Influence of a low-level contractile response from the soleus, gastrocnemius and tibialis anterior muscles on viscoelastic stress-relaxation of aged human calf muscle-tendon units

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 ≥1% and <10%, or in the TA ≥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 (R2=0.581). The G SEMG from 0–15 s accounted for 56% (r2=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 (R2=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.