An in vivo analysis of the effect of transcutaneous electrical stimulation of the quadriceps and hamstrings on anterior cruciate ligament deformation

Transcutaneous electrical muscle stimulation (TEMS) has been advocated as a method to rehabilitate the postoperative ACL repaired/reconstructed lower extremity. Isolated quadriceps contraction can potentially disrupt the ACL repair/reconstruction; to minimize this risk simultaneous quadriceps and hamstring stimulation has been used. This study measured the in vivo deformation of the ACL during TEMS of the quadriceps and hamstrings. Six legs in four Rhesus monkeys were immobilized in 0 degrees, 45 degrees, and 90 degrees of flexion in neutral rotation using a Hoffman frame and pins placed through the proximal femur and distal tibia. The hamstrings and quadriceps muscles were stimulated with a dual channel electrical stimulator individually and simultaneously at each point of flexion, and ACL deformation was measured using a Hall effect device placed on the anterior medial fibers of the ACL. The following conclusions were made: 1) Isolated quadriceps contraction produces ACL elongation at 0 degrees and 45 degrees of knee flexion and produces ACL shortening at 90 degrees of knee flexion. 2) Isolated hamstrings contraction produces ACL shortening at 45 degrees and 90 degrees of knee flexion and negligible effects at full knee extension. 3) It is not possible to simultaneously contract the quadriceps and hamstrings using separate stimulator pads for each muscle group. 4) At 45 degrees of knee flexion when the quadriceps muscles are stimulated before the hamstring muscles and simultaneous contraction of both is then sustained, ACL lengthening occurs. 5) When the hamstring muscles are fired before the quadriceps muscles and simultaneous contraction of both is sustained, ACL shortening occurs.     

Outcome of sports injuries treated in a casualty department.

The present investigation analyses 2493 patients with a sports injury treated in a casualty department during a one-year period. Of the patients 73% were men, the age of the patients averaging 26 years and the mean follow-up time was 24 months. Soccer and indoor ball games caused 24% and 23% of the injuries respectively, these being followed by injuries in ice hockey in 14%. Track and field injuries scored low with 2% out of all injuries. Injuries to the lower extremity predominated. At follow-up, ligamentous injuries of the lower extremity were the major cause of discomfort. Further, in the group of patients with persistent discomfort 36% had suffered a fracture or a dislocation, 13% a contusion and 10% a wound. The mean period of sports incapacity after a sustained injury was 3 weeks. In track and field events the injury seldom disturbed training for more than one week, but in soccer, indoor ball games, skiing and skating the mean sports incapacity period varied between 6 and 3 weeks. Out of the total injured, 2% had to give up their sports activity completely. An injury of the lower extremity demanded on average 4 weeks' rest, an injury of the upper extremity and the trunk 2 weeks and injuries of the head and neck one week's rest. According to the present investigation sports injuries were in the majority of cases of a relatively benign nature and sick leave from work seldom exceeded 2 weeks.     

Evaluation of hamstring muscle strength and morphology after anterior cruciate ligament reconstruction

This study aimed to clarify the relationship between knee flexor strength and hamstring muscle morphology after anterior cruciate ligament (ACL) reconstruction using the semitendinosus (ST) tendon and to determine the causative factors of decreased knee flexor muscle strength. Fourteen male and ten female patients who resumed sports activities after surgery participated in the experiment. Isometric knee flexion torque was measured at 30°, 45°, 60°, 90°, and 105° of knee flexion. Magnetic resonance imaging (MRI) was used to calculate ST muscle length and hamstring muscle volume, and to confirm the status of ST tendon regeneration. The correlation between the MRI findings and flexor strength was analyzed. Regenerated ST tendon was confirmed in 21 of the 24 patients, but muscle volume (87.6%) and muscle length (74.5%) of the ST in the operated limb were significantly smaller than those in the normal limb. The percentage of the knee flexion torque of the operated limb compared with that of the normal was apparently lower at 105° (69.1%) and 90° (68.6%) than at 60° (84.4%). Tendon regeneration, ST muscle shortening, and ST muscle atrophy correlated with decreased knee flexion torque. These results indicated that preserving the morphology of the ST muscle‐tendon complex is important.     

Effect of the vastus medialis obliquus on the patellofemoral joint

In this study, the effect of dynamic stabilizers on the patellofemoral (PF) joint was investigated in normal volunteers (group I) and in patients with patellar pain (group II) or instability (group III) by using computed tomography (CT) analysis and integrated electromyography (iEMG) of the quadriceps muscle. Nine subjects (16 knees) from group I, 10 patients (12 knees) from group II and 8 patients (12 knees) from group III were included in the study. CT scans of the PF joint with quadriceps contracted (QC) and uncontracted (QU) and iEMG of vastus medialis obliquus (VMO), vastus lateralis (VL) and rectus femoris (RF) were obtained with the aid of a specially designed jig at 0°, 15°, 30° and 45° of knee flexion. The same muscle contraction pattern simulating closed kinetic chain exercise was used for both CT and iEMG. The difference between the congruence angles (CA) and tilt angles (PTA) in QC and QU positions and VMO:VL ratio from the iEMG were calculated separately for each flexion angle. CA was increased in all groups with quadriceps contraction at 0° and 15° of flexion. PTA was decreased in group I and increased in groups II and III with quadriceps contraction at the same flexion angles. This difference was statistically significant in group III at 0° and 15° of flexion. Quadriceps contraction did not affect the patellar position significantly even in the instability group at 45° of flexion. In all flexion angles the balanced VMO:VL activity ratio was observed only in group I. In the other goups, VL activity was higher than VMO activity except at 45° of flexion. These findings do not support the hypothesis of dominant centralizing effect of VMO on the patella in extension, but the effect of the VMO may be more clearly demonstrated by measuring PTA in both QC and QU positions. Received: 30 December 1996 Accepted: 22 August 1997     

Conditions of isokinetic knee flexion that enhance isokinetic knee extension

The results of two experiments are reported. The purpose of the initial investigation was to determine the effect of isokinetic knee flexion contraction intensity on subsequent knee extension contraction. Seven subjects performed ten isokinetic knee flexion-extension cycles at six isokinetic velocities under two antagonist contraction conditions. In the first condition, isokinetic knee flexion and extension were speed-matched, and, in the second condition, the high-speed condition, knee flexion was 7.85 rad.s-1 irrespective of knee extension velocity, which ranged from 0.52 to 7.85 rad.s-1. Significantly greater isokinetic knee extension measures were observed at low isokinetic velocities and were associated with the high-speed contraction condition. Enhancement of the knee extension contraction was in the initial work phase of the isokinetic contraction. The second experiment was conducted to determine whether the enhancement of the initial work phase could be associated, in part, with passive elastic qualities of the involved musculature. In this study the range of motion for the knee involved musculature. In this study the range of motion for the knee extension-flexion cycles was reduced from the previous 2.01 rad to 1.57 rad. Two isokinetic knee extension velocities were studied (1.57 and 7.85 rad.s-1) under five conditions: initiated from rest, initiated from isometric knee flexion MVC, and with preceding isokinetic knee flexion at 0.52, 4.19, and 7.85 rad.s-1. The hypothesis that knee extension contraction measures would not increase was supported. Based upon the results of the two experiments, it is suggested that the increases in knee extension contraction measures observed in the first experiment are at least partially mediated by the contribution to net torque by passive elastic musculotendinous elements. Further study of this phenomenon with the inclusion of electromyographic measures will allow determination of the presence and contribution of increased neural drive.     

Cardiovascular responses to isometric neck muscle contractions: results after dynamic exercise with various headgear loading configurations

Experiments were conducted to quantify the cardiovascular response (blood pressure and heart rate) elicited by sustained isometric contractions of the neck muscles. The response was secondary to dynamic exercise with various headgear loading combinations. The neck muscles were loaded by the head itself (CON), the standard U.S. Army SPH-4 helmet (HEL), and a combination of the SPH-4 helmet with Night Vision Goggles (H/NVG). During two exercise periods of 5 min and 35 min, each of the five subjects would rotate the head from side-to-side in the CON, HEL, or H/NVG configuration. Immediately thereafter, the subject would position his head in an isometric head dynamometer and exert a sustained right lateral (LAT) neck contraction or forward (FOR) neck contraction at 70% of a maximal voluntary contraction (MVC). During this isometric neck muscle contraction, the subject's endurance time to fatigue was recorded, the blood pressure was manually recorded, and the heart rate was continuously recorded. Characteristic increases in the systolic and diastolic blood pressure and heart rate occurred with sustained isometric neck muscle contractions. There was an average 40% increase in the systolic blood pressure, an average 50% increase in the diastolic blood pressure, and an average 45% increase in the heart rate from resting to the end of a fatiguing 70% MVC (p less than 0.05). These responses appear to be relatively independent of the duration of the exercise period, the loading during the exercise period, and the specific muscle mass involved. The mechanisms for the pressor response and the heart rate response are reviewed.     

Phosphocreatine resynthesis during recovery in different muscles of the exercising leg by 31P‐MRS

To investigate the high‐energy phosphate metabolism by ³¹P‐nuclear magnetic resonance spectroscopy during off‐transition of exercise in different muscle groups, such as calf muscles and biceps femoris muscles, seven male long‐distance runners (LDR) and nine untrained males (UT) performed both submaximal constant and incremental exercises. The relative exercise intensity was set at 60% of the maximal work rate (60%Wmax) during both knee flexion and plantar flexion submaximal constant load exercises. The relative areas under the inorganic phosphate (P_i) and phosphocreatine (PCr) peaks were determined. During the 5‐min recovery following the 60%Wmax, the time constant for the PCr off‐kinetics was significantly faster in the plantar flexion (LDR: 17.3 ± 3.6 s, UT: 26.7 ± 6.7 s) than in the knee flexion (LDR: 29.7 ± 4.7 s, UT: 42.7 ± 2.8 s, P < 0.05). In addition, a significantly faster PCr off‐kinetics was observed in LDR than in UT for both exercises. The ratio of P_i to PCr (P_i/PCr) during exercise was significantly lower during the plantar flexion than during the knee flexion (P < 0.01). These findings indicated that the calf muscles had relatively higher potential for oxidative capacity than that of biceps femoris muscles with an association of training status.     

Muscle training by means of combined electrical stimulation and volitional contraction

INTRODUCTION: A variety of techniques have been used with variable success in an attempt to reverse the loss of muscle strength, bulk, and endurance that occurs during spaceflight. This study was designed to evaluate a new "hybrid" approach that uses the resistance provided by an electrically stimulated antagonist muscle to resist the volitional contraction of its agonist. METHODS: There were 20 subjects who were randomized into 3 groups that received either the new hybrid program (HYB), isotonic weight training (WT), or isometric electrical stimulation (ES). Subjects trained 3 times per week for 8 wk with each session consisting of their non-dominant upper extremity performing 10 sets of 10 reciprocal 2-s elbow flexion and extension contractions separated by 1-min rest intervals. Elbow flexion\extension torques and biceps\triceps cross-sectional areas (CSA) were measured at the beginning, midpoint, and end of training as well as at follow-up 4 wk later. RESULTS: The HYB group demonstrated statistically significant increases in elbow flexion and extension torques (56% and 31%, respectively) at the end of training that were similar to or larger than the gains in the other groups with benefits that persisted at follow-up 4 wk later. Muscle CSA increases in the HYB group (10%) were comparable or larger than those in WT and ES subjects. CONCLUSIONS: These results suggest that HYB training may be an effective way to maintain and increase muscle bulk and strength for patients during bed rest, as well as astronauts in space.     

Mechanical output about the ankle joint in isokinetic plantar flexion and jumping

The purpose of this study was to compare for a group of ten subjects the mechanical output about the ankle during isokinetic plantar flexion with that during one-legged vertical jumps. For evaluation of the mechanical output the plantar flexion moment of force was related to the angular velocity of plantar flexion. The relationship for isokinetic plantar flexion was obtained using an isokinetic dynamometer; that for plantar flexion in jumping was obtained by combining kinematics and ground reaction forces. It was found that, at any given angular velocity of plantar flexion above 1 rad.s-1, the subjects produced much larger moments during jumping than during isokinetic plantar flexion. In order to explain the observed differences in mechanical output about the ankle, a model was used to simulate isokinetic plantar flexion and plantar flexion during jumping. The model represented both m. soleus and m. gastrocnemius as a complex composed of elastic tissue in series with muscle fibers. The force of the muscle fibers depended on fiber length, shortening velocity (Vfibers), and active state. The input variables of the model were histories of shortening velocities of the complexes, determined from kinematics, and active state. Among the output variables were Vfibers and plantar flexion moment. The simulation results were very similar to the experimental findings. According to the simulation results there are two reasons why at the same angular velocity of plantar flexion larger moments were produced during jumping than during isokinetic plantar flexion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of real and sham whole‐body mechanical vibration on spinal excitability at rest and during muscle contraction

We examined the effects of whole‐body mechanical vibration (WBV) on indices of motoneuronal excitability at rest and during muscle contraction in healthy humans. Real and sham WBV at 30 Hz had no effect on reflexes measured during muscle contraction. Real WBV at 30 and 50 Hz depressed the H‐reflex ～45%. These depressions diminished across the five inter‐bout rest intervals. The depression converted to 27% and 7% facilitation over the 15‐min long recovery period following real WBV at 30 and 50 Hz, respectively. The depression, measured during the inter‐bout rest, correlated r = 0.48 (P = 0.007) with the subsequent facilitation, measured during the follow‐up. The depression produced by sham vs real WBV was significant but less (23%), recovered faster, and the facilitation was absent in the 15‐min long follow‐up period. WBV produced time‐varying depression followed by facilitation of the H‐reflex at rest. A lack of change in volitional wave suggests that WBV did not affect the efferent neural drive.     

Alterations of the proton-T2 time in relaxed skeletal muscle induced by passive extremity flexions

PURPOSE: To demonstrate reciprocal changes of the apparent proton-T2 time in the biceps and triceps due to passive contraction and extension of the muscle fibers. MATERIALS AND METHODS: The contraction state of the upper arm muscles of six healthy volunteers was passively changed by alternating the forearm position between the straight-arm position and an elbow flexion of 90 degrees. The relaxation of the muscle during passive contraction and extension was measured with the use of muscle electromyography (EMG) experiments. Spin-echo (SE) MRI with increasing echo times (TEs) of 12-90 msec was used to acquire the averaged signal decay of the segmented biceps and triceps. The apparent T2 was deduced using monoexponential least-square fitting. RESULTS: The median T2 alterations in biceps and triceps among all volunteers were found to be 1.2 and -1.3 msec in the straight and bent forearm positions, respectively. The confidence intervals (0.5 to 1.7 msec in biceps, and -2.6 to -1.1 msec in triceps) clearly indicate that proton-T2 in MR images is significantly (P < 0.05) prolonged with muscle contraction. CONCLUSION: The observed increase of the proton-T2 time was correlated with a passive contraction of skeletal muscle fibers. This passive effect can be attributed to changes in the intracellular water mobility corresponding to the well-known "active" T2 increase that occurs after stimulation of muscle.     

The relationship of the kicking action in soccer and anterior ankle impingement syndrome. A biomechanical analysis.

Two different hypotheses have been advanced to explain the formation of talotibial osteophytes in the anterior ankle impingement syndrome. We investigated how frequently hyperplantar flexion occurs during kicking and whether the site of impact of the ball coincides with the reported location of the osteophytes. We also measured the magnitude of the impact force. We studied 150 kicking actions performed by 15 elite soccer players by using mobile sensors and high-speed video. In 39% of the kicking actions, the plantar flexion angle exceeded the maximum static plantar flexion angle. Ball impact was predominantly made with the anteromedial aspect of the foot and ankle, with impact between the ball and the base of the first metatarsal bone in 89% of the kicking actions and between the ball and the anterior part of the medial malleolus in 76%. Postimpact ball velocity averaged 24.6 m/s, with a corresponding average contact force of 1025 N. Hyperplantar flexion was reached in only the minority of the kicking actions. The data on impact location and impact force support the hypothesis that spur formation in anterior ankle impingement syndrome is related to recurrent ball impact, which can be regarded as repetitive microtrauma to the anteromedial aspect of the ankle.     

Effect of patellar tendon shortening on tracking of the patella

BACKGROUND: Although 10% postoperative patellar tendon shortening after bone-patellar tendon-bone autograft reconstruction of the anterior cruciate ligament has been reported, there are no published studies assessing the effect of shortening on patellofemoral joint biomechanics under physiological loading conditions. PURPOSE: To investigate the influence of patellar tendon shortening on patellofemoral joint biomechanics. STUDY DESIGN: Controlled laboratory study. METHODS: The authors evaluated the patellofemoral contact area, the location of contact, and the patellofemoral joint reaction force and contact stresses in 7 cadaveric knees before and after 10% patellar tendon shortening. Shortening was achieved using a specially designed device. Experimental conditions simulating those occurring during level walking were employed: physiological quadriceps loads and corresponding angles of tibial rotation were applied at 15 degrees , 30 degrees , and 60 degrees flexion of the knee. Patellofemoral joint contact areas were measured before and after shortening using the silicone oil-carbon black powder suspension squeeze technique. RESULTS: After patellar tendon shortening, patellofemoral joint contact areas were displaced proximally on the patellar surface and distally on the femoral surface. Although the contact area increased by 18% at 15 degrees of knee flexion (P = .04), no significant change occurred at 30 degrees or 60 degrees of knee flexion (P > .05). Patellofemoral contact stress remained unchanged after patellar tendon shortening (P > .05) at each flexion angle. CONCLUSION: Our results suggest that a 10% shortening of the patellar tendon does not alter patellar contact stresses during locomotion. It is not clear whether apparent changes in contact location in all positions and contact area at 15 degrees would have clinical consequences.     

Resistance training during unweighting maintains muscle size and function in human calf

PURPOSE: A 20-d 6 degrees head-down tilt bed rest project was conducted to evaluate the effect of dynamic leg press and plantar flexion resistance training on muscle size and function in human plantar flexors (PF) throughout the prolonged bed rest. METHODS: Twelve healthy men participated in this study and were divided two groups: resistance training (BR-Tr group: N = 6, age: 23 +/- 2 yr, height: 170 +/- 3 cm, weight: 66 +/- 7 kg) and nontraining (BR-Cont group: N = 6, age: 23 +/- 1 yr, height: 170 +/- 3 cm, weight: 67 +/- 6 kg) during the bed rest. Physiological cross-sectional area (PCSA) and peak torque of the PF muscle group was determined. Spin-spin relaxation times (T2) of the medial (MG) and lateral gastrocnemius (LG) and soleus (Sol) muscle was measured at rest and immediately after unilateral calf-raising exercise (5 sets of 10 reps). RESULTS: PCSA of the PF muscle group did not show any significant change in BR-Tr group; however, for the BR-Cont group, PCSA decreased by 13% after bed rest (P < 0.05). There was no significant change in exercise-induced T2 change of the MG, LG, or Sol muscles between before and after the bed rest in BR-Tr group; however, in the BR-Cont group, significant increases in T2 were found in these three muscles after the bed rest (P < 0.05 to 0.01). CONCLUSION: We conclude that dynamic leg press and plantar flexion resistance training during bed rest maintains muscle size and function (torque and T2), and that this training could be useful for prevention of progressive muscle deconditioning during spaceflight.     

The effects of rest interval and resistance training on quadriceps femoris muscle. Part II: EMG and perceived exertion

AIM: The purpose of this study was to examine the effects of rest interval on quadriceps femoris muscle activation and perceived exertion, during short-term resistance training. METHODS: Vastus medialis (VM) and vastus lateralis (VL) muscle electromyograms (EMG) were assessed in 15 males during a sustained 80% maximal voluntary contraction (MVC). During the pre-training evaluation, the absolute value of the 80% MVC (N.m) and contraction duration (s) was performed at 2, 4, and 6 weeks during the training period. Perceived exertion was measured via the Borg category-ratio scale every 5 s during the 80% MVC. Subjects were randomly assigned to 3 groups: group 1 received a 40 s rest interval in between exercise sets, group 2 received a rest period of 160 s, and the control group did not participate in training. Groups 1 and 2 performed isokinetic knee extensions at 180 deg.s(-1) 2 days per week for 6 weeks. RESULTS: The results demonstrated a significant decrease in VM EMG within the initial portion of the 80% MVC across the training period in the short rest interval group. The long rest interval and control groups showed no significant changes in VM EMG during 1st part of the contraction across the training period, whereas the control group exhibited a significant reduction in VL EMG across weeks 4 to 6. VL EMG increased during the 80% MVC in the control group across the training period. VM EMG increased during the sustained contractions in the long rest interval and control groups across the training period. The perceived exertion response was lower in the 1st part of the 80% MVC in the short and long rest interval groups, but not in the control group, across the training period. The results also showed a significant decrease in perceived exertion at the end of the sustained contraction in the short rest interval group, but not in the long rest interval group or the control group. CONCLUSIONS: The findings from this study suggest that the application of relatively short rest intervals in between sets of resistance exercise induced a greater neuromuscular response of the VM muscle during short-term training.     

Plantar flexion torque as a function of time of day

The possible peripheral and/or central origin in the mechanisms responsible for day-time fluctuation in maximal torque of the triceps surae muscle were investigated with a special emphasis on antagonist muscle coactivation. Eleven 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 h and 08:00 h in the morning and between 17:00 h and 19:00 h in the evening. To investigate peripheral mechanisms, the posterior tibial nerve was stimulated at rest, using percutaneous electrical stimuli, to evoke single twitch, double twitch, and maximal tetanic contraction (100 Hz). Maximal voluntary contraction of the plantar flexors was also assessed by means of the relative electromyographic activity of respective agonist and antagonist muscles (soleus, gastrocnemius medialis, gastrocnemius lateralis, and tibialis anterior). A double twitch was delivered during maximal voluntary plantar flexion to record muscle activation (i.e., interpolated twitch technique). The coactivation level of the tibialis anterior muscle during plantar flexion was calculated. The results indicated a significant decrease in maximal voluntary muscle torque of triceps surae in the evening as compared with the morning (-7.0 %; p < 0.05). Concerning the central command, when extrapolated by the twitch interpolation technique, the decrease in mean activation level of -6.8 % was consistent with the fluctuation in torque (-7.0 %). Soleus muscle electromyographic activity (normalized to the M-wave) showed a significant decline (21.6 %; p < 0.001). Moreover, individual changes in MVC percentage were significantly related to those of normalized electromyographic activity of the soleus muscle (r = 0.688; p < 0.01). Thus, it indicated that the subject's capacity to activate the soleus muscle was affected by the time of day. The coactivation level in the tibialis anterior muscle during plantar flexion did not change significantly in the evening. Concerning peripheral mechanisms, we observed a decrease in maximal M-wave amplitude for soleus and gastrocnemii, associated with unchanged single twitch and tetanus torque. To conclude, impairment in soleus muscle central command seemed to be the mechanism in the origin of torque failure. Such information would be of importance in the investigation of day-time fluctuations in complex motor task performances implicating the triceps surae muscle.     

Electromechanical delay during knee extensor contractions.

The purpose of this study was to investigate the magnitude of electromechanical delay (EMD) and its possible dependence on muscle type, type of contraction, fatigue, level of force, initial muscle length, and muscle contraction velocity. This was achieved using an experiment that measured voluntary knee extensor torques and surface EMG activity for a variety of different contractile conditions in seven male subjects. EMD values were obtained using a cross-correlation technique in three experimental KIN-COM dynamometer conditions of vastus medialis, rectus femoris, and vastus lateralis. In the first condition, a series of 10 repetitive submaximal (50% and 70% MVC) isometric knee extensor contractions were performed at knee angles of 90 degrees and 130 degrees extension. In the second condition, 10 maximal isokinetic knee extensor contractions were performed during passive shortening and lengthening. As such, the dynamometer was used to passively move the knee joint at 30 degrees.s-1 and 60 degrees.s-1. Both during lengthening and shortening, the contractions occurred at an angular position of 110 degrees. In the last condition, a repetitive submaximal isometric knee extensor fatigue test was performed for 100 s (150 contractions). At 10, 40, and 90 s during the time course of this fatigue test, a series of 10 contractions were recorded. To avoid a phase lag, which is introduced with one-way filtering, the EMG was processed with a bidirectional low-pass filter application. A significant main effect in EMD for the factor level of force was found. The EMD values obtained at a force level of 50% MVC were longer than at 70% MVC (107 vs 98 ms).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of massage of the hamstring muscles on selected electromyographic characteristics of biceps femoris during sub-maximal isometric contraction

The aim of this study was to investigate whether a single massage of the hamstring muscles would alter selected electromyographic characteristics of biceps femoris during a sub-maximal isometric contraction. Eleven healthy young males participated in this crossover study. They were randomly assigned to two groups, receiving either a 15-minute hamstring muscle massage or a 15-minute prone rest. One week later they returned to receive the alternate intervention. Immediately pre- and post-intervention, participants were instructed to contract their muscles just sufficiently to maintain right knee flexion (5 degrees from full extension) for 30 seconds. Simultaneous to this contraction, surface electromyography (EMG) was used to record the electrical activity of right biceps femoris. Electromyograms were analyzed by calculating the averaged/integrated EMG (aEMG) and median frequency (MF) for each of 14 one-second windows, sampled every two seconds throughout the 30-second contractions per participant. Mean values of aEMG and MF were calculated per participant (aEMGmean and MFmean), pre- and post-massage or rest intervention. Group means (n = 11) of aEMGmean and MFmean were then calculated and compared using a repeated measures analysis of variance (ANOVA) procedure (p < 0.05). Rates of change of aEMG and MF throughout each 30-second contraction were represented by their respective gradients as functions of time. These gradients were similarly compared pre- to post-intervention using repeated measures ANOVA (p < 0.05). For the young healthy males considered in this study, a single massage of the hamstring muscles had no statistically significant effects on selected electromyographic characteristics of biceps femoris during sub-maximal isometric contractions (p > 0.05).     

Neck muscle activity in helicopter pilots: effect of position and helmet-mounted equipment

BACKGROUND: Helicopter pilots usually work in unfavorable ergonomic positions, often with bulky head-worn equipment during flying missions. The purpose of this study was to evaluate and compare immediate muscle response in the dorsal neck muscles to different positions with a variety of head-worn equipment. METHODS: Fourteen healthy male helicopter pilots volunteered for this study. EMG activity in the upper and lower dorsal neck muscles and the trapezius muscle was measured in a laboratory situation for 5 s in different sitting positions (neutral, trunk inclined 20 degrees, neck flexed 20 degrees), including registration of a 30 degrees left and right rotation in every position; all measurements were performed while wearing a helmet, a helmet and night vision goggles (hNVG), and a helmet, night vision goggles, and counterweight (hCW), in random order. RESULTS: There was significant higher EMG activity in the upper neck with hNVG and hCW than with the helmet only when comparing the mean activity level of all positions. However, there was no significant difference in EMG activity between any variations of head-worn equipment when comparing activity levels during each position separately. In the upper and lower neck, respectively, there was significantly higher muscle activity during the ipsilateral rotated positions plus neck flexion and trunk inclination than in most other positions. CONCLUSION: The increased load caused by different positions seems to have a greater influence on muscle activity than the increased load of the head-worn equipment, which must be considered when designing helicopter work-places.     

Effect of acute static stretching on force, balance, reaction time, and movement time

PURPOSE: The purpose of the study was to investigate the effect of an acute bout of lower limb static stretching on balance, proprioception, reaction, and movement time. METHODS: Sixteen subjects were tested before and after both a static stretching of the quadriceps, hamstrings, and plantar flexors or a similar duration control condition. The stretching protocol involved a 5-min cycle warm-up followed by three stretches to the point of discomfort of 45 s each with 15-s rest periods for each muscle group. Measurements included maximal voluntary isometric contraction (MVC) force of the leg extensors, static balance using a computerized wobble board, reaction and movement time of the dominant lower limb, and the ability to match 30% and 50% MVC forces with and without visual feedback. RESULTS: There were no significant differences in the decrease in MVC between the stretch and control conditions or in the ability to match submaximal forces. However, there was a significant (P < 0.009) decrease in balance scores with the stretch (decreasing 9.2%) compared with the control (increasing 17.3%) condition. Similarly, decreases in reaction (5.8%) and movement (5.7%) time with the control condition differed significantly (P < 0.01) from the stretch-induced increases of 4.0% and 1.9%, respectively. CONCLUSION: In conclusion, it appears that an acute bout of stretching impaired the warm-up effect achieved under control conditions with balance and reaction/movement time.