Viscoelastic stress relaxation in human skeletal muscle

MCHUGH, M. P., S. P. MAGNUSSON, G. W. GLEIM, and J. A. NICHOLAS. Viscoelastic stress relaxation in human skeletal muscle. Med. Sci. Sports Exerc., Vol. 24, No. 12, pp. 1375–1382, 1992. Viscoelastic stress relaxation refers to the decrease in tensile stress over time that occurs when a body under tensile stress is held at a fixed length. The purpose of this study was to demonstrate viscoelastic stress relaxation in human skeletal muscle. Resistance to stretch (tensile force), hip flexion range of motion (ROM), and reflex contractile activity (IEMG) of the hamstring muscle group were measured during a passive straight leg raise. The testing protocol involved a first stretch to the maximum tolerated ROM with the lower extremity held at that point for 45 s (test 1). All 15 subjects tested-(9 men, 6 women) had a stretch induced EMG response. The onset of a sustained EMG response occurred at a specific hip flexion angle in 10 subjects. These 10 subjects (6 men, 4 women) underwent a second straight leg raise stretch (test 2) to a ROM 5 degrees below the ROM at which the onset of EMG activity occurred in test 1. The stretch was held at this hip flexion angle for 45 s. There was a significant decrease in force at final ROM during the 45 s in test 1 (11.35 ± 1.75 N, P < 0.0001) and in test 2 (4.2 ± 1.55 N, P < 0.05). The percent decrease from the force at the respective final ROM was not significantly different between the tests (14.4 ± 2.2% in test 1 and 13 ± 2.3% in test 2). In test 1 there was a significant decrease over time in IEMG of 59.71 ± 16.01 μV.s (P < 0.01) which was not significantly correlated to the decrease in force. In conclusion, the data from test 2 demonstrates viscoelastic stress relaxation independent of detectable EMG activity. When the stretch was sufficient to elicit an EMG response the relaxation response was similar but had a larger absolute magnitude.