Central activation and force-frequency responses of the lumbar extensor muscles

Abstract

Purpose: 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).

Methods: 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.

Results: 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).

Conclusions: 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.