Neuromuscular adaptations to detraining following resistance training in previously untrained subjects

Resistance training has been shown to considerably increase strength and neural drive during maximal eccentric muscle contraction; however, less is known about the adaptive change induced by subsequent detraining. The purpose of the study was to examine the effect of dynamic resistance training followed by detraining on changes in maximal eccentric and concentric isokinetic muscle strength, as well as to examine the corresponding adaptations in muscle cross-sectional area (CSA) and EMG activity. Maximal concentric and eccentric isokinetic knee extensor moment of force was measured in 13 young sedentary males (age 23.5±3.2 years), before and after 3 months of heavy resistance training and again after 3 months of detraining. Following training, moment of force increased during slow eccentric (50%, P<0.001), fast eccentric (25%, P<0.01), slow concentric (19%, P<0.001) and fast concentric contraction (11%, P<0.05). Corresponding increases in EMG were observed during eccentric and slow concentric contraction. Significant correlations were observed between the training-induced changes in moment of force and EMG (R²=0.33–0.77). Muscle CSA (measured by MRI) increased by 10% (P<0.001). After 3 months of detraining maximal muscle strength and EMG remained preserved during eccentric contraction but not concentric contraction. The present findings suggest that heavy resistance training induces long-lasting strength gains and neural adaptations during maximal eccentric muscle contraction in previously untrained subjects.     

Exercise-induced muscle damage and the repeated bout effect: evidence for cross transfer

We examined whether a prior bout of eccentric exercise in the elbow flexors provided protection against exercise-induced muscle damage in the contralateral arm. Fifteen males (age 22.7 ± 2.1 years; height 178.6 ± 6.8 cm, mass 75.8 ± 9.3 kg) were randomly assigned to two groups who performed two bouts of 60 eccentric contractions (30°/s) separated by 2 weeks: ipsilateral (n = 7, both bouts performed in the same arm), contralateral (n = 8, one bout performed in each arm). Strength, muscle soreness and resting arm angle (RAA) were measured at baseline and at 1, 24 and 48 h post exercise. Surface electromyography was recorded during both bouts of exercise. The degree of strength loss was attenuated (p < 0.05) in the ipsilateral group after the second bout of eccentric exercise (−22 cf. −3% for bout 1 and 2 at 24 h, respectively). Strength loss following eccentric exercise was also attenuated (p < 0.05) at 24 h in the contralateral group (−30 cf. 13% for bout 1 and 2, respectively). Muscle soreness (≈34 cf 19 mm) and change in RAA (≈5 cf. 3%) were also lower following the second bout of eccentric exercise (p < 0.05), although there was no difference in the overall change in these values between groups. Median frequency (MF) was decreased by 31% between bouts, with no difference between groups. Data support observations that the repeated bout effect transfers to the opposite (untrained) limb. The similar reduction in MF between bouts for the two groups provides evidence for a centrally mediated, neural adaptation.     

Changes in force,cross-sectional area and neural activation during strength training and detraining of the human quadriceps

Summary Four male subjects aged 23–34 years were studied during 60 days of unilateral strength training and 40 days of detraining. Training was carried out four times a week and consisted of six series of ten maximal isokinetic knee extensions at an angular velocity of 2.09 rad·s⁻¹. At the start and at every 20th day of training and detraining, isometric maximal voluntary contraction (MVC), integrated electromyographic activity (iEMG) and quadriceps muscle cross-sectional area (CSA) assessed at seven fractions of femur length (L_f), by nuclear magnetic resonance imaging, were measured on both trained (T) and untrained (UT) legs. Isokinetic torques at 30° before full knee extension were measured before and at the end of training at: 0, 1.05, 2.09, 3.14, 4.19, 5.24 rad·s⁻¹. After 60 days T leg CSA had increased by 8.5%±1.4% (mean±SEM,n=4,p<0.001), iEMG by 42.4%±16.5% (p<0.01) and MVC by 20.8%±5.4% (p<0.01). Changes during detraining had a similar time course to those of training. No changes in UT leg CSA were observed while iEMG and MVC increased by 24.8%±10% (N.S.) and 8.7%±4.3% (N.S.), respectively. The increase in quadriceps muscle CSA was maximal at 2/10 L_f (12.0%±1.5%,p<0.01) and minimal, proximally to the knee, at 8/10 L_f (3.5%±1.2%, N.S.). Preferential hypertrophy of the vastus medialis and intermedius muscles compared to those of the rectus femoris and lateralis muscles was observed. Isoangular torque of T leg increased by 20.9%±5.4% (p<0.05), 23.8%±7.8% (p<0.05) and 22.5%±6.7% (p<0.05) at 0, 1.05 and 2.09 rad·s⁻¹ respectively; no significant change was observed at higher velocities and in the UT leg. Hypertrophy produced by strength training accounts for 40% of the increase in force while the remaining 60% seems to be attributable to an increased neural drive and possibly to changes in muscle architecture.     

Neuro-Physiological Adaptations Associated with Cross-Education of Strength

Cross-education of strength is the increase in strength of the untrained contralateral limb after unilateral training of the opposite homologous limb. We investigated central and peripheral neural adaptations associated with cross-education of strength. Twenty-three right-handed females were randomized into a unilateral training group or an imagery group. A sub-sample of eight subjects (four training, four imagery) was assessed with functional magnetic resonance imaging (fMRI) for patterns of cortical activation during exercise. Strength training was 6 weeks of maximal isometric ulnar deviation of the right arm, four times per week. Peak torque, muscle thickness (ultrasound), agonist–antagonist electromyography (EMG), and fMRI were assessed before and after training. Strength training was highly effective for increasing strength in trained (45.3%; P < 0.01) and untrained (47.1%; P < 0.01) limbs. The imagery group showed no increase in strength for either arm. Muscle thickness increased only in the trained arm of the training group (8.4%; P < 0.001). After training, there was an enlarged region of activation in contralateral sensorimotor cortex and left temporal lobe during muscle contractions with the untrained left arm (P < 0.001). Training was associated with a significantly greater change in agonist muscle EMG pooled over both limbs, compared to the imagery group (P < 0.05). These results suggest that cross-education of strength may be partly controlled by adaptations within sensorimotor cortex, consistent with previous studies of motor learning. However, this research demonstrates the involvement of temporal lobe regions that subserve semantic memory for movement, which has not been previously studied in this context. We argue that temporal lobe regions might play a significant role in the cross-education of strength.     

High-intensity unilateral dorsiflexor resistance training results in bilateral neuromuscular plasticity after stroke

Hemiparesis after stroke decreases ability to dorsiflex the more-affected ankle during walking. Increased strength would be beneficial, but the more-affected limb is often too weak to be trained. In neurologically intact participants, training one limb induces strength gains in the contralateral, untrained limb. This approach remains unexplored post-stroke. The aim of this study was to test the hypothesis that unilateral dorsiflexor high-intensity resistance training on the less-affected side increases strength and motor output bilaterally following stroke. 19 participants (84.1 ± 77.6 months post-infarct) performed 6 weeks of maximal isometric dorsiflexion training using the less-affected leg. Voluntary isometric strength (dorsiflexion torque, muscle activation), reciprocal inhibition (RI), walking ability (gait speed, kinematics, EMG patterns), and clinical function were measured within 1 week before and 4 days following training. Post-intervention, dorsiflexion torque increased by ~31 % (p < 0.05) in the more-affected (untrained) and by ~34 % (p < 0.05) in the less-affected (trained) legs. Muscle activation significantly increased bilaterally, by ~59 and ~20 % in the trained and untrained legs, respectively. Notably, 4 participants who were unable to generate functional dorsiflexion on the more-affected side before training could do so post-intervention. Significant correlations between muscle activation and size of RI were noted across muscle groups before and after training, and the relation between size of RI and level of muscle activation in the more-affected tibialis anterior muscle was significantly altered by training. Thus, significant gains in voluntary strength and muscle activation on the untrained, more-affected side after stroke can be invoked through training the opposite limb. We demonstrate residual plasticity existing many years post-stroke and suggest clinical application of the cross-education effect where training the more-affected limb is not initially possible.     

Buffer capacity and lactate accumulation in skeletal muscle of trained and untrained men

Buffer capacity (β) of skeletal muscle has been determined in trained (n=7) and in sedentary subjects (n=8). The trained subjects were active in ball games where a high degree of anaerobic energy utilization is required. Percentage fibre type occurrence in the thigh muscle was not significantly different in the two groups. However, there was a tendency towards a higher proportion of type I (slow-twitch) fibres (61.5±11.6% vs. 50.2±12.5%) and a lower proportion of type IIB fibres (2.1±3.5% vs 14.1±16.3%) in the trained subjects. The proportion of the cross-sectional area of the muscle biopsies that was made up of type I or type II fibres was not different in the two groups. All subjects performed an isometric contraction of the knee extensors to fatigue at 61% of their maximal voluntary contraction force. Muscle biopsies were taken from the quadriceps femoris muscle at rest and immediately after contraction. The buffer capacity of muscle was calculated from: β= (Muscle lactate (work)-Muscle lactate (rest))/(Muscle pH (rest) -Muscle pH (work)). A higher buffer capacity (p<0.05) was observed in the trained subjects (β=194±30 mmolxpH^-1xkg^-1 dry wt.) compared to the sedentary group (β=164±20) (mean±SD). An unexpected finding was that muscle lactate after contraction to fatigue was lower (30%, p<0.01) and muscle pH was higher (6.80±0.06 vs. 6.61±0.12, p<0.01) in the trained subjects than in the sedentary controls. Creatine phosphate stores were almost completely depleted in both groups. Post-exercise lactate values were related to the proportion of type II fibres in the muscle (p<0.01). There was, however, no statistical correlation betwe β and fibre type occurrence (p>0.05). In summary, the present results indicate that skeletal muscle buffer capacity can be changed by training in man. Furthermore, it is concluded that the lower lactate accumulation and pH decline after an isometric contraction to fatigue that was observed in the trained compared to the sedentary subjects is related to the training per se. However, the tendency towards a lower type I (slowtwitch) fibre percentage in the trained subjects is likely to have contributed to the observed differences.     

Effects of detraining following short term resistance training on eccentric and concentric muscle strength

Healthy males were examined before and after 12 weeks of accommodated resistance training (three week^-1) and after 12 weeks of detraining. Training consisted of four to five sets of six coupled maximum voluntary bilateral concentric and eccentric (Grp ECCON; n= 10) or 12 concentric (Grp CON; n= 8) quadriceps muscle actions. Concentric and eccentric peak torque at various constant angular velocities and three repetition maximum half-squat and vertical jump height were measured. Grp ECCON showed greater (P < 0.05) overall increase in peak torque after training and detraining than Grp CON. Thus, concentric peak torque (0.52 rad s^-1) increased more (P < 0.05) over the experimental period in Grp ECCON and increases in eccentric peak torque were preserved in Grp ECCON only. Increases in peak torque in response to training were greater (P < 0.05) at 0.52 than at 2.62 rad s^-1. Alterations in the torque-velocity patterns induced by training remained after detraining in Grp ECCON but not in Grp CON. The retained increases (P < 0.05) in half-squat were 12 and 18% in Grps CON and ECCON, respectively. Neither group showed increased vertical jump height after detraining. This study showed greater preservation of concentric and eccentric peak torque after detraining following coupled concentric and eccentric than concentric resistance training. Only the former regime induced a change in the shape of torque-velocity curves that was manifest after detraining. These results suggest that the performance of eccentric muscle actions is critical to optimize increases in muscular strength in response to heavy resistance training, because it probably induce greater and more long-lived neural adaptations than the performance of concentric actions.     

Muscle performance,morphology and metabolic capacity during strength training and detraining: A one leg model

Summary To investigate biochemical, histochemical and contractile properties associated with strength training and detraining, six adult males were studied during and after 10 weeks of dynamic strength training for the quadriceps muscle group of one leg, as well as during and after a subsequent 12 weeks of detraining. Peak torque outputs at the velocities tested (0–270·s^–1) were increased (p<0.05) by 39–60% and 12–37% after training for the trained and untrained legs, respectively. No significant changes in peak torques were observed in six control subjects tested at the same times. Significant decreases in strength performance of the trained leg (16–21%) and untrained leg (10–15%) were observed only after 12 weeks of detraining. Training resulted in an increase (p0.05) in the area of FTa (21%) and FTb (18%) fibres, while detraining was associated with a 12% decrease in FTb fibre cross-sectional area. However, fibre area changes were only noted in the trained leg. Neither training nor detaining had any significant effect on the specific activity of magnesium-activated myofibrillar ATPase or on the activities of enzymes of phosphagen, glycolytic or oxidative metabolism in serial muscle biopsy samples from both legs. In the absence of any changes in muscle enzyme activities and with only modest changes in FT fibre areas in the trained leg, the significant alterations in peak torque outputs with both legs suggest that neural adaptations play a prominent role in strength performance with training and detraining.This study was funded by a grant from the Natural Sciences and Engineering Research Council of Canada     

Skin microvascular reactivity in trained adolescents

Whilst endothelial dysfunction is associated with a sedentary lifestyle, enhanced endothelial function has been documented in the skin of trained individuals. The purpose of this study was to investigate whether highly trained adolescent males possess enhanced skin microvascular endothelial function compared to their untrained peers. Seventeen highly and predominantly soccer trained boys ( [(V)\dot]\textO_2 \textpeak \dot{V}{\text{O}}_{{2\,{\text{peak}}}} : 55 ± 6 mL kg⁻¹ min⁻¹) and nine age- and maturation-matched untrained controls ( [(V)\dot]\textO_2 \textpeak \dot{V}{\text{O}}_{{2\,{\text{peak}}}} : 43 ± 5 mL kg⁻¹ min⁻¹) aged 13–15 years had skin microvascular endothelial function assessed using laser Doppler flowmetry. Baseline and maximal thermally stimulated skin blood flow (SkBF) responses were higher in forearms of trained subjects compared to untrained participants [baseline SkBF: 11 ± 4 vs. 9 ± 3 perfusion units (PU), p < 0.05; SkBF_max: 282 ± 120 vs. 204 ± 68 PU, p < 0.05]. Similarly, cutaneous vascular conductance (CVC) during local heating was superior in the forearm skin of trained versus untrained individuals (CVC_max: 3 ± 1 vs. 2 ± 1 PU mmHg⁻¹, p < 0.05). Peak hyperaemia following arterial occlusion and area under the reactive hyperaemia curve were also greater in forearm skin of the trained group (peak hyperaemia: 51 ± 21 vs. 35 ± 15 PU, p < 0.05; area under curve: 1596 ± 739 vs. 962 ± 796 PUs, p < 0.05). These results suggest that chronic exercise training in adolescents is associated with enhanced microvascular endothelial vasodilation in non-glabrous skin.     

The effect of eccentric training at different velocities on cross-education

The purpose of this study was to determine whether cross-education, defined as the increase in strength of an untrained limb after training of the contralateral homologous limb, is specific to low and high velocity eccentric training. Twenty-six subjects were randomized into two groups (n=13 each) that performed unilateral eccentric training of the elbow flexors on an isokinetic dynamometer at velocities of either 30° s^-1 (0.52 rad s^-1) or 180° s^-1 (3.14 rad s^-1 ). Subjects trained three times per week for 8 weeks. Ten subjects served as controls and did not train. Subjects were tested before and after training for peak torque of the elbow flexors during eccentric and concentric contractions at 30° s^-1 and 180° s^-1 . Eccentric peak torque at the velocity of 180° s^-1 in the untrained arm increased only for the group that trained at that velocity (P<0.05). There were no other changes in untrained arms for any of the groups at velocities of 30° s^-1 or 180° s^-1. For the trained arm, the increase in eccentric torque (pooled over velocities) was greatest for the group training at 180° s^-1, whereas the increase in concentric torque was similar for the groups training at 30° s^-1 and 180° s^-1. For the trained arm, there was no specificity for velocity or contraction type. We conclude that cross-education was specific to contraction type and velocity when fast (but not slow) eccentric contractions were used during training; whereas there was no specificity of training in the trained arm.     

Hypertrophy with unilateral resistance exercise occurs without increases in endogenous anabolic hormone concentration

We aimed to gain insight into the role that the transitory increases in anabolic hormones play in muscle hypertrophy with unilateral resistance training. Ten healthy young male subjects (21.8 ± 0.4 years, 1.78 ± 0.04 m, 75.6 ± 2.9 kg; mean ± SE) engaged in unilateral resistance training for 8 week (3 days/week). Exercises were knee extension and leg press performed at 80–90% of the subject’s single repetition maximum (1RM). Blood samples were collected in the acute period before and after the first training bout and following the last training bout and analyzed for total testosterone, free-testosterone, luteinizing hormone, sex hormone binding globulin, growth hormone, cortisol, and insulin-like growth factor-1. Thigh muscle cross sectional area (CSA) and muscle fibre CSA by biopsy (vastus lateralis) were measured pre- and post-training. Acutely, no changes in systemic hormone concentrations were observed in the 90 min period following exercise and there was no influence of training on these results. Training-induced increases were observed in type IIx and IIa muscle fibre CSA of 22 ± 3 and 13 ± 2% (both P < 0.001). No changes were observed in fibre CSA in the untrained leg (all P > 0.5). Whole muscle CSA increased by 5.4 ± 0.9% in the trained leg (P < 0.001) and remained unchanged in the untrained leg (P = 0.76). Isotonic 1RM increased in the trained leg for leg press and for knee extension (P < 0.001). No changes were seen in the untrained leg. In conclusion, unilateral training induced local muscle hypertrophy only in the exercised limb, which occurred in the absence of changes in systemic hormones that ostensibly play a role in muscle hypertrophy.     

The effects of eccentric and concentric training at different velocities on muscle hypertrophy

The purpose of this study was to examine the effect of isokinetic eccentric (ECC) and concentric (CON) training at two velocities [fast, 180° s⁻¹ (3.14 rad s⁻¹) and slow,30° s⁻¹(0.52 rad s⁻¹)] on muscle hypertrophy. Twenty-four untrained volunteers (age 18–36 years) participated in fast- (n=13) or slow- (n=11) velocity training, where they trained one arm eccentrically for 8 weeks followed by CON training of the opposite arm for 8 weeks. Ten subjects served as controls (CNT). Subjects were tested before and after training for elbow flexor muscle thickness by sonography and isokinetic strength (Biodex). Overall, ECC training resulted in greater hypertrophy than CON training (P<0.01). No significant strength or hypertrophy changes occurred in the CNT group. ECC (180° s⁻¹) training resulted in greater hypertrophy than CON (180° s⁻¹) training and CON (30° s⁻¹) training (P<0.01). ECC (30° s⁻¹) training resulted in greater hypertrophy than CON (180° s⁻¹) training (P<0.05), but not CON (30° s⁻¹) training. ECC (180° s⁻¹) training resulted in the greatest increases in strength (P<0.01). We conclude that ECC fast training is the most effective for muscle hypertrophy and strength gain.     

The effect of strength training and short-term detraining on maximum force and the rate of force development of older men

This study examined the effect of strength training (ST) and short-term detraining on maximum force and rate of force development (RFD) in previously sedentary, healthy older men. Twenty-four older men (70–80 years) were randomly assigned to a ST group (n = 12) and C group (control, n = 12). Training consisted of three sets of six to ten repetitions on an incline squat at 70–90% of one repetition maximum three times per week for 16 weeks followed by 4 weeks of detraining. Regional muscle mass was assessed before and after training by dual-energy X-ray absorptiometry. Training increased RFD, maximum bilateral isometric force, and force in 500 ms, upper leg muscle mass and strength above pre-training values (14, 25, 22, 7, 90%, respectively; P < 0.05). After 4 weeks detraining all neuromuscular variables were significantly (P < 0.05) lower than after 16 weeks training but remained significantly (P < 0.05) higher than pre-training levels except for RFD which had returned to pre-training levels. These findings demonstrate that high-intensity ST can improve maximum force and RFD of older men. However, older individuals may lose some neuromuscular performance after a period of short-term detraining and that resistance exercise should be performed on a regular basis to maintain training adaptations.     

Exercise training-induced enhancement in myocardial mechanics is lost after 2 weeks of detraining in rats

Exercise training is assumed to improve myocardial function; however, the role of detraining and its effect on myocardial parameters are still unclear. The aim of the present study was to evaluate the effect of detraining on ventricular remodeling and myocardial mechanical parameters after an 8 week (5 days/week, 60 min/day) swimming training period. Forty-three female Wistar rats were distributed into six groups: trained (T, n = 9), detrained 2 weeks (D2, n = 8), detrained 4 weeks (D4, n = 8) and their respective controls: untrained (U, n = 5), untrained 2 weeks (U2, n = 5) and untrained 4 weeks (U4, n = 5). Detrained rats underwent training and then remained sedentary (i.e., “detraining”) for 2 or 4 weeks. After training, the T group demonstrated increased physical capacity, left ventricular (LV) posterior wall thickness, and LV end-diastolic diameter, along with decreased heart rate, as evaluated by echocardiogram. In addition, the inotropism and lusitropism parameters studied on papillary muscles showed improvement in the T group (P < 0.05). However, after just 2 weeks of detraining, all parameters regressed back to values which were similar to those of the untrained groups. In conclusion, our results confirmed that exercise training is capable of inducing myocardial remodeling and improving contractile performance; however, these changes are completely lost after a short period of detraining.     

Pulmonary oxygen uptake and muscle deoxygenation kinetics during recovery in trained and untrained male adolescents

Previous studies have demonstrated faster pulmonary oxygen uptake ( [(V)\dot]\textO₂ \dot{V}{\text{O}}_{2} ) kinetics in the trained state during the transition to and from moderate-intensity exercise in adults. Whilst a similar effect of training status has previously been observed during the on-transition in adolescents, whether this is also observed during recovery from exercise is presently unknown. The aim of the present study was therefore to examine [(V)\dot]\textO₂ \dot{V}{\text{O}}_{2} kinetics in trained and untrained male adolescents during recovery from moderate-intensity exercise. 15 trained (15 ± 0.8 years, [(V)\dot]\textO_2max \dot{V}{\text{O}}_{2\max} 54.9 ± 6.4 mL kg⁻¹ min⁻¹) and 8 untrained (15 ± 0.5 years, [(V)\dot]\textO_2max \dot{V}{\text{O}}_{2\max } 44.0 ± 4.6 mL kg⁻¹ min⁻¹) male adolescents performed two 6-min exercise off-transitions to 10 W from a preceding “baseline” of exercise at a workload equivalent to 80% lactate threshold; [(V)\dot]\textO₂ \dot{V}{\text{O}}_{2} (breath-by-breath) and muscle deoxyhaemoglobin (near-infrared spectroscopy) were measured continuously. The time constant of the fundamental phase of [(V)\dot]\textO₂ \dot{V}{\text{O}}_{2} off-kinetics was not different between trained and untrained (trained 27.8 ± 5.9 s vs. untrained 28.9 ± 7.6 s, P = 0.71). However, the time constant (trained 17.0 ± 7.5 s vs. untrained 32 ± 11 s, P < 0.01) and mean response time (trained 24.2 ± 9.2 s vs. untrained 34 ± 13 s, P = 0.05) of muscle deoxyhaemoglobin off-kinetics was faster in the trained subjects compared to the untrained subjects. [(V)\dot]\textO₂ \dot{V}{\text{O}}_{2} kinetics was unaffected by training status; the faster muscle deoxyhaemoglobin kinetics in the trained subjects thus indicates slower blood flow kinetics during recovery from exercise compared to the untrained subjects.     

Muscle damage protection by low-intensity eccentric contractions remains for 2 weeks but not 3 weeks

This study investigated the hypothesis that the protective effect conferred by a low-intensity eccentric exercise against maximal eccentric exercise would not last more than a week. Untrained men (21.3 ± 1.6 years) were allocated into either a control or one of four repeated bout groups (n = 13 per group). The repeated bout groups performed 30 low-intensity eccentric contractions (ECC) of the elbow flexors with a dumbbell set at 10% of maximal isometric strength (10%-ECC) either 2 days, 7 days (1 week), 14 days (2 weeks) or 21 days (3 weeks) before 30 maximal eccentric contractions (Max-ECC). The control group performed Max-ECC only. Changes in maximal voluntary contraction strength, optimum angle, range of motion, upper arm circumference, muscle soreness, plasma creatine kinase activity and myoglobin concentration, and ultrasound echo-intensity following 10%-ECC were analysed by a one-way repeated measures ANOVA. Changes in the variables following Max-ECC were compared among the groups by a two-way repeated measures ANOVA. The 10%-ECC did not change any variables, showing no indication of muscle damage. The changes in all variables following Max-ECC were smaller (P < 0.05) for 2-day, 1- and 2-week groups than control group, without significant differences between 2-day and 1-week groups. The 2-week group showed greater (P < 0.05) changes in all variables compared with 2-day and 1-week groups. Changes in the variables were similar between 3-week and control groups, except for muscle soreness showing smaller (P < 0.05) changes for 3-week group. These results suggest that non-damaging eccentric exercise confers a protective effect against Max-Ecc, but the effect is attenuated between 1 and 2 weeks.     

Effects of endurance training on oxidative capacity and structural composition of human arm and leg muscles

Six healthy subjects performed endurance training of the same duration with legs and arms consecutively. Performance and muscle structure were measured before and after training in lower and upper limbs. Training induced similar increases in maximal oxygen consumption (6 ± 1 vs. 7 ± 2 mL min^?1 kg^?1: legs vs. arms, P > 0.05) and mitochondrial volume in leg and arm muscles (42 ± 12 vs. 31 ± 11%: legs vs. arms, P > 0.05). The gain in mitochondrial volume after training was achieved solely by increasing the fraction of mitochondria (+40 ± 11%, P < 0.05) in the same muscle volume (+2 ± 2%, P > 0.05) in the legs. In contrast, increased muscle volume (+14 ± 3%, P < 0.05), in addition to a tendency for an increase in mitochondrial fraction (+16 ± 11%, P > 0.05), occurred in the arms after training. Thus, similar improvements in muscle oxidative capacity in upper and lower limbs were brought about by different mechanisms. It is suggested that due to infrequent use and a lack of load-bearing function, arm muscle volume is underdeveloped in untrained, sedentary or detrained/injured subjects and that the mode of endurance training used in this study is sufficient to enlarge arm muscle volume as well as aerobic capacity.     

Muscle damage responses of the elbow flexors to four maximal eccentric exercise bouts performed every 4 weeks

Since little is known about the repeated bout effect of more than two eccentric exercise bouts, this study compared muscle damage responses among four exercise bouts. Fifteen young (21.8 ± 1.9 years) men performed four bouts of 30 maximal isokinetic eccentric contractions of the elbow flexors every 4 weeks. Maximal voluntary elbow flexion isometric and concentric strength, range of motion at the elbow joint (ROM), upper arm circumference, blood markers of muscle damage, and muscle soreness were measured before and up to 120 h following each bout. Changes in all measures following the second to fourth bouts were significantly (P < 0.05) smaller than those after the first bout. The decreases in strength and ROM immediately after the fourth bout were significantly (P < 0.05) smaller than other bouts. It is concluded that the first bout confers the greatest adaptation, but further adaptation is induced when the exercise is repeated more than three times.     

Cross education of muscular strength during unilateral resistance training and detraining

Abstract. The purpose of the present study was to examine the changes in maximum voluntary isometric contraction (MVC) in the contralateral untrained limb during unilateral resistance training and detraining, and to examine the factors inducing these changes by means of electrophysiological techniques. Nine healthy males trained their plantar flexor muscles unilaterally 4 days·week^–1 for 6 weeks using 3 sets of 10–12 repetitions at 70–75% of one-repetition maximum a day, and detrained for 6 weeks. Progressive unilateral resistance training significantly (P<0.05) increased MVC, integrated electromyogram (iEMG), and voluntary activation in the trained and contralateral untrained limbs. The changes in MVC after training were significantly correlated with the changes in iEMG in both limbs. No significant changes occurred in MVC, voluntary activation, and iEMG in the contralateral limb after detraining. The changes in MVC after detraining did not correlate with the changes in voluntary activation or iEMG in either limb. Training and detraining did not alter twitch and tetanic peak torques in either limb. These results suggest that the mechanisms underlying cross education of muscular strength may be explained by central neural factors during training, but not solely so during detraining. Electronic Publication     

Muscle triacylglycerol and hormone-sensitive lipase activity in untrained and trained human muscles

During exercise, triacylglycerol (TG) is recruited in skeletal muscles. We hypothesized that both muscle hormone-sensitive lipase (HSL) activity and TG recruitment would be higher in trained than in untrained subjects in response to prolonged exercise. Healthy male subjects (26 ± 1 years, body moss index 23.3 ± 0.5 kg m⁻²), either untrained (N = 8, VO_2max 3.8 ± 0.2 l min⁻¹) or trained (N = 8, VO_2max 5.1 ± 0.1 l min⁻¹), were studied. Before and after 3-h exercise (58 ± 1% VO_2max), a biopsy was taken. Muscle citrate synthase (32 ± 2 vs. 47 ± 6 μmol g⁻¹ min⁻¹ d.w.) and β-hydroxy-acyl-CoA-dehydrogenase (38 ± 3 vs. 52 ± 5 μmol g⁻¹ min⁻¹ d.w.) activities were lower in untrained than in trained subjects (p < 0.05). Throughout the exercise, fat oxidation was higher in trained than in untrained subjects (p < 0.05). Muscle HSL activity was similar at rest (0.72 ± 0.08 and 0.74 ± 0.03 mU mg⁻¹ protein) and after exercise (0.71 ± 0.1 and 0.68 ± 0.03 mU mg⁻¹ protein) in untrained and trained subjects. At rest, the chemically determined muscle TG content (37 ± 8 and 26 ± 5 mmol g⁻¹ d.w.) was similar (p > 0.05), and after exercise it was unchanged in untrained and lower (p < 0.05) in trained subjects (41 ± 9 and 10 ± 2 mmol g⁽¹ d.w.). Determined histochemically, TG was decreased (p < 0.05) after exercise in type I and II fibres. Depletion of TG was not different between fibre types in untrained, but tended to be higher (p = 0.07) in type I compared with type II fibres in trained muscles. In conclusion, HSL activity is similar in untrained and trained skeletal muscles both before and after prolonged exercise. However, the tendency to higher muscle TG recruitment during exercise in the trained subjects suggests a difference in the regulation of HSL or other lipases during exercise in trained compared with untrained subjects.