Dexterous Manipulation Is Poorer at Older Ages and Is Dissociated From Decline of Hand Strength

The worsening of fine motor skills for manipulation, whereas not a health risk in the elderly people like loss of strength (1) and risk of falls (2), is nevertheless a critical and little understood contributor to the decline of quality of life, social participation, and work capabilities (3). Pick-and-place standardized tests of dexterity (4–6) reveal behavioral impairments in upper extremity and hand function after the age of 65 (7) and have been of some clinical utility. These methods, however, lack the specificity and resolution to quantify in detail the dynamic control of fingertip force vectors that is so critical to dexterous manipulation. Given this unmet need, we developed the strength–dexterity (SD) test to measure dynamic dexterous manipulation, which we define as the dynamic regulation of fingertip force vector magnitude and direction. It is based on the mechanical perspective of evaluating the dynamical ability of the fingertips to stabilize unstable objects (8–14). This simple test consists of measuring a person’s ability to compress slender springs with the fingertips. The physical characteristics of each spring define how stiff, compliant, stable, and prone to buckling it is. As such, the SD test has been shown to assess the interactions among sensory, motor, and neural processing capabilities in the behavioral context of dexterous manipulation (9,11,12,14). The SD test differs from available tests of static forces and pick-and-place tasks in important ways (see Discussion section). For example, the Nine Hole Peg test (4) measures the time it takes to pick up, insert into holes, and remove nine pegs. Given that it studies the use of the whole upper extremity to grasp and orient rigid objects, an individual can apply excessive forces or adapt their kinematic trajectories to achieve the same score, and the directional control of the fingertips forces is not measured in detail. Using the SD test to quantify dynamic dexterous manipulation is also relevant to fine manipulation in activities of daily living. The dynamic control of fingertip force vector magnitude and direction is important functionally for manipulating fragile, deformable, or small objects in daily life. Basic research studies have shown that when older people are asked to produce static fingertip forces against a rigid surface, they tend to misdirect (15) the force vector and fail to precisely regulate its magnitude (16,17), especially at low magnitudes (17). This has been attributed to differential weakening of hand musculature or changes in the neural coupling among muscles (18). Such deficits in the control of static fingertip force vectors could compromise force and moment balance of the object being manipulated (19). However, these directional deficits are not observed or implicated consistently (20,21)—and even if present, it is unclear whether and how they would affect dynamic manipulation of hand-held objects. Here we extend those studies of static force regulation and pick-and-place tasks by quantifying the dynamical control of fingertip force vector magnitude and direction in middle-aged and older adults.