Age-related weakness of proximal muscle studied with motor cortical mapping: a TMS study

Abstract

Aging-related weakness is due in part to degeneration within the central nervous system. However, it is unknown how changes to the representation of corticospinal output in the primary motor cortex (M1) relate to such weakness. Transcranial magnetic stimulation (TMS) is a noninvasive method of cortical stimulation that can map representation of corticospinal output devoted to a muscle. Using TMS, we examined age-related alterations in maps devoted to biceps brachii muscle to determine whether they predicted its age-induced weakness. Forty-seven right-handed subjects participated: 20 young (22.6 ± 0.90 years) and 27 old (74.96 ± 1.35 years). We measured strength as force of elbow flexion and electromyographic activation of biceps brachii during maximum voluntary contraction. Mapping variables included: 1) center of gravity or weighted mean location of corticospinal output, 2) size of map, 3) volume or excitation of corticospinal output, and 4) response density or corticospinal excitation per unit area. Center of gravity was more anterior in old than in young (p<0.001), though there was no significant difference in strength between the age groups. Map size, volume, and response density showed no significant difference between groups. Regardless of age, center of gravity significantly predicted strength (β = -0.34, p = 0.005), while volume adjacent to the core of map predicted voluntary activation of biceps (β = 0.32, p = 0.008). Overall, the anterior shift of the map in older adults may reflect an adaptive change that allowed for the maintenance of strength. Laterally located center of gravity and higher excitation in the region adjacent to the core in weaker individuals could reflect compensatory recruitment of synergistic muscles. Thus, our study substantiates the role of M1 in adapting to aging-related weakness and subtending strength and muscle activation across age groups. Mapping from M1 may offer foundation for an examination of mechanisms that preserve strength in elderly.

Figure 1. Effects of age and gender on strength.

Findings of two-way analyses of variance exploring effect of age group and gender on (a) mean force of left elbow flexion and (b) mean EMG of biceps brachii. Older adults were not significantly weaker than young, but females were weaker than males in both age groups (* p≤0.05; ** p≤0.001).

Figure 2. Center of gravity in young and old individuals.

Location of center of gravity across all participants in both groups. As the averages demonstrate, center of gravity in older participants was more anterior.

Figure 3. Representative motor maps in young and old individuals.

Representative examples from a young and an older individual showing the size and spatial distribution of MEPs across the motor map of left biceps muscle in right hemisphere. The size (number of sites) did not vary significantly across groups (see also Fig. 4a). Note that map in the older individual is positioned anteriorly, which potentially explains why the center of gravity in older individuals was anterior too (Fig. 2).

Figure 4. Map size and volume in young and old individuals.

Differences between young and old subjects with respect to (a) map size or sites on scalp that are included in the motor map for left biceps muscle and (b) map volume. The groups did not differ significantly upon either measure although there is a seemingly higher count and volume in the younger group.

Figure 5. Age and corticospinal excitation.

5a,b shows 3-D function plots of the representative maps from a young and an old participant. These maps illustrate the spread of excitation across maps. Response densities did not significantly differ between age groups. The old subject has a response density of 25.39. The young subject has a response density of 21.63.