All participants completed the three experimental sessions without adverse effects. We were unable to record AP MEPs in four older participants (3 female) due to high threshold of activation (AP RMT not recordable). In addition, we were unable to record PA MEPs in one older male participant 5 min following iTMS due to coil overheating. To more clearly address the specific interests of the current study, the results will first detail effects within the older group. Age-related comparisons relative to our previously published dataset (Opie et al. 2021) will then be reported.

Effects of iTMS in older adultsBaseline

Baseline stimulation intensities, MEP amplitude and SICF are presented in Table 1. There were no differences between sessions for all stimulation intensities (all P > 0.355). Baseline AP MEP1.0mV was increased during iTMS4.5 session compared to iTMS4.0 (EMD = 40.2% [0.4, 98.7], P = 0.0007) and iTMS5.0 (EMD = 56.5% [12.4, 118.1], P < 0.0001). MEP amplitude during B1 of iTMS4.5 was also increased compared to iTMS4.0 (EMD = 11.6% [-5.6, 31.8], P = 0.014) and iTMS5.0 (EMD = 26.0% [-3.0, 63.7], P = 0.0006). There were no other significant effects at baseline (all P > 0.416).

Table 1 Baseline TMS intensities, MEP1.0mV, SICF, and first block of iTMS between sessions of older adultsCorticospinal excitability during iTMS

MEP amplitude during iTMS differed between sessions (X2 (2) = 14.5, P = 0.0007), with increased MEP amplitude during iTMS4.0 (EMD = 7.6% [1.9, 13.7], P = 0.001) and iTMS4.5 (EMD = 12.0% [6.1, 18.2], P < 0.0001) compared to iTMS5.0. MEP amplitude during iTMS also varied between blocks (X2 (9) = 170.1, P < 0.0001), with increased MEP amplitude during blocks 6–10 compared to block 1 (all P < 0.002). There were also a two-factor interaction between session and block (X2 (18) = 71.5, P < 0.0001; Fig. 2A). Post-hoc analysis for the interaction revealed increased MEP amplitude during B8-10 of iTMS4.0 compared to B1 (all P < 0.0006) and during B7 and B10 of iTMS5.0 compared to B1 (both P < 0. 021).

Corticospinal excitability following iTMS

PA MEP1.0mV amplitude did not vary between sessions (X2 (2) = 1.5, P = 0.473), but varied between time points (X2 (2) = 23.1, P < 0.0001), with increased MEP amplitude relative to baseline following the intervention (both time points P < 0.0001). There was no interaction between session and time point (X2 (4) = 7.4, P = 0.115; Fig. 2B).

AP MEP1.0mV amplitude differed between sessions (X2 (2) = 27.0, P < 0.0001), with increased MEP amplitude during iTMS4.0 (EMD = 20.4% [4.9, 38.2], P = 0.001) and iTMS4.5 (EMD = 27.2% [10.8, 46.0], P < 0.0001) compared to iTMS5.0. AP MEP1.0mV amplitude also varied between time points (X2 (2) = 21.3, P < 0.0001), with increased MEP amplitude 30 min following iTMS compared to baseline (EMD = 24.1% [8.3, 42.2], P = 0.0001). There was also a session by time point interaction (X2 (4) = 16.3, P = 0.003; Fig. 2C), which revealed increased AP MEP1.0mV amplitude relative to baseline at both time points following iTMS4.0 (both P < 0.01) and 30 min following iTMS5.0 (EMD = 29.7% [-2.6, 72.6], P = 0.009). AP MEP1.0mV amplitude 30 min following iTMS4.0 was also increased compared to iTMS5.0 (EMD = 28.7% [-3.5, 71.5], P = 0.013).

Intracortical excitability following iTMS in older adults

PA SICF did not vary between sessions (X2 (2) = 2.7, P = 0.258), but differed between time points (X2 (2) = 20.6, P < 0.0001), with increased SICF at both post-iTMS time points compared to baseline (both P < 0.0001). PA SICF varied between SICF ISIs (X2 (2) = 9.0, P = 0.011), with increased PA SICF at 4.5 ms (EMD = 16.3% [9.6, 23.4], P < 0.0001) and 5.0 ms (EMD = 11.9% [5.5, 18.7], P < 0.0001) compared to 4.0 ms. There were no interactions between factors (all P > 0.062; Fig. 2D shows session by time point values).

AP SICF did not differ between sessions (X2 (2) = 1.2, P = 0.555) or SICF ISI (X2 (2) = 1.27, P = 0.529), but varied between time points (X2 (2) = 10.5, P = 0.005), with responses increasing throughout each session (all P < 0.033). There was also a session by time point effect (X2 (4) = 15.5, P = 0.004; Fig. 2E), with post-hoc contrasts showing increased AP SICF at both time points following iTMS4.0 and iTMS5.0 compared to baseline (all P < 0.008), and increased AP SICF 30 min following iTMS4.5 compared to 5-minutes (EMD = 16.5% [-1.4, 37.8], P = 0.009) and baseline (EMD = 13.4% [-4.0, 34.0], P = 0.041). AP SICF was also increased following iTMS4.0 and iTMS5.0 relative to iTMS4.5 at both time points (all P < 0.036). There were no other interactions between factors (all P > 0.107).

Age-related differences in the effects of iTMSAge-related differences in corticospinal excitability during iTMS

MEP amplitude during iTMS did not vary between age groups (X2 (1) = 3.0, P = 0.081), but differed between sessions (X2 (2) = 15.1, P = 0.0005) and between blocks (X2 (9) = 176.3, P < 0.0001). There were also two-factor interactions between session and block (X2 (18) = 74.1, P < 0.0001), age and block (X2 (9) = 25.5, P = 0.002), and a three-factor interaction (X2 (18) = 39.2, P = 0.003). There was no interaction between session and age group (X2 (2) = 4.6, P = 0.1). For all iTMS sessions, on-line changes in the mean MEP amplitude during blocks 2–10 were reduced in older adults compared to young adults (all P < 0.005; Fig. 3A).

Age-related differences in corticospinal excitability following iTMS

PA MEP1.0mV amplitude (Fig. 3B) did not vary between sessions (X2 (2) = 5.6, P = 0.062) or age groups (X2 (1) = 2.2, P = 0.141), but varied between time points (X2 (2) = 79.0, P < 0.0001). In addition, there was an interaction between session and time point (X2 (4) = 12.1, P = 0.017). Given these effects failed to interact with the age factor, post-hoc comparisons will not be described in detail. There were no other interactions between factors (P > 0.1).

AP MEP1.0mV amplitude did not vary between age groups (X2 (1) = 3.7, P = 0.053), but differed between sessions (X2 (2) = 44.8, P < 0.0001) and between time points (X2 (2) = 15.6, P = 0.0004). There was also a session by age group interaction (X2 (2) = 21.1, P < 0.0001), a session by time point interaction (X2 (4) = 30.1, P < 0.0001), and a three-factor interaction between session, time point, and age group (X2 (4) = 11.4, P = 0.023). There was no interaction between time and age group (P > 0.09). Post-hoc comparisons for the three-factor interaction showed that off-line changes in AP MEP1.0mV amplitude (Fig. 3C) were increased following iTMS4.0 compared to iTMS4.5 and iTMS5.0 in young adults (both P < 0.002), as reported previously (Opie et al. 2021). In contrast, off-line changes in AP MEP1.0mV amplitude of older adults were increased following both iTMS4.0 (EMD = 36.6% [-2.1, 91.3], P = 0.017) and iTMS5.0 (EMD = 53.1% [9.2, 114.6], P = 0.002) compared to iTMS4.5. Off-line changes AP MEP1.0mV amplitude following iTMS5.0 were also increased in older adults compared to young (EMD = 45.8% [5.6, 101.1], P = 0.0006).

Age-related differences in intracortical excitability following iTMS

SICF ISIs at baseline did not vary between young and older adults (all P > 0.055). PA SICF differed between sessions (X2 (2) = 9.2, P = 0.010), time points (X2 (2) = 194.9, P < 0.0001), SICF ISIs (X2 (2) = 44.6, P < 0.0001), but not between age groups (X2 (1) = 3.8, P = 0.052). There was a session by age group interaction (X2 (2) = 6.7, P = 0.034), an age group by SICF ISI interaction (X2 (2) = 18.1, P = 0.0001), and a three-factor interaction between session, time point, and age group (X2 (4) = 20.8, P = 0.0003). There were no other interactions between factors (P > 0.06). Post-hoc comparisons for the three-factor interaction showed that off-line changes in PA SICF (Fig. 3D) of young adults following iTMS4.0 were increased compared to iTMS4.5 (EMD = 13.9% [-0.8, 30.7], P = 0.014). In contrast, off-line changes in PA SICF of older adults following iTMS4.5 were increased compared to young adults (EMD = 18.5% [3.2, 35.9], P = 0.0003).

AP SICF varied between sessions (X2 (2) = 48.1, P < 0.0001), time points (X2 (2) = 16.8, P = 0.0002), SICF ISIs (X2 (2) = 23.8, P < 0.0001), but not between age groups (X2 (1) = 0.3, P = 0.600). There was a session by age group effect (X2 (2) = 64.1, P < 0.0001), a session by time point effect (X2 (4) = 43.0, P < 0.0001), an age group by time point effect (X2 (2) = 23.2, P < 0.0001), a time point by SICF ISI effect (X2 (4) = 15.7, P = 0.003), and a three-factor interaction between session, age group, and time point (X2 (4) = 73.4, P < 0.0001). There were no interactions between other factors (all P > 0.051). Post-hoc comparisons for the three-factor interaction showed that off-line changes in AP SICF (Fig. 3E) of young adults following iTMS4.0 were increased compared to iTMS4.5 (EMD = 41.4% [20.6, 65.7], P < 0.0001) and iTMS5.0 (EMD = 49.9% [28.1, 75.5], P < 0.0001). In contrast, AP SICF in older adults was increased following iTMS5.0 compared to iTMS4.0 (EMD = 14.6% [3.2, 35.9], P = 0.04) and iTMS4.5 (EMD = 24.1% [4.9, 28.4], P = 0.0004). Importantly, while off-line changes in AP SICF of young adults were increased compared to older adults following iTMS4.0 (EMD = 15.0% [-2.1, 35.1], P = 0.011), AP SICF was increased in older adults compared to young following iTMS4.5 (EMD = 13.5% [-3.5, 33.5], P = 0.022) and iTMS5.0 (EMD = 49.4% [27.7, 74.9], P < 0.0001).

Age-related differences in MEP onset latency at baseline

Baseline MEP latencies (Table 2) varied between age groups (X2 (1) = 23.1, P < 0.0001) and coil orientations (X2 (1) = 54.0, P < 0.0001), with comparisons revealing longer MEP latencies in older adults compared to young (EMD = 2.4 ms [1.1, 3.6], P = 0.0001), and longer AP MEP latencies compared to PA (EMD = 1.9 ms [1.2, 2.5], P < 0.0001).

Table 2 Baseline MEP latencies between sessions of young and older adultsCorrelation analyses

Spearman’s rank correlation analysis did not reveal a relationship between the magnitude of baseline SICF recorded within each ISI and changes in corticospinal or intracortical excitability following iTMS within the same session for older adults (all ρ < 0.676, P > 0.264). There was also no correlation between maximum baseline SICF and changes in corticospinal or intracortical excitability following iTMS of the same session (all ρ < 0.516, P > 0.071).