Adaptations in motor unit discharge activity with force control training in young and older human adults

Abstract

Six young (aged 18-22 years) and six older (aged 66-76 years) healthy humans participated in a visually guided isometric force modulation training program designed to improve accurate control of force during ankle dorsiflexion. Isometric force and the discharge activity of motor units (MU) supplying the tibialis anterior muscle were sampled concurrently at the beginning of the study, following 2 weeks of force modulation training and again after a 4 week retention period which followed immediately. The initial maximal voluntary force (MVC) and MU discharge rates were similar between young and older adults at 10-60% MVC while MU discharge rates during maximal effort were significantly reduced in older adults. Following the 2 weeks of force modulation training, both young and older adults demonstrated significant improvements in force accuracy (44% young, 48% older) and significantly reduced MU discharge rates at 30%, 40%, and 60% MVC. Young adults also demonstrated increased MVC force (11%), while older adults demonstrated significantly increased (30%) maximal MU discharge rate. Thus, following 2 weeks of force modulation training, young and older individuals demonstrated similar MU discharge rates at all force levels. The MU discharge rate adaptations were retained after the 4 week retention period. In young adults, improved force accuracy and increased MVC force were accompanied by significantly reduced MU recruitment thresholds. In the older subjects, improved force accuracy was accompanied by an increase in the difference between the recruitment-derecruitment force threshold and significantly reduced antagonist co-contraction. Age-related alterations in force regulation and MU discharge activity cannot be explained solely on the basis of contractile changes in senescent muscle. Rather, reliance on compensatory neuromuscular changes including antagonist muscle co-contraction is suggested.

Low-frequency Force Steadiness Practice in Plantar Flexor Muscle Reduces Postural Sway during Quiet Standing

“…Keen et al (1994) found that 4 weeks of strength training with the first dorsal interosseous muscle with heavy load (80% of maximum) by 10 young adults (aged 18-27 years) reduced the force fluctuations during isometric contraction at 50% MVC. A study by Patten and Kamen (2000) found that the ability of 6 young adults (aged 18-22 years) to match a force trajectory requiring force modulation up to 60% MVC in the dorsiflexor muscles improved after 2 weeks of force-tracking tasks. These previous findings agree with the results of the present study.…”

“…Similar effects have been observed in young adults. Patten and Kamen (2000) reported that the ability to match force trajectory in the dorsiflexor muscles improved after 2 weeks of force-tracking tasks.…”

Motor unit activity when young and old adults perform steady contractions while supporting an inertial load

“…The adaptation presumably involves both intrinsic motoneuron properties (Christie and Kamen 2010;Piotrkiewicz et al 2007) and the magnitude and distribution of synaptic inputs (Earles et al 2001;Eisen et al 1996;Erim et al 1999;Kido et al 2004;Oliviero et al 2006;Patten and Kamen 2000), which together influence the transformation of synaptic inputs into trains of motor unit action potentials. The present findings suggest that the integration of synaptic inputs as the membrane potential approached voltage threshold for generating action potentials differed for the two groups of subjects.…”

Functional Significance of Force Fluctuation During Voluntary Muscle Contraction

“…Keen et al (1994) found that 4 weeks of strength practice with the first dorsal interosseous muscle with a heavy load (80% of maximum) in 10 young adults (aged 18-27 years) reduced force fluctuation during isometric contraction at 50% MVC. Patten and Kamen (2000) report that the ability of 6 young adults (aged 18-22 years) to match a force trajectory requiring force modulation up to 60% MVC in the dorsiflexor muscles improved after 2 weeks of isometric force modulation training. Table 1.…”

“…For example, 2 weeks of practicing a steadiness task with a light load (10% of maximum) on the index finger reduces force fluctuations and the discharge rate variability of single motor units in the hand muscles of older adults (Kornatz et al, 2005); similar effects are observed in young adults. Patten and Kamen (2000) report that the ability to match force trajectory in the dorsiflexor muscles improves after 2 weeks of isometric force modulation training. However, the effect of low-frequency steadiness practice (1 day of practice per week) on force fluctuations during isometric contraction is currently unknown.…”