Influence of isometric training at short and long muscle-tendon unit lengths on the history dependence of force.

The history dependence of force is an intrinsic property of muscle whereby a muscle actively shortened or lengthened to an isometric steady-state produces less (residual force depression; rFD) or more force (residual force enhancement; rFE), respectively, than a purely isometric contraction at the same muscle length and level of activation. Previous studies on the modifiability of the history dependence of force have been inconclusive, and none have attempted to modify rFD and rFE through isometric resistance training biased to short vs long muscle-tendon unit (MTU) lengths. We tested maximal voluntary rFD and rFE in seven males and six females before and after 8 weeks of maximal isometric dorsiflexion training 3 days/wk. Participants trained one leg at 0° of plantar flexion (short-MTU training) and one at 40° of plantar flexion (long-MTU training). Ultrasonography of the tibialis anterior assessed resting muscle architecture. Tibialis anterior fascicle length decreased by ~3% following short-MTU training (P = .03) and increased by ~4% following long-MTU training (P = .01). rFD did not change following training at either MTU length (absolute rFD: P = .53; percent rFD: P = .51), nor did rFE (absolute rFE: P = .78; percent rFE: P = .80), with no relationships between the change in fascicle length and the change in percent rFD (R2  = .01, P = .62) nor rFE (R2  = .001, P = .88). Our data indicate that voluntary rFD and rFE were not modified by isometric training and not related to the fascicle length adaptations we observed.

Ankle antagonist coactivation in the double-support phase of walking: Stroke vs. healthy subjects.

INTRODUCTION: Lesions in ipsilateral systems related to postural control in the ipsilesional side may justify the lower performance of stroke subjects during walking. PURPOSE: To analyze bilateral ankle antagonist coactivation during double support in stroke subjects. METHODS: Sixteen (8 females; 8 males) subjects with a first isquemic stroke and 22 controls (12 females; 10 males) participated in this study. The double-support phase was assessed through ground reaction forces and the electromyography of ankle muscles was assessed in both limbs. RESULTS: The ipsilesional limb presented statistically significant differences from the control when assuming specific roles during double support. The tibialis anterior and soleus pair was the one in which this atypical behavior was more pronounced. CONCLUSION: The ipsilesional limb presents a dysfunctional behavior when a higher postural control activity was demanded.

Muscle size, activation, and coactivation in adults with cerebral palsy.

INTRODUCTION: Muscle weakness is present in the paretic limbs of individuals with cerebral palsy (CP). We aimed to determine what neuromuscular factors contribute to weakness in adults with CP during isometric maximal voluntary contractions (iMVCs). METHODS: Gastrocnemius anatomical cross-sectional area (ACSA) and agonist and antagonist activation were measured in 11 CP and 11 control adult men during plantarflexion iMVC. RESULTS: Plantarflexion iMVC torque of the paretic leg was 42% and 52% less than in the non-paretic and control limbs, respectively. The paretic gastrocnemius ACSA was smaller than in the control group only. Paretic agonist activation was less than the non-paretic and control groups, whereas antagonist coactivation was higher. Multiple regression analysis revealed muscle activation accounted for 57% of variation in paretic plantarflexion iMVC torque. CONCLUSIONS: In individuals with CP, muscle weakness in the paretic limb is attributed primarily to impaired neural activation and, to a lesser degree, ACSA.

Operant conditioning to increase ankle control or decrease reflex excitability improves reflex modulation and walking function in chronic spinal cord injury.

Ankle clonus is common after spinal cord injury (SCI) and is attributed to loss of supraspinally mediated inhibition of soleus stretch reflexes and maladaptive reorganization of spinal reflex pathways. The maladaptive reorganization underlying ankle clonus is associated with other abnormalities, such as coactivation and reciprocal facilitation of tibialis anterior (TA) and soleus (SOL), which contribute to impaired walking ability in individuals with motor-incomplete SCI. Operant conditioning can increase muscle activation and decrease stretch reflexes in individuals with SCI. We compared two operant conditioning-based interventions in individuals with ankle clonus and impaired walking ability due to SCI. Training included either voluntary TA activation (TA↑) to enhance supraspinal drive or SOL H-reflex suppression (SOL↓) to modulate reflex pathways at the spinal cord level. We measured clonus duration, plantar flexor reflex threshold angle, timed toe tapping, dorsiflexion (DF) active range of motion, lower extremity motor scores (LEMS), walking foot clearance, speed and distance, SOL H-reflex amplitude modulation as an index of reciprocal inhibition, presynaptic inhibition, low-frequency depression, and SOL-to-TA clonus coactivation ratio. TA↑ decreased plantar flexor reflex threshold angle (-4.33°) and DF active range-of-motion angle (-4.32°) and increased LEMS of DF (+0.8 points), total LEMS of the training leg (+2.2 points), and nontraining leg (+0.8 points), and increased walking foot clearance (+ 4.8 mm) and distance (+12.09 m). SOL↓ decreased SOL-to-TA coactivation ratio (-0.21), increased nontraining leg LEMS (+1.8 points), walking speed (+0.02 m/s), and distance (+6.25 m). In sum, we found increased voluntary control associated with TA↑ outcomes and decreased reflex excitability associated with SOL↓ outcomes.

Reaction time in healthy elderly is associated with chronic low-grade inflammation and advanced glycation end product.

Chronic inflammation and Advanced Glycation End products (AGE) are associated with sarcopenia. Decreased voluntary muscle activation and increased antagonist coactivation can contribute to age-related muscle weakness. The influence of chronic inflammation and AGE in these neuromuscular mechanisms is not clear. We studied whether a relation exists between circulating levels of inflammatory cytokines and AGEs as well as the interplay between agonist and antagonist muscle activation. We studied 64 community-dwelling old subjects, during a maximal isometric voluntary contraction (MVC) and a reaction-time (RT) test of the upper limb. Twenty-five circulating inflammatory biomarkers were determined. Linear regression showed significant relationships between chronic inflammation and six muscle activation parameters. MIP-1ß showed a significant negative relation with antagonist coactivation (during MVC) and antagonist muscle activity during pre-movement time (PMT) and movement time (MT) (during RT). A higher level of pentosidine (AGE) was predictive for a longer PMT. We conclude that in older relatively healthy persons antagonist muscle activation is influenced by chronic inflammation, contributing to age-related muscle weakness. Our results also suggest a mechanical and inflammatory influence of pentosidine in upper limb slowing of movement. These findings show novel insight in underlying mechanisms of age-related muscle weakness.

Antagonist co-activation during short and medium latency responses in subjects with chronic ankle instability.

PURPOSE: This study aims to evaluate bilateral ankle antagonist co-activation during compensatory postural responses, including short latency responses (SLR) and medium latency responses (MLR), in response to a unilateral sudden ankle inversion in subjects with chronic ankle instability (CAI). METHODS: Twenty-four participants with CAI and 20 controls participated in the present study. Bilateral eletromyography of peroneus longus (PL), peroneus brevis (PB), tibialis anterior (TA) and soleus (SOL) muscles was collected during a unilateral sudden inversion perturbation (30°) to assess antagonist co-activation of SOL/TA and TA/P pairs during SLR and MLR in both support and perturbed positions. RESULTS: Compared to control group, participants with CAI presented: (1) bilateral increased antagonist co-activation of SOL/TA MLR in the support position; and (2) decreased antagonist co-activation of TA/P MLR of uninjured limb in the support position and of injured limb in the perturbed position. CONCLUSIONS: CAI involves deregulation of ankle antagonist co-activation of MLR in both injured and uninjured limbs.

Reduced cortical voluntary activation during bilateral knee extension.

INTRODUCTION: Reduced neural drive is mainly thought to explain the bilateral deficit phenomenon, i.e. the difference in maximal isometric voluntary contraction (MVC) between unilateral and bilateral contractions. The aim of the present study was to further document if bilateral knee extension is associated with changes in voluntary activation level assessed by both peripheral nerve electrical stimulation and transcranial magnetic stimulation. METHODS: Fourteen subjects performed unilateral and bilateral knee extensions with both superimposed femoral electrical nerve stimulation and transcranial magnetic stimulation in order to assess voluntary activation (VAFNES) and cortical voluntary activation (VATMS), respectively. RESULTS: There was no difference in MVC force of the tested leg when involved in unilateral and bilateral knee extensions (p=0.87). However, a significantly reduced VAFNES (-2.1±2.4%; p=0.01) and VATMS (-1.6±2.7%; p=0.04) have been evidenced during bilateral knee extension. DISCUSSION: It is hypothesized that counterbalances could have masked the decrease of voluntary activation during bilateral contraction.

Influence of prolonged wearing of unstable shoes on upright standing postural control.

OBJECTIVE: To study the influence of prolonged wearing of unstable shoes on standing postural control in prolonged standing workers. METHODS: The participants were divided into two groups: one wore unstable shoes while the other wore conventional shoes for 8weeks. Stabilometry parameters related to centre of pressure (CoP), rambling (RM) and trembling (TR) as well as the total agonist/antagonist muscle activity, antagonist co-activation and reciprocal activation were evaluated during upright standing, before and after the 8weeks period. In both moments, the subjects were evaluated wearing the unstable shoes and in barefoot. RESULTS: The unstable shoe condition presented increased CoP displacement related variables and decreased co-activation command compared to barefoot before and after the intervention. The prolonged wearing of unstable shoes led to: (1) reduction of medial-lateral CoP root mean square and area; (2) decreased anteroposterior RM displacement; (3) increased anteroposterior RM mean velocity and mediolateral RM displacement; (4) decreased anteroposterior TR RMS; and (5) increased thigh antagonist co-activation in the unstable shoe condition. CONCLUSION: The unstable shoe condition is associated to a higher destabilising effect that leads to a selection of more efficient and accurate postural commands compared to barefoot. Prolonged wearing of unstable shoes provides increased effectiveness and performance of the postural control system, while wearing of unstable shoes in upright standing, that are reflected by changes in CoP related variables and by a reorganisation of postural control commands.

Training-related changes in the EMG-moment relationship during isometric contractions: Further evidence of improved control of muscle activation in strength-trained men?

The possibility of using electromyography (EMG) to track muscle activity has raised the question of its relationship with the effort exerted by the muscles around the joints. However, the EMG-moment relationship is yet to be fully defined, and increasing knowledge of this topic could contribute to research in motor control and to the development of EMG-based algorithms and devices. With regards the training-related adaptations at the peripheral and central level, the present study investigated the effect of strength training on EMG-moment relationship. Our aim was to clarify its nature and gain further understanding of how morphological and neural factors may affect its form. The EMG-moment relationship was determined during knee flexion and extension isometric contractions performed by strength-trained male athletes and untrained male participants. The results showed that strength training induced linearity of the EMG-moment relationship concomitantly with enhanced maximum force production capacity and decreased co-activation of knee agonist-antagonist muscle pair. These results clarified discordant results regarding the linear or curved nature of the EMG-moment in isometric conditions and suggested that the remarkable linearity of the EMG-moment found in trained participants could indicate improved control of muscle activation.

Co-activation of upper limb muscles during reaching in post-stroke subjects: an analysis of the contralesional and ipsilesional limbs.

The purpose of this study was to analyze the change in antagonist co-activation ratio of upper-limb muscle pairs, during the reaching movement, of both ipsilesional and contralesional limbs of post-stroke subjects. Nine healthy and nine post-stroke subjects were instructed to reach and grasp a target, placed in the sagittal and scapular planes of movement. Surface EMG was recorded from postural control and movement related muscles. Reaching movement was divided in two sub-phases, according to proximal postural control versus movement control demands, during which antagonist co-activation ratios were calculated for the muscle pairs LD/PM, PD/AD, TRIlat/BB and TRIlat/BR. Post-stroke's ipsilesional limb presented lower co-activation in muscles with an important role in postural control (LD/PM), comparing to the healthy subjects during the first sub-phase, when the movement was performed in the sagittal plane (p<0.05). Conversely, the post-stroke's contralesional limb showed in general an increased co-activation ratio in muscles related to movement control, comparing to the healthy subjects. Our findings demonstrate that, in post-stroke subjects, the reaching movement performed with the ipsilesional upper limb seems to show co-activation impairments in muscle pairs associated to postural control, whereas the contralesional upper limb seems to have signs of impairment of muscle pairs related to movement.

Task complexity and maximal isometric strength gains through motor learning.

This study compared the effects of a simple versus complex contraction pattern on the acquisition, retention, and transfer of maximal isometric strength gains and reductions in force variability. A control group (N = 12) performed simple isometric contractions of the wrist flexors. An experimental group (N = 12) performed complex proprioceptive neuromuscular facilitation (PNF) contractions consisting of maximal isometric wrist extension immediately reversing force direction to wrist flexion within a single trial. Ten contractions were completed on three consecutive days with a retention and transfer test 2-weeks later. For the retention test, the groups performed their assigned contraction pattern followed by a transfer test that consisted of the other contraction pattern for a cross-over design. Both groups exhibited comparable increases in strength (20.2%, P < 0.01) and reductions in mean torque variability (26.2%, P < 0.01), which were retained and transferred. There was a decrease in the coactivation ratio (antagonist/agonist muscle activity) for both groups, which was retained and transferred (35.2%, P < 0.01). The experimental group exhibited a linear decrease in variability of the torque- and sEMG-time curves, indicating transfer to the simple contraction pattern (P < 0.01). The control group underwent a decrease in variability of the torque- and sEMG-time curves from the first day of training to retention, but participants returned to baseline levels during the transfer condition (P < 0.01). However, the difference between torque RMS error versus the variability in torque- and sEMG-time curves suggests the demands of the complex task were transferred, but could not be achieved in a reproducible way.