Validation of Muscle Ultrasound Speckle Tracking and the Effect of Nordic Hamstring Exercise on Biceps Femoris Displacement

Objective: This study aimed to validate the ultrasound speckle tracking (UST) algorithm, determine the optimal probe location by comparing normalized cross-correlation (NCC) values of muscle displacement at two locations (proximal vs. middle) of the biceps femoris long head (BFlh) using the UST, and investigate the effects of Nordic hamstring curl exercise (NHE) training on BFlh displacement. Methods: UST efficacy was verified with ex vivo uniaxial testing of porcine leg muscles. Ten participants (mean age 23.4 y) were recruited for comparison of NCC values between the proximal and middle BFlh during maximal knee flexor eccentric contraction using an ultrasound device and isokinetic dynamometer. Using the above devices, electromyography and shear wave elastography, the effects of an 8-wk NHE program on the morphomechanical profiles, displacement and activation of the middle BFlh and eccentric torque of the knee flexor were investigated in 20 males (mean age 23.5 y). Results: The validity of UST was confirmed by comparing UST and ex vivo test results (r = 0.99). The NCC values of the middle BFlh were greater than those of the proximal BFlh. The caudal-direction displacements of the BFlh in the dominant leg were reduced after the NHE training (from 3.98 ± 3.84 to 1.50 ± 4.17 mm, p < 0.05). The magnitude of reduction was associated with improved eccentric strength of the knee flexor muscle in the dominant leg (r = 0.63). Conclusions: UST is a validated tool for measuring muscle displacement. NHE training decreased caudal-direction muscle displacement in the BFlh and increased eccentric strength.