This difference between fascicle and muscle-tendon behaviour suggests that the neural input in fast stretch-shortening cycle exercises minimizes the length changes in muscle fascicle and enables storage and recoil of energy from elastic components that contributes to the enhanced mechanical output of the MTU during the push-off phase.

Nine healthy men (22-45 yr) completed 100 repetitive maximal isometric contractions of the ankle plantar flexor muscles in two knee positions of full extension (K0) and flexion at 90 degrees (K90), positions that varied the contribution of the gastrocnemii. Electromyographic activity was recorded from the medial and lateral gastrocnemii and soleus muscles by using surface electrodes. Plantar flexion torque in K0 was greater and decreased more rapidly than in K90. The electromyographic amplitude decreased over time, and there were no significant differences between muscles and knee joint positions. The level of voluntary effort, assessed by a supramaximal electrical stimulation during every 10th contraction, decreased from 96 to 70% (P < 0.05) with no difference between K0 and K90. It was suggested that a decrease in plantar flexion torque was attributable to both central and peripheral fatigue and that greater fatigability in K0 than in K90 would result from a greater contribution and hence more pronounced fatigue of the gastrocnemius muscle. Further support for this possibility was provided from changes in twitch torque.

The cross-sectional areas (CSA) of fat, muscle and bone tissues of the limb as well as maximal voluntary isokinetic strength were measured in untrained men (n = 27) and women (n = 26) aged 18-25 years. Anatomical CSA of the three tissues were determined by ultrasound on the upper arm and thigh. The isokinetic strength of the elbow and knee extensor and flexor muscles were measured by an isokinetic dynamometer (Cybex II) at 1.05 rad.s-1. The women had significantly (P < 0.001) larger fat CSA and smaller bone and muscle CSA than the men in both the upper arm and thigh. Among tissue CSA, the largest difference between the women and men was found in fat CSA regardless of the measurement sites. The sex differences in bone and muscle CSA were found largely in the upper arm compared to the thigh, even when expressed per unit second power of the limb length. Regression analyses of the data for respective samples for the men and women showed significant correlations (r = 0.411-0.707, P < 0.05-P < 0.001) between CSA and strength in all muscle groups except for the elbow extensors of the men (r = 0.328, P > 0.05) and the elbow flexors of the women (r = 0.388, P > 0.05). No significant difference between sexes was observed when strength was expressed per unit of muscle CSA (F.CSA-1) for the elbow flexors and extensors. However, the men showed significantly higher F.CSA-1 than the women for the knee flexors and extensors (P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

The influences of age and sex on the cross-sectional area (CSA) and isometric strength of the ankle dorsiflexors and plantarflexors (PF) were investigated in four age groups of 121 boys and 121 girls aged: 7-9, 10-12, 13-15, and 16-18 years. A single anatomical cross-section was determined at 30% of the distance from the articular cleft between the femur and tibiacondyles by using an ultrasonic apparatus. In both sexes, the increase in age was associated with significant increases in the CSA and strength (ST) of these opposing muscle groups. The sex differences became apparent in the 13-15 year group for CSA and in the 16-18 year group for ST but the differences reduced considerably when CSA and ST were expressed per unit of the second power of the lower leg length (CSA.LL-2) and the product of CSA and the lower leg length (ST.CSA-1.LL-1), respectively. However, CSA.LL-2 of both muscles had a tendency to be increased at and over the age of 10-12 years, and was the highest at 16-18 years, and ST.CSA-1.LL-1 of PF showed higher values in the older boys than in the younger. Thus, it appeared that, at least in the reciprocal muscle groups of the ankle joint, the sex differences in muscle CSA and ST during growth could be accounted for by differences in LL and muscle mass, respectively. However, other factors must also be involved to explain completely the age differences in these variables.

The purpose of this study was to investigate the development in muscle cross-sectional area (CSA) and strength capability of the reciprocal muscle groups in the upper arm and thigh. Subjects were 130 boys aged 7 to 18 years; 7-9 yr aged-group (n = 30), 10-12 yr (n = 30), 13-15 yr (n = 35), and 16-18 yr (n = 35). Anatomical CSAs of elbow flexors (EF), elbow extensors (EE), knee flexors (KF) and knee extensors (KE) were determined with a B-mode ultrasound apparatus. Isokinetic strength during elbow or knee extension as well as elbow or knee flexion was measured by using a Cybex II isokinetic dynamometer at the pre-set constant velocity of 60 degrees/s. The effect of age associated with a significant increase in both CSA and strength with a marked increase in 13-15 yr. The elder aged-groups had a significantly higher ratio of strength to muscle CSA than in the lower aged-groups. The effect of age on EF/EE CSA ratio was not significant, but EF/EE strength ratio was the highest in 16-18 yr and the lowest in 7-9 yr. With advancing age, KF/KE CSA ratio had a tendency to increase, but KF/KE strength ratio remained almost unchanged. These results indicate that (1) children in the pre-puberty or the early stage of puberty do not develop strength in proportion to their muscle CSA, and (2) the flexors and extensors in limbs have reciprocally different growth rates in either CSA or strength even in the same region.