Unilateral Lower Limb Suspension Research Articles

Neuromuscular function following muscular unloading and blood flow restricted exercise

The aim of the study is to evaluate central and peripheral neuromuscular function in the knee extensors (KE) and plantar flexors (PF) after 30days of unilateral lower limb suspension (ULLS) and to examine the effects of low-load blood flow restricted (BFR) resistance training on the KE during ULLS. Strength, cross-sectional area (CSA), central activation, evoked force, and rates of force development and relaxation were assessed in the KE and PF before and after ULLS in sixteen subjects (9M, 7F; 18-49years). Eight of those subjects participated in BFR on the KE three times per week during ULLS (ULLS+Exercise). The ULLS group had decrements in strength and CSA of the KE (16 and 7%, respectively) and PF (27 and 8%, respectively) and the ULLS+Exercise maintained strength and CSA of the KE (P>0.05), but significantly lost strength andCSA in the PF (21 and 5%; P>0.05). KE central activation declined 6% in the ULLS group and was maintained in the ULLS+Exercise group, but a time×group interaction was not evident (P=0.31). PF central activation was reduced in both groups (ULLS: -7.6±9.9 and -7.9±11.6%; time main effect P=0.01). A time×group interaction for KE-evoked twitch force (P=0.04) demonstrated a 9% decline in the ULLS+Exercise group following the intervention. Evoked PF doublet torque decreased 12% in both groups (P=0.002). Central and peripheral neuromuscular function is compromised during unloading. While BFR resistance training on the KE during unloading can maintain muscle mass and strength, it may only partially attenuate neuromuscular dysfunction.

Early changes in costameric and mitochondrial protein expression with unloading are muscle specific.

We hypothesised that load-sensitive expression of costameric proteins, which hold the sarcomere in place and position the mitochondria, contributes to the early adaptations of antigravity muscle to unloading and would depend on muscle fibre composition and chymotrypsin activity of the proteasome. Biopsies were obtained from vastus lateralis (VL) and soleus (SOL) muscles of eight men before and after 3 days of unilateral lower limb suspension (ULLS) and subjected to fibre typing and measures for costameric (FAK and FRNK), mitochondrial (NDUFA9, SDHA, UQCRC1, UCP3, and ATP5A1), and MHCI protein and RNA content. Mean cross-sectional area (MCSA) of types I and II muscle fibres in VL and type I fibres in SOL demonstrated a trend for a reduction after ULLS (0.05 ≤ P < 0.10). FAK phosphorylation at tyrosine 397 showed a 20% reduction in VL muscle (P = 0.029). SOL muscle demonstrated a specific reduction in UCP3 content (−23%; P = 0.012). Muscle-specific effects of ULLS were identified for linear relationships between measured proteins, chymotrypsin activity and fibre MCSA. The molecular modifications in costamere turnover and energy homoeostasis identify that aspects of atrophy and fibre transformation are detectable at the protein level in weight-bearing muscles within 3 days of unloading.

Skeletal muscle adaptations to physical inactivity and subsequent retraining in young men

Skeletal muscle structure and function are markedly affected by chronic disuse. With unloading, muscle mass is lost at rate of about 0.4 %/day but little is known about the recovery of muscle mass and strength following disuse. Here we report an extensive data set describing in detail skeletal muscle adaptations in structure and function in response to both disuse and retraining. Eight young men (23 ± 2.2 years) underwent 3 weeks of unilateral lower limb suspension (ULLS) followed by a 3-week resistance training recovery program. Knee extensor isometric torque, voluntary activation, quadriceps femoris (QF) muscle volume (QFvol), fascicle length (Lf) and pennation angle (θ), physiological cross-sectional area (PCSA) of all four heads of the QF muscle, were measured before, after ULLS, and post-ULLS-resistance training. Needle biopsies were taken from the vastus lateralis muscle of a subgroup (n = 6) of the same subjects and cross sectional area of individual muscle s and myosin content of muscle samples were determined. Following 3 weeks of ULLS, isometric torque decreased by 26 %, PCSA by 3 %, QFvol by 10 %. Lf and θ of all four heads of QF significantly decreased (p ≤ 0.05). Following the 3-week retraining period, isometric torque, PCSA, QFvol, Lf and θ of all four heads of QF were all fully restored to pre ULLS values. CSA of individual muscle fibres and myosin content of muscle samples decreased by 26 and 35 % respectively (post-ULLS) and recovered to almost pre-ULLS values following retraining. There were no significant changes in voluntary activation of the quadriceps muscles in response to either ULLS or subsequent retraining. These results indicate that: (1) the loss of muscle force with 3-week unloading in humans is mostly explained by muscle atrophy and by a decrease in myosin content and, (2) all the neuromuscular changes induced by this model of disuse can be fully restored after a resistance training intervention of equal duration.

Influence of 10 days of unilateral lower limb suspension and combined exercise training on human vastus lateralis and soleus muscles

The unilateral lower limb suspension (ULLS) model is used to study disuse conditions, such as spaceflight, which typically induce a slow‐to‐fast phenotype transition. The vastus lateralis (VL) and soleus (SOL) are both antigravity muscles that are very sensitive to changes in loading states. We investigated whether fiber type changes associated with 10 days of ULLS could be ameliorated with combined aerobic and resistance exercise (EX) on a gravity independent device. Total RNA concentration was increased only in EX by 14% and 19% in both the VL and SOL respectively. In the VL, exercise increased type IIa mRNA expression by 25% while exhibiting a 58% decline in type IIx mRNA. Type IIx mRNA expression in the SOL was increased from 5% to 10% for ULLS and from 3% to 9% for EX. No significant changes were seen in protein expression. Increases in VL IGF1 and COL3A mRNA were seen for EX only and in both groups for SOL. These data show that EX can produce favorable MHC transitions during unloading in the mixed muscle VL but not the slow type I dominant soleus. Supported by NIH Postdoctoral Training grant 2T32AR047752 to VJC. Funded by NSBRI grant NCC9‐58‐70 to GRA.

Nutrition and Resistance Exercise During Reconditioning From Unloading

The recovery of muscle size and function following musculoskeletal unloading has received little attention in the scientific literature. Nutritional factors such as total energy, protein intake, and the pre- and/or post-exercise consumption of amino acid-carbohydrate (AACHO) have been shown to be important for enhancing training adaptations in recreational exercisers. A preliminary study was conducted to explore the interaction between nutrition and resistance exercise during reconditioning from unloading. Muscle CSA, strength, and endurance were measured during a control period following 30 d of unilateral lower limb suspension (Post-ULLS) and after 18 d of reconditioning (ReCon). Six participants consumed either AACHO (979 kJ, 36 g carbohydrate, 22.5 g protein) or placebo (PLAC) prior to resistance exercise (3 d x wk(-1)) during reconditioning. Total daily energy and macronutrient intake were evaluated from dietary journals. From Post-ULLS to ReCon, muscle endurance increased 1.1 +/- 0.6 min in AACHO and decreased 1.3 +/- 0.7 min in PLAC. Muscle CSA (6 +/- 2 vs. 5 +/- 3 cm2) and strength (105 +/- 53 vs. 81 +/- 37 N) increased similarly in AACHO and PLAC, respectively. When groups were pooled there was a significant correlation between daily protein intake and the recovery of muscle CSA (r = 0.81). Although our findings are preliminary, timing AACHO intake during reconditioning was beneficial for muscular endurance, while overall protein intake was associated with increased muscle size. A systematic evaluation into the synergistic relationship between nutrition and exercise during muscular recovery from prolonged unloading is warranted.

Unilateral lower limb suspension: integrative physiological knowledge from the past 20 years (1991–2011)

In 1991, Hans Berg and colleagues published the first research investigation using unilateral lower limb suspension (ULLS) as a human model to study the influence of unloading on skeletal muscle. ULLS requires a participant to perform all activities with axillary crutches while wearing one thick-soled shoe. The elevated shoe eliminates ground contact with the adjacent foot, thereby unloading the lower limb. Today, ULLS is a well-known ground-based analog for microgravity. The present review will synthesize the physiological findings from investigations using ULLS to study the deleterious effects of unloading. Compromised human performance and the neuromuscular, musculoskeletal and circulatory mechanisms leading to altered function will be a major emphasis of the work. Results from prolonged bed rest will also be included in order for general comparisons to be made between analogs. Finally, the efficacy of exercise to mitigate the negative consequences of unloading is presented.

Changes in muscle contractile characteristics and jump height following 24 days of unilateral lower limb suspension

We measured changes in maximal voluntary and electrically evoked torque and rate of torque development because of limb unloading. We investigated whether these changes during single joint isometric muscle contractions were related to changes in jump performance involving dynamic muscle contractions and several joints. Six healthy male subjects (21 ± 1 years) underwent 3 weeks of unilateral lower limb suspension (ULLS) of the right limb. Plantar flexor and knee extensor maximal voluntary contraction (MVC) torque and maximal rate of torque development (MRTD), voluntary activation, and maximal triplet torque (thigh; 3 pulses at 300 Hz) were measured next to squat jump height before and after ULLS. MVC of plantar flexors and knee extensors (MVCke) and triplet torque decreased by 12% (P = 0.012), 21% (P = 0.001) and 11% (P = 0.016), respectively. Voluntary activation did not change (P = 0.192). Absolute MRTD during voluntary contractions decreased for plantar flexors (by 17%, P = 0.027) but not for knee extensors (P = 0.154). Absolute triplet MRTD decreased by 17% (P = 0.048). The reduction in MRTD disappeared following normalization to MVC. Jump height with the previously unloaded leg decreased significantly by 28%. No significant relationships were found between any muscle variable and jump height (r < 0.48), but decreases in torque were (triplet, r = 0.83, P = 0.04) or tended to be (MVCke r = 0.71, P = 0.11) related to decreases in jump height. Thus, reductions in isometric muscle torque following 3 weeks of limb unloading were significantly related to decreases in the more complex jump task, although torque in itself (without intervention) was not related to jump performance.

The Effect of Intense Interval Cycle-training on Unloading-Induced Dysfunction and Atrophy in the Human Calf Muscle

We investigated whether intense interval training on a cycle ergometer would prevent loss of muscle strength and atrophy in the human calf during unilateral lower limb suspension (ULLS). The present study involved 11 healthy men. We defined unloading leg and contralateral leg as ULLS-leg and CONT-leg, respectively. The subjects were divided into 2 groups: one with single-leg cycling training (Tr-UL, n=6); the other as a control (UL, n=5). The Tr-UL group performed an intense 25-min interval cycling training up to 80% of peak oxygen uptake on alternate days during 20-d ULLS. It was found that: 1) in maximal voluntary contraction (MVC) and the cross-sectional area of the planter flexor, there was a significant time- (pre-ULLS and post-ULLS) by-leg (ULLS-leg and CONT-leg) interaction; 2) in voluntary activation during MVC evaluated by the twitch interpolation technique, no significant time-by-leg interaction was detected but the trend of change from before to after ULLS tended to be different between ULLS-leg and CONT-leg; and 3) regarding ULLS-leg, the change in any parameters was not significantly different between the Tr-UL and UL groups. These results suggest that unloading induces dysfunction and atrophy in the human calf and that high-intensity interval training on a cycle ergometer cannot significantly prevent unloading-induced deconditioning in the human calf.

Aerobic Exercise Capacity and Muscle Volume After Lower Limb Suspension with Exercise Countermeasure

The purpose of the present study was to elucidate the effect of unilateral lower limb suspension (ULLS) on aerobic capacity during one-legged cycle exercise and whether the change in aerobic exercise capacity after ULLS with or without intensive interval training is related to the change in skeletal muscle volume. There were 13 young men who underwent 20 d of ULLS and were divided into 2 groups based on some physical characteristics: the control group (CON; N=7) and the trained group (TRN; N=6). Subjects in TRN underwent interval cycle training on alternate days during ULLS. The respiratory and circulatory responses to one-legged incremental cycling and muscle volume of the thigh were measured before and after 20 d of ULLS. : One-legged peak oxygen uptake (VO2peak) and total muscle volume of the thigh in CON decreased significantly after 20 d of ULLS (-11.0 +/- 3.5% in VO2peak, -6.1 +/- 1.8% in total muscle volume). However, these parameters were maintained in TRN (+0.2 +/- 5.2% in VO2peaka, -1.0 +/- 1.8% in total muscle volume). Circulatory variables at rest and during submaximal exercise were unchanged in both groups after ULLS. Changes in one-legged VO2peak were significantly correlated with change in total muscle volume of the thigh in CON. Our findings indicated that peripheral adaptations after ULLS could relate to the change in aerobic exercise capacity during one-legged exercise. Our results also suggest that intensive interval training prevents ULLS-induced deconditioning of both aerobic exercise capacity and skeletal muscle volume.

Forty-eight hours of unloading and 24 h of reloading lead to changes in global gene expression patterns related to ubiquitination and oxidative stress in humans

Although short-term disuse does not result in measurable muscle atrophy, studies suggest that molecular changes associated with protein degradation may be initiated within days of the onset of a disuse stimulus. We examined the global gene expression patterns in sedentary men (n = 7, mean age ± SD = 22.1 ± 3.7 yr) following 48 h unloading (UL) via unilateral lower limb suspension and 24 h reloading (RL). Biopsy samples of the left vastus lateralis muscle were collected at baseline, 48 h UL, and 24 h RL. Expression changes were measured by microarray and gene clustering; identification of enriched functions and canonical pathways were performed using the Database for Annotation, Visualization and Integrated Discovery (DAVID) and Ingenuity Pathway Analysis (IPA). Four genes were validated with quantitative RT-PCR (qRT-PCR), and protein levels were measured with Western blot. Of the upregulated genes after UL, the most enriched functional group and highest ranked canonical pathway were related to protein ubiquitination. The oxidative stress response pathway was the second highest ranked canonical pathway. Of the downregulated genes, functions related to mitochondrial metabolism were the most highly enriched. In general, gene expression patterns following UL persisted following RL. qRT-PCR confirmed increases in mRNA for ubiquitin proteasome pathway-related E3 ligase Atrogin1 (but not accompanying increases in protein products) and stress response gene heme oxygenase-1 (HMOX, which showed a trend toward increases in protein products at 48 h UL) as well as extracellular matrix (ECM) component COL4A3. The gene expression patterns were not reversed on RL, suggesting that molecular responses to short-term periods of skeletal muscle inactivity may persist after activity resumes.

Reduction in tendon elasticity from unloading is unrelated to its hypertrophy

Tendinous tissues respond to chronic unloading with adaptive changes in mechanical, elastic, and morphological properties. However, little is known about the changes in the detailed structures of the entire tendinous tissue and whether the change in tendon stiffness is related to morphology. We investigated changes in dimensional (volume, cross-sectional area, segmented lengths) and elastic (Young's modulus) properties of the Achilles tendon and distal aponeurosis in response to chronic unilateral lower limb suspension (ULLS) using velocity encoded phase contrast (VE-PC) and three-dimensional morphometric magnetic resonance imaging (MRI). Five healthy subjects underwent ULLS for 4 wk. Axial morphometric MRI was acquired along the entire length from the calcaneous to the medial gastrocnemius insertion. An oblique sagittal VE-PC MRI was also acquired. The Young's modulus could be calculated from this cine dynamic sequence of velocity encoded images from the slope of the stress-strain curve during the submaximal isometric plantar flexion. After 4 wk of ULLS, we found significant (46.7%) decrease in maximum plantar flexion torque. The total volumes of entire tendinous tissue (determined as the sum of the Achilles tendon and distal aponeurosis) increased significantly by 6.4% (11.9 vs. 12.7 ml) after ULLS. In contrast, Young's modulus decreased significantly by 10.4% (211.7 vs. 189.6 MPa) for the Achilles tendon and 29.0% for the distal aponeurosis (158.8 vs. 113.0 MPa) following ULLS. There was no significant correlation between relative change in volume and Young's modulus with 4 wk of ULLS. It is suggested that, although tendon hypertrophy can be expected to adversely affect tendon stiffness, the absence of any significant correlation between the magnitude of tendon hypertrophy and reduced Young's modulus indicates that dimensional factors were not critical to the elastic properties.

Effects of 3 days unloading on molecular regulators of muscle size in humans

Changes in skeletal muscle mass are controlled by mechanisms that dictate protein synthesis or degradation. The current human study explored whether changes in activation of the phosphoinositide 3-kinase (PI3K)-Akt1, p38, myostatin, and mRNA expression of markers of protein degradation and synthesis occur soon after withdrawal of weight bearing. Biopsies of the vastus lateralis muscle (VL) and soleus muscle (Sol) were obtained from eight healthy men before and following 3 days of unilateral lower limb suspension (ULLS). Akt1, Forkhead box class O (FOXO)-1A, FOXO-3A, p38, and eukaryotic translation initiation factor 4E binding protein 1 (4E-BP1) phosphorylation and protein levels and myostatin protein level were analyzed by Western blot. Levels of mRNA of IGF1, FOXO-1A, FOXO-3A, atrogin-1, MuRF-1, caspase-3, calpain-2, calpain-3, 4E-BP1, and myostatin were measured using real-time PCR. The amounts of phosphorylated Akt1, FOXO-1A, FOXO-3A, and p38 were unaltered (P>0.05) after ULLS. Similarly, mRNA levels of IGF1, FOXO-1A, FOXO-3A, caspase-3, calpain-2, and calpain-3 showed no changes (P>0.05). The mRNA levels of atrogin-1 and MuRF-1, as well as the mRNA and protein phosphorylation of 4E-BP1, increased (P<0.05) in VL but not in Sol. Both muscles showed increased (P<0.05) myostatin mRNA and protein following ULLS. These results suggest that pathways other than PI3K-Akt stimulate atrogin-1 and MuRF-1 expression within 3 days of ULLS. Alternatively, transient changes in these pathways occurred in the early phase of ULLS. The increased myostatin mRNA and protein expression also indicate that multiple processes are involved in the early phase of muscle wasting. Further, the reported difference in gene expression pattern across muscles suggests that mechanisms regulating protein content in human skeletal muscle are influenced by phenotype and/or function.

Skeletal muscle adaptations following blood flow-restricted training during 30 days of muscular unloading

This study evaluated the effectiveness of low-load resistance training with a blood flow restriction (LL(BFR)) to attenuate muscle loss and weakness after 30 days of unilateral lower limb suspension (ULLS). Sixteen subjects (ages 18-50 yr) underwent 30 days of ULLS. Measurements of muscle strength, cross-sectional area, and endurance on the knee extensors and plantar flexors were collected before and after ULLS. Plasma concentrations of IGF-1 and IGFBP-3 were also assessed. During ULLS, eight subjects (5 males, 3 females) participated in LL(BFR) three times per week (ULLS + Exercise) while eight subjects (4 males, 4 females) did not exercise (ULLS). The blood flow-restricted exercise consisted of dynamic knee extension at 20% of the subject's isometric maximum voluntary contraction coupled with a suprasystolic blood flow restriction. After 30 days of limb suspension, the ULLS + Exercise group experienced minimal and insignificant losses in knee extensor cross-sectional area and strength (1.2% and 2.0%, respectively; P </= 0.05), while the ULLS group demonstrated significant reductions in cross-sectional area and strength (7.4% and 21%, respectively). Decrements in plantar flexor strength (23.7%) and cross-sectional area (7.4%) were observed after ULLS (P < 0.05) and were of similar magnitude between the experimental groups (P > 0.05). Muscular endurance in the knee extensors improved 31% in the ULLS + Exercise group, while it decreased 24% in the ULLS group (P = 0.01). No changes were seen in hormone concentrations throughout the study. In conclusion, LL(BFR) of the knee extensors is effective in maintaining muscle strength and size during 30 days of ULLS and results in improved knee extensor muscular endurance.

Evaluation of Bone Markers during Unilateral Lower Limb Suspension and Blood Flow Restricted Exercise

Evaluation of Bone Markers during Unilateral Lower Limb Suspension and Blood Flow Restricted Exercise

The effect of physical deconditioning and exercise on VEGF expression and vascular function

Vascular endothelial growth factor (VEGF) has been identified as a key growth factor that modulates vascular adaptations. The aim of our study was to assess the impact of physical deconditioning and exercise training on vascular structure and function, and to determine expression of genes involved in the VEGF‐pathway. Six healthy men underwent 3 weeks of Unilateral Lower Limb Suspension (ULLS), while eight men with a spinal cord injury performed an 8‐week Functional Electrical Stimulation (FES) exercise training. Before and after the intervention, vascular characteristics of the superficial femoral artery were measured using ultrasound, and biopsies were taken from the vastus lateralis and soleus muscles. Baseline diameter was decreased after ULLS (P&lt;0.05), and increased after FES training (P&lt;0.01). Flow‐mediated dilation was unaltered after ULLS and FES exercise. VEGFB, VEGF receptor Flt‐1 and co‐receptor NRP‐1 mRNA were downregulated after ULLS (P&lt;0.05), while FES exercise upregulated VEGFB, NRP‐1 and co‐receptor HSPG‐2 mRNA (P&lt;0.05). For the first time, human translational data is available to simultaneously evaluate adaptations in vascular health and VEGF related gene expression to physical deconditioning and exercise. Our study indicates that vascular adaptations do occur parallel to alterations in expression of VEGFB and its (co‐)receptors Flt‐1 and NRP‐1.

The adaptive responses in several mediators linked with hypertrophy and atrophy of skeletal muscle after lower limb unloading in humans

To determine the adaptive changes in several molecules regulating muscle hypertrophy and atrophy after unloading, we examined whether unilateral lower limb suspension changes the mRNA and protein levels of SRF-linked (RhoA, RhoGDI, STARS and SRF), myostatin-linked (myostatin, Smad2, Smad3 and FLRG) and Foxo-linked (P-Akt, Foxo1, Foxo3a and Atrogin-1) mediators. A single lower limb of each of eight healthy men was suspended for 20 days. Biopsy specimens were obtained from the vastus lateralis muscle pre- and post-suspension. The volume of the vastus lateralis muscle was significantly decreased after unloading. The amount of RhoA, RhoGDI or SRF protein in the muscle was not significantly changed post-suspension. An RT-PCR semiquantitative analysis showed increased levels of myostatin mRNA but not Smad2, Smad3 or FLRG mRNA. Unloading did not elicit significant changes in the amount of p-Smad3 or myostatin protein in the muscle. The amount of p-Akt protein was markedly reduced in the unloaded muscle. Lower limb SUSPENSION DID NOT INFLUENCE THE EXPRESSION PATTERN OF FOXO1, FOXO3A OR ATROGIN-1. Unloading inducing a mild degree of muscle atrophy may decrease p-Akt and increase myostatin but not SRF-linked mediators.

Cycle Ergometer Exercise to Counteract Muscle Atrophy During Unilateral Lower Limb Suspension

Atrophy of the leg muscles is a serious consequence of disuse and occurs with prolonged exposure to microgravity. We investigated whether intense interval training on a cycle ergometer would prevent muscle changes during unilateral lower limb suspension (ULLS). This study involved 11 healthy men who were divided into 2 groups, one with training on a cycle ergometer (TRN, N=6) and the other a control group (CON, N=5). TRN performed intense interval training (up to 90% of maximum heart rate) on alternate days during 20 d of unloading. Maximum voluntary contraction (MVC) of the quadriceps femoris and volume of the thigh muscle were measured before and after unloading. Muscle functional magnetic resonance imaging (mfMRI), which reflects the contractile activity of the activating muscle, were acquired during submaximal dynamic knee extension exercises. The MVC was significantly decreased in TRN (-11.1%) and CON (-18.7%). The total volume of the thigh muscles was maintained in TRN (-0.2%); however, a significant decrease was found in CON (-6.9%). The mfMRI signal increase for submaximal exercise did not change for TRN; however, it significantly increased for CON. Although the power of this study was limited, these results suggest that training on a cycle ergometer counteracts some of the effects of unloading and merits further study as a potential countermeasure for muscle changes in microgravity.

Increased H‐reflex excitability is not accompanied by changes in neural drive following 24 days of unilateral lower limb suspension

The purpose of this study was to determine whether the gain in soleus H-reflex excitability induced by unilateral lower limb suspension (ULLS) is associated with changes in neural drive to the plantar flexor muscles. Six male subjects (23 ± 2 years, 187 ± 7 cm, 79 ± 9 kg) underwent 24 days of ULLS of the dominant limb. Plantar flexor maximal voluntary contraction (MVC) torque, activation capacity (twitch interpolation), soleus maximal electromyographic (EMG) activity, Hoffman (H)-reflex, and the first volitional (V) wave normalized to the compound muscle action potential (M-wave) were quantified before and after ULLS. Following ULLS, MVC torque decreased by 15% (P < 0.05). However, neither activation capacity nor EMG activity was significantly altered after the suspension. The V-wave remained unchanged consistently after ULLS, whereas the H-reflex increased significantly (+20%). Furthermore, there was no significant relationship between changes in H-reflex and V-wave over the ULLS period. These findings indicate that 24 days of ULLS can result in a substantial reduction of muscle strength without any apparent change in voluntary activation capacity. H-reflex and V-wave findings suggest that the spinal adaptations that underlie the unloading-induced increase in resting soleus H-reflex excitability did not significantly affect the efferent motor output to the plantar flexor muscles.

Soleus T reflex modulation in response to spinal and tendinous adaptations to unilateral lower limb suspension in humans

To investigate the influence of tendinous and synaptic changes induced by unilateral lower limb suspension (ULLS) on the tendon tap reflex. Eight young men underwent a 23-day period of ULLS. Muscle cross-sectional area (CSA), torque and electromyographic (EMG) activity of the plantar flexor muscles (normalized to the M wave), Achilles tendon-aponeurosis mechanical properties, soleus (SOL) H and T reflexes and associated peak twitch torques were measured at baseline, after 14 and 23 days of ULLS, and 1 week after resuming ambulatory activity. Significant decreases in muscle CSA (-9%), in maximal voluntary torque (-10%) and in the associated SOL EMG activity (-16%) were found after ULLS (P < 0.05). In addition to a 36% (P < 0.01) decrease in tendon-aponeurosis stiffness, normalized H reflex increased by 35% (P < 0.05). An increase in the slope (28%, P < 0.05) and intercept (85%, P < 0.05) of the T reflex recruitment curve pointed to an increase in the gain and to a decrease in the sensitivity of this reflex, possibly resulting from the decrease in the tendon-aponeurosis stiffness at low forces. Following ULLS, changes in tendinous stiffness correlated with changes in neuromuscular efficiency (peak twitch torque to reflex ratio) at higher tendon tap forces. These findings point out the dual and antagonistic influences of spinal and tendinous adaptations upon the tendon tap reflex in humans under conditions of chronic unloading. These observations have potential implications for the sensitivity of the short-latency Ia stretch response involved in rapid compensatory contractions to unexpected postural perturbations.

Reply to Rennie, Phillips, and Smith

to the editor: Several points need to be made to allow the readers of the Journal of Applied Physiology to put the comments made by Drs. Rennie, Phillips, and Smith ([3][1]) into proper context. 1 ) We will begin by addressing the two specific points raised about methodology. First, the authors of