Brian L. Tracy

Mechanisms that contribute to differences in motor performance between young and old adults

This paper examines the physiological mechanisms responsible for differences in the amplitude of force fluctuations between young and old adults. Because muscle force is a consequence of motor unit activity, the potential mechanisms include both motor unit properties and the behavior of motor unit populations. The force fluctuations, however, depend not only on the age of the individual but also on the muscle group performing the task, the type and intensity of the muscle contraction, and the physical activity status of the individual. Computer simulations and experimental findings performed on tasks that involved single agonist and antagonist muscles suggest that differences in force fluctuations are not attributable to motor unit twitch force, motor unit number, or nonuniform activation of the agonist muscle, but that they are influenced by the variability and common modulation of motor unit discharge in both the agonist and antagonist muscles. Because the amplitude of the force fluctuations does not vary linearly with muscle activation, these results suggest that multiple mechanisms contribute to the differences in force fluctuations between young and old adults, although the boundary conditions for each mechanism remain to be determined.

Variability of motor unit discharge and force fluctuations across a range of muscle forces in older adults

Variability of motor unit discharge is a likely contributor to the greater force fluctuations observed in old adults at low muscle forces. We sought to determine whether the variability of motor unit discharge rate underlies the fluctuations in force during steady contractions across a range of forces in young (n = 11) and old (n = 14) adults. The coefficient of variation (CV) for discharge rate and force were measured during a force‐matching task as the first dorsal interosseous muscle performed isometric contractions. The recruitment thresholds of the 78 motor units ranged from 0.04% to 34% of maximal voluntary contraction (MVC) force. The CV for discharge rate ranged from 7.6% to 46.2% and was greater (P < 0.05) for old adults (21.5% ± 7.7%) than young adults (17.3% ± 8.1%). Although the CV for force was similar for young and old subjects (2.53% ± 1.6%) across all target forces, it was greater for old adults at the lowest forces. Furthermore, there was a positive relation (r2 = 0.20, P < 0.001) between the CV for force and the CV for discharge rate across the range of recruitment thresholds. This relation was significant for old adults (r2 = 0.30, P < 0.001), but not for young adults (r2 = 0.06, P > 0.05). Thus, the normalized variability (CV) of motor unit discharge was greater in old adults and was related to the amplitude of force fluctuations across a broader range of forces than previously examined. These findings underscore the contribution of variability of motor unit activity to motor output in normal human aging. Muscle Nerve, 2005

A more efficient magnetic resonance imaging-based strategy for measuring quadriceps muscle volume

PURPOSE To determine the accuracy of several magnetic resonance imaging (MRI)-based strategies for assessment of quadriceps muscle volume (MV) and changes in MV with training. METHODS Images were acquired along the length of both thighs from young (26 +/- 3 yr, N= 23) and older (69 +/- 3 yr, N= 24) men and women before and after strength training. The quadriceps cross-sectional area (QCSA) of each section was measured before and after training. MV was directly assessed using all of the sections (each 9-mm thick with a 1-mm gap). Alternative estimates of MV were calculated using increasingly greater intervals between sections: every 1.1 cm (MV2), 3.1 cm (MV4), 5.1 cm (MV6), 7.1 cm (MV8), 9.1 cm (MV10), and a single QCSA (L1). The 95% limits of agreement (LOA, +/- 2 SD) between each alternative measure and the criterion measure MV were determined with Bland and Altman plots. Regression was used to predict MV from L1 and to obtain the standard error of the estimate (SEE). RESULTS Before training, the 95% LOA with MV for the alternative measures ranged from 0.7% to 6.36% of MV, and the prediction of MV from L1 yielded a SEE x 2 of 14.1% of MV. For change in the alternative measures, the 95% LOA ranged from 10.3% to 26.3% of the total change in MV, and the prediction of deltaMV from deltaL1 yielded a SEE x 2 of 60% of the change in MV. CONCLUSION Increasingly greater intervals between axial MRI sections result in substantially reduced agreement with a criterion measure of MV. The use of one axial section results in relatively higher error and thus should be used only when large effect sizes are expected.

The relationship of motor unit size, firing rate and force

OBJECTIVE Using a clinical electromyographic (EMG) protocol, motor units were sampled from the quadriceps femoris during isometric contractions at fixed force levels to examine how average motor unit size and firing rate relate to force generation. METHODS Mean firing rates (mFRs) and sizes (mean surface-detected motor unit action potential (mS-MUAP) area) of samples of active motor units were assessed at various force levels in 79 subjects. RESULTS MS-MUAP size increased linearly with increased force generation, while mFR remained relatively constant up to 30% of a maximal force and increased appreciably only at higher force levels. A relationship was found between muscle force and mS-MUAP area (r2 = 0.67), mFR (r2 = 0.38), and the product of mS-MUAP area and mFR (mS-MUAP x mFR) (r2 = 0.70). CONCLUSIONS The results support the hypothesis that motor units are recruited in an orderly manner during forceful contractions, and that in large muscles only at higher levels of contraction ( > 30% MVC) do mFRs increase appreciably. MS-MUAP and mFR can be assessed using clinical EMG techniques and they may provide a physiological basis for analyzing the role of motor units during muscle force generation.

Functional repetitive transcranial magnetic stimulation increases motor cortex excitability in survivors of stroke.

OBJECTIVE To determine if repetitive transcranial magnetic stimulation (rTMS) applied to the motor cortex with simultaneous voluntary muscle activation, termed functional-rTMS, will promote greater neuronal excitability changes and neural plasticity than passive-rTMS in survivors of stroke. METHODS Eighteen stroke survivors were randomized into functional-rTMS (EMG-triggered rTMS) or passive-rTMS (rTMS only; control) conditions. Measures of short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF), force steadiness (coefficient of variation, CV) at 10% of maximum voluntary contraction, and pinch task muscle activity were assessed before and after rTMS. Functional-rTMS required subjects to exceed a muscle activation threshold to trigger each rTMS train; the passive-rTMS group received rTMS while relaxed. RESULTS Significant interactions (time × condition) were observed in abductor pollicis brevis (APB) SICI, APB ICF, CV of force, and APB muscle activity. Functional-rTMS decreased APB SICI (p < 0.05) and increased ICF (p < 0.05) after stimulation, whereas passive-rTMS decreased APB muscle activity (p < 0.01) and decreased CV of force (p < 0.05). No changes were observed in FDI measures (EMG, ICF, SICI). CONCLUSION(S) Functional-rTMS increased motor cortex excitability, i.e., less SICI and more ICF for the APB muscle. Passive stimulation significantly reduced APB muscle activity and improved steadiness. SIGNIFICANCE Functional-rTMS promoted greater excitability changes and selectively modulated agonist muscle activity.

Muscle Force Steadiness in Older Adults Before and After Total Knee Arthroplasty

The ability to control submaximal muscle forces has been shown to be associated with age-related decreases in physical function, such as increased tendency to fall. This study compared quadriceps muscle force steadiness (MFS) in individuals with knee OA before and after total knee arthroplasty (TKA) to an age-matched group of controls. Lower extremity MFS was measured in 13 subjects with knee OA before and at six months after TKA (TKA-GROUP) and compared to an age-matched control group (CONTROL-GROUP). MFS was significantly more impaired in the TKA-GROUP at the pre-operative, but not post-operative visit, and significantly improved between the pre-operative and post-operative visits. Further research is warranted to evaluate the relation between this MFS measurement and physical functional performance in those at high risk for falling.

Aging-Related Cocontraction Effects During Ankle Strategy Balance Recovery Following Tether Release in Women

ABSTRACT Increased antagonistic muscle activation during balance recovery has been documented during proprioceptive reliant responses in older adults. The authors examined ankle muscle cocontraction levels in young and older adults during balance recovery from a tether-release-induced, vestibular-input-reliant perturbation. Nine healthy young adult and 9 older women without history of falls performed maximum isometric plantar flexion and dorsiflexion trials followed by balance recovery trials using the ankle strategy. Surface electromyography data normalized to isometric conditions were analyzed during the 100 ms prior to release, the 50 ms immediately after release, and the 100-ms epochs from 100–400 ms following release. No differences existed in gastrocnemius and soleus activity levels (p > .05), though antagonistic tibialis anterior activity was greater in the older adults during the 300–400 ms epoch (young: 23.5 ± 5.8%, older: 38.7 ± 9.4%; p = .001). Vestibular-dominated perturbations may increase antagonistic activity during the recovery phase in older adults, inhibiting efficient balance recovery during proprioceptive and vestibular reliant perturbations.

Greater glucose uptake heterogeneity in knee muscles of old compared to young men during isometric contractions detected by [18F]-FDG PET/CT

We used positron emission tomography/computed tomography (PET/CT) and [18F]-FDG to test the hypothesis that glucose uptake (GU) heterogeneity in skeletal muscles as a measure of heterogeneity in muscle activity is greater in old than young men when they perform isometric contractions. Six young (26 ± 6 years) and six old (77 ± 6 years) men performed two types of submaximal isometric contractions that required either force or position control. [18F]-FDG was injected during the task and PET/CT scans were performed immediately after the task. Within-muscle heterogeneity of knee muscles was determined by calculating the coefficient of variation (CV) of GU in PET image voxels within the muscles of interest. The average GU heterogeneity (mean ± SD) for knee extensors and flexors was greater for the old (35.3 ± 3.3%) than the young (28.6 ± 2.4%) (P = 0.006). Muscle volume of the knee extensors were greater for the young compared to the old men (1016 ± 163 vs. 598 ± 70 cm3, P = 0.004). In a multiple regression model, knee extensor muscle volume was a predictor (partial r = −0.87; P = 0.001) of GU heterogeneity for old men (R2 = 0.78; P < 0.001), and MVC force predicted GU heterogeneity for young men (partial r = −0.95, P < 0.001). The findings demonstrate that GU is more spatially variable for old than young men and especially so for old men who exhibit greater muscle atrophy.

Repeated sessions of functional repetitive transcranial magnetic stimulation increases motor cortex excitability and motor control in survivors of stroke.

OBJECTIVE To determine the impact of a single-session of repetitive transcranial magnetic stimulation (rTMS) and an rTMS intervention on neurophysiology and motor control in survivors of stroke. METHODS Twelve stroke survivors were randomized into functional-rTMS or passive-rTMS conditions. Measures of short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF), and force steadiness (coefficient of variation, CV) at 10 and 20% of maximum voluntary contraction were assessed at baseline and after a single-session of rTMS (post single-session), and again following an intervention of 8 rTMS sessions (2 sessions per day; post-intervention). Functional-rTMS required subjects to exceed a muscle activation threshold assessed by surface electromyography to trigger each rTMS train; the passive-rTMS group received rTMS while relaxed. RESULTS ICF scores significantly increased following the single-session of functional-rTMS compared to the decrease following passive-rTMS. The increase in APB SICI and ICF scores following the intervention was significantly greater for the functional-rTMS group compared to the decreases following passive-rTMS. The groups were significantly different in the CV of force (20%) following the single-session of rTMS, and in the 10 and 20% tasks following the intervention. The functional-rTMS group increased steadiness overtime, whereas the passive group demonstrated a return to baseline following the intervention session. No differences were observed in first dorsal interosseus (FDI) measures (SICI and ICF) between groups. CONCLUSIONS The functional-rTMS protocol enhanced cortical excitability following a single-session and after repeated sessions and improved steadiness, whereas the passive stimulation protocol tended to decrease excitation and no improvements in steadiness were observed.

Acute Improvement in Intraoperative EMG Following Common Fibular Nerve Decompression in Patients with Symptomatic Diabetic Sensorimotor Peripheral Neuropathy: 1. EMG Results

Background and Study Aims Electromyographic (EMG) recordings of the fibularis longus (FL) and tibialis anterior (TA) muscles were performed intraoperatively during common fibular nerve (CFN) nerve decompression (ND) in patients with symptomatic diabetic sensorimotor peripheral neuropathy (DSPN) and clinical nerve compression. Materials and Methods Forty‐six legs in 40 patients underwent surgical ND by external neurolysis; FL and TA muscles were monitored intraoperatively. Evoked EMGs were recorded just prior to and within 1 minute after ND. Results Thirty‐eight legs (82.6%) demonstrated EMG improvement 1 minute after ND. Sixty muscles (31 FL, 29 TA) were monitored, with 44 (73.3%) improving in EMG amplitude. Mean change in EMG amplitude represented a 73.6% improvement (p < 0.0001). Changes in EMG amplitudes correlated with visual analog scale pain improvement (p = 0.03). Conclusion This is the first report of acute changes in objective EMG responses during ND of CFN in DSPN patients and demonstrates that patients with symptomatic DSPN and clinical nerve entrapment have latent but functional axons that surgical ND can improve immediately.