- The purpose of the current study was to establish that the measurements taken during dynamic contractions could provide insight into differences/similarities that evolve between muscle mechanics and it's neuromuscular (electrical) source as one aging. The present study assesses the differences in strength that is resistance to fatigue (RTF) of the knee extensors (vastus lateralis (VL) and vastus medialis(VM)) as a function of age and gender. Eighty subjects (45 males and 35 females) with a-symptomatic knees were tested; Males: Group A, 20-29 years old (N = 20); Group B, 30-39 years old (N = 8); Group C, 40-49 years old (N = 9); Group D, 50-60 years old (N = 8) and the Females: Group E, 20-39 years old (N = 25); Group F, 40-60 years old (N = 10). All subjects were tested for their dominant knee extensor strength by performing a maximal voluntary isometric contraction (MVIC) and fatiguing bouts at four isokinetic velocities of 300°/sec, 180°/sec, 60°/sec, 30°/sec. The muscular fatigue was assessed by comparing the median frequency (MF) of the surface EMG (SEMG) activity of VL and VM during the 2nd contraction and the 20th (last) contraction of the bouts. The results showed distinct differences between groups A, B and C, D similarly between group E and F in muscle strength, when performing isokinetic concentric contraction. There was no age and gender difference in isokinetic fatigue ratio (IFR) and in electromyographic measurements in MF of VM and VL. The decrease in knee extensor strength occurred in the 4th decade for the males and in the 3rd for the females. These results of this study provide quantitative evaluation of knee strength of sedentary adults at various ages. The knowledge of the critical age group allow us to construct a fast, reliable and valid test in assessing the condition of main anti-gravity muscles and alert the person before any stability deficiencies may crop out. The isokinetic findings were in line with the SEMG findings that is the phasic (strength) component of knee extensors decreased with age while there is no significant difference in tonic (endurance) component up to 60 years old. It is anticipated that the proposed protocol as administered may be useful in occupational settings.