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.     

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.     

Estimation of errors in mechanical efficiency

Summary Errors in measurements of mechanical work, net energy expenditure and mechanical efficiency (ME) were calculated, when subjects performed isolated eccentric or concentric muscle actions and combinations of these actions [stretch-shortening cycle (SSC) exercises] with a special sledge apparatus. The relative error of mechanical work was 6.1%. When estimating the error of energy metabolism from oxygen consumption the error would be about 4% (McArdle et al. 1981). The maximum error of ME was the sum of these two values (10.1%). Obviously the error of ME was less than 5%, because 30 muscle actions were averaged and, in addition, the errors of mechanical work and energy expenditure were not in the same direction every time. It was concluded that mechanical work can be determined accurately when the force is measured as a function of the moved distance of the sledge. Thus calculation of ME can be performed quite reliably in isolated eccentric and concentric exercises. The greatest problems were, however, in the SSC exercises, where the errors were higher, because of the problems of dividing the net energy expenditure into eccentric and concentric phases. Therefore, further developments must be made to minimize the errors in measurement and calculation during SSC-exercise.     

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.     

Contraction-specific differences in maximal muscle power during stretch-shortening cycle movements in elderly males and females

Elderly people (age 75 years; n=48 males and 34 females) were studied in order to elucidate gender differences in elderly subjects on the determinants of muscle power (force and velocity) during a stretch-shortening cycle. All subjects performed three maximal counter-movement vertical jumps using both legs, on a force platform (Kistler 9281 B). The eccentric (Ep) and concentric (Cp) phases of the jumps were analyzed. The Ep was further divided into an acceleration phase (Ep_acc: from the start of the downward movement to the maximal negative velocity) and deceleration phase (Ep_dec: from the maximal negative velocity to the end of the downward movement). Jump height for the men was higher than for the women (P < 0.001). During both Ep_acc and Ep_dec no significant differences were observed between males and females in force and power generation. However, the men had a higher peak muscle power during the Cp, which may be explained exclusively by the velocity determinant (P < 0.001). No specific gender-related strategy appeared to influence the motor pattern of the movement. The comparable eccentric force generation of the leg extensors in both genders suggests a similar ability to cope with eccentric muscle actions during everyday activities. In contrast, the marked lower capacity for concentric contractions in women may result in an impaired performance, especially in activities where intense and rapid movements are essential, for example when reversing a forward fall. This may be one reason why elderly women are more prone to falls than are elderly men. Accepted: 19 September 2000     

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.     

Effects of the transition time between muscle-tendon stretch and shortening on mechanical efficiency

The net mechanical efficiency of positive work (η_pos) has been shown to increase if it is immediately preceded by negative work. This phenomenon is explained by the storage of elastic energy during the negative phase and its release during the subsequent positive phase. If a transition time (T) takes place, the elastic energy is dissipated into heat. The aim of the present study was to investigate the relationship between η_pos and T, and to determine the minimal T required so that η_pos reached its minimal value. Seven healthy male subjects were tested during four series of lowering–raising of the body mass. In the first series (S ₀), the negative and positive phases were executed without any transition time. In the three other series, T was varied by a timer (0.12, 0.24 and 0.56 s for series S ₁, S ₂ and S ₃, respectively). These exercises were performed on a force platform sensitive to vertical forces to measure the mechanical work and a gas analyser was used to determine the energy expenditure. The results indicated that η_pos was the highest (31.1%) for the series without any transition time (S ₀). The efficiencies observed with transition times (S ₁, S ₂ and S ₃) were 27.7, 26.0 and 23.8%, respectively, demonstrating that T plays an important role for mechanical efficiency. The investigation of the relationship between η_pos and T revealed that the minimal T required so that η_pos reached its minimal value is 0.59 s.     

Acute and prolonged reduction in joint stiffness in humans after exhausting stretch-shortening cycle exercise

The purpose of the present study was to examine the acute and long-term fatigue effects of exhausting stretch-shortening cycle (SSC) exercise on the stiffness of ankle and knee joints. Five subjects were fatigued on a sledge apparatus by 100 maximal rebound jumps followed by continuous submaximal jumping until complete exhaustion. Neuromuscular fatigue effects were examined in submaximal hopping (HOP) and in maximal drop jumps (DJ) from 35 (DJ35) and 55 cm (DJ55) heights on a force plate. Additional force and reflex measurements were made using an ankle ergometer. Jumping tests and ankle ergometer tests were carried out before, immediately after, 2 h (2H), 2 days and 7 days (7D) after the SSC exercise. Kinematics, force and electromyography (EMG) recordings were complemented with inverse dynamics, which was used to calculate joint moments. The quotient of changes in joint moment divided by changes in joint angle was used as a value of joint stiffness (JS). In addition, blood lactate concentrations and serum creatine kinase activities were determined. The exercise induced a clear decrease in knee joint stiffness by [mean (SD)] 29 (13)% (P<0.05) in HOP, 31 (6)% (P<0.05) in DJ35 and 34 (14)% (P<0.05) in DJ55. A similar trend was observed in the ankle joint stiffness with significant post-exercise reductions of 22 (8)% (P<0.05) in DJ35 and of 27 (19)% (P<0.05) at 2H in DJ55. The subsequent recovery of JS was slow and in some cases incomplete still at 7D. Generally, all the EMG parameters were fully recovered by 2H, whereas the force recovery was still incomplete at this time. These data indicate that the immediate reduction in JS was probably related to the effects of both central (neural) and peripheral (metabolic) fatigue, whereas the prolonged impairment was probably due to peripheral fatigue (muscle damage). Electronic Publication     

Changes in upper body muscle activity with increasing double poling velocities in elite cross-country skiing

The purpose of this study was to investigate whether the stretch-shortening cycle (SSC) contraction is integrated in neuromuscular activation in upper body muscles during double poling in cross-country skiing. Thirteen elite skiers performed double poling roller-skiing at increasing treadmill velocities of 9, 15, 21, 27 km h⁻¹ and their individual maximal velocity. Elbow angle, axial pole force and surface EMG in the triceps brachii, pectoralis major, latissimus dorsi and teres major muscle were recorded. Increases in peak pole force, rate of force development and elbow flexion angular velocities were identified (P < 0.05). The mean MVC-normalized EMG amplitudes increased during the pre-activation phase before pole plant, elbow flexion and the reflex-mediated phase between 30 and 120 ms after pole plant due to velocity increases (P < 0.05). It is thus suggested that elite cross-country skiers use SSC during double poling, particularly in the triceps muscle in order to generate high forces.     

The influence of agonist premotor silence and the stretch-shortening cycle on contractile rate in active skeletal muscle

Summary Agonist premotor silence (PMS), a brief period of relative quiescence in active skeletal muscle prior to phasic activation, was investigated in subjects performing maximal contractions. The frequency of occurrence and potential function of the silent period were examined for elbow flexions and extensions. PMS was evident for movements in both directions, indicating that the mechanism is not primarily limited to extensors as previously hypothesized. Flexions demonstrating PMS exhibited increased velocity and acceleration; however, kinematic facilitation was only evident on trials exhibiting the muscular stretch-shortening cycle (SSC). The SSC was present on trials lacking PMS, demonstrating that biceps and triceps silence are not the sole determinants of preparatory agonist lengthening for elbow flexions and extensions, respectively. Taken together, the data indicate that agonist PMS is a mechanism under apparent central control that acts concomitantly with mechanical factors to potentiate elbow flexor contractions.     

Use of a Kin-Com dynamometer to study the stretch-shortening cycle during plantar flexion

Summary The aim of this study was to evaluate the Kin-Com II dynamometer in the study of the stretch-shortening cycle (a concentric muscle action preceded by an eccentric muscle action). Measurements were made of plantar flexion at different angular velocities (120° · s^–1 and 240° · s^–1) with the knee at two different angles (0° and 90°). Ten healthy women ranging in age from 22 to 41 years were studied. Torque values were recorded simultaneously with surface electromyograms (EMG); maximal voluntary concentric torque values were recorded and, after a short rest, the torque values of the concentric action which followed immediately after an eccentric action of the same velocity, both with maximal effort. Mean values were taken at different ankle positions and also averaged over different ranges. A concentric action preceded by an eccentric action generated a torque value on an average about 100% larger than a concentric action alone. The EMG activity was lower or unchanged. It was concluded that the present method could be useful in the study of the stretch-shortening cycle in plantar flexion and in the testing of the behaviour of the elastic components in people with disabilities in the lower limbs.     

Stretch shortening cycle fatigue: interactions among joint stiness,reflex, and muscle mechanical performance in the drop jump

The purpose of the present study was to investigate the effect of strenuous stretch-shortening cycle exercise on the relationship between reflex and stiffness regulation during the drop jump. Ten healthy male subjects performed submaximal stretch-shortening cycle exercise on a special sledge apparatus. Exhaustion occurred on average within 3 min. A drop jump test from a 50-cm height was performed immediately before and after the sledge exercise, as well as 2 h, 2 days and 4 days later. The fatigue exercise showed relatively high blood lactate concentrations 12.5 (SD 2.6) mmol·l^–1 and a 2-day delayed increase of serum cretaine kinase concentration. In drop jumps, the short latency M1 component of the vastus lateralis muscle electromyogram (EMG) response showed a continuous decline throughout the entire follow-up period after fatigue (NS), whereas the medium latency EMG component increased 2 days after the postfatigue sessions (P < 0.05). Immediately after the fatigue exercise a positive correlation (P < 0.05) was found between the changes in the short latency EMG response and in the amount of knee joint stiffness during the early post-landing phase of the drop jump. This suggests that the M1 response was closely related to the stiffness changes during the initial braking phase of the drop jump. Increase of creatine kinase concentration on the 2nd day correlated negatively with the changes in the drop jump performance (P < 0.05). Since the short latency EMG component has almost recovered on the 2nd day, impairment of the mechanical function of the muscle might have taken place. In conclusion, exhausting stretch-shortening cycle exercise induced local muscle impairment, which resulted in modulation of the reflex and stiffness interaction in the drop jump as well as compensation by central motor command.     

Interaction between pre-landing activities and stiffness regulation of the knee joint musculoskeletal system in the drop jump: implications to performance

The purpose of the present study was to investigate the interaction between the pre-landing activities and the stiffness regulation of the knee joint musculoskeletal system and the takeoff speed during a drop jump (DJ). Nine healthy male subjects performed a DJ test from the height of 50 cm. The surface electromyographic (EMG) activity of the vastus lateralis (VL) muscle was recorded to evaluate both the pre-landing and post-landing muscle activation levels. Simultaneous recording of the jumping motion and ground reaction force was performed by a high-speed video camera (100 frames·s^–1), and a force platform was employed to allow joint moment analysis. Joint stiffness was calculated by a linear regression of the knee joint moment/angle relationship. Elasticity of the knee extensor muscle during DJ was estimated by means of a four-element muscle model consisting of a parallel elastic component, a series elastic component (SEC), a viscous damper, and a contractile element. DJ performance correlated positively with the positive peak power of the knee joint (P<0.01) and with the moment of the knee joint at the end of stretch (P<0.01). However, there was no significant relationship between DJ performance and the positive peak power of the ankle joint. The knee joint moment at the end of stretch correlated with the SEC stiffness during the transmission phase from the end of the initial impact to the onset of the concentric action (P<0.01) and with the maximum rate of isometric force development of the knee extensors (P<0.01). Multiple regression analysis showed that the SEC stiffness during the transmission phase of the knee joint can be explained by a combination of the pre-activity of the VL muscle and the knee joint angular velocity at touchdown (F=5.76, P<0.05). These results seem to emphasize the functional significance of the pre-programmed activity for controlling the subsequent stiffness regulation and then contributing to the performance in DJ. Thus, it can be suggested that the centrally pre-programmed activity and the associated elastic behavior of the SEC in the knee extensor muscle in conjunction with the muscle contractile property play a major role in regulating the performance in DJ. Electronic Publication     

Mechanical activation and isometric oscillation in insect fibrillar muscle

Summary In isolated contractile structures of insect fibrillar muscle (DLM of Lethocerus) a quick extension causes a delayed rise of tension which is often followed by (damped) isometric oscillations. Similar phenomena may be also observed after a quick release which causes an immediate tension fall followed by a small and nearly immediate tension recovery passing into oscillation. The oscillation frequency has a temperature coefficient (Q ₁₀) of about 3.5 but depends also on the nature of the muscle (8 Hz in DLM of Lethocerus maximus at 20° C; 14 Hz in DLM of Lethocerus annulipes); it is further affected by the ATPase reaction products but not by calcium ions (range 10^–6–10^–5 M) nor by stretch, and it corresponds to the optimum myogenic oscillation frequency in driven oscillation experiments at 0.5% length change as well as to the wingbeat frequency of Lethocerus. All these findings agree with a previous hypothesis (Pringle) that myogenic oscillation — at least at low amplitude and optimal frequency — may be due to a more or less synchronized cross-bridge action. Evidence for partial cross-bridge synchronization during isometric oscillation stems from ATPase estimations in conjunction with measurements of the immediate elasticity (stiffness); these indicate that a maximal number of cross-bridges attaches almost immediately after stretch activation, and nearly in synchrony. The immediate series elasticity determined by quick releases suggests cross-bridge movements of the order 100 Å, in an oscillatory cycle. It is also concluded that contraction speed but not contractile tension is dependent on the turnover frequency of cross-bridges, if, indeed, the latter is reflected in the isometric oscillation frequency.Supported by the Deutsche Forschungsgemeinschaft (Grant RU 154/3-6).     

Neuromuscular function and mechanical efficiency of human leg extensor muscles during jumping exercises

The influence of prestretch amplitude on the mechanical efficiency was examined with 5 subjects, who performed 5 different series of vertical jumps, each of which differed with respect to the mechanics of the knee joint action during the prestretch (eccentric) phase of the contact on the floor. Electromyographic activity was recorded from the major extensor muscles during the entire work period of 1 min per series. In addition, expired air was collected during the test and recovery for determination of energy expenditure. Mechanical work was calculated from the vertical displacement of the body during the jumps. The results indicated that high net efficiency of 38.7% was observed in condition where amplitude of knee bending in eccentric phase was small. In large range motion the corresponding net efficiency was 30.1%. In jumps where no prestretching of extensor muscles ocurred the net efficiency was 19.7%. The high efficiency of small amplitude jumps was characterized by low myoelectrical activity of the leg extensor muscles during the positive (concentric) work phase. In addition, the small amplitude jumps had shorter transition time in the stretch-shortening cycle, high average eccentric force and high stretching speed. Therefore the results suggest that the restitution of elastic energy, which was also related to the length change and stiffness of the muscles during stretch, plays an important role in regulating the mechanical efficiency of work.     

Effects of long- and short-term fatiguing stretch-shortening cycle exercises on reflex EMG and force of the tendon-muscle complex

This study examined the fatigue effects of stretch-shortening cycle exercises of different intensity and duration on stretch reflex EMG and mechanical responses of the triceps surae muscle. Twelve subjects performed either a 10-km run (n=6) or short but exhaustive rebound exercise on a sledge apparatus (n=6). Passive reflex tests (mechanically induced ankle dorsiflexions) were examined before, after as well as 2 h, 2 and 7 days after exercise. Mechanical reflex responses were recorded from the ergometer torque signal. An acute contractile failure was observed as large reductions in twitch responses, especially in the sledge subgroup who showed high post-exercise peak blood lactate and an increased EMG/torque ratio. Independently of the exercise, the delayed fatigue analysis revealed strong relationships between the reflex-induced EMG and mechanical changes. In addition to muscle damage, these results may be explained by inhibitory effects via the sensitisation of small muscle afferents particularly during the exercise-induced delayed recovery process.     

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     

Limitation of jaw movement by antagonist muscle stiffness during unloading of human jaw closing muscles

Summary The unloading reflex in the jaw closing muscles in man was investigated with a view to correlating the jaw closing movement with the timing of the electrical activity of the agonist and antagonist muscles. When the resistance to a forceful isometric bite was suddenly and unexpectedly withdrawn, the closing movement of the mandible was always arrested before the teeth came together. The rapid arrest of the jaw closing movement was not adequately accounted for by the timing of the inhibition of the jaw closing muscles and reflex excitation of the jaw opening muscles. It was observed that the jaw opening muscles, as well as the jaw closing muscles, were always active during the phase of isometric biting on an object between the teeth. It is therefore concluded that the resulting stiffness of the antagonist muscles is the mechanism which is principally responsible for limiting the jaw closing movement after unloading of the agonist muscles.Supported by the National Health and Medical Research Council of Australia, and the Australian Dental Education and Research Trust     

Absence of an effect of fatigue on muscle efficiency during high-intensity exercise in rat skeletal muscle

The effect of fatigue was studied on rat skeletal muscle efficiency during maximal dynamic exercise of 10s duration. After the initial 4s of exercise, power output decreased rapidly to 46.2 ± 6.7% (mean ± SD; n=6) after 6s of stimulation and further to 17.5 ± 5.8% in the last contraction. Both the rates of total work output and high-energy phosphate consumption decreased with increasing exercise duration. As a result muscle efficiency was not affected by exercise time in the present experiments. This result indicates that fatigue in severe maximal exercise is induced by a feed-back mechanism, which in the case of high ATP utilisation rates will reduce ATP splitting probably by reducing Ca²⁺-release from the sarcoplasmic reticulum.     

Aging process of leg muscle endurance in males and females

Summary In order to estimate muscle endurance during knee extension (KME) and plantar flexion (PME) using a simple method, holding times for the half-squatting posture for KME and for the calf-raising posture for PME were measured in 3,792 males and 3,620 females aged between 6 and 79 years. The regression lines of the aging process for KME and PME in both sexes were determined. Peak values were found at 13 years in males and 12 years in females for KME, and at 15 years in males and 16 years in females for PME. The mean values increased sharply with age up to each peak and then decreased abruptly to a low level at around 20 years, thereafter declining gradually. The values of KME as a percentage of the peak value were 71, 75 and 34 at the ages of 6, 20 and 70–79 years respectively in males, while the corresponding values were 80, 77 and 36 for females. The values of PME at these ages were 34, 93 and 69 in males and 54, 91 and 73 in females. The mean values for males were generally higher than those for females. These results suggest that endurance of the leg muscles differs according to age, sex and the region of the leg muscle. These data will be useful as standard values for evaluating endurance and for prescribing suitable exercise for the promotion of physical fitness.