Scand zyxwvutsrqpon J Med Sci Sports 1997: 7: 195-202 Printed in Denmark zyxwvutsrqponm . All rights reserved Copyright zyxw 0 Munksgaard 1997 Scandinavian Journal of zyx MEDICINE & SCIENCE IN SPORTS zyx ISSN 0905-7188 zyx Determinants of musculoskeletal flexibility: viscoelastic properties, cross-sectional area, EMG and stretch tolerance Magnusson zyxwvutsrq SP, Simonsen EB, Aagaard P, Boesen J, Johannsen E Kjaer M. Determinants of musculoskeletal flexibility: viscoelastic properties, cross-sectional area, EMG and stretch tolerance. Scand J Med Sci Sports 1997: 7: 195-202. 0 Munksgaard, 1997 Cross-sectional area, stiffness, viscoelastic stress relaxation, stretch toler- ance and EMG activity of the human hamstring muscle group were examined in endurance-trained athletes with varying flexibility. Subjects were defined as tight (n=10) or normal (n=8) based on a clinical toe-touch test. Cross-sectional area was computed from magnetic resonance imagin- ing (MRI) images. Torque (Nm) offered by the hamstring muscle group, electromyographic (EMG) activity, knee joint angle and velocity were con- tinuously monitored during two standardized stretch protocols. Protocol 1 consisted of a slow stretch at 0.087 radls (dynamic phase) to a pre-deter- ' mined final angle followed by a 90-s static phase. In the dynamic phase final angle and stiffness was lower in tight (28.052.9 Ndrad) than nor- mal subjects (54.956.5 Ndrad), P<O.Ol. In the static phase tight subjects had lower peak (15.4212 Nm) and final torque (10.8k1.6 Nm) than nor- mal subjects (31.654.1 Nm, 24.153.7 Nm, respectively)(P<O.Ol), but torque decline was similar. Protocol 2 consisted of a slow stretch to the point of pain and here tight subjects reached a lower maximal angle!, torque, stiffness and energy than normal subjects (fYO.01). On the other hand, stiffness was greater in tight subjects in the common range (P<O.Ol). Cross-sectional area of the hamstring muscles and EMG ac- tivity during the stretch did not differ between the groups. However, lat- eral hamstring cross-sectional area was positively related to mid-range stiff- ness (P<0.05), but inversely related to final stiffness, peak torque and the toe-touch test (P<O.Ol). Final angle and peak torque in protocol 1 com- bined to improve the predictability of the toe-touch test (R2=0.77, P<O.OOl). These data show that the toe-touch test is largely a measure of hamstring flexibility. Further, subjects with a restricted joint range of movement on a clinical toe-touch test have stiffer hamstring muscles and a I lower stretch tolerance. Restricted musculoskeletal flexibility, defined as the maximal available joint range of motion (l), may be related to injury (2-7). Nicholas (7) suggested that musculoskeletal tightness in football players may be associated with an increased likelihood of muscle strain injury. Subsequently, retrospective studies have shown that athletes with previous hamstring strains have limited flexibility (5, 8) and prospective investi- gations have suggested that lower extremity tightness predisposes to injury (3, 6). It is commonly implied that the relationship of skeletal muscle flexibility to injury is in part related to the passive properties of the muscle zyxwvutsr (4, 9, 10). However, clinical investigations S. P. Magnussonl E. B. Simonsen2, P. Aagaardl, J. Boesen3, F. Johannsen4, M. Kjael.4 'Team Danmark Test Center, Rigshospitalet, 21nstitute of Medical Anatomy, Panum Institute, University of Copenhagen, 3Department of Radiology, Frederiksberg Hospital, 4Copenhagen Muscle Research Center, University of Copenhagen, Rigshospitalet and Department of Rheumatology H, Bispebjerg Hospital, Copenhagen, Denmark Key words: cross-sectional area: stiffness; hamstring muscle; stretch tolerance; flexibility Peter Magnusson, Project Coordinator, Team Danmark Test Center, Rigshopitalet, Afd 2001, Blegdamsvej 9, DK-2100 Copenhagen, Denmark Tel.: +45 35 45 28 01; fax: +45 35 45 22 51 Accepted for publication 9 January 1997 on flexibility in humans have largely relied on maxi- mal joint range of motion as the dependent variable rather than its passive properties (3, 5, 8). It was re- cently suggested that acute and long term increases in maximal attainable joint range of motion is related to the subject's tolerance to the stretch, rather than the passive properties of the muscle (1 1, 12). How- ever, it remains unclear if subjects with varying flexi- bility have a different tolerance to stretch. In contrast to clinical studies animal models have been used to describe the behavior of the muscle- tendon during stretch in biomechanical terms, such as stiffness and viscoelastic stress relaxation (1 0, 13- 195