Table 2.

Task and discharge characteristics of biceps brachii motor units at recruitment when old and young adults performed isometric contractions with elbow flexors either against a rigid restraint or supporting an inertial load at two target forces

	Old		Young
	Rigid	Inertial	Rigid	Inertial
Recruitment threshold force, % MVC	25.4 ± 16.6	26.1 ± 12.4	32.8 ± 9.7	30.0 ± 10.9
Target force, % MVC
Small	18.4 ± 7.9	15.5 ± 9.3	27.1 ± 10	17.5 ± 6.7
Large	13.5 ± 7.0*	9.24 ± 7.2*	22.3 ± 10*	11.6 ± 5.1*
Target force difference, % MVC
Small	7.05 ± 4.2	10.7 ± 4.2	5.72 ± 2.0	12.7 ± 4.4
Large	11.9 ± 5.2*	17.0 ± 6.9*	10.5 ± 2.4*	18.4 ± 6.2*
Time to onset, s
Small	17.9 ± 27	28.5 ± 59	22.5 ± 22	17.4 ± 28
Large	160 ± 145*	202 ± 265*	37.1 ± 50	172 ± 187*
Contraction duration, s
Small	84.4 ± 29	185 ± 219		138 ± 21
Large	223 ± 147*	325 ± 266*		295 ± 195*
Mean discharge rate, pps
Small	8.3 ± 2.5	9.1 ± 2.9	13.2 ± 3.5	10.9 ± 3.3
Large	8.4 ± 3.3	9.3 ± 4.8	13.9 ± 4.3	8.6 ± 2.5*
CV for ISI, %
Small	26.3 ± 15	21.0 ± 9.8	18.7 ± 7.9	27.3 ± 13
Large	24.0 ± 13	23.7 ± 9.1	35.0 ± 10*	27.9 ± 12

Values are means ± SD. Data for the rigid restraint condition were obtained from Pascoe et al. (2011) for the old adults and from Riley et al. (2008a) for the young adults. “Small” and “large” refer to the difference between the target force and recruitment threshold force of the motor unit.

^*P < 0.007 compared with small target force difference for the same age and type of load.