Plantar flexor activation capacity and H reflex in older adults: adaptations to strength training

Lifelong strength training mitigates the age-related decline in efferent drive

Recently, we documented age-related attenuation of efferent drive to contracting skeletal muscle. It remains elusive if this indication of reduced muscle strength is present with lifelong strength training. For this purpose, we examined evoked potentials in the calf muscles of 11 [71 ± 4 (SD) yr] strength-trained master athletes (MA) contrasted with 10 (71 ± 4 yr) sedentary (SO) and 11 (73 ± 6 yr) recreationally active (AO) old subjects, as well as 9 (22 ± 2 yr) young controls. As expected, MA had higher leg press maximal strength (MA, 185 ± 32 kg; AO, 128 ± 15 kg; SO, 106 ± 11 kg; young, 147 ± 22 kg, P < 0.01) and rate of force development (MA, 5,588 ± 2,488 N/s; AO, 2,156 ± 1,100 N/s; SO, 2,011 ± 825 N/s; young, 3,663 ± 1,140 N/s, P < 0.05) than the other groups. MA also exhibited higher musculus soleus normalized V waves during maximal voluntary contractions (MVC) [maximal V wave amplitude/maximal M wave during MVC (Vsup/Msup); 0.28 ± 0.15] than AO (0.13 ± 0.06, P < 0.01) and SO (0.11 ± 0.05, P < 0.01), yet lower than young (0.45 ± 0.12, P < 0.01). No differences were apparent between the old groups in H reflex recorded at rest or during MVC [maximal H reflex amplitude/maximal M wave during rest (Hmax/Mmax); maximal H reflex amplitude during MVC/maximal M wave during MVC (Hsup/Msup)], and all were lower ( P < 0.01) than young. MA (34.4 ± 2.1 ms) had shorter ( P < 0.05) H reflex latency compared with AO (36.4 ± 3.7 ms) and SO (37.3 ± 3.2 ms), but longer ( P < 0.01) than young (30.7 ± 2.0 ms). Using interpolated twitch analysis, MA (89 ± 7%) had plantar flexion voluntary activation similar to young (90 ± 6%), and this was higher ( P < 0.05), or tended to be higher ( P = 0.06–0.09), than SO (83 ± 10%) and AO (84 ± 5%). These observations suggest that lifelong strength training has a protective effect against age-related attenuation of efferent drive. In contrast, no beneficial effect seems to derive from habitual recreational activity, indicating that strength training may be particularly beneficial for counteracting age-related loss of neuromuscular function.

Neural Activation During Submaximal Contractions Seems More Reflective of Neuromuscular Ageing than Maximal Voluntary Activation

This study aimed at testing the hypothesis that differences in neural activation strategy during submaximal but not maximal plantarflexions exist between young and older men. Eleven young men (YM, 26 ± 4 years) and thirteen old men (OM, 76 ± 3 years) volunteered for the investigation. Maximal voluntary torque (MVT) was 38.2%, lower (p < 0.001) in OM than in YM, while voluntary activation was equivalent (~97%). The relationship between the interpolated twitch-torque and the voluntary torque (IT-VT relationship) was composite (curvilinear + exponential) for both age-groups. However, the OM showed accentuated concavity, as attested by the occurrence of the deviation from linearity at a lower contraction intensity (OM: 54.9 vs. YM: 71.9% MVT). In conclusion, ageing does not affect the capacity to fully activate the plantar flexors during maximal performances, but it alters the activation pattern for submaximal levels of effort. The greater age-related concavity of the IT-VT relationship suggests that, during submaximal contractions, OM need to reach a level of activation higher than YM to develop an equivalent relative torque.

Inter-repetition rest training and traditional set configuration produce similar strength gains without cortical adaptations

This study compared the functional and neural effects of two strength training programmes differing in set configuration. Thirteen participants performed 10 sessions, over a period of 5 weeks, of unilateral leg extensions with different set configurations but with identical work-to-rest ratios for each limb: a traditional configuration (4 sets of 8 repetitions, 10RM load, 3-min pause between sets) and an inter-repetition rest configuration (32 repetitions, 10RM load, 17.4 s of rest between each repetition). Mean propulsive velocity of the traditional sessions was lower than for inter-repetition rest sessions (0.48 ± 0.06 vs. 0.54 ± 0.06 m · s(-1); P < 0.001), while perceived exertion was higher (8.3 ± 0.9 and 6.56 ± 1.6 for traditional training and IRT; P = 0.002). One repetition maximum (RM), work with 10RM load, maximum mean propulsive power, maximum voluntary contraction and time to failure with 50% of maximum isometric force improved similarly in both legs (time effect, P < 0.001; effect size range, 0.451-1.190). Time and set configuration did not show significant main effects or interactions for cortical adaptations (motor-evoked potentials, short-interval intracortical inhibition, intracortical facilitation). There were no significant correlations between changes in cortical and peripheral neural adaptations and strength improvement. In conclusion, inter-repetition rest configuration was as effective as traditional training in improving muscle performance.

Strength training-induced responses in older adults: attenuation of descending neural drive with age

Although reductions in resting H-reflex responses and maximal firing frequency suggest that reduced efferent drive may limit muscle strength in elderly, there are currently no reports of V-wave measurements in elderly, reflecting the magnitude of efferent output to the muscle during maximal contraction. Furthermore, it is uncertain whether potential age-related neural deficiencies can be restored by resistance training. We assessed evoked reflex recordings in the triceps surae muscles during rest and maximal voluntary contraction (MVC), rate of force development (RFD), and muscle mass in seven elderly (74 ± 6 years) males before and after 8 weeks of heavy resistance training, contrasted by seven young (24 ± 4 years) male controls. At baseline, m. soleus (SOL) V/M ratio (0.124 ± 0.082 vs. 0.465 ± 0.197, p < 0.05) and H/M ratio (0.379 ± 0.044 vs. 0.486 ± 0.101 p = 0.07) were attenuated in elderly compared to young. Also, SOL H-reflex latency (33.29 ± 2.41 vs. 30.29 ± 0.67 ms, p < 0.05) was longer in elderly. The reduced neural drive was, despite similar leg muscle mass (10.7 ± 1.2 vs. 11.5 ± 1.4 kg), mirrored by lower MVC (158 ± 48 vs. 240 ± 54 Nm, p < 0.05) and RFD (294 ± 126 vs. 533 ± 123 Nm s(-1), p < 0.05) in elderly. In response to training SOL V/M ratio (0.184 ± 0.092, p < 0.05) increased in the elderly, yet only to a level ~40 % of the young. This was accompanied by increased MVC (190 ± 70 Nm, p < 0.05) and RFD (401 ± 147 Nm[Symbol: see text]s(-1), p < 0.05) to levels of ~80 % and ~75 % of the young. H/M ratio remained unchanged. These findings suggest that changes in supraspinal activation play a significant role in the age-related changes in muscle strength. Furthermore, this motor system impairment can to some extent be improved by heavy resistance training.

Weaker Seniors Exhibit Motor Cortex Hypoexcitability and Impairments in Voluntary Activation

BACKGROUND

Weakness predisposes seniors to a fourfold increase in functional limitations. The potential for age-related degradation in nervous system function to contribute to weakness and physical disability has garnered much interest of late. In this study, we tested the hypothesis that weaker seniors have impairments in voluntary (neural) activation and increased indices of GABAergic inhibition of the motor cortex, assessed using transcranial magnetic stimulation.

METHODS

Young adults (N = 46; 21.2±0.5 years) and seniors (N = 42; 70.7±0.9 years) had their wrist flexion strength quantified along with voluntary activation capacity (by comparing voluntary and electrically evoked forces). Single-pulse transcranial magnetic stimulation was used to measure motor-evoked potential amplitude and silent period duration during isometric contractions at 15% and 30% of maximum strength. Paired-pulse transcranial magnetic stimulation was used to measure intracortical facilitation and short-interval and long-interval intracortical inhibition. The primary analysis compared seniors to young adults. The secondary analysis compared stronger seniors (top two tertiles) to weaker seniors (bottom tertile) based on strength relative to body weight.

RESULTS

The most novel findings were that weaker seniors exhibited: (i) a 20% deficit in voluntary activation; (ii) ~20% smaller motor-evoked potentials during the 30% contraction task; and (iii) nearly twofold higher levels of long-interval intracortical inhibition under resting conditions.

CONCLUSIONS

These findings indicate that weaker seniors exhibit significant impairments in voluntary activation, and that this impairment may be mechanistically associated with increased GABAergic inhibition of the motor cortex.

H-index is important for postural control for people with impaired foot sole sensation

INTRODUCTION

People with Peripheral Neuropathy (PN), especially those with impaired sensory inputs through the small-afferent fiber (type II afferent fibers) reflex loop (SAF), might depend more on the large-afferent fiber (type I afferent fibers) reflex loop (LAF) for postural control.

PURPOSE

To examine whether the function of the LAF reflex loop, reflected by the H-reflex and ankle joint proprioception, influences postural control when the SAF reflex loop is impaired, as indicated by reduced foot sole cutaneous sensation.

METHODS

Thirteen participants (8 women, 5 men) diagnosed with PN and 12 age-matched controls (7 women, 5 men) completed the testing protocol. Measures of interest included the H-index, active (AAP) and passive (PAP) ankle proprioception, plantar pressure sensitivity (PPS), average sway velocity (VAVG) and area (A95) during 30 seconds eyes-closed standing, 6-minute walk distance (6MWD) and timed up-and-go duration (TUG).

RESULTS

Statistically significant group-dependent regression was observed between VAVG and H-index. Compared to the control group, the PN group demonstrated reduced PPS (2.0 ± 1.9 vs. 4.2 ± 1.2, P < .05) and H-index (63.6 ± 10.9 vs. 76.4 ± 16.0, P < .05), greater VAVG (3.5 ± 2.1 vs. 1.6 ± 0.6 cm/s, P < .05) and A95 (10.0 ± 10.1 vs. 2.5 ± 1.5 cm2, P < .05), shorter 6MWD (442.2 ± 93.0 vs. 525.3 ± 68.2 m, P < .05), and longer TUG (9.4 ± 1.6 vs. 6.5 ± 1.3 s, P < .05). Within the PN group, but not the control group, the H-index was correlated with VAVG (r = -.56, P < .05). Moreover, within the PN group only, PAP scores were correlated with 6MWD (r = -.68, P < .05) and TUG (r = -.59, P < .05) performance. No other statistically significant group difference, correlation or group-dependent regression was observed.

CONCLUSION

VAVG, 6MWD, and TUG correlated with LAF reflex loop function observed among those with impaired functioning of the SAF reflex loop. This observation suggests that the LAF reflex loop may be critical to the control of balance in those individuals suffering from small-fiber PN.

Force steadiness during a co-contraction task can be improved with practice, but only by young adults and not by middle-aged or old adults

NEW FINDINGS

What is the central question of this study? Does the capacity to modulate afferent input to spinal motor neurons during steady submaximal contractions change with advancing age? What is the main finding and its importance? After practising a co-contraction task involving lower leg muscles, young subjects improved force steadiness by reducing the amount of Ia presynaptic inhibition as indexed by D1 inhibition. Middle-aged and old adults both found the task challenging, and force steadiness even worsened for old adults after practising the co-contraction task. Despite similar muscle strength for young and middle-aged adults, the capacity to modulate a spinal reflex pathway was reduced in middle-aged adults. This study compared the changes in steadiness and the modulation of presynaptic inhibition of soleus Ia afferents in young, middle-aged and old adults before and after a single session of practising a task that involved concurrent contraction of dorsiflexor and plantarflexor muscles. The hypothesis was that young subjects would be able to improve steadiness with practice by modulating Ia afferent feedback as indicated by changes in a measure of presynaptic inhibition (D1 inhibition), but that middle-aged and older subjects would exhibit a lesser ability to augment steadiness. There were no differences in steadiness between groups during an initial co-contraction trial (P = 0.713). Maximal voluntary contraction force for the plantarflexors was not significantly different between young and middle-aged subjects (P > 0.05), but it was significantly less in old subjects (P < 0.05). The main finding of the study was that young adults were able to improve steadiness by ∼19% (P < 0.001) during a co-contraction task after 50 min of practice, whereas there was no change for the middle-aged adults, and old adults became less steady by ∼15% (P < 0.05). The improvement in steadiness by young adults was accompanied by a significant reduction in the amount of Ia presynaptic inhibition as indexed by D1 inhibition (P < 0.01). Conversely, neither of the other two groups exhibited any change in D1 inhibition after practising the co-contraction task. In contrast to young subjects, middle-aged and old adults found the co-contraction task challenging and were not able to improve steadiness after practising the low-force isometric contraction.

Age-associated differences in triceps surae muscle composition and strength - an MRI-based cross-sectional comparison of contractile, adipose and connective tissue

Background

In human skeletal muscles, the aging process causes a decrease of contractile and a concomitant increase of intramuscular adipose (IMAT) and connective (IMCT) tissues. The accumulation of non-contractile tissues may contribute to the significant loss of intrinsic muscle strength typically observed at older age but their in vivo quantification is challenging. The purpose of this study was to establish MR imaging-based methods to quantify the relative amounts of IMCT, IMAT and contractile tissues in young and older human cohorts, and investigate their roles in determining age-associated changes in skeletal muscle strength.

Methods

Five young (31.6 ± 7.0 yrs) and five older (83.4 ± 3.2 yrs) Japanese women were subject to a detailed MR imaging protocol, including Fast Gradient Echo, Quantitative Fat/Water (IDEAL) and Ultra-short Echo Time (UTE) sequences, to determine contractile muscle tissue and IMAT within the entire Triceps Surae complex, and IMCT within both heads of the Gastrocnemius muscle. Specific force was calculated as the ratio of isometric plantarflexor force and the physiological cross-sectional area of the Triceps Surae complex.

Results

In the older cohort, total Triceps Surae volume was smaller by 17.5%, while the relative amounts of Triceps Surae IMAT and Gastrocnemius IMCT were larger by 55.1% and 48.9%, respectively. Differences of 38.6% and 42.1% in plantarflexor force and specific force were observed. After subtraction of IMAT and IMCT from total muscle volume, differences in intrinsic strength decreased to 29.6%.

Conclusions

Our data establishes that aging causes significant changes in skeletal muscle composition, with marked increases in non-contractile tissues. Such quantification of the remodeling process is likely to be of functional and clinical importance in elucidating the causes of the disproportionate age-associated decrease of force compared to that of muscle volume.

Reliability of nerve function assessments for people with peripheral neuropathy

Examining degenerative sensory and reflexive activity among people with peripheral neuropathy (PN) is important for clinical examinations and understanding relevant neural impairments. However, other than the test of presynaptic inhibition, the reliability of other related tests is largely unknown. The purpose of this project was to examine the test-retest reliability of lower extremity sensory and reflexive measures in people with PN. Twelve participants (8 women, 4 men, age = 72.5 ± 9.2 years) diagnosed with PN were assessed on two occasions at least 7 d apart. Plantar sensitivity, H-reflex and active and passive ankle proprioception (AAP and PAP) were tested. Paired student's t-test and Intraclass correlation coefficients (ICC) were used for statistical analysis. Significant difference was observed in AAP at 0º position (p = 0.04). No other significant differences were observed. Moderate to high reliability was observed at measures of Plantar sensitivity (0.92), peak to peak H (0.71) and M (0.84) waves, latency between H and M (0.78), H-Index (0.85), AAP (0.62) and PAP index (0.60). Low reliability was observed of other parameters. The measures of Hmax, Mmax, latency, H-Index, plantar sensitivity and AAP & PAP indices of proprioception tests can be measured reliably for further study in this population.

Gait disturbances as specific predictive markers of the first fall onset in elderly people: a two-year prospective observational study

Falls are common in the elderly, and potentially result in injury and disability. Thus, preventing falls as soon as possible in older adults is a public health priority, yet there is no specific marker that is predictive of the first fall onset. We hypothesized that gait features should be the most relevant variables for predicting the first fall. Clinical baseline characteristics (e.g., gender, cognitive function) were assessed in 259 home-dwelling people aged 66 to 75 that had never fallen. Likewise, global kinetic behavior of gait was recorded from 22 variables in 1036 walking tests with an accelerometric gait analysis system. Afterward, monthly telephone monitoring reported the date of the first fall over 24 months. A principal components analysis was used to assess the relationship between gait variables and fall status in four groups: non-fallers, fallers from 0 to 6 months, fallers from 6 to 12 months and fallers from 12 to 24 months. The association of significant principal components (PC) with an increased risk of first fall was then evaluated using the area under the Receiver Operator Characteristic Curve (ROC). No effect of clinical confounding variables was shown as a function of groups. An eigenvalue decomposition of the correlation matrix identified a large statistical PC1 (termed “Global kinetics of gait pattern”), which accounted for 36.7% of total variance. Principal component loadings also revealed a PC2 (12.6% of total variance), related to the “Global gait regularity.” Subsequent ANOVAs showed that only PC1 discriminated the fall status during the first 6 months, while PC2 discriminated the first fall onset between 6 and 12 months. After one year, any PC was associated with falls. These results were bolstered by the ROC analyses, showing good predictive models of the first fall during the first six months or from 6 to 12 months. Overall, these findings suggest that the performance of a standardized walking test at least once a year is essential for fall prevention.

Practice improves motor control in older adults by increasing the motor unit modulation from 13 to 30 Hz

Practice of a motor task decreases motor output variability in older adults and is associated with adaptations of discharge activity of single motor units. In this study we were interested in the practice-induced modulation of multiple motor units within 13-30 Hz because theoretically it enhances the timing of active motoneurons. Our purpose, therefore, was to determine the neural adaptation of multiple motor units and related improvements in movement control following practice. Nine healthy older adults (65-85 yr) performed 40 practice trials of a sinusoidal task (0.12 Hz) with their index finger (10° range of motion). Multi-motor unit activity was recorded intramuscularly from the first dorsal interosseus muscle. The mean spike rate (MSR), spike rate variability (CV(ISI)), and frequency modulation (5-60 Hz) of the spike rate were calculated from the multi-motor unit activity and were correlated with movement accuracy and variability of index finger position. A decrease in movement trajectory variability was associated with an increase in MSR (R(2) = 0.58), a decrease in CV(ISI) (R(2) = 0.58), and an increase in total power within a 13- to 30-Hz band (R(2) = 0.48). The increase in total power within a 13- to 30-Hz band was associated significantly (P < 0.005) with an increase in MSR (R(2) = 0.75) and the decrease in CV(ISI) (R(2) = 0.70). We demonstrate that practice-induced improvements in movement control are associated with changes in activity of multiple motor units. These findings suggest that practice-induced improvements in movement steadiness of older adults are associated with changes in the modulation of the motoneuron pool from 13 to 30 Hz.

The effects of isometric resistance training on stretch reflex induced tremor in the knee extensor muscles

This study examines the effect of 4 wk of high-intensity isometric resistance training on induced tremor in knee extensor muscles. Fourteen healthy volunteers were assigned to either the training group (n = 7) or the nontraining control group (n = 7). Induced tremor was assessed by measuring force fluctuations during anisometric contractions against spring loading, whose compliance was varied to allow for preferential activation of the short or long latency stretch reflex components. Effects of high-intensity isometric resistance training on induced tremor was assessed under two contraction conditions: relative force matching, where the relative level of activity was equal for both pre- and post-training sessions, set at 30% maximum voluntary contraction (MVC), and absolute force matching, where the level of activity was set to 30% pretrained MVC. The training group experienced a 26.5% increase in MVC in contrast to the 0.8% for the control group. For relative force-matching contractions, induced tremor amplitude and frequency did not change in either the training or control group. During absolute force-matching contractions, induced tremor amplitude was decreased by 37.5% and 31.6% for the short and long components, respectively, with no accompanying change in frequency, for the training group. No change in either measure was observed in the control group for absolute force-matching contractions. The results are consistent with high-intensity isometric resistance training induced neural changes leading to increased strength, coupled with realignment of stretch reflex automatic gain compensation to the new maximal force output. Also, previous reported reductions in anisometric tremor following strength training may partly be due to changed stretch reflex behavior.

Are H-reflex and M-wave recruitment curve parameters related to aerobic capacity?

Soleus Hoffmann reflex (H-reflex) amplitude is affected by a training period and type and level of training are also well known to modify aerobic capacities. Previously, paired changes in H-reflex and aerobic capacity have been evidenced after endurance training. The aim of this study was to investigate possible links between H- and M-recruitment curve parameters and aerobic capacity collected on a cohort of subjects (56 young men) that were not involved in regular physical training. Maximal H-reflex normalized with respect to maximal M-wave (H(max)/M(max)) was measured as well as other parameters of the H- or M-recruitment curves that provide information about the reflex or direct excitability of the motoneuron pool, such as thresholds of stimulus intensity to obtain H or M response (H(th) and M(th)), the ascending slope of H-reflex, or M-wave recruitment curves (H(slp) and M(slp)) and their ratio (H(slp)/M(slp)). Aerobic capacity, i.e., maximal oxygen consumption and maximal aerobic power (MAP) were, respectively, estimated from a running field test and from an incremental test on a cycle ergometer. Maximal oxygen consumption was only correlated with M(slp), an indicator of muscle fiber heterogeneity (p < 0.05), whereas MAP was not correlated with any of the tested parameters (p > 0.05). Although higher H-reflex are often described for subjects with a high aerobic capacity because of endurance training, at a basic level (i.e., without training period context) no correlation was observed between maximal H-reflex and aerobic capacity. Thus, none of the H-reflex or M-wave recruitment curve parameters, except M(slp), was related to the aerobic capacity of young, untrained male subjects.

Age-related changes in motor cortical properties and voluntary activation of skeletal muscle

Aging is associated with dramatic reductions in muscle strength and motor control, and many of these agerelated changes in muscle function result from adaptations in the central nervous system. Aging is associated with widespread qualitative and quantitative changes of the motor cortex. For example, advancing age has been suggested to result in cortical atrophy, reduced cortical excitability, reduced cortical plasticity, as well as neurochemical abnormalities.The associated functional effects of these changes likely influence numerous aspects of muscle performance such as muscle strength and motor control. For example, there is evidence to suggest that the muscle weakness associated with aging is partially due to impairments in the nervous system's ability to fully activate motor neurons- particularly in the larger proximal muscle groups. In this review article we discuss age-related changes in the motor cortex, as well as the abilityor lack thereof- of older adults to voluntarily activate skeletal muscle. We also provide perspectives on scientific and clinical questions that need to be addressed in the near future.

Effects of prolonged vibration on H-reflexes, muscle activation, and dynamic strength

Neural activation is generally lower during maximal voluntary lengthening compared with shortening and isometric muscle actions, but the mechanisms underlying these differences are unclear. In maximal voluntary isometric actions, reduced Ia-afferent input induced by prolonged tendon vibration has been shown to impair neural activation and strength.

PURPOSE

This study aimed to investigate whether reducing Ia-afferent input influences neural activation in maximal voluntary dynamic muscle actions and, if so, whether it affects shortening and lengthening muscle actions differently.

METHODS

Eight women participated in three familiarization sessions and two randomly ordered experiments. In one experiment, 30-min vibration at 100 Hz was applied to the Achilles tendon to decrease Ia-afferent input as measured by the H-reflex. In the control experiment, rest substituted the vibration. Root mean square EMG from plantar and dorsiflexor muscles and plantar flexor strength were measured during maximal voluntary plantar flexor shortening and lengthening actions (20°·s(-1)) before and after vibration and rest, respectively. Soleus H-reflexes and M-waves were elicited before each set of strength tests.

RESULTS

The vibration caused a decrease in H-reflex amplitude by, on the average, 33%, but root mean square EMG and plantar flexor strength remained largely unaffected in both action types.

CONCLUSIONS

The findings suggest that Ia-afferent input may not substantially contribute to maximal voluntary dynamic muscle strength of the plantar flexor muscles, as tested here, and thus, the results do not support the notion that Ia-afferent excitation would contribute differently to neural activation in maximal voluntary lengthening and shortening muscle actions.

Spinal reflex properties in the long term after stroke

In the long term after stroke, secondary functional deterioration may be observed while patients also get older. Possible underlying mechanisms are largely unknown. We aimed to assess neuromuscular degeneration represented by alterations in peripheral reflex loop characteristics as a function of follow-up time after stroke, controlled for age. Twenty-one stroke survivors within a small age range (62-67 years) but large variance in follow-up time after stroke (1-15 years) and both five age matched (59-62 years) and young subjects (28-36 years) participated. Short and long latency reflexes evoked by ramp and hold stretches were identified from EMG traces of the m. flexor carpi radialis. Short latency reflex onset time was not enhanced (mean difference 1.6ms compared to age matched controls) and did not relate to follow-up time after stroke (p=0.81). Young controls showed significantly lower reflex delay times (mean difference 7.2ms with respect to older subjects, p=0.009). No evidence was found for peripheral neuromuscular deterioration as a function of follow up time after stroke. Functional deterioration as a result of ageing of stroke patients that may interact with post stroke follow-up time is of further interest.

Neural adaptations to electrical stimulation strength training

This review provides evidence for the hypothesis that electrostimulation strength training (EST) increases the force of a maximal voluntary contraction (MVC) through neural adaptations in healthy skeletal muscle. Although electrical stimulation and voluntary effort activate muscle differently, there is substantial evidence to suggest that EST modifies the excitability of specific neural paths and such adaptations contribute to the increases in MVC force. Similar to strength training with voluntary contractions, EST increases MVC force after only a few sessions with some changes in muscle biochemistry but without overt muscle hypertrophy. There is some mixed evidence for spinal neural adaptations in the form of an increase in the amplitude of the interpolated twitch and in the amplitude of the volitional wave, with less evidence for changes in spinal excitability. Cross-sectional and exercise studies also suggest that the barrage of sensory and nociceptive inputs acts at the cortical level and can modify the motor cortical output and interhemispheric paths. The data suggest that neural adaptations mediate initial increases in MVC force after short-term EST.

Neural adaptations to strength training: moving beyond transcranial magnetic stimulation and reflex studies

It has long been believed that training for increased strength not only affects muscle tissue, but also results in adaptive changes in the central nervous system. However, only in the last 10 years has the use of methods to study the neurophysiological details of putative neural adaptations to training become widespread. There are now many published reports that have used single motor unit recordings, electrical stimulation of peripheral nerves, and non-invasive stimulation of the human brain [i.e. transcranial magnetic stimulation (TMS)] to study neural responses to strength training. In this review, we aim to summarize what has been learned from single motor unit, reflex and TMS studies, and identify the most promising avenues to advance our conceptual understanding with these methods. We also consider the few strength training studies that have employed alternative neurophysiological techniques such as functional magnetic resonance imaging and electroencephalography. The nature of the information that these techniques can provide, as well as their major technical and conceptual pitfalls, are briefly described. The overall conclusion of the review is that the current evidence regarding neural adaptations to strength training is inconsistent and incomplete. In order to move forward in our understanding, it will be necessary to design studies that are based on a rigorous consideration of the limitations of the available techniques, and that are specifically targeted to address important conceptual questions.

Dynapenia and aging: an update

In 2008, we published an article arguing that the age-related loss of muscle strength is only partially explained by the reduction in muscle mass and that other physiologic factors explain muscle weakness in older adults (Clark BC, Manini TM. Sarcopenia =/= dynapenia. J Gerontol A Biol Sci Med Sci. 2008;63:829-834). Accordingly, we proposed that these events (strength and mass loss) be defined independently, leaving the term "sarcopenia" to be used in its original context to describe the age-related loss of muscle mass. We subsequently coined the term "dynapenia" to describe the age-related loss of muscle strength and power. This article will give an update on both the biological and clinical literature on dynapenia-serving to best synthesize this translational topic. Additionally, we propose a working decision algorithm for defining dynapenia. This algorithm is specific to screening for and defining dynapenia using age, presence or absence of risk factors, a grip strength screening, and if warranted a test for knee extension strength. A definition for a single risk factor such as dynapenia will provide information in building a risk profile for the complex etiology of physical disability. As such, this approach mimics the development of risk profiles for cardiovascular disease that include such factors as hypercholesterolemia, hypertension, hyperglycemia, etc. Because of a lack of data, the working decision algorithm remains to be fully developed and evaluated. However, these efforts are expected to provide a specific understanding of the role that dynapenia plays in the loss of physical function and increased risk for disability among older adults.

Short-term strength training does not change cortical voluntary activation

PURPOSE

The neural mechanisms responsible for strength improvement in the early phase of strength training are unknown. One hypothesis is that strength increases because of increased neural drive to the trained muscles. Here, we used twitch interpolation to assess voluntary activation before and after a 4-wk strength training program.

METHODS

Twelve volunteers performed unilateral strength training for the right wrist abductors (three times per week). Control subjects (n = 11) practiced the same movement without resistance. We assessed voluntary activation of the trained muscles during wrist abduction and extension contractions using twitch interpolation with motor nerve and motor cortical stimulation.

RESULTS

Strength training increased wrist abduction maximal voluntary contraction (MVC) force for the trained hand by 11.0% (+/-8.7, P < 0.01). MVC of the untrained wrist was unchanged. There were no significant changes in wrist extension MVC force in either group. During submaximal wrist abduction, but not extension contractions, the average size of the superimposed twitches produced by cortical stimulation was significantly larger after strength training (P < 0.01). Furthermore, the direction of the twitches produced by cortical stimulation during wrist abductions and maximal wrist extension shifted toward abduction (P = 0.04). There were neither significant changes in voluntary activation measured during MVC with motor nerve or motor cortical stimulation nor changes in the amplitude of evoked EMG responses to motor cortical or motor nerve stimulation.

CONCLUSIONS

Four weeks of strength training produced a small increase in MVC that was specific to the training direction. Although maximal voluntary activation did not change with short-term strength training, the changes in direction and amplitude of cortically evoked twitches suggest that motor cortical stimulation (and presumably volition) can generate motor output more effectively to the trained muscles.

Role of the nervous system in sarcopenia and muscle atrophy with aging: strength training as a countermeasure

Aging is characterized by loss of spinal motor neurons (MNs) due to apoptosis, reduced insulin-like growth factor I signaling, elevated amounts of circulating cytokines, and increased cell oxidative stress. The age-related loss of spinal MNs is paralleled by a reduction in muscle fiber number and size (sarcopenia), resulting in impaired mechanical muscle performance that in turn leads to a reduced functional capacity during everyday tasks. Concurrently, maximum muscle strength, power, and rate of force development are decreased with aging, even in highly trained master athletes. The impairment in muscle mechanical function is accompanied and partly caused by an age-related loss in neuromuscular function that comprise changes in maximal MN firing frequency, agonist muscle activation, antagonist muscle coactivation, force steadiness, and spinal inhibitory circuitry. Strength training appears to elicit effective countermeasures in elderly individuals even at a very old age (>80 years) by evoking muscle hypertrophy along with substantial changes in neuromuscular function, respectively. Notably, the training-induced changes in muscle mass and nervous system function leads to an improved functional capacity during activities of daily living.

Differential age-related changes in motor unit properties between elbow flexors and extensors

AIM

Healthy adult ageing of the human neuromuscular system is comprised of changes that include atrophy, weakness and slowed movements with reduced spinal motor neurone output expressed by lower motor unit discharge rates (MUDRs). The latter observation has been obtained mostly from hand and lower limb muscles. The purpose was to determine the extent to which elbow flexor and extensor contractile properties, and MUDRs in six old (83 +/- 4 years) and six young (24 +/- 1 years) men were affected by age, and whether any adaptations were similar for both muscle groups.

METHODS

Maximal isometric voluntary contraction (MVC), voluntary activation, twitch contractile properties, force-frequency relationship and MUDRs from sub-maximal to maximal intensities were assessed in the elbow flexors and extensors.

RESULTS

Both flexor and extensor MVCs were significantly (P < 0.05) less (approximately 42% and approximately 46% respectively) in the old than in the young. Contractile speeds and the force-frequency relationship did not show any age-related differences (P > 0.05). For the elbow flexors contraction duration was approximately 139 ms and for the extensors it was approximately 127 ms for both age groups (P > 0.05). The mean MUDRs from 25% MVC to maximum were lower (approximately 10% to approximately 36%) in the old than in the young (P < 0.01). These age-related differences were larger for biceps (Cohen's d = 8.25) than triceps (Cohen's d = 4.79) brachii.

CONCLUSION

Thus, at least for proximal upper limb muscles, mean maximal MUDR reductions with healthy adult ageing are muscle specific and not strongly related to contractile speed.

Improvements in dynamic plantar flexor strength after resistance training are associated with increased voluntary activation and V-to-M ratio

The aim of this study was to investigate if, and via what mechanisms, resistance training of the plantar flexor muscles affects voluntary activation during maximal voluntary eccentric and concentric muscle actions. Twenty healthy subjects were randomized into a resistance training group ( n = 9) or a passive control group ( n = 11). Training consisted of 15 sessions of unilateral mainly eccentric plantar flexor exercise over a 5-wk period. During pre- and posttraining testing, dynamic plantar flexor strength was measured and voluntary activation was calculated using the twitch interpolation technique. The soleus Hoffman reflex (H-reflex) was used to assess motoneurone excitability and presynaptic inhibition of Ia afferents, whereas the soleus V-wave was used to test for changes in both presynaptic inhibition of Ia afferents and supraspinal inputs to the motoneurone pool. H-reflexes, V-waves, supramaximal M-waves, and twitches were evoked as the foot was moved at 5°/s through an angle of 90° during passive ankle rotations (passive H-relexes and M-waves) and during maximal voluntary concentric and eccentric plantar flexions [maximal voluntary contraction (MVC) H-reflexes, M-waves, and V-waves]. Training induced significant improvements in plantar flexor strength and voluntary activation during both concentric and eccentric maximal voluntary actions. Soleus passive and MVC H-to-M ratios remained unchanged after training, whereas the soleus V-to-M ratio was increased during both concentric and eccentric contractions after training. No changes were found in the control group for any of the parameters. The enhanced voluntary strength could be attributed partly to an increase in voluntary activation induced by eccentric training. Since the passive and MVC H-to-M ratios remained unchanged, the increase in activation is probably not due to decreased presynaptic inhibition. The increased V-to-M ratio for both action types indicates that increased voluntary drive from supraspinal centers and/or modulation in afferents other than Ia afferents may have contributed to such an increase in voluntary activation.

Resistance training increases in vivo quadriceps femoris muscle specific tension in young men

AIM

The present study investigated whether in vivo human quadriceps femoris (QF) muscle specific tension changed following strength training by systematically determining QF maximal force and physiological cross-sectional area (PCSA).

METHODS

Seventeen untrained men (20 +/- 2 years) performed high-intensity leg-extension training three times a week for 9 weeks. Maximum tendon force (F(t)) was calculated from maximum voluntary contraction (MVC) torque, corrected for agonist and antagonist muscle activation, and moment arm length (d(PT)) before and after training. QF PCSA was calculated as the sum of the four component muscle volumes, each divided by its fascicle length. Dividing F(t) by the sum of the component muscle PCSAs, each multiplied by the cosine of the respective fascicle pennation angle, provided QF specific tension.

RESULTS

MVC torque and QF activation increased by 31% (P < 0.01) and 3% (P < 0.05), respectively, but there was no change in antagonist co-activation or d(PT). Subsequently, F(t) increased by 27% (P < 0.01). QF volume increased by 6% but fascicle length did not change in any of the component muscles, leading to a 6% increase in QF PCSA (P < 0.05). Fascicle pennation angle increased by 5% (P < 0.01) but only in the vastus lateralis muscle. Consequently, QF specific tension increased by 20% (P < 0.01).

CONCLUSION

An increase in human muscle specific tension appears to be a real consequence of resistance training rather than being an artefact of measuring errors but the underlying cause of this phenomenon remains to be determined.

Effects of ageing on motor unit activation patterns and reflex sensitivity in dynamic movements

Both contraction type and ageing may cause changes in H-reflex excitability. H reflex is partly affected by presynaptic inhibition that may also be an important factor in the control of MU activation. The purpose of the study was to examine age related changes in H-reflex excitability and motor unit activation patterns in dynamic and in isometric contractions. Ten younger (YOUNG) and 13 elderly (OLD) males performed isometric (ISO), concentric (CON) and eccentric (ECC) plantarflexions with submaximal activation levels (20% and 40% of maximal soleus surface EMG). Intramuscular EMG data was analyzed utilizing an intramuscular spike amplitude frequency histogram method. Average H/M ratio was always lowest in ECC (n.s.). Mean spike amplitude increased with activation level (P<.05), whereas no significant differences were found between contraction types. Both H-reflex excitability, which may be due to an increase in presynaptic inhibition, and mean spike frequency were higher in YOUNG compared to OLD. In OLD the mean spike frequency was significantly smaller in CON compared to ISO. Lack of difference in mean spike amplitude and frequency across contraction types in YOUNG would imply a similar activation strategy, whereas the lower frequency in dynamic contractions in OLD could be related to synergist muscle behavior.

Sarcopenia: characteristics, mechanisms and functional significance

Sarcopenia reflects a progressive withdrawal of anabolism and an increased catabolism, along with a reduced muscle regeneration capacity. Muscle force and power decline more than muscle dimensions: older muscle is intrinsically weak. Sarcopenic obesity (SO) among the elderly corroborates to the loss of muscle mass increasing the risk of metabolic syndrome development. Recent studies on the musculoskeletal adaptations with ageing and key papers on the mechanisms of muscle wasting, its functional repercussions and on SO are included. Neuropathic, hormonal, immunological, nutritional and physical activity factors contribute to sarcopenia. Selective fast fibre atrophy, loss of motor units and an increase in hybrid fibres are typical findings of ageing. Satellite cell number decreases reducing muscle regeneration capacity. SO promotes further muscle wasting and increases risk of metabolic syndrome development. The proportion of fast to slow fibres seems maintained in old age. In elderly humans, nuclear domain is maintained constant. Basal protein synthesis and breakdown show little changes in old age. Instead, blunting of the anabolic response to feeding and exercise and of the antiproteolytic effect of insulin is observed. Further understanding of the mechanisms of sarcopenia requires disentangling of the effects of ageing alone from those of disuse and disease. The causes of the greater anabolic resistance to feeding and exercise of elderly women need elucidating. The enhancement of muscle regeneration via satellite cell activation via the MAPK/notch molecular pathways seems particularly promising.

Assessment of plantar flexors activation capacity: nerve versus muscle stimulation by single versus double pulse

This study was designed to investigate if the relationship between the interpolated twitch-torque (IT) and voluntary torque (VT) is affected by the number of electrical stimuli (single vs. double) and the stimulation site (nerve trunk vs. muscle). The results showed that the IT-VT relationship of the plantar flexors is appropriately described by a composite (linear + curvilinear) model. Indeed, whatever the stimulation method, the IT-VT relationship was linear between approximately 25 and 75% of the maximal voluntary torque (MVT) and curvilinear for higher contraction intensities. The four stimulation conditions are equivalent in assessing the maximal voluntary activation (VA% range 96.2 +/- 5.0 to 98.5 +/- 3.1%) as well as in determining the true maximal torque expected for total twitch occlusion (MT(exp) range 171.4 +/- 21.2 to 179.0 +/- 26.8 Nm). The gap between the MVT and MT(exp) should be viewed as an index of muscle inactivation. This gap was comparable for the four stimulation methods (2-6%) and close to the deficit in VA% (2-4%). No pulse-number effect was found on the IT-VT relationship when the nerve was stimulated but an effect on the concavity of the composite relationship was observed when the stimulation was applied over the muscle. Even though the four stimulation techniques are equivalent in assessing the maximal activation capacity our results demonstrate that the neural stimulation method is the most consistent as it guarantees the same motor pool recruitment independently from the number of pulses.

Ageing of human muscles and tendons

At whole muscle level, the reduction in intrinsic force observed with ageing is probably the result of the combined effect of changes in: (i) muscle architecture, (ii) tendon mechanical properties, (iii) neural drive (reduced agonist and increased antagonist muscles' activity), and (iv) single fibre specific tension. Only recently have alterations in muscle architecture and in tendon mechanical properties been shown to contribute to the reduction in intrinsic muscle force, and tendon stiffness changes play an important role. Of note is the fact that most of these changes may be reversed by 14 weeks of resistive training, for both fibre fascicle length and tendon stiffness were found to be increased by 10% and 64%, respectively. Surprisingly, however, training had no effect on the estimated relative length-tension properties of the muscle, indicating that the effects of increased tendon stiffness and increased fascicle length cancelled out each other. It seems that natural strategies may be in place to ensure that the relative operating range of muscle remains unaltered by changes in physical activity, and perhaps age.

Chronic low-frequency rTMS of primary motor cortex diminishes exercise training-induced gains in maximal voluntary force in humans

Although there is consensus that the central nervous system mediates the increases in maximal voluntary force (maximal voluntary contraction, MVC) produced by resistance exercise, the involvement of the primary motor cortex (M1) in these processes remains controversial. We hypothesized that 1-Hz repetitive transcranial magnetic stimulation (rTMS) of M1 during resistance training would diminish strength gains. Forty subjects were divided equally into five groups. Subjects voluntarily (Vol) abducted the first dorsal interosseus (FDI) (5 bouts x 10 repetitions, 10 sessions, 4 wk) at 70-80% MVC. Another group also exercised but in the 1-min-long interbout rest intervals they received rTMS [Vol+rTMS, 1 Hz, FDI motor area, 300 pulses/session, 120% of the resting motor threshold (rMT)]. The third group also exercised and received sham rTMS (Vol+Sham). The fourth group received only rTMS (rTMS_only). The 37.5% and 33.3% gains in MVC in Vol and Vol+Sham groups, respectively, were greater (P = 0.001) than the 18.9% gain in Vol+rTMS, 1.9% in rTMS_only, and 2.6% in unexercised control subjects who received no stimulation. Acutely, within sessions 5 and 10, single-pulse TMS revealed that motor-evoked potential size and recruitment curve slopes were reduced in Vol+rTMS and rTMS_only groups and accumulated to chronic reductions by session 10. There were no changes in rMT, maximum compound action potential amplitude (M(max)), and peripherally evoked twitch forces in the trained FDI and the untrained abductor digiti minimi. Although contributions from spinal sources cannot be excluded, the data suggest that M1 may play a role in mediating neural adaptations to strength training.

Recovery from supraspinal fatigue is slowed in old adults after fatiguing maximal isometric contractions

This study compared the contribution of supraspinal fatigue to muscle fatigue in old and young adults. Transcranial magnetic stimulation (TMS) of motor cortex was used to assess voluntary activation during maximal voluntary contractions (MVCs) of elbow flexor muscles in 17 young adults (25.5 +/- 3.6 yr; mean +/- SD) and 7 old adults (73.0 +/- 3.3 yr). Subjects performed a fatigue task involving six sustained MVCs (22-s duration, separated by 10 s). Young adults exhibited greater reductions in maximal voluntary torque (67 +/- 15% of baseline) than the old (37 +/- 6%; P < 0.001). Increments in torque (superimposed twitch) generated by TMS during sustained MVCs increased for the young and old (P < 0.001) but were larger for the old adults at the start of the sustained contractions and during recovery (P < 0.05). Voluntary activation was less for the old adults at the start of some sustained contractions and during recovery (P = 0.02). Motor-evoked potential area increased similarly with age during the fatiguing task but was greater for the old adults than young during recovery. Silent period duration lengthened less for the old adults during the fatigue task. At the end of the fatiguing task, peak relaxation rate of muscle fibers had declined more in the young than the old adults. The greater endurance with age is largely due to a difference in mechanisms located within the muscle. However, recovery from the fatiguing exercise is impaired for old adults because of greater supraspinal fatigue than in the young.