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The literature generally supports the observation of less muscle fatigue in older (в‰Ґ65 years) compared with young (в‰¤40 years) adults in response to isometric contractions. Mechanisms of fatigue can include transient impairments at any site along the pathway of force or power production, from the brain to the myofilaments. Muscle fatigue is defined as the reduced capacity to produce torque or power in response to a period of contractile activity. To date, the extent to which architectural remodeling affects age-related differences in muscle fatigue is not known. Thus, age-related differences in fascicle length could place older muscles at a disadvantage for generating power during high-velocity contractions. Changes in fascicle length may account for ~20% of the age-related difference in maximal shortening velocity of the medial gastrocnemius in men. observed an age-related decrease in fascicle length in the vastus lateralis in men but not women. Given the apparent lack of sex-related differences in the decline of quadriceps muscle volume with age, similar age-related changes in muscle architecture may be expected in women. Previous work has shown fascicle length and pennation angle of the vastus lateralis to be ~12% shorter and ~25% smaller, respectively, in older compared with young men. In aging, the loss of muscle mass results in architectural remodeling fascicles can become shorter and less pennate.

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Conceptually, a greater number of sarcomeres in parallel allows for greater torque production, while more sarcomeres in series allows for greater velocity of shortening. Generally, muscles with a greater pennation angle have more sarcomeres in parallel, whereas muscles with longer fascicles contain more sarcomeres in series. The decline in maximal torque and power with old age might be explained, at least in part, by changes in muscle architecture such as muscle thickness, pennation angle, and fascicle length. In particular, power production by the knee extensor muscles is important due to its association with physical function in aging, as well as the concept that rapid contraction of the knee extensors appears to be crucial for preventing falls following a balance perturbation. These changes have significant consequences for older adults because deficits in muscular power are associated with an increased likelihood of falling, which may place older adults at risk of injury, poor mobility and subsequent loss of independence. Maximal torque and power production are lower in older compared with younger adults, due to factors such as a loss of muscle mass, slower maximal contraction velocities, and lower maximal motor unit discharge rates. The inverse association between baseline T 1/2 and fatigue in older women suggests that factors related to slower muscle contractile properties may be protective against fatigue during fast, repetitive contractions in aging. These results indicate that differences in muscle architecture, voluntary activation, and low-frequency fatigue do not explain the greater fatigue of older compared with young women during high-velocity contractions. Baseline T 1/2 was inversely associated with fatigue in older (r 2 = 0.584, p = 0.045), but not young women (r 2 = 0.147, p = 0.348). Low-frequency fatigue (О”10:80Hz torque) occurred in both groups (p<0.001), as did slowing of T 1/2 (p = 0.001), with no differences between groups. 55.2В±4.2% initial torque, respectively p = 0.004), with no evidence of voluntary activation failure (О”MVIC:80Hz torque) in either group (pв‰Ґ0.317). Older women fatigued more than young (to 33.6В±4.7% vs. Architecture variables did not differ between groups (pв‰Ґ0.209), but the half-time of torque relaxation (T 1/2) was longer in older than young women at baseline (151.9в–.0 vs. Maximal voluntary dynamic (MVDC) and isometric (MVIC), and stimulated (80Hz and 10Hz, 500ms) isometric contractions were performed before and immediately after 120 MVDCs (240В°. Vastus lateralis muscle architecture was determined in quiescent muscle by ultrasonography in 8 young (23.4в‘.8 yrs) and 8 older women (69.6в‘.1). We examined the potential roles of resting muscle architecture and baseline contractile properties, as well as changes in voluntary activation and low-frequency fatigue in response to high-velocity knee extensor work. Although high-velocity contractions elicit greater muscle fatigue in older than young adults, the cause of this difference is unclear.

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