Measuring viscoelastic stress-relaxation (VSR) as the decline in passive tension over time when the skeletal muscle-tendon unit (MTU) is held in a lengthened position may be difficult in older people who may have difficulty relaxing. This study examined the VSR of the aged calf MTUs with a low-level involuntary contractile response from the soleus (S), gastrocnemius (G) and tibialis anterior (TA) muscles. Calf MTUs of 29 men and women (65-90 years) were stretched to maximal dorsiflexion and held for 60 s while torque (Nm) and surface electromyograms (SEMG) were recorded. Subjects with normalized SEMG (% of maximal voluntary contraction SEMG) of the S, G or TA <1% were assigned to Group 1 (n=14) and subjects with SEMG of the S or G > or =1% and <10%, or in the TA > or =1% were assigned to Group 2 (n=15). Although the mean total percent torque decline for Group 1 (17.9%) and Group 2 (16.6%) did not differ statistically, multiple regression analysis within Group 2 indicated that the G and TA SEMG accounted for 58% of its variation (R (2)=0.581). The G SEMG from 0-15 s accounted for 56% (r (2)=0.563) of the variation in the percent decline normalized to total percent decline (100%). From 15-30 s Group 2 had less percent decline (2%) than Group 1 (14%) (P=0.043) and the S and G SEMG accounted for 67% of its variation (R (2)=0.673). The results indicated that a low-level involuntary contractile response from the S, G and TA muscles may be related to the VSR process of aged calf MTUs. Measuring VSR within time intervals normalized to the total percent decline offers a new method to study VSR, which should be measured with SEMG in the muscles <1% MVC SEMG to ensure valid measurements.