Central Activation and Force-Frequency Responses of the Lumbar Extensor Muscles

Abstract

To determine the central activation and force-frequency properties of a muscle group in which these data have not previously been reported (i.e., the lumbar extensors). Fifteen young healthy participants were tested. Maximal voluntary isometric contraction (MVIC) of the trunk was collected using a custom apparatus with a cable-mounted load cell. Central activation was determined by delivering a supramaximal 100-ms, 100-Hz train to the lumbar muscles during the MVIC. The MVIC and the stimulated forces were used to calculate the central activation ratio (CAR) for each subject using the formula CAR = MVIC force / (MVIC + stimulated force), with a value of 1 indicating full central activation. Force-frequency relationship of the muscle group was determined by stimulating the muscles every 10 s with two 500-ms trains of the following frequencies: 1, 5, 10, 20, 40, 60, 80, and 100 Hz. The force-frequency data were fit with a four-parameter Hill equation to obtain the frequency at which 50% of the range of forces generated by the various stimulation trains is produced (F50) and the Hill coefficient, which indicates the steepness of the linear portion of the relationship. Mean MVIC was 345.4 N (SD = 126.7), and mean CAR was 0.95 (SD = 0.06). The force-frequency data showed a mean F50 of 16.40 Hz (SD = 3.15) and a mean Hill coefficient of 2.21 (SD = 0.50). Central activation and force-frequency testing of the lumbar extensor muscles is feasible, and the data reported here represent, to our knowledge, the first of their kind in this muscle group.