Paavo V. Komi

Electromechanical delay in human skeletal muscle under concentric and eccentric contractions

SummaryIn contraction of skeletal muscle a delay exists between the onset of electrical activity and measurable tension. This delay in electromechanical coupling has been stated to be between 30 and 100 ms. Thus, in rapid movements it may be possible for electromyographic (EMG) activity to have terminated before force can be detected. This study was designed to determine the dependence of the EMG-tension delay upon selected initial conditions at the time of muscle activation. The rigth forearms of 14 subjects were passively oscillated by a motor-driven dynamometer through flexion-extension cycles of 135 deg at an angular velocity of ≈0.5 rad/s. Upon presentation of a visual stimulus the subjects maximally contracted the relaxed elbow flexors during flexion, extension, and under isometric conditions. The muscle length at the time of the stimulus was the same in all three conditions. An on-line computer monitoring surface EMG (Biceps and Brachioradialis) and force calculated the electromechanical delay. The mean value for the delay under eccentric condition, 49.5 ms, was significantly different (p<0.05) from the delays during isometric (53.9 ms) and concentric activity (55.5 ms). It is suggested that the time required to stretch the series elastic component (SEC) represents the major portion of the measured delay and that during eccentric muscle activity the SEC is in a more favorable condition for rapid force development.

Strength and power in sport

List of Contributors. Preface. Units of Measurement and Terminology. Part 1: Definitions. 1 Basic Definitions for Exercise. H.G. KNUTTGEN AND P.V. KOMI. . Part 2: Biological Basis for Strength and Power. 2 Neuronal control of functional movement. VOLKER DIETZ. 3 Motor Unit and Motor Neuron Excitability during Explosive Movement. TOSHIO MORITANI. 4 Muscular Basis of Strength. R. BILLETER AND H. HOPPELER. 5 Hormonal Mechanisms Related to the Expression of Muscular Strength and Power. WILLIAM J. KRAEMER AND SCOTT A. MAZZETTI. 6 Exercise--related Adaptations in Connective Tissue. RONALD F. ZERNICKE AND BARBARA LOITZ--RAMAGE. 7 Contractile Performance of Skeletal Muscle Fibres. K.A. PAUL EDMAN. 8 Skeletal Muscle and Motor Unit Architecture: Effect on Performance. RONALD R. ROY, RYAN J. MONTI, ALEX LAI AND V. REGGIE EDGERTON. 9 Mechanical Muscle Models and Their Application to Force and Power Production. WALTER HERZOG. 10 Stretch--shortening Cyle. PAAVO V. KOMI. 11 Stretch--shortening Cycle Fatigue and its Influence on Force and Power Production. CAROLINE NICOL AND PAAVO. V. KOMI. . Part 3: Mechanism for Adaptation in Strength and Power Training. 12 Cellular and Molecular Aspects of Adaptation in Skeletal Muscle. GEOFFREY GOLDSPINK AND STEPHEN HARRIDGE. 13 Hypertrophy and Hyperplasia. J. DUNCAN MACDOUGALL. 14 Acute and Chronic Muscle Metabolic Adaptations to Strength Training. PER A. TESCH AND BJORN A. ALKNER. 15 Neural Adaptation to Strength Training. DIGBY G. SALE. 16 Mechanism of Muscle and Motor Unit Adaptation to Explosive Power Training. JACQUES DUCHATEAU AND KARL HAINAUT. 17 Proprocetive Training: Considerations for Strength and Power Production. ALBERT GOLLHOFER. 18 Connective Tissue and Bone Response to Strength Training. MICHAEL H. STONE AND CHRISTINA KARATZAFERI. 19 Endocrine Responses and Adaptations to Strength and Power Training. WILLIAM KRAEMER AND NICHOLASS A. RATAMESS. 20 Cardiovascular Responses to Training. STEVEN J. FLECK. . Part 4: Special Problems in Strength and Power Training. 21 Aging and Neuromuscular Adaptation to Strength Training. KEIJO HAKKINEN. 22 Use of Electrical Stimulation in Strength and Power Training. GARY A. DUDLEY AND SCOTT W. STEVENSON. Part 5: Strength and Power Training for Sports. 23 Biomechanics of Strength and Strength Training. VLADIMIR M. ZATSIORSKY. 24 Vibration Loads: Potential for Strength and Power Development. JOACHIM MESTER P.(?) SPITZENPFEIL AND ZENGYOUAN YUE. 25 Training for Weightlifting. JOHN GARHAMMER AND BOB TAKANO

Biomechanics of sprint running. A review.

SummaryUnderstanding of biomechanical factors in sprint running is useful because of their critical value to performance. Some variables measured in distance running are also important in sprint running. Significant factors include: reaction time, technique, electromyographic (EMG) activity, force production, neural factors and muscle structure. Although various methodologies have been used, results are clear and conclusions can be made.The reaction time of good athletes is short, but it does not correlate with performance levels. Sprint technique has been well analysed during acceleration, constant velocity and deceleration of the velocity curve. At the beginning of the sprint run, it is important to produce great force/ power and generate high velocity in the block and acceleration phases. During the constant-speed phase, the events immediately before and during the braking phase are important in increasing explosive force/power and efficiency of movement in the propulsion phase. There are no research results available regarding force production in the sprint-deceleration phase. The EMG activity pattern of the main sprint muscles is described in the literature, but there is a need for research with highly skilled sprinters to better understand the simultaneous operation of many muscles. Skeletal muscle fibre characteristics are related to the selection of talent and the training-induced effects in sprint running.Efficient sprint running requires an optimal combination between the examined biomechanical variables and external factors such as footwear, ground and air resistance. Further research work is needed especially in the area of nervous system, muscles and force and power production during sprint running. Combining these with the measurements of sprinting economy and efficiency more knowledge can be achieved in the near future.

EMG frequency spectrum, muscle structure, and fatigue during dynamic contractions in man.

SummaryFatigue of the vastus lateralis muscle was studied in healthy well-conditioned students, who differed considerably regarding their muscle fibre type distribution. Muscle force decline during repeated maximum voluntary knee extensions at a constant angular velocity (180‡×s−1 or π rad×s−1), using isokinetic equipment, was taken as the criterion for the degree of fatigue. In an attempt to study quantitative as well as qualitative changes in the EMG pattern, integrated EMG (IEMG) and the frequency of the mean power (MPF), computed from the power spectral density function (PSDF), were analysed. It was found that individuals with muscles made up of a high proportion of fast twitch (FT) muscle fibres demonstrated higher peak knee extension torque, and a greater susceptibility to fatigue than did individuals with muscles mainly composed of slow twitch (ST) muscle fibres. An IEMG decline (p<0.01) was demonstrated during 100 contractions in individuals rich in FT fibres. Only a slight, but not significant, reduction in IEMG occurred in individuals with a high percentage of ST fibres. Concomitantly, MPF decreased (p<0.001) in individuals with a high percentage of FT fibres, while their opposites demonstrated only a slight decrease (non-significant). It is suggested that muscle contraction failure might also be related to qualitative changes in the motor unit recruitment pattern, and that these changes occur more rapidly in muscles composed of a high proportion of FT muscle fibres than in muscles composed of a high proportion of ST fibres.

Electromyographic changes during strength training and detraining.

Fourteen male subjects (20-30 yr) accustomed to weight training went through progressive strength training of combined concentric and eccentric contractions three times per week for 16 wk. The training was followed by the 8-wk detraining period. The training program consisted mainly of dynamic exercises for leg extensors with the loads of 80-120% of one maximum concentric repetition. Significant improvements in muscle function were observed in early conditioning; however, the increase in maximal force during the very late training period was greatly limited. Marked improvements (P less than 0.001) in muscle strength were accompanied by significant (P less than 0.01) increases in the neural activation (IEMG) of the leg extensor muscles. The relationship between IEMG and high absolute forces changed (P less than 0.01) during the training period. The occurrence of these changes varied during the course of training. It was concluded that the early change in strength may be accounted for largely by neural factors with a gradually increasing contribution of hypertrophic factors as the training proceeds. It was suggested that the magnitudes and occurrence of these changes may vary due to the differences in conditioning periods, in individual muscles of muscle groups, in subject material, and in conditioning methods. During detraining, the decrease in muscle force seemed to be explainable also by the neural and muscular adaptations caused by the inactivity.

Effect of Eccentric and Concentric Muscle Conditioning on Tension and Electrical Activity of Human Muscle

The effects of seven weeks of eccentric or concentric muscle conditioning on muscle tension and. integrated electrical activity (IEMG) were investigated on human subjects by using a special electrical dynamometer as a testing and training apparatus. The eccentric conditioning caused, on the average, a greater improvement in muscle tension than did the concentric conditioning. In early conditioning those in the eccentric group experienced soreness in their exercised muscles. This caused a concomitant drop in maximum strength. After the disappearance of pain symptoms, ability to develop tension increased in a linear fashion. Neither method was able to cause statistically significant changes in the maximum IEMG associated with any type of muscle contraction. The regression lino expressing the relationship between IEMG ( μ.v. per sec.) and isometric tension (in percent of maximal voluntary contraction) was parabolic. In this relationship muscle conditioning failed to cause any significant changes in IEMG per ...

Knee and ankle joint stiffness in sprint running

INTRODUCTION Stiffness has often been considered as a regulated property of the neuromuscular system. The purpose of this study was to examine the ankle and knee joint stiffness regulation during sprint running. METHODS Ten male sprinters ran at the constant relative speeds of 70, 80, 90, and 100% over a force platform, and ground reaction forces, kinematic, and EMG parameters were collected. RESULTS The results indicated that with increasing running speed the average joint stiffness (change in joint moment divided by change in joint angle) was constant (7 N x m x deg(-1)) in the ankle joint and increased from 17 to 24 N x m x deg(-1) (P < 0.01) in the knee joint. CONCLUSION The observed constant ankle joint stiffness may depend on (constant) tendon stiffness because of its dominating role in triceps surae muscle-tendon unit. Thus, we conclude that in sprint running the spring-like behavior of the leg might be adjusted by changing the stiffness of the knee joint. However, in complicated motor task, such as sprint running, ankle and knee joint stiffness might be controlled by the individual mechanical and neural properties.

Relevance of in vivo force measurements to human biomechanics

The function and mechanical behaviour of human skeletal muscle are in many ways unknown during natural locomotion. To gain more insight into these questions a method was developed to record directly in vivo forces from the human Achilles tendon (AT). The paper focuses on the details of the various techniques including the design, surgical implantation and calibration of the transducers. The implantation is performed under local anaesthesia and the measurements can last up to three hours, after which the transducer is removed. Exemplar results are presented from the measurements during walking, running and jumping. The loading of AT reached in some cases values as high as 9 KN, corresponding to 12.5 times the body weight or, when expressed per cross-sectional area of the tendon, the value was 11,100 N cm-2. During the early contact phase of running the rate of AT force development increased linearly with the increase of running speed. Indirect measurements of the length changes of the muscle-tendon complex was used to plot the force-length and force-velocity relationships in the various activity situations. The observed results demonstrated that in normal locomotion involving the stretch-shortening cycle (SSC) muscle actions, the mechanical response of the triceps surae muscle is very different from the classical curves obtained in isolated muscle preparations. In agreement with the animal experiments using a similar in vivo technique, the natural locomotion with primarily SSC actions may produce muscle outputs which can be very different from the various conditions of the isolated preparations, where activation levels are held constant and storage and utilization of strain energy is limited. It is suggested that despite some limitations (due to e.g. difficulties in obtaining volunteers for AT force measurements, possible inaccuracies in transducer calibration and in muscle length estimates) the in vivo force measurement technique has an important role in studying the mechanical behaviour of muscle and its control under normal movement conditions.

Mechanical power test and fiber composition of human leg extensor muscles

SummaryThe present study was undertaken to assess the relationship between the mechanical power developed during new anaerobic power test and muscular fiber distribution. Ten track and field male athletes were used as subjects, whose muscle fiber composition (m. vastus lateralis) varied from 25 to 58 fast twitch (FT) fibers. The test consisted of measuring the flight time with a special timer during 60 s continuous jumping. A formula was derived to allow the calculation of mechanical power during a certain period of time (e.g., in the present study every 15 s during 60 s of jumping performance). The relationship between the mechanical power for the first 15 s period correlated best with fast twitch (FT) fiber distribution (r=0.86,p<0.005). However, the power output during the successive 15 s periods demonstrated lower correlation with FT, and this relationship became statistically non-significant after 30 s of work. The sensitivity to fatigue of the test was supported by the relationship observed between the decrease of power during 60 s jumping performance and the percentage of FT fibers (r=0.73,p<0.01). Thus, the present findings suggest that muscular performance, as determined by the new jumping test, is influenced by skeletal muscle fiber composition. The new test, which primarily evaluates maximal short term muscular power, also proved sensitive in assessing fatigue patterns during 60 s of strenuous work.

Reduced stretch-reflex sensitivity after exhausting stretch-shortening cycle exercise

The stretch-shortening cycle (SSC) is an effective and natural form of muscle function but, when repeated with sufficient intensity or duration, it may lead to muscle damage and functional defects. A reduced tolerance to impact has been reported, which may be partly attributed to a reduced stretch-reflex potentiation. The aim of the present study was to examine the influence of SSC-induced metabolic fatigue and muscle damage on the efficacy of stretch reflexes, as judged by the electromyograph (EMG) response of two shank muscles (lateral gastrocnemius LG, soleus SOL) to controlled ramp stretches. These EMG responses were recorded before and immediately after exhausting SSC-type leg exercise and 2 h, 2 days and 4 days later. Serum concentrations of creatine kinase ([CK]), myoglobin and lactate were measured repetitively along the protocol. Two maximal vertical drop jumps and counter-movement jumps were performed after each reflex test. The exhausting SSC-type exercise induced an immediate reduction (P < 0.05) with a delayed short-term recovery of the LG peak-to-peak reflex amplitude. This was not accompanied by significant changes in the reflex latency. The drop jump performance remained slightly but significantly reduced (P < 0.05) until the 2nd day postexercise. Peak [CK] appeared for all the subjects on the 2nd day, suggesting the presence of muscle damage. The increase in [CK] between the 2nd h and the 2nd day postexercise was found to be negatively related (P < 0.001) to the relative changes in the drop jump height. Furthermore, a significant relationship (P < 0.05) was found between recovery of the stretch reflex in LG and the decrease of [CK] between the 2nd and the 4th day. hese findings support the hypothesis of a reduced stretch-reflex sensitivity. While the exact mechanisms of the reflex inhibition remain unclear, it is emphasized that the delayed recovery of the reflex sensitivity could have resulted from the progressive inflammation that develops in cases of muscle damage.