Sex differences in muscle fatigability and activation patterns of the human quadriceps femoris

The purposes of this study were to determine if the fatigability of the quadriceps femoris varies by biological sex under conditions of normal muscle blood flow and ischemia, and if differences in neuromuscular activation patterns exist. Young men and women (n=11/group; age 20–39 years) performed a sustained knee extension contraction at 25% of maximal force under conditions of occluded (OCC) and normal muscle blood flow (NON-OCC). Electromyographic (EMG) activity was recorded from the vastus lateralis (VL), rectus femoris (RF), vastus medialis (VM) and biceps femoris (BF) muscles, and analyzed for fatigue-induced changes in the amplitude and burst rate and duration (transient changes in motor unit recruitment) of the signal. Additionally, force fluctuations during the sustained contraction were quantified. Women had a longer time to task failure during the NON-OCC task [214.9±20.5 vs. 169.1±20.5 (SE) s] (P=0.02), but not during the OCC task (179.6+19.6 vs. 165.2±19.6 s). EMG data demonstrated sex differences in the neuromuscular activation pattern of the RF muscle and the collectively averaged QF muscles. During the NON-OCC and OCC tasks women achieved a higher relative activation of the RF at task failure than men (NON-OCC: 40.68±4.57 vs. 24.49±4.19%; OCC: 36.80±5.45 vs. 24.41±2.12%) (P=0.02 and 0.05, respectively). Also, during both tasks, they demonstrated a greater relative activation at task failure than men when an average of the VL, VM and RF was considered. Additionally, women exhibited a greater coefficient of variation in force fluctuations during the last-third of the fatiguing NON-OCC task (6.21±0.567 vs. 4.56±0.56%) (P=0.001). No sex differences in EMG burst rate or duration were observed, although there was a trend towards greater EMG burst rate of the RF in association with muscle fatigue in the women (P=0.09). Interestingly, the only neuromuscular activation variable that displayed a significant relationship with the time to task failure was the average relative EMG of the QF at task failure, and this relationship was observed under both experimental blood flow conditions (NON-OCC: r=0.47, P=0.03; OCC: r=0.44, P=0.04). These results indicate that sex differences in muscle blood flow and/or muscle metabolism are in part responsible for the female advantage in fatigue-resistance. Additionally, these findings suggest that men synergistically recruit the RF compartment to a lesser extent than women in association with muscle fatigue, and that women achieve an overall greater relative activation of the QF at task failure than men. However, the implications of these sex differences in neuromuscular activation patterns during fatiguing muscular contractions on the ability to withstand muscle fatigue (prolonged time to task failure) does not appear to be causally related.     

Comparison of conventional resistance training and the fly-wheel ergometer for training the quadriceps muscle group in patients with unilateral knee injury

A fly-wheel ergometer (FWE) offering resistance training of the knee extensors has been designed for space travel and found to be effective during bed rest. The possibility exists that this device is also effective in training the knee extensors after knee injury. The purpose of this study was to compare the FWE to standard knee extensor training equipment for their effects on individuals with a history of knee injury, a group who commonly suffer from weakness of the knee extensors that effects their function. Twenty-nine subjects completed the study, which included tests of knee self-assessment, knee extensor static and dynamic muscle strength, size and neural activation as well as single leg power output, standing balance and vertical jump performance. Both groups showed statistically significant (P < 0.05) improvements in these variables over the 3-month training period but no differences were noted between the groups. The FWE appears to be as effective as standard resistance training equipment for improving knee extensor muscle group size and performance after knee injury.     

The relationship between muscle kinetic parameters and kinematic variables in a complex movement

Summary Kinematic variables of the vertical jump (jumping height, jump phase durations and joint angles) were measured on 39 male physical education students. In addition, kinetic parameters of the hip and knee extensors, and of the plantar flexors (maxima voluntary force and its rate of development) were recorded on the same subjects, in isometric conditions. The results demonstrated significant positive correlations between kinetic parameters of the active muscle groups and jumping height (r=0.217−0.464). The dominant effect on these correlations was due to the knee extensors. Correlations between these parameters and the duration of the jump phases were much weaker. Correlation coefficients between kinetic parameters and limb angles in the lowest body position showed that fast force production in one muscle group was related to a significant decrease in the joint angles of distant body segments. Multiple correlation coefficients between leg extensor parameters and kinematic variables (ranging between 0.256 for the duration of the counter-movement phase and 0.616 for jump height) suggested that kinetic parameters could explain more than a quarter of the variability of this complex human movement. Therefore, the conclusion was drawn that an extended set of measurements of the relevant musculo-skeletal system parameters could predict a considerable amount of the variability of human movement. However, high correlation coefficients between the same kinetic parameters of different muscle groups suggest that not all active muscle groups have to be included in the measurements.     

In vivo human triceps surae and quadriceps femoris muscle function in a squat jump and counter movement jump

An optic fibre method was used to measure in humans in vivo Achilles (ATF) and patellar tendon forces (PTF) during submaximal squat jumps (SJ) and counter movement jumps (CMJ). Normal two-legged jumps on a force plate and one-legged jumps on a sledge apparatus were made by four volunteers. Kinetics, kinematics, and muscle activity from seven muscles were recorded. The loading patterns of the tendomuscular system differed among the jumping conditions, but were similar when the jumping height was varied. Peak PTF were greater than ATF in each condition. In contrast to earlier simulation studies it was observed that tendomuscular force could continue to increase during the shortening of muscle-tendon unit in CMJ. The concentric tendomuscular output was related to the force at the end of the stretching phase while the enhancement of the output in CMJ compared to SJ could not be explained by increases in muscle activity. The stretching phase in CMJ was characterised by little or no electromyogram activity. Therefore, the role of active stretch in creating beneficial conditions for the utilisation of elastic energy in muscle was only minor in these submaximal performances. The modelling, as used in the present study, showed, however, that tendon underwent a stretch-shortening cycle, thus having potential for elastic energy storage and utilisation. In general, the interaction between muscle and tendon components may be organised in a manner that takes advantage of the basic properties of muscle at given submaximal and variable activity levels of normal human locomotion. Accepted: 28 June 2000     

Active contraction of cardiac muscle: in vivo characterization of mechanical activation sequences in the beating heart

Progressive alterations in cardiac wall strains are a classic hallmark of chronic heart failure. Accordingly, the objectives of this study are to establish a baseline characterization of cardiac strains throughout the cardiac cycle, to quantify temporal, regional, and transmural variations of active fiber contraction, and to identify pathways of mechanical activation in the healthy beating heart. To this end, we insert two sets of twelve radiopaque beads into the heart muscle of nine sheep; one in the anterior-basal and one in the lateral-equatorial left ventricular wall. During three consecutive heartbeats, we record the bead coordinates via biplane videofluoroscopy. From the resulting four-dimensional data sets, we calculate the temporally and transmurally varying Green–Lagrange strains in the anterior and lateral wall. To quantify active contraction, we project the strains onto the local muscle fiber directions. We observe that mechanical activation is initiated at the endocardium slightly after end diastole and progresses transmurally outward, reaching the epicardium slightly before end systole. Accordingly, fibers near the outer wall are in contraction for approximately half of the cardiac cycle while fibers near the inner wall are in contraction almost throughout the entire cardiac cycle. In summary, cardiac wall strains display significant temporal, regional, and transmural variations. Quantifying wall strain profiles might be of particular clinical significance when characterizing stages of left ventricular remodeling, but also of engineering relevance when designing new biomaterials of similar structure and function.