a-c: Different counterforce units. 1a shows a side view on the spiral tension spring unit. A spiral tension spring (2) is attached to the rotational axle (1a) via a helical roller (1). The radius of the roller decreases (angle ϕ) when the spring is elongated. The spring does not touch the roller tangentially, but with a tracking error angle ψ: 1b shows a side view on an electromagnetic plunger-coil unit. The rotating axle (1) is connected via a translating gear wheel (1a) to a linear axle (1b). Thus the coil (2) is inserted in the magnet core (2a). 1c shows the brushless unipolar motor unit (2, magenta) in connection to the other elements of the device. With the help of laser beamers (1a, light gray), the subject's hand can be set onto the setting device (8, blue) and the thumb can be attached to the thumb lever (7, dark gray). A vertical axle (1, yellow) connects the thumb lever to the length-adjusting mechanism (4) and the motor (2). An adjustable friction clutch (3) prevents overstretching of the muscle. Sensors measure muscle length (5, green) and force (6, green).