Endpoint accuracy for a small and a large hand muscle in young and old adults during rapid, goal-directed isometric contractions

Abstract

The minimum variance theory proposes that stronger (larger) muscles produce less variable trajectories compared with weaker (smaller) muscles and thus can accomplish more accurate contractions. The purpose of the study was to determine the influence of muscle size and trajectory variability on the endpoint accuracy of goal-directed isometric contractions. Twelve young (25 +/- 5 years) and 12 old adults (76 +/- 6 years) performed 100 trials with each of two muscles in both hands. Subjects were instructed to match the peak of a force trajectory to a target force by controlling either the abduction (first dorsal interosseus muscle; FDI) or adduction force (second palmar interosseus muscle; SPI) exerted by the index finger of each hand. The time to peak force was 150 ms and the peak force required was 25% of the maximal force that could be achieved in 150 ms. Endpoint accuracy and variability in force and time along with intramuscular EMG activity of the agonist muscle (FDI and SPI) involved in each task were quantified for each set of 100 trials. The MVC force was less for the SPI muscle, and the force endpoint error and variance were greater in the SPI muscle compared with the FDI muscle. Conversely, endpoint measures that included timing were similar for the two muscles. Trajectory variability was greater for the FDI muscle, but did not influence endpoint error for either muscle. The young and old adults had similar strength values, but the old adults were less accurate and more variable than the young subjects. Nonetheless, the accuracy and variability displayed by the old adults for the two muscles was the same as that observed for the young adults. The force accuracy and variability findings are consistent with the predictions of the minimum variance theory that motor-output variability is inversely related to muscle size, strength, and motor unit number.

Motor asymmetry reduction in older adults

“…Normal aging has been associated with deficits in coordination during goal-directed movements, such as those studied here [18, 22, 23]. We therefore expected that age-related reductions in lateralization might explain the genesis of age-related coordination deficits, rather than revealing an in improvement in coordination of non-dominant arm movements.…”

Non-monotonicity on a spatio-temporally defined cyclic task: evidence of two movement types?

“…However, rather than a further drop in accuracy when tracing the tall ellipse on the phase plane, participants’ accuracy scores were significantly ( p  < 0.0125) higher in that block compared to the medium one. Aging often affects motor performance in terms of speed as well as accuracy (e.g., Smith et al 1999; Poston et al 2008). In order to test whether age plays a role in the observed differences among the blocks, we separated the participant population into two groups, by age: a young group (22–39 years, 10 participants) and an old group (65–81 years, 13 participants), and repeated the analysis separately for each group.…”

Changes in muscle fascicles of tibialis anterior during anisometric contractions are not associated with motor-output variability of the ankle dorsiflexors in young and old adults

“…Although the consequences of sarcopenia and a worsening of motor-output variability are typically observed concurrently (Barry et al 2007;Carville et al 2007;Christou and Carlton 2002c;Christou et al 2003;Graves et al 2000;Seynnes et al 2005;Tracy and Enoka 2002;Tracy et al 2007), it is possible for one adaptation to appear without the other. For example, there can be no decline in the strength of a hand muscle, yet the same muscle can have a reduced ability to perform steady contractions (Burnett et al 2000;Laidlaw et al 2002;Poston et al 2008a, b). However, it appears highly improbable for older adults to experience a decrease in muscle strength without an increase in motor-output variability.…”