Neural adaptations underlying cross-education after unilateral strength training

The purpose of this study was to investigate the effects of 4-week (16 sessions) unilateral, maximal isometric strength training on contralateral neural adaptations. Subjects were randomised to a strength training group (TG, n = 15) or to a control group (CG, n = 11). Both legs of both groups were tested for plantar flexion maximum voluntary isometric contractions (MVCs), surface electromyogram (EMG), H-reflexes and V-waves in the soleus (SOL) and gastrocnemius medialis (GM) superimposed during MVC and normalised by the M-wave (EMG/M_SUP, H_SUP/M_SUP, V/M_SUP, respectively), before and after the training period. For the untrained leg, the TG increased compared to the CG for MVC torque (33%, P < 0.01), SOL EMG/M_SUP (32%, P < 0.05) and SOL V/M_SUP (24%, P < 0.05). For the trained leg, the TG increased compared to the CG for MVC torque (40%, P < 0.01), EMG/M_SUP (SOL: 38%, P < 0.05; GM: 60%, P < 0.05) and SOL V/M_SUP (72%, P < 0.01). H_SUP/M_SUP remained unchanged for both limbs. No changes occurred in the CG. These results reinforce the concept that enhanced neural drive to the contralateral agonist muscles contributes to cross-education of strength.     

Changes in muscle function in response to 10 days of lower limb unloading in humans

Force-generating capacity and electromyographic (EMG) activity of the knee extensor muscles were studied before and after short-term (10 d) unilateral lower limb unloading and during 4 days of recovery. Ten healthy males used crutches to prevent one of their lower limbs from weight-bearing while maintaining joint mobility as well as daily ambulatory activities. Knee extensor torque and quadriceps rectified EMG during maximal voluntary isometric contraction (MVC) was measured repeatedly before and after the intervention. Also, EMG at a fixed submaximal level (100 Nm; 30–45% MVC) and maximal angular velocity (AV_max), during unresisted knee extension, were assessed. Maximum torque decreased (P<0.05) by 13±8% in response to unloading while maximum EMG activity did not change after unloading or during recovery (P=0.35). Submaximum EMG increased (P<0.05) by 25±16% after unloading. Maximum and submaximum torque/EMG ratios decreased (P<0.05) after unloading. AV_max decreased (P<0.05) by 9±8% after unloading. The post value, however, was not different from that of the weight-bearing limb. Torque, EMG and AV_max were recovered (P>0.05) after 4 days of resumed weight-bearing. The pronounced decrease and the rapid recovery in maximum torque appears not to be attributed to a change in muscle mass alone. Because the findings of unchanged maximum EMG and increased EMG at a submaximal force level suggest no change in neural drive, we propose that unspecific tissue factors in part impair muscle function in response to short-term loss of weight-bearing activity. Results also indicate that recovery in muscle function after short-term unloading seems to be completed in a time span shorter than the period of preceding inactivity.     

Neural drive preservation after detraining following neuromuscular electrical stimulation training

The purpose of the study was to investigate the behaviour of the central nervous system when 5 weeks of neuromuscular electrical stimulation (NMES) training was followed by 5 weeks of detraining. Nineteen males were divided into the neuromuscular electrostimulated group (EG, n = 12) and the control group (CG, n = 7). The training program consisted of 15 sessions of isometric NMES over a 5-week period. The EG subjects were tested before training (PRE), after 5 weeks of NMES training (POST) and after 5 weeks of detraining (DE) while CG subjects were only tested at PRE and at POST. Soleus (SOL) and gastrocnemii (GAS) maximal H-reflex and M-wave potentials were evoked at rest (i.e., H_max and M_max, respectively) and during maximal voluntary contraction (MVC) (i.e., H_sup and M_sup, respectively). SOL and GAS V-wave were recorded by supramaximal stimulation delivered during MVC. SOL and GAS electromyographic (EMG) activity as well as muscle activation were also assessed during MVC. After training, plantar flexor MVC increased significantly by 22% (P < 0.001). Torque gains were associated with an increase in muscle activation (P < 0.05), SOL and GAS normalized EMG activity (P < 0.01 and P < 0.05, respectively) and V/M_sup ratios (P < 0.01 and P < 0.05, respectively). No significant changes occurred in any of these parameters between POST and DE. H_max/M_max and H_sup/M_sup ratios for both muscles were unchanged after both the training and detraining periods. In conclusion, the NMES training-induced neural adaptations were maintained after detraining, suggesting that neural changes are long-lasting and did not affect the elements of H-reflex pathways.     

Central fatigue during a long-lasting submaximal contraction of the triceps surae

Our purpose was to study central fatigue and its dependence on peripheral reflex inhibition during a sustained submaximal contraction of the triceps surae. In 11 healthy subjects, superimposed twitches, surface electromyograms (EMG) from the medial head of the gastrocnemius (MG) and soleus (SOL) muscles, maximal compound motor action potentials (M_max), tracking error and tremor were recorded during sustained fatiguing contractions at a torque level corresponding to 30% of maximal voluntary contraction (MVC). When the endurance limit (401±91 s) of the voluntary contraction (VC-I) was reached, the triceps surae could be electrically stimulated to the same torque level for an additional 1 min in 10 of the 11 subjects. These subjects were then able to continue the contraction voluntarily (voluntary contraction II, VC-II) for another 85±48 s. At the endurance limit of VC-I, the superimposed twitch was larger than during the unfatigued MVC, while there was no significant difference between the twitch at the endurance limit of VC-II and MVC. The EMG amplitude of both MG and SOL at the endurance limit of VC-I was significantly less than that during the MVC. While the EMG amplitude of MG increased further during VC-II, SOL EMG remained unchanged, neither muscle reaching their unfatigued MVC values. This difference was diminished for SOL by taking into account its decrease in M_max found during VC-II, and relative EMG levels approached their MVC values. These results clearly indicate that a higher voluntary muscle activation was achievable after 1 min of electrical muscle stimulation, which continued metabolic stress and contractile fatigue processes but allowed for supraspinal, muscle spindle and/or motoneuronal recovery. It is concluded that peripheral reflex inhibition of -motoneurons via small-diameter muscle afferents is of minor significance for the development of the central fatigue that was found to occur during the first voluntary contraction.     

Increased spinal excitability does not offset central activation failure

We hypothesized that if reduced spinal excitability contributes to central activation failure, then a caffeine-induced increase in spinal excitability would enhance postfatigue maximal voluntary activation and maximal voluntary contraction (MVC). Ten male volunteer subjects attended two laboratory sessions separated by at least 1 week. Contractile and electrical properties were assessed before, and 1 h after oral administration of caffeine (6 mg/kg) or placebo (all-purpose flour), and again following a fatigue protocol. The slope of the H reflex recruitment curve, normalized to that of the M wave (H _slp/M _slp), was used to estimate spinal excitability. Maximal voluntary activation was assessed using maximal EMG (EMG_max) and twitch interpolation. Postfatigue, MVC torque declined (P<0.05) to 75.2±12.7 and 70.2±9.3% of the prefatigue values in the placebo (PL) and caffeine (CF) trials, respectively, and remained depressed throughout the recovery period. This was accompanied by a decline in % activation (P<0.05) from 99.6±0.3% (PL) and 99.8±0.3% (CF) to 94.8±3.5% (PL) and 95.3±5.0% (CF), indicating the presence of central activation failure. Caffeine offset the decline in H _slp/M _slp observed in the placebo trial (P<0.05), but it did not prevent the decline in maximal voluntary activation or MVC torque. Furthermore, although the decline in spinal excitability was correlated to the decline in EMG_max (r=0.55, P<0.05) it was not correlated with the decline in % activation or MVC torque. Thus a fatigue-induced decline in spinal excitability did not limit maximal activation.This study was supported by a Reebok Research Grant on Human Performance and Injury Prevention from the American College of Sports Medicine Foundation and a NSERC PGS-B scholarship to J. Kalmar as well as NSERC grant A-6655 to E. Cafarelli.     

Changes in the soleus muscle architecture after exhausting stretch-shortening cycle exercise in humans

This study focused on the architectural changes in the muscle–tendon complex during the immediate and secondary (delayed) reductions of performance (bimodal recovery) caused by an exhaustive rebound type stretch-shortening cycle (SSC) exercise. The isometric plantar flexor torque during maximum voluntary contraction (MVC) was measured together with recording of electromyography (EMG) and ultrasonography from the soleus muscle before (BEF), after (AFT), 2 h (2H), 2 and 8 days (2D, 8D) after the SSC exercise (n = 8). The performance variables (MVC torque and EMG activation) followed the bimodal recovery patterns. This was not the case in the changes of the fascicle length and muscle thickness. The relative torque changes in MVC correlated positively (R = 0.78, P = 0.02) to the corresponding averaged EMG changes between BEF and 2H (BEF → 2H); the significance disappeared in the comparison between 2H and 2D (2H → 2D), during which period MVC showed a secondary reduction. The relative torque changes in MVC showed no correlation with the changes in muscle thickness between BEF–2H. However, this correlation between 2H–2D was negative (R = –0.85, P < 0.01). The fascicle shortening/average EMG ratio in MVC increased at 2H, and then decreased more at 2D than 2H (P < 0.05). Thus, the secondary performance decline was not related to the corresponding EMG reduction but to the increased muscle thickness, which peaked at 2D. The results suggest clearly that the secondary decline in MVC could be related to the increase in muscle volume.     

Muscle activation during maximal voluntary eccentric and concentric knee extension

Summary The aim of this investigation was to study the relationships among movement velocity, torque output and electromyographic (EMG) activity of the knee extensor muscles under eccentric and concentric loading. Fourteen male subjects performed maximal voluntary eccentric and concentric constant-velocity knee extensions at 45, 90, 180 and 360° · s^–1. Myoelectric signals were recorded, using surface electrodes, from the vastus medialis, vastus lateralis and rectus femoris muscles. For comparison, torque and full-wave rectified EMG signals were amplitude-averaged through the central half (30°–70°) of the range of motion. For each test velocity, eccentric torque was greater than concentric torque (range of mean differences: 20%–146%,P < 0.05). In contrast, EMG activity for all muscles was lower under eccentric loading than velocity-matched concentric loading (7%–31%,P < 0.05). Neither torque output nor EMG activity for the three muscles changed across eccentric test velocities (P > 0.05). While concentric torque increased with decreasing velocity, EMG activity for all muscles decreased with decreasing velocity (P < 0.05). These data suggest that under certain high-tension loading conditions (especially during eccentric muscle actions), the neural drive to the agonist muscles was reduced, despite maximal voluntary effort. This may protect the musculoskeletal system from an injury that could result if the muscle was to become fully activated under these conditions.     

Influence of muscle architecture on the torque and power–velocity characteristics of young and elderly men

This study investigated the contribution of muscle architecture to the differences in the torque–velocity and power–velocity relationships between older (OM n = 9, aged 69–82 years) and younger men (YM n = 15, aged 19–35 years). Plantarflexors’ (PF) maximal isometric and concentric torques were recorded at 0.87, 1.75, 2.62, 3.49 and 4.36 rad s⁻¹. Physiological cross-sectional area (PCSA) was calculated as the ratio of muscle volume (determined by magnetic resonance imaging) to muscle fascicle length (L _f, measured by ultrasonography). GM PCSA and L _f of the OM were, respectively, 14.3% (P < 0.05) and 19.3% (P < 0.05) smaller than of the YM. In the OM, GM maximum isometric torque and maximum contraction velocity (V _max), estimated from Hill’s equation were, respectively, 48.5 and 38.2% lower (P < 0.001) than in the YM. At all contraction velocities, the OM produced less torque than the YM (46.3% of YM at 0.87 rad s⁻¹ to 14.7% at 4.36 rad s⁻¹, P < 0.001). Peak power (PP) of the OM was 80% lower than that of the YM and normalisation of PP to muscle volume only reduced this difference by 10%. Normalisation of torque to PCSA reduced, but did not eliminate, differences in torque between YM and OM (9.6%) and differences in torque/PCSA increased with contraction velocity (P < 0.05). After normalisation of velocity to L _f, the difference in V _max between the OM and the YM was reduced to 15.9%. Thus, although muscle architecture contributes significantly to the differences in the torque– and power–velocity properties of OM and YM, other contractile factors, intrinsic to the muscle, seem to play a role. It is noteworthy that the deficit in PP between OM and YM is far greater than that of muscle torque, even after normalisation of PP to muscle volume. This finding likely plays an important role in the loss of mobility in old age.     

Early structural adaptations to unloading in the human calf muscles

Aim: The present study investigated the influence of muscle architectural changes on muscle torque during 3‐week unilateral lower limb suspension (ULLS). Methods: Plantarflexion maximal voluntary contraction (MVC), soleus (SOL), gastrocnemius medialis (GM) and lateralis (GL) muscle volume (VOL), GL fascicle length (L_f) and pennation angle (θ), physiological cross‐sectional area (PCSA), and electromyographic (EMG) activity were assessed in eight healthy men (aged 19 ± 0 years) after days 14 and 23 of ULLS. Results: After 14 day of ULLS, MVC and SOL EMG decreased (P < 0.05) by 10% and 29%, respectively, but did not further decline between days 14 and 23. SOL, GM and GL muscle VOL decreased by 5%, 6% and 5%, respectively (P < 0.05), on day 14, and by 7% (SOL), 10% (GM) and 6% (GL) on day 23. In GL, θ and L_f were reduced by 3% (P < 0.05) and 2% (NS), respectively, on day 14, and by 5% (P < 0.05) and 4% (P < 0.05), respectively, on day 23. Consequently, GL PCSA declined by 3% (P < 0.05) on day 14, but did not further decrease on day 23. Similarly, the 7% (P < 0.05) loss in GL force/PCSA observed on day 14 persisted until the end of the unloading period. Conclusion: These findings suggest that rapid muscle architecture remodelling occurs with lower limb unloading in humans, with changes occurring within 14 days of weight bearing removal. These adaptations, mitigating the decline in muscle PCSA, might protect from a larger loss of muscle force.     

Synergists activation pattern of the quadriceps muscle differs when performing sustained isometric contractions with different EMG biofeedback

The aims of the present study were to examine (1) endurance time and (2) activation pattern of vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF) muscles during fatiguing isometric knee extensions performed with different EMG biofeedbacks. Thirteen men (27 ± 5 year) volunteered to participate in three experimental sessions. Each session involved a submaximal isometric contraction held until failure at an EMG level corresponding to 40% maximal voluntary contraction torque (MVC), with visual EMG biofeedback provided for either (1) RF muscle (RF task), (2) VL and VM muscles (Vasti task) or (3) the sum of the VL, VM and RF muscles (Quadriceps task). EMG activity of VL, VM and RF muscles was recorded during each of the three tasks and further analyzed. Time to task failures and MVC loss (P < 0.001) after exercises were similar (P > 0.05) between the three sessions (182 s and ∼28%, respectively) (P > 0.05). Moreover, the magnitude of central and peripheral fatigue was not different at failure of the three tasks. Activation pattern was similar for knee extensors at the beginning of each task (P > 0.05). However, RF EMG activity decreased (P < 0.05) during the Vasti and the Quadriceps tasks (from ∼33 to ∼25% maximal EMG), whereas vasti EMG activity remained constant during the RF task (∼41% maximal EMG). These findings suggest that (1) task failure occurs when sustaining a submaximal level of EMG activity for as long as possible and (2) CNS is not able to differentiate descending drive to the different heads of the quadriceps at the beginning of a sustained contraction, despite a different activation pattern for the bi-articular RF muscle compared to the mono-articular vasti muscles during fatigue.     

Muscle activation of the knee extensors following high intensity endurance exercise in cyclists

This study was conducted to assess the effects in trained cyclists of exhausting endurance cycle exercise (CE) on maximal isometric force production, surface electromyogram (EMG) and activation deficit (AD) of the knee extensors. Ten male subjects made four isometric maximal voluntary contractions (MVC) of the knee extensor muscles immediately prior (pre), 10 min after (post) and 6 h after completion of CE. The CE consisted of 30 min of exercise on a stationary cycle ergometer at an intensity corresponding to 80% of maximal oxygen uptake (V˙O_2max) followed by four × 60-s periods at 120% of V˙O_2max. Two MVC were performed with recording of surface EMG from the knee extensors, whilst an additional two MVC were completed with percutaneous electrical muscle stimulation (EMS; 25 pulses at 100 Hz with the maximal tolerable current) superimposed over the maximal voluntary contraction force (MVF) but without EMG (to avoid interference). The MVF, integrated EMG (iEMG), and AD [calculated as the difference between MVF and the electrically stimulated force (ESF) during the EMS contractions] were statistically analysed. The MVF was significantly reduced (P < 0.05) post and 6-h post compared to pre-CE level. The iEMG was significantly reduced (P < 0.05) post and 6 h post CE. The ESF was also reduced, whilst AD was significantly increased (P < 0.05) post and 6-h post CE compared to the pre CE. These results suggest that the level of exercise stress administered in this study was sufficient to impair the central and peripheral mechanisms of force generation in knee extensors for a period of 6-h. Athletes engaged in concurrent training (strength and endurance) should consider this effect in exercise programming. Accepted: 22 September 1999     

Effect of skin temperature on the ion reabsorption capacity of sweat glands during exercise in humans

The effect of skin temperature on the ion reabsorption capacity of sweat glands during exercise in humans is unknown. In this study, eight healthy subjects performed a 60-min cycling exercise at a constant intensity (60% VO_2max) under moderate (25°C) and cool (15°C) ambient temperatures at a constant relative humidity of 40%. The sweating rate (SR), index of sweat ion concentration (ISIC) by using sweat conductivity, esophageal temperature (Tes), mean skin temperature, and heart rate (HR) were measured continuously under both ambient temperatures. The SR and ISIC were significantly lower at the cool ambient temperature versus the moderate temperature. There were no significant differences in the changes in HR and esophageal temperature between these ambient temperature conditions, while the mean skin temperature was significantly lower at the cool ambient temperature by almost 3°C (P<0.05). The slopes of the relationships between Tes and the SR and ISIC were significantly lower and the thresholds of these relationships were significantly higher at the cool ambient temperature (P<0.05). The ion reabsorption capacity of the sweat glands was significantly lower (P<0.05) in a cool environment (0.21±0.04 vs. 0.52±0.06 mg/cm²/min at 15 and 25°C, respectively) as evaluated using the relationships for SR and ISIC. The results suggest that the ion reabsorption capacity of the sweat glands is influenced by skin temperature during exercise in humans.     

Reduced plantarflexor specific torque in the elderly is associated with a lower activation capacity

Previous studies have reported a decrease in muscle torque per cross-sectional area in old age. This investigation aimed at determining the influence of agonists muscle activation and antagonists co-activation on the specific torque of the plantarflexors (PF) in recreationally active elderly males (EM) and, for comparison, in young men (YM). Twenty-one EM, aged 70–82 years, and 14 YM, aged 19–35 years, performed isometric maximum voluntary contractions (MVC). Activation was assessed by comparing the amplitude of interpolated supramaximal twitch doublets at MVC, with post-tetanic doublet peak torque. Co-activation of the tibialis anterior (TA) was evaluated as the ratio of TA-integrated EMG (IEMG) activity during PF MVC compared to TA IEMG during maximal voluntary dorsiflexion. Triceps surae muscle volume (VOL) was assessed using magnetic resonance imaging (MRI), and PF peak torque was normalised to VOL (PT/VOL) since the later approximates physiological cross-sectional area (CSA) more closely than anatomical CSA. Also, physical activity level, assessed by accelerometry, was significantly lower (21%) in the elderly males. In comparison to the YM group, a greater difference in PT (39%) than VOL (19%) was found in the EM group. PT/VOL and activation capacity were respectively lower by 25% and 21% in EM compared to YM, whereas co-activation was not significantly different. In EM PT/VOL correlated with activation (R²=0.31, P<0.01). In conclusion, a reduction in activation capacity may contribute significantly to the decline in specific torque in the plantar flexors of elderly males. The hypothesis is put forward that reduced physical activity is partialy responsible for the reduced activation capacity in the elderly.     

Electromyogram power spectrum during dynamic contractions at different intensities of exercise

Summary During dynamic contractions performed on a cycle ergometer, we studied the influence of motor unit (MU) recruitment on the electromyographic (EMG) spectral content by exerting mechanical power of different intensities, which was chosen to remain below the maximal aerobic power (VO_2max). The spectral parameters: EMG total power (P_EMG), mean (MPF) and median (MED) power frequencies, which are the most representative of the EMG spectral content, were calculated according to the EMG activity of the vastus medialis muscle (VM) and soleus muscle (SOL) of the right leg. For VM and SOL, PEMG increased linearly with exerted power demonstrating an enhancement of MU recruitment. Moreover these relationships were less scattered when exerted power was expressed as a percentage of VO_2max. Changes in MPF and MED with varying exercise intensities were different from one subject to another. For a set of subjects, MPF and MED were found to be independent of exerted power. Although VM and SOL muscles are different in fibre type composition, similar results were obtained for both EMG activities. We have concluded that for dynamic contractions performed at different intensities below VO_2max, the recruitment of the MU has a poor effect on the EMG spectral content whatever the predominant type of fibre.     

Morning to evening changes in the electrical and mechanical properties of human soleus motor units activated by H reflex and M wave

The aim of the present study was to compare the relative contribution of the soleus motor units (MUs) activated by H and M waves to the plantar-flexion torque in the morning and in the evening. Twelve healthy male subjects (physical education students) took part in this investigation. The electromechanical properties of the plantar flexor muscles were recorded at two different times of day: between 06:00 and 08:00 h and between 17:00 and 19:00 h. Plantar-flexion torque and concomitant electromyographic activity of soleus muscle were assessed under voluntary and evoked conditions. The results indicated a significant decrease in maximal voluntary muscle torque of triceps surae and associated soleus EMG in the evening as compared with the morning. The mean values of MVC ranged from 131.6±9.6 N m in the morning to 125.1±9.0 N m in the evening. Peak-to-peak values of soleus H _max and M _max potentials were comparable in the morning and in the evening (2.97 vs 3.18 mV and 7.95 vs 7.44 mV for H _max and M _max, respectively). The H _max/M _max ratio was not modified between the two experimental test sessions (34.8 vs 41.3%). The peak amplitude of the twitch produced by the H _max wave decreased significantly. When estimating the mechanical contribution to of the slowest and fastest-twitch MUs reflexively and directly activated, we observed that the contribution of the slowest MUs did not change while those of the fastest decreased significantly in the evening. To conclude, a weaker reflex twitch torque caused by higher fatigue state of the MUs directly activated by the M wave which accompanied H _max in the evening may be regarded as a possible explanation of the weaker plantar-flexion torque production in the evening.     

Passive hyperthermia reduces voluntary activation and isometric force production

It has been suggested that a critically high body core temperature may impair central neuromuscular activation and cause fatigue. We investigated the effects of passive hyperthermia on maximal isometric force production (MVC) and voluntary activation (VA) to determine the relative roles of skin (T_sk) and body core temperature (T_c) on these factors. Twenty-two males [O_2max=64.2 (8.9) ml kg^–1 min^–1, body fat=8.2 (3.9)%] were seated in a knee-extension myograph, then passively heated from 37.4 to 39.4°C rectal temperature (T_re) and then cooled back to 37.4^oC using a liquid conditioning garment. Voluntary strength and VA (interpolated twitch) were examined during an isometric 10-s MVC at 0.5°C intervals during both heating and cooling. Passive heating to a T_c of 39.4^oC reduced VA by 11 (11)% and MVC by 13 (18)% (P<0.05), but rapid skin cooling, with a concomitant reduction in cardiovascular strain [percentage heart rate reserve decreased from 64 (11)% to 29 (11)%] and psychophysical strain did not restore either of these measures to baseline. Only when cooling lowered T_c back to normal did VA and MVC return to baseline (P<0.05). We conclude that an elevated T_c reduces VA during isometric MVC, and neither T_sk nor cardiovascular or psychophysical strain modulates this response. Results are given as mean (SD) unless otherwise stated.     

Electrically evoked contractions of the triceps surae during and following 21 days of voluntary leg immobilization

Summary The effects of 21 days voluntary leg (plaster) immobilization on the mechanical properties of the triceps surae have been studied in 11 young female subjects, mean age 19.4 years. The results show that during the period of immobilization the mean time to peak tension (TPT) and half relaxation time (1/2RT) and tension (P_t) of the maximal twitch increased significantly (p<0.001) but the effects were short lived. Maximal tension and contraction times of the twitch recovered within 2–14 days following the removal of the plaster cast. The electrically evoked tetanic tensions at 10 Hz and 20 Hz did not change significantly (p>0.1) during immobilization, but the 50 Hz tetanic tension (P_°50) and maximal voluntary contraction (MVC) were reduced (p<0.05). The fall in P_°50 and MVC was associated with 10% decrease in the estimated muscle (plus bone) cross-sectional area. The relative (%) change in P_°50 and MVC following immobilization was related to the initial physiological status (as indicated by the response of the triceps surae to a standard fatigue test prior to immobilization) of the muscle. The rate of rise and recovery fall of the tetanus were slightly but significantly (p<0.01) reduced on day 7 of immobilization, but thereafter remained constant. The isokinetic properties of the triceps surae as reflected in the measured torque/velocity relation of the muscle in 4 subjects did not change significantly if account was taken of the slight degree of atrophy present following immobilization. It was concluded that short term voluntary leg immobilization produces atrophy and some loss of isometric twitch and tetanic function, but has little effect on the isokinetic properties of the triceps surae. The changes in the twitch characteristics during and immediately following immobilization may be indicative of a prolongation of the active state of the muscle.     

Isometric axial rotation of the human trunk from pre-rotated postures

The purpose of this investigation was to study the torque and electromyogram (EMG) in axial rotation from pre-rotated postures. A group of 50 young adults (27 men and 23 women) volunteered for the study. These prepared subjects carried out axial rotation with pre-rotated postures in the direction of pre-rotation and away from it. Torque and EMG were recorded bilaterally from latissimus dorsi, erector spinae at L₃ and T₁₀ levels, pectoralis, rectus abdominis, external and internal oblique. In 15° pre-rotated posture the axial rotation in the direction of pre-rotation reduced the torque by between 11% and 17% and away from it increased the torque by 12% to 16%. In 30° pre-rotated posture the decrement in torque in the direction of pre-rotation was 24%–33%, and in the opposite direction the gain was between 21% and 32%. Even with decreased torque with rotation in the pre-rotation direction the EMG increased up to 123%. The EMG magnitude and slopes of EMG in these activities demonstrated significant increases while in the opposite direction slight decreases were observed. The EMG of each muscle was significantly different from all other muscles (P<0.001). A significant (P<0.01) but low correlation between EMG and torque was obtained. Significant linear regressions between torque and EMG of different muscles were obtained (P<0.01; r=up to 0.70). Electronic Publication     

Effect of isokinetic cycling versus weight training on maximal power output and endurance performance in cycling

The aim of this study was to compare the effects of a weight training program for the leg extensors with isokinetic cycling training (80 rpm) on maximal power output and endurance performance. Both strength training interventions were incorporated twice a week in a similar endurance training program of 12 weeks. Eighteen trained male cyclists (VO_2peak 60 ± 1 ml kg⁻¹ min⁻¹) were grouped into the weight training (WT n = 9) or the isokinetic training group (IT n = 9) matched for training background and sprint power (P _max), assessed from five maximal sprints (5 s) on an isokinetic bicycle ergometer at cadences between 40 and 120 rpm. Crank torque was measured (1 kHz) to determine the torque distribution during pedaling. Endurance performance was evaluated by measuring power, heart rate and lactate during a graded exercise test to exhaustion and a 30-min performance test. All tests were performed on subjects’ individual race bicycle. Knee extension torque was evaluated isometrically at 115° knee angle and dynamically at 200° s⁻¹ using an isokinetic dynamometer. P _max at 40 rpm increased in both the groups (~15%; P < 0.05). At 120 rpm, no improvement of P _max was found in the IT training group, which was possibly related to an observed change in crank torque at high cadences (P < 0.05). Both groups improved their power output in the 30-min performance test (P < 0.05). Isometric knee extension torque increased only in WT (P < 0.05). In conclusion, at low cadences, P _max improved in both training groups. However, in the IT training group, a disturbed pedaling technique compromises an improvement of P _max at high cadences.     

Changes in conduction velocity,median frequency,and root mean square-amplitude of the electromyogram during 25% maximal voluntary contraction of the triceps brachii muscle,to limit of endurance

Summary The objective of the present study was to investigate whether isometric contraction of the right triceps brachii muscle, of maximal duration and at 25% of the maximal voluntary contraction (MVC), would reduce mean fibre conduction velocity (CV) for the active motor units (MU). In addition to the cross-correlation of surface electromyograms (EMG) for CV determination, median frequency (f _m) and root-mean-square amplitude (rms-amplitude) were calculated. The initial 5 min of the recovery of the three parameters was also investigated. The MVC were performed before and after the sustained contraction. Seven males — six in their twenties and one aged 43 - participated in the investigation. Mean CV for the unfatigued muscle was 4.5 m·s^–1, SD 0.38. On average, CV decreased less than 10% during the sustained contraction (P<0.05). Thef _m decreased almost linearly (46%) during the endurance time, while three quarters of the 250% increase in rms-amplitude took place during the last 50% of the contraction (P<0.001, both parameters). The MVC was reduced by 39% immediately after exhaustion (P<0.05). During the 1st min of recovery the rms-amplitude decreased by 50%, and the fm increased from 54% to 82% of the initial value (bothP<0.05. No measurable simultaneous CV restitution occurred. A parallel 15% increase inf _m and CV took place during the last 4 min of recovery (bothP<0.001), while the amplitude remained constant. Since mean CV was essentially unchanged during the last 50% of the endurance time where large changes inf _m and rms-amplitude occurred, factors supplementary to CV probably caused the striking changes in fatigue EMG, notably —MU recruitment, synchronization of MU activity, and lowering of MU firing frequencies. Nevertheless, during the last 4 min of recovery the entire increase inf _m could be accounted for by the simultaneous increase in CV.