Figure 1.

Representative data for younger and older individuals performing three tasks. (A) Constant position task (positional variability) during a low-intensity contraction of the first dorsal interosseus muscle (10% 1-repetition maximum). Position (or force) variability during constant contractions is quantified from the detrended signal. (B) Tracing (position signal, gray line) of a slow sinusoidal task (0.1 Hz, black line) during shortening (ascending limb) and lengthening (descending limb) contractions while lifting a light load with the same muscle (10% 1-repetition maximum). Movement (or force) variability is quantified as the variations (SD) in the movement trajectory filtering out the frequency of the task (bottom trace of B). (C) A single trial during a goal-directed force task, where subjects attempt to match a force-time target by exerting 20% of their maximum force in 200 milliseconds (placing their peak force on the center of the force-time target). Each black dot represents their peak force for 20 different trials. Variability in force (or displacement) can be quantified as the SD of peak force across trials (y axis), whereas variability in time to peak force can be quantified as the SD of time to peak force across trials (x axis). End-point accuracy for force is quantified as the average of the shortest distance of the peak force across trials from the targeted force, whereas end-point accuracy for time is quantified as the average of the shortest distance of the exerted time to peak force across trials from the targeted time to peak force. Trajectory variability is quantified from the variations of the detrended force from the start to the peak force of each trial (bottom trace of C).