Hill’s classic force-velocity curve (52), demonstrates the inverse, curvilinear relationships between contractile force and velocity

60%. Viewed differently, boys required >400ms to reach 80% of their maximal torque, whereas men needed Table 1 ). However, at the https://hookupdate.net/lovoo-review/ same time they fully conform to expectations from a smaller proportion of activated type-II motor units in children’s muscles.

Force-Velocity Curve

This relationship has been repeatedly demonstrated in adults, but only to a very limited extent in children. Asai & Aoki (4), had young boys and adults perform horizontal elbow flexions, as rapidly as possible. Forearm velocity was measured at 90° elbow angle. Repeated testing under various resistance loads produced a series of force-dependent contraction velocities used to create child- and adult-specific force-velocity curves. To facilitate group comparisons, force was normalized to maximal strength (100%) and velocity to forearm length. Both men and boys exhibited Hill’s characteristic curvilinear inverse force-velocity relationship. However, at any given percentage of normalized force level, the boys’ contraction velocity was

Barrett & Harrison (7) also depict lower velocity at any force (torque) level in boys vs. men. However, for reasons that have not been elucidated, their force-velocity relationship, whether absolute or normalized, is highly incongruent with Hill’s original (52) and others’ subsequent findings (e.g., 4). That is, in spite of a wide velocity range (30–300°·s ?1 ), children’s torque does not appreciably fall off with increasing velocity, while in adults, it drops off only slightly. Consequently, and contrary to its well-established behavior, power does not peak within this velocity range. Therefore, the apparent child-adult normalized-power identity is difficult to interpret and the conclusions hard to accept.

High-Velocity Isokinetic Torque

Maximal isometric or low-velocity force production depends on both, slow- and fast-twitch motor-unit recruitment. High-velocity force production or high power production, on the other hand, must heavily depend on the extent of fast-twitch motor-unit utilization (24). Presumably, this may be due to a significant portion of the type-I motor-unit pool either not being recruited in the first place, or more likely, being unable to effectively contribute to the fast-contracting muscle.

16 years of age. Concentric and eccentric quadriceps torque was tested on an isokinetic dynamometer at 45, 90, and 180°·s ?1 . Among the boys, aside from the concentric, low-velocity (45°·s ?1 ) values, all normalized torque values were significantly higher at age 16 compared with 11. More importantly, the increase in torque was larger with increasing contractile velocity. Girls exhibited similar trends, although differences did not reach statistical significance. The latter could be due to the smaller sample size. However, we too found girl-woman differences (34) to be much smaller than the corresponding ones for males (37), suggesting sex differences in muscle functional development. These findings support the idea that fast-twitch motor units are being increasingly employed as children, boys in particular, go from prepubescence to adolescence.

Maximal, Short-Term Power Output

Children’s size-normalized power output in short-term, supra-maximal exercise (e.g., Wingate Anaerobic Test), is significantly lower than that of adults (91). These differences persist even after child-adult differences in relative muscle-mass proportions are corrected for. A seeming exception is a study by Beneke et al. (15) who showed Wingate-Test size-normalized power output of 16.3-yr-old adolescents to be 31 P MRS during progressive exercise of the quadriceps muscle. They found the intracellular threshold of the Pi/PCr (inorganic phosphate to phosphocreatine) ratio to occur at

20% higher body-mass-normalized power output in boys than in men. Although, by themselves, these findings cannot distinguish differential motor-unit activation from differences in the muscle’s metabolic profile, they provide tissue-level corroboration of children’s lower lactate response.

Phosphocreatine (PCr) Recovery

Faster PCr recovery following intense muscular exercise is regarded as an indication of more oxidative / less glycolytic metabolic profile, or a greater reliance on oxidative motor units (3). Following intense exercise, PCr was shown to recover considerably faster in children (particularly boys) than in adults (particularly men; 89, 94).

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