Influence of aging on the mechanical behavior of leg extensor muscles

Summary Age dependence of the mechanical behavior of leg extensor muscle was investigated using vertical jumps with and without a stretch-shortening cycle on the force-platform. A total 226 subjects (113 females and 113 males) ranging in age from 4–73 years were examined. The results indicated in general that performance in males was better than that in females. This difference was reduced when body weight was taken into consideration. The peak performance of the various parameters, such as average force, height of rise of center of gravity, net impulse, and also the average power output, was reached in both sexes between the ages of 20 and 30 years. For example, the average vertical force in squatting had the following mean values in the various age groups of the male subjects: 114 N (4–6 years), 402 N (13–17), 618 N (18–28), 508 N (29–40), 435 N (41–49), 320 N (54–65), 315 N (71–73 years). When the jumps were performed using the stretch-shortening cycle, the potential of the mechanical performance after prestretching was also sensitive to aging in a similar manner. The results suggest that it is not only the performance of pure concentric contraction that is influenced by the maturation and aging processes but, the that elastic behavior of muscle and reflex potentiation are also affected by the same processes.Supported in part by a grant No. 8318/78/78 from the Ministry of Education, Finland     

Store and recoil of elastic energy in slow and fast types of human skeletal muscles

Stretch-shortening cycle refers to the mechanical condition in which store and recoil of elastic energy occur in the skeletal muscle. This leads to a greater work output when compared to a simple shortening contraction. The subjects performed vertical jumps with and without preliminary counter-movement and with small and large knee angular displacement. The results indicated that those subjects who had more fast twitch (FT) fibers benefited more from the stretching phase performed with high speed and short angular displacement. The amounts of elastic energy stored in this phase were 30 and 26 N × kgBW^-1, respectively, for FT and slow twitch (ST) type subjects. The recoil of elastic energy was proportional to the amount of energy storage. In large amplitude jumps where transient period between stretch and shortening is long the both types of subjects demonstrated similar amount of storage of elastic energy (17 and 16 N × kgBW^-1, respectively). However, the re-use of this elastic energy was greater in ST group (24%) as compared to the FT group (17%). The results can be interpreted through differences in sarcomere cross-bridge life times between fast and slow muscle fibers. The slow type muscle may be able to retain the cross-bridge attachment for a longer period of time and therefore it may utilize elastic energy better in a slow type ballistic motion.     

Relationship between the efficiency of muscular work during jumping and the energetics of running

Summary The running economy of seventeen athletes was studied during running at a low speed (3.3 m · s^–1) on a motor-driven treadmill. The net energetic cost during running expressed in kJ·kg^–1·km^–1 was on average 4.06. As expected, a positive relationship was found between the energetic cost and the percentage of fast twitch fibres (r=0.60,n=17,p<0.01). In addition, the mechanical efficiency during two different series of jumps performed with and without prestretch was measured in thirteen subjects. The effect of prestretch on muscle economy was represented by the ratio between the efficiency of muscular work performed during prestretch jumps and the corresponding value calculated in no prestretch conditions. This ratio demonstrated a statistically significant relationship with energy expenditure during running (r=–0.66,n=13,P<0.01), suggesting that the elastic behaviour of leg extensor muscles is similar in running and jumping if the speeds of muscular contraction during eccentric and concentric work are of similar magnitudes.     

Prestretch potentiation of human skeletal muscle during ballistic movement

The conditions associated prior to and during the transition from prestretch to shortering may have considerable influence on the final performance of muscle. In the present study male subjects of good physical condition performed vertical jumps on the force-platform with and without preliminary counter movement. In the counter movement jump (CMJ) the amplitude of the knee bending, velocity of the prestretch and the force attained at end of prestretch were the primary parameters of interest. In addition the coupling time indicating the transition from the eccentric (prestretch) phase to the concentric phase was recorded from the angular displacement and reaction force curves. In the final calculation the mechanical performance parameters of CMJ were always compared with those of the jumps performed without counter movement. The results indicated in general first that CMJ enhanced the average concentric force and average mechanical power by 423 N (66%) and 1158 W (81%), respectively. This potentiation effect was the higher the higher was the force at end of prestretch (p<0.001). Similarly, the prestretch speed (p<0.001) and short coupling time (p<0.01) were associated with enhanced performance during the concentric phase. The average coupling time was 23 ms. The results are interpreted through changes in the prestretch conditions to modify the acto-myosin cross-bridge formation so that the storage and utilization of elastic energy is associated with high prestretch speed, high eccentric force and short coupling time. The role of the reflex potentiation is also suggested as additional enhancement of the final performance.     

Force-time characteristics and fiber composition in human leg extensor muscles

Summary Recording of the force-time (f-t) characteristics of muscular contraction expresses the rate at which tension is developed. To further understand the problems involved in force production during voluntary contraction, the f-t curve was registered during maximal voluntary isometric extension of both legs performed in the sitting position with the knee angle at 107 degrees. 38 athletes representing various sport events, five pairs of monozygous, and ten pairs of dizygous twins were used as subjects. The reference group consisted of eight normal men. The data disclosed that the time to produce certain force levels showed good trial-to-trial and satisfactory day-to-day reproducibility below tension levels of 0.9×P ₀. At these force levels the time of tension development was positively (p<0.05) related to the per cent distribution of slow twitch fibers in the vastus lateralis muscle. This result is consistent with animal experiments concerning the mechanical characteristics of slow and fast muscles. In addition, it was observed that the athletic groups had f-t curves different from the other subjects. Genetic factors had only slight influence on the f-t measurement.     

Mechanical efficiency during repetitive vertical jumping

The purpose of this study was to compare mechanical efficiency between repeated static jumps (SJ), countermovement jumps (CMJ), drop jumps from 75% of maximum CMJ jump height (75DJ) and drop jumps from 125% of maximum CMJ height (125DJ). Subjects included eight jump-trained males. All subjects completed 30 continuous repetitions in the SJ, CMJ, 75DJ, and 125DJ. Oxygen consumption, peak force and center of mass displacement for each repetition during the four jumping patterns were measured. ME was calculated from a combination of force-time curves, displacement-time curves and lactate-corrected oxygen consumption values. In addition, muscle activity was recorded from the vastus medialis, vastus lateralis and biceps femoris using surface electromyography (EMG). 125DJ and 75DJ resulted in significantly (P ≤ 0.05) greater ME in comparison to CMJ and SJ. CMJ resulted in significantly greater ME in comparison to SJ. In addition, braking phase muscle activity was significantly greater in 125DJ and 75DJ in comparison to CMJ. Negative work was significantly different between 125DJ, 75DJ and CMJ (125DJ > 75DJ > CMJ). There was a significant positive correlation (r = 0.68) between ME and negative work performed across 125DJ, 75DJ and CMJ. These findings suggest that stretch-shortening cycle movements, which include a strenuous braking phase combined with simultaneous high muscle activity, increase ME. This may be due to optimal muscle-tendon unit kinetics and usage of stored elastic energy.     

Combined effect of elastic energy and myoelectrical potentiation during stretch-shortening cycle exercise

In addition to the utilization of muscle's elastic energy enhancement of performance in exercise involving stretch-shortening cycle might be also due to simultaneous increase of myoelectrical activity. This hypothesis was tested by examining three athletes during jumping exercise on force-platform. Vertical jumps were performed with and without preliminary counter-movement, and the jumps were called counter-movement jump (CMJ) and squatting jump (SJ), respectively. In both conditions several jumps were performed also with extra loads on the shoulders (15–220% of b. wt.). Additional droppingjumps (DJ) were executed from different heights (20–100 cm). During jumping exercise myoelectrical activity of selected muscles from the quadriceps femoris was monitored with surface electrodes. The results obtained were similar to those reported in isolated muscle and as expected, the prestretch in CMJ shifted the force-velocity curve of concentric work to the right. In two cases enhancement of performance was attributed primarily to restitution of elastic energy because myoelectrical activity was similar to that observed in SJ. In one subject increased myoelectrical activity was observed during the concentric phase of CMJ. In DJ condition the EMG activity during eccentric phase was much higher than in SJ. Therefore the high performance in this condition was attributed to both elastic energy and reflex potentiation. In eccentric work of CMJ the average force decreased with the increase of stretching speed. This phenomenon was associated with a light increase of EMG activity. The observed results emphasize that both elastic energy and reflex potentiation may operate effectively during stretch-shortening cycle activity.     

The influence of muscle pain and fatigue on the activity of synergistic muscles of the leg

The purpose of this study was to investigate the effect of experimentally induced muscle pain on the motor-control strategies of synergistic muscles during submaximal fatiguing isometric contractions. The root mean square (RMS) and median frequency (MF) of the surface electromyographic (EMG) signal from synergistic plantarflexors and dorsiflexors were assessed to exhaustion. Ten subjects performed sustained dorsiflexions and plantarflexions at two contraction levels, 50% and 80% of maximum voluntary contraction, with or without muscle pain, induced by injection of 6% hypertonic saline in one synergist. In the painful contractions, the RMS of the EMG signal was decreased compared to the control condition in the initial phase of the contraction, in the muscles where pain was induced as well as in the nonpainful synergists. Moreover, the EMG signal MF decreased faster during muscle pain than in the control condition. The endurance time was shorter during muscle pain, and some of the nonpainful synergists showed increased compensatory activity at the end of the contractions to maintain the target force. The decreased EMG activation during pain was coupled with significantly decreased torque levels during the painful condition that would partly explain the results. However, the ratio between force and EMG amplitude was decreased for both the painful and nonpainful synergists, so other mechanisms might explain the present findings. This study shows that localized muscle pain can reorganize the EMG activity of synergists where no pain is present. These findings may have implications for the understanding of manifestations seen in relation to painful musculoskeletal disorders.     

Differences in mechanical efficiency between power- and endurance-trained athletes while jumping

Mechanical efficiency (ME) of jumping exercises was compared between power-trained (n = 11) and endurance-trained athletes (n = 10) using both a biomechanical and a physiological approach. In drop jumps and in stretch-shortening cycle exercise on a special sledge (sledge jumps), the subjects performed 60 muscle actions from a dropping height of optimum minus 40 cm (O – 40), as well as from dropping heights of optimum (O) and optimum plus 40 cm (O + 40). Thus, they were tested in six different tests which lasted for a total of 3 min for each. The mean ME values in the drop jumps from the lowest dropping height upwards were as follows: 23.8 (SD 5.3)%, 35.5 (SD 10.8)% and 39.2 (SD 6.6)% for the power group, and 30.8 (SD 6.5)%, 37.5 (SD 8.7)% and 41.4 (SD 7.0)% for the endurance group. In the sledge jumps the ME values were 37.0 (SD 5.6)%,48.4 (SD 4.0)% and 54.9 (SD 8.5)% for the power group, and 40.2 (SD 5.9)%, 46.9 (SD 5.7)% and 58.5 (SD 5.5)% for the endurance group. As can be seen, the ME values increased with increasing stretch load. However, the groups did not differ from each other except in the drop jump condition of O – 40 (P < 0.05). The higher power (P < 0.001) among the power athletes in every measured condition was associated with a faster rate of electromyogram development during the pre-activity, and smoother muscle activity patterns in the ground contact. On the other hand, the endurance athletes had a lower blood lactate concentration after every test, and in addition a lower heart rate and ventilation during the sledge jumps than their power counterparts. Therefore, it would seem that the similar mean ME values between the subject groups could be explained by improved function of the neuromuscular system among the power group and improved metabolism among the endurance group.     

Electrical stimulation-induced changes in performance and fiber type proportion of human knee extensor muscles

The purpose of the present study was to look at the changes in the performance of human knee extensor muscles (KEM) induced by 6 weeks of low-frequency (8 Hz) electrical stimulation (LFES). KEM performance of 20 sedentary (before and after stimulation), ten active, and five endurance-trained subjects was evaluated during 25 consecutive 10-s isometric contractions, each separated by a rest period of 5 s. The mean force maintained during six consecutive 10-s contractions was expressed as a relative percentage of that of the first contraction. The mean performance of the first series of six contractions was not altered in response to stimulation, whereas that of the other four series was significantly increased. No significant difference was noted among the three groups in terms of KEM performance during the first series of six contractions. However, after the first series of six contractions, KEM performance of endurance-trained subjects was better in comparison to the other groups. Citrate synthase (CS) activity, capillary number per type IIA and IIB fibers, and the percentage of type IIA muscle fibers determined from vastus lateralis samples were significantly increased in response to the stimulation protocol. No significant change was observed in the proportion or capillary number of type I fibers, or in the areas of type I, IIA, and IIB fibers. The present study provides evidence that resistance to fatigue of human skeletal muscle can be significantly altered in response to 6 weeks of transcutaneous low-frequency electrical stimulation. The improvement in KEM resistance to fatigue of the sedentary subjects was such that, at the end of the stimulation protocol, resistance to fatigue was similar to that of active subjects. However, the ability of endurance-trained subjects to withstand fatigue was still superior compared to that of the other untrained or active subjects.     

Effects of power training on mechanical efficiency in jumping

The present study investigates the effects of power training on mechanical efficiency (ME) in jumping. Twenty-three subjects, including ten controls, volunteered for the study. The experimental group trained twice a week for 15 weeks performing various jumping exercises such as drop jumps, hurdle jumps, hopping and bouncing. In the maximal jumping test, the take-off velocity increased from 2.56 (0.24) m·s^–1 to 2.77 (0.18) m·s^–1 (P<0.05). In the submaximal jumping of 50% of the maximum, energy expenditure decreased from 660 (110) to 502 (68) J·kg^–1·min^–1 (P<0.001) while, simultaneously, ME increased from 37.2 (8.4)% to 47.4 (8.2)% (P<0.001). Some muscle enzyme activities of the gastrocnemius muscle increased during the training period: citrate synthase from 35 (8) to 39 (7) mol·g^–1 dry mass·min^–1 (P<0.05) and -hydroxyacyl CoA dehydrogenase from 21 (4) to 23 (5) mol·g^–1 dry mass·min^–1 (P<0.05), whereas no significant changes were observed in phosphofructokinase and lactate dehydrogenase. In the control group, no changes in ME or in enzyme activities were observed. In conclusion, the enhanced performance capability of 8% in maximal jumping as a result of power training was characterized by decreased energy expenditure of 24%. Thus, the increased neuromuscular performance, joint control strategy, and intermuscular coordination (primary factors), together with improved aerobic capacity (secondary factor), may result in reduced oxygen demands and increased ME.     

Potentiation of concentric plantar flexion torque following eccentric and isometric muscle actions

In a stretch-shortening cycle (SSC) the concentric muscle action is enhanced by a preceding eccentric muscle action. The hypothesis of the present study is that a preceding isometric action can also have an effect on a following concentric action, but to a lesser degree. A KINetic-COMmunicator II dynamometer was used to test muscle strength of the plantar flexion of the right foot in 20 healthy women. Maximal voluntary torque measurements were made at different angular velocities (120^o s^-1 and 240^o s^-1) and the range of motion of the ankle joint was 78–125^o. The assessment was based on concentric torque output and EMG recording from the gastrocnemius muscle under three different types of testing conditions (concentric actions with and without preceding eccentric or isometric actions, all with maximal efforts). The results showed that preceding muscle actions led to greater concentric torque output (P < 0.01) between 90 and 99^o plantar flexion. However, the increase in the concentric action was significantly (P < 0.01) larger with eccentric than with isometric preceding action, regardless of velocity. The EMG activity of the concentric action showed unchanged or lower values when preceded by a muscle action. In this model our conclusion is that the main reason for larger concentric torque values after a preceding muscle action is that time is sufficient for maximal muscle tension development; in addition, elastic energy is stored, particularly during the preceding eccentric action. Our results show that the effect of preceding muscle actions should be taken into account when measuring isokinetic muscle strength at relatively small angular movements.     

Mechanical characteristics and fiber composition of human leg extensor muscles

Summary To investigate the influence of skeletal muscle fiber composition on the mechanical performance of human skeletal muscle under dynamic conditions, 34 physical education students with differing muscle fiber composition (M. vastus lateralis) were used as subjects to perform maximal vertical jumps on the force-platform. Two kinds of jumps were performed: one from a static starting position (SJ), the other with a preliminary counter-movement (CMJ). The calculated mechanical parameters included height of rise of center of gravity (h), average force (¯F), net impulse (NI) and average mechanical power (W). It was observed that the percentage of fast twitch fibers was significantly related (p< 0.05-0.01) to these variables in SJ condition and also to h and NI of the positive work phase in CMJ. It is concluded that skeletal muscle fiber composition also determines performance in a multijoint movement. The result is explainable through the differences in the mechanical characteristics of the motor units and their respective muscle fibers.     

Activity of mono- and biarticular leg muscles during sprint running

Summary A cinematographic recording of the movements of the lower limbs together with simultaneous emg tracings from nine lower limb muscles were obtained from two male track sprinters during three phases of a 100 m sprint run. The extensor muscles of the hip joint were found to be the primary movers by acceleration of the body's center of gravity (C.G.) during the ground phase of the running cycle. The extensors of the knee joint were also important in this, but to a minor extent, while the plantar flexors of the ankle joint showed the least contribution. The biarticular muscles functioned in a way different from the monoarticular muscles in the sense that they perform eccentric work during the flight and recovery phases and concentric work during the whole ground phase (support), whereas the monoarticular muscles are restricted first to eccentric work and then to concentric work during the ground phase. Furthermore, the biarticular muscles show variation (and rate of variation) in muscle length to a larger extent than the monoarticular muscles. Paradoxical muscle actions appear to take place around the knee joint, where the hamstring muscles, m. gastrocnemius, m. vastus laterialis and m. vastus medialis act as synergists by extending the knee joint during the last part of the ground phase.     

Assessment of human knee extensor muscles stress from in vivo physiological cross-sectional area and strength measurements

Summary The physiological cross-sectional areas (CSAp) of the vastus lateralis (VL), vastus intermedius (VI), vastus medialis (VM) and rectus femoris (RF) were obtained, in vivo, from the reconstructed muscle volumes, angles of pennation and distance between tendons of six healthy male volunteers by nuclear magnetic resonance imaging (MRI). In all subjects, the isometric maximum voluntary contraction strength (MVC) was measured at the optimum angle at which peak force occurred. The MVC developed at the ankle was 746.0 (SD 141.8) N and its tendon component (F _t) given by a mechanical advantage of 0.117 (SD 0.010), was 6.367 (SD 1.113) kN. To calculate the force acting along the fibres (F _f) of each muscle, F _t was divided by the cosine of the angle of pennation and multiplied for (CSAp · CSAp^–1), where CSAp was the sum of CSAp of the four muscles. The resulting F _f values of VL, VI, VM and RF were: 1.452 (SD 0.531) kN, 1.997 (SD 0.187) kN, 1.914 (SD 0.827) kN, and 1.601 (SD 0.306) kN, respectively. The stress of each muscle was obtained by dividing these forces for the respective CSAp which was: 6.24 × 10^–3 (SD 2.54 × 10^–3) m² for VL, 8.35 × 10^–3 (SD 1.17 × 10^–3) m² for VI, 6.80 × 10^–3 (SD 2.66 × 10^–3) m² for VM and 6.62 × 10^–3 (SD 1.21 × 10^–3) m² for RE The mean value of stress of VL, VI, VM and RF was 250 (SD 19) kN m²; this value is in good agreement with data on animal muscle and those on human parallel-fibred muscle.     

Force-velocity-power characteristics and fiber composition in human knee extensor muscles

Summary To investigate the influence of skeletal muscle fiber composition on the mechanical characteristics of human skeletal muscle under isometric and dynamic conditions, ten well-trained track athletes with different muscle compositions (m. vastus lateralis) were used. The subjects were instructed to perform maximum isometric and dynamic knee extensions at maximal speed against increasing load. To determine the mechanical properties of the knee extensors a special dynamometer was used. The load was increased by adding weight discs to the electromagnetic part of the dynamometer. The load increased from 0.33–2.33 kg² moment of inertia in six stages. The data were handled on the basis of Hill's characteristic equation. The subjects were divided into two equal groups, one with more and one with less than 50% of fast twitch fibers. The force-velocity curve was found to be different in the two groups, the subjects with a predominance of fast twitch fibers being able to develop higher power output at a given load. Significant correlations between percentage of fast twitch fiber and power, as well as velocity, were found at the four greater loads only. However, when the percentage of fast twitch fiber and fast twitch fiber cross-sectional areas were combined, significant correlations were also found between this combined parameter and power output at the two lowest loads.     

The mechanical efficiency of locomotion in men and women with special emphasis on stretch-shortening cycle exercises

Summary The mechanical efficiency of the leg extensor musculature of men and women was examined with a special sledge ergometer. The subjects (ten males and ten females) performed (a) pure positive work, (b) pure negative work and (c) a combination of negative and positive work (strech-shortening cycle). The mechanical efficiency of pure positive work was on average 19.8±1.2% for female subjects and 17.4±1.2% for male subjects (t=4.12, P<0.001), although the work intensity was equal in both groups. The mechanical efficiency of pure negative work was slightly lower in women than in men (59.3±14.4% vs 75.6±29.3%). The mechanical efficiency of positive work (₊) in a stretch-shortening cycle exercise was 38.1±6.8% in men and 35.5±6.9% in women. The utilization of prestretch was better for female subjects at low prestretch levels, whereas males showed greater potentiation of elastic energy at higher prestretch levels. Regarding absolute W _el (work due to elasticity) values, male subjects showed greater (P<0.001) values than females (189±44 J vs 115±36 J, respectively). Fundamental differences in neuromuscular functions in men and women might cause the differences in the results obtained.     

EMG-activity and muscular performance of lower leg during stretch-shortening cycle after cooling

To test the effect of cooling on EMG-activity of muscles working as an agonist and antagonist in the lower leg, 12 men dressed in shorts and jogging shoes performed a drop-jump exercise after 60 min exposures to 27 °C and 10 °C. Cooling decreased mean skin temperature 5.6±0.4 °C (mean±SD, P<0.001), whereas rectal temperature was unaffected. The muscle temperature measured from m. gastrocnemius medialis decreased 4.1±0.3 °C (P<0.01) at the depth of 30 mm below skin surface. To find the optimal stretching velocity for potentiation of elastic energy, the drop-jump exercise was performed from six different bench heights (10, 20, 30, 40, 50 and 60 cm). The optimal velocity was not altered on account of cooling. In cooled subjects during the stretch phase of the drop jumps the EMG-activity of m. triceps surae complex (agonist) increased (P<0.05–0.001) while the activity of m. tibialis anterior (antagonist) remained unchanged. After cooling during the shortening phase of the jumps the EMG-activity of m. triceps surae complex decreased (P<0.05–0.001), whereas the activity of m. tibialis anterior increased (P<0.05–0.001). In addition, after cooling the peak EMG-activity appeared on the average 28 ms earlier, which shifted the peak activity from the shortening phase (at 27 °C) to the stretch phase (at 10 °C). Cooling increased the mean duration of stretch and shortening phases by 28±3 ms (P<0.001) and 23±2 ms (P<0.001), respectively. The average force production during the shortening phase was 26% less (P<0.05) after cooling, which resulted in a decreased rise of body centre of gravity (P<0.05–0.01). It is concluded that during a stretch-shortening cycle cooling alters the EMG-activity of agonist and antagonist muscles on a contradictory manner and results in an earlier peak EMG-activity. Therefore, alterations in motor unit recruitment could be responsible for the prolonged muscle contraction and decreased force production on account of cooling.     

Torque and surface mechanomyogram parallel reduction during fatiguing stimulation in human muscles

The purpose of the study was to verify, by means of torque and mechanomyogram (MMG) compared analysis, the validity of MMG as a tool to investigate the contractile changes due to localized muscular fatigue induced by stimulation protocols usually employed for sport training and rehabilitation programs. Ten healthy sedentary subjects participated in the study. Torque produced by the dominant biceps brachii (BB) and vastus lateralis (VL) during transcutaneous stimulated contractions has been recorded by a load cell strapped to the subjects’ wrist and distal one-third of the tibia, respectively. MMG was detected over the muscle bellies during a monopolar supramaximal stimulation of the main motor point. After potentiation, the fatiguing stimulation was administered. It consisted of 50 cycles, with 2 s of 50 Hz and 25 s of 2 Hz. Averaged normalized values of peak torque (pT) and MMG peak-to-peak (MMG-pp) of the subjects group decreased from their initial 100% values to 55% (pT) and 60% (MMG-pp) for BB and to 43% (pT) and 47% (MMG-pp) for VL. The pT% and MMG-pp% changes throughout the stimulation protocol presented high correlation (BB: R=0.95, P<0.001; VL: R=0.94, P<0.001). This correlation suggests that MMG could be used to follow muscle mechanical fatigue development when torque output is not or hardly detectable such as during electrical stimulation programs employed for sport training or rehabilitation protocols.