1. Characterization of the human quadriceps muscle in active elders
Jennifer E. Stevens, MPT, Stuart Binder-Macleod, PhD, PT, Lynn Snyder-Mackler, ScD, PT 
Archives of Physical Medicine and Rehabilitation 
Volume 82, Issue 7, Pages (July 2001)
DOI: /apmr
Copyright © 2001 American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation Terms and Conditions

2. Fig. 1
Experimental setup. Subjects sat secured to a force dynamometer with the hip flexed to 85° and the knee flexed to 90°. For maximal voluntary isometric contraction (MVIC) testing, current was delivered to electrodes placed over the rectus femoris and vastus medialis portions of the quadriceps femoris muscles. Visual feedback of quadriceps force was provided and subjects performed maximal voluntary contractions.
Archives of Physical Medicine and Rehabilitation  , DOI: ( /apmr )
Copyright © 2001 American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation Terms and Conditions

3. Fig. 2
Central activation ratio (CAR) determination. A supramaximal 100-ms train of stimuli (100pps) was delivered to the quadriceps femoris muscles during a MVIC. (A) The CAR was equal to 1.0 if the superimposed stimulation did not increase or caused a decrease in the volitional force. (B) If the superimposed stimulation increased force above the volitional contraction, then the CAR was less than 1.0. Arrows indicate the response to the superimposed electric stimulation.
Archives of Physical Medicine and Rehabilitation  , DOI: ( /apmr )
Copyright © 2001 American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation Terms and Conditions

4. Fig. 3
Testing sequence. Order of testing for MVIC, force-frequency, and fatigue tests.
Archives of Physical Medicine and Rehabilitation  , DOI: ( /apmr )
Copyright © 2001 American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation Terms and Conditions

5. Fig. 4
MVIC force (mean ± standard deviation [SD]) produced by young (n = 20) and elderly (n = 20) subjects. A train of electric pulses superimposed on the maximal contraction verified that the forces produced were maximal. NOTE. p <.01, Student's t test.
Archives of Physical Medicine and Rehabilitation  , DOI: ( /apmr )
Copyright © 2001 American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation Terms and Conditions

6. Fig. 5
Raw data from force-frequency testing for young and elderly subjects. (A) Young subject during fresh and fatigue conditions (8pps). (B) Elderly subject during fresh condition and fatigue conditions (8pps). (C) Young subject during fresh and fatigue conditions (100pps). (D) Elderly subject during fresh and fatigue conditions (100pps).
Archives of Physical Medicine and Rehabilitation  , DOI: ( /apmr )
Copyright © 2001 American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation Terms and Conditions

7. Fig. 6
Force-frequency relationship for the quadriceps femoris muscle of young (n = 20) and elderly (n = 20) subjects during the (A) fresh, (B) fatigue, and (C) recovery conditions. The solid line, bisecting the y axis, indicates the normalized force from which F50 values were calculated. A significant difference existed in F50 values (mean ± SD) under all 3 conditions. (A) Fresh (t = 2.866, p ≤.01): young, 14.7 ± 3.1; elderly, 12.3 ± 2.1. (B) Fatigue (t = 2.014, p ≤.05): young, 24 ± 5.4; elderly, 21.1 ± 3.6. (C) Recovery (t = 2.925, p ≤.01): young, 22.1 ± 4; elderly, 18.8 ± 3.2.
Archives of Physical Medicine and Rehabilitation  , DOI: ( /apmr )
Copyright © 2001 American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation Terms and Conditions

8. Fig. 7
Fatigability during electrically elicited fatigue test (20 young, 20 elderly). For clarity, only 1 of every 4 trains is presented. Error bars represent SDs. No significant differences existed in the average, normalized amount of fatigue ± SD over the final 30 contractions (t =.624, p ≤.05): young, 49.8% ± 2.6%; elderly, 51.1% ± 2.8%.
Archives of Physical Medicine and Rehabilitation  , DOI: ( /apmr )
Copyright © 2001 American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation Terms and Conditions