steadiness exhibited by the old adults during these tasks was not associated with differences in the average level of agonist muscle electromyogram or with coactivation of the antagonist muscle.

The purpose of this study was to compare the steadiness and discharge rate of motor units during submaximal contractions performed by young and old adults. Subjects performed isometric and slow shortening and lengthening contractions with the first dorsal interosseous muscle. The steadiness of the isometric and slow anisometric contractions was less for the old subjects compared with young subjects, especially at the lower target forces and with the lightest loads. Furthermore, the steadiness of the lengthening contractions was less compared with the shortening contractions for the old subjects. Although the mean discharge rates of motor units were not different for the two groups of subjects, the variability of the discharge rates was greater for the old subjects during the isometric and anisometric contractions. We conclude that a more variable discharge by single motor units probably contributes to the reduced ability of old adults to perform steady muscle contractions.

When old adults participate in a strength-training program with heavy loads, they experience an increase in muscle strength and an improvement in the steadiness of submaximal isometric contractions. The purpose of this study was to determine the effect of light- and heavy-load strength training on the ability of old adults to perform steady submaximal isometric and anisometric contractions. Thirty-two old adults (60-91 yr) participated in a 4-wk training program of a hand muscle. Both the light- and heavy-load groups increased one-repetition maximum and maximal voluntary contraction (MVC) strength and experienced similar improvements in the steadiness of the isometric and shortening and lengthening contractions. The increase in MVC strength was greater for the heavy-load group and could not be explained by changes in muscle activation. Before training, the lengthening contractions were less steady than the shortening contractions with the lightest loads (10% MVC). After training, there was no difference in steadiness between the shortening and lengthening contractions, except with the lightest load. These improvements were associated with a reduced level of muscle activation, especially during the lengthening contractions.

The purpose of this study was to determine the minimum number of contractions that are needed to detect an increase in the muscle proton spin-spin relaxation time (T2) at a given exercise intensity. Five healthy human subjects performed five sets of an exercise that included concentric and eccentric contractions of the elbow-flexor muscles with loads that were 25 or 80% of maximum. With the 80% load, the five sets involved 1, 2, 5, 10, or 20 repetitions of the exercise; with the 25% load the five sets were 2, 5, 10, 20, or 40 repetitions. The upper arm of each subject was imaged before and immediately after each set of the exercise. Spin-echo images (repetition time/echo time = 2,000 ms/30, 60, 90, and 120 ms) were collected using an extremity coil, and T2 values were calculated. The signal intensity was measured from the elbow-flexor and -extensor muscles and from the bone marrow of the humerus. With the 80% load, T2 increased in the short head of the biceps brachii after two repetitions of the elbow exercise and after five repetitions in the brachialis and the long head of the biceps brachii. With the 25% load, T2 became longer after five repetitions of the exercise for the short head of the biceps brachii and after 10 repetitions for the brachialis and the long head of the biceps brachii. T2 varied linearly with the number of contraction repetitions for each of the elbow-flexor muscles at either load (r2 > or = 0.97, P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

No abstract

This study examined the patterns of activation in the superficial and deep parts of the first dorsal interosseus muscle and in the antagonist muscle, second palmar interosseus, during postural tasks (position holding) and slow movements (position tracking) of the index finger performed by young and old adults. The position-tracking task involved the index finger lifting light loads (2.5, 10, and 35% of maximum) with shortening and lengthening contractions as steadily as possible. Steadiness was quantified in both tasks as the standard deviation of index finger acceleration. The fluctuations in acceleration during the two tasks were greater for the old subjects (62-72 yr) compared with young subjects (19-27 yr), especially with the lightest loads. The two groups of subjects activated the superficial and deep parts of first dorsal interosseus at similar intensities during the position-holding task, whereas the deep part was more active during the shortening and lengthening contractions of the position-tracking task. The nonuniform activation of first dorsal interosseus, therefore, was not associated with the difference in the standard deviation of acceleration between the young and old subjects. Furthermore, there was no association between the average level of coactivation by the antagonist muscle and the standard deviation of acceleration for either group of subjects across these tasks. Thus the greater variability in motor output exhibited by the older adults could not be explained by either the nonuniform activation of the agonist muscle or the average level of coactivation by the antagonist muscle.

A description is provided of the fiber-type composition of several hindlimb muscles of the adult turtle, Pseudemys (Trachemys) scripta elegans. In addition, cross-section areas of each fiber type and an estimation of the relative (weighted) cross-section area (wCSA) occupied by the different fiber types are also provided. Seven muscles were selected for study, based on their suitability for future neurophysiological analysis as components of the segmental motor system, and on their homologies with muscles in other vertebrates. The test muscles were iliofibularis (ILF), ambiens (AMB), external gastrocnemius (EG), extensor digitorum communis (EDC), flexor digitorum longus (FDL), tibialis anterior (TA), and peroneus anterior (PA). Serial sections of these muscles were stained for myosin adenosine triphosphatase (ATPase), NADH-diaphorase, and alpha-glycerophosphate dehydrogenase (alpha-GPDH), thereby enabling fiber-type classification on the basis of indirect markers for contraction speed and oxidative (aerobic) vs. glycolytic (anaerobic) metabolism. All muscles contained three fiber types: slow oxidative (SO; possibly including some non-twitch tonic fibers); fast oxidative glycolytic (FOG); and fast glycolytic (Fg). There were at least 30% FOG and 50% FOG + Fg fibers in the seven muscles, the extreme distributions being the predominantly glycolytic ILF vs. the predominantly oxidative FDL muscle (ILF--15.5% SO, 35.2% FOG, 49.3% Fg vs. FDL--49.1% SO, 41.1% FOG, 9.8% Fg). As in other species, the test muscles exhibited varying degrees of regional concentration (compartmentalization) of the different fiber types. This feature was most striking in ILF. Pronounced compartmentalization was also observed in AMB, EG, PA, TA, and EDC, whereas the distribution of fiber types in the highly oxidative FDL was homogeneous. In five of the seven muscles, fiber size was ranked with Fg > FOG > SO. In terms of wCSA, which provides a coarse-grain measure of the different fiber types' potential contribution to whole muscle peak force, all muscles exhibited a higher Fg and lower SO contribution to cross-section area than suggested by their corresponding fiber-type composition. The largest relative increase in wCSA vs. fiber-type composition were in the ILF and AMB muscles. We conclude that the turtle hindlimb provides some interesting possibilities for testing for a division of labor among different muscles during different movements (e.g., sustained vs. ballistic), and for study of the behavior of the different fiber (and motor unit) types under normal and perturbed conditions. The relationships between the present results and previous findings on homologous muscles of the mammalian (cat, rat) and reptilian (lizard) hindlimb are discussed.