The effect of fast and slow motor unit activation on whole-muscle mechanical performance: the size principle may not pose a mechanical paradox.

The output of skeletal muscle can be varied by selectively recruiting different motor units. However, our knowledge of muscle function is largely derived from muscle in which all motor units are activated. This discrepancy may limit our understanding of in vivo muscle function. Hence, this study aimed to characterize the mechanical properties of muscle with different motor unit activation. We determined the isometric properties and isotonic force-velocity relationship of rat plantaris muscles in situ with all of the muscle active, 30% of the muscle containing predominately slower motor units active or 20% of the muscle containing predominately faster motor units active. There was a significant effect of active motor unit type on isometric force rise time (p < 0.001) and the force-velocity relationship (p < 0.001). Surprisingly, force rise time was longer and maximum shortening velocity higher when all motor units were active than when either fast or slow motor units were selectively activated. We propose this is due to the greater relative effects of factors such as series compliance and muscle resistance to shortening during sub-maximal contractions. The findings presented here suggest that recruitment according to the size principle, where slow motor units are activated first and faster ones recruited as demand increases, may not pose a mechanical paradox, as has been previously suggested.

Modeling muscle function using experimentally determined subject-specific muscle properties.

Muscle models are commonly based on intrinsic properties pooled across a number of individuals, often from a different species, and rarely validated against directly measured muscle forces. Here we use a rich data set of rat medial gastrocnemius muscle forces recorded during in-situ and in-vivo isometric, isotonic, and cyclic contractions to test the accuracy of forces predicted using Hill-type muscle models. We identified force-length and force-velocity parameters for each individual, and used either these subject-specific intrinsic properties, or population-averaged properties within the models. The modeled forces for cyclic in-vivo and in-situ contractions matched with measured muscle-tendon forces with r2 between 0.70 and 0.86, and root-mean square errors (RMSE) of 0.10 to 0.13 (values normalized to the maximum isometric force). The modeled forces were least accurate at the highest movement and cycle frequencies and did not show an improvement in r2 when subject-specific intrinsic properties were used; however, there was a reduction in the RMSE with fewer predictions having higher errors. We additionally recorded and tested muscle models specific to proximal and distal regions of the muscle and compared them to measures and models from the whole muscle belly: there was no improvement in model performance when using data from specific anatomical regions. These results show that Hill-type muscle models can yield very good performance for cyclic contractions typical of locomotion, with small reductions in errors when subject-specific intrinsic properties are used.

The influence of strain and activation on the locomotor function of rat ankle extensor muscles.

The ankle extensor muscles of the rat have different mechanical and physiological properties, providing a means of studying how changes in locomotor demands influence muscle fascicle behaviour, force and mechanical power output in different populations of muscle fibre types. Muscle fascicle strain, strain rate and activation patterns in the soleus, plantaris and medial gastrocnemius muscles of the rat were quantified from sonomicrometric and myoelectric data, collected during treadmill locomotion under nine velocity/incline conditions. Significant differences in peak-to-peak muscle fascicle strains and strain rates were identified between the three muscles (P<0.001, all cases), with much smaller strains (<0.1) and strain rates (<0.5 s(-1)) occurring in soleus and plantaris compared with medial gastrocnemius (>0.2 and >1.0 s(-1), respectively). The proportion of stride duration that each muscle was active (duty cycle) differed between locomotor conditions as did the timing of the activation and deactivation phases. A simple Hill-based muscle model was used to determine the influence of muscle activation relative to maximum fascicle strain and duty cycle on total force production and mechanical power output, from a slow and a fast muscle fibre, simulated through two peak-to-peak strain cycles (0.1 and 0.3). The predictions of the model did not complement conclusions that may be drawn from the observation of myoelectric timing and fascicle strain trajectories in each of the muscles. The model predicted that changes in mechanical power output were more sensitive to changes in activation parameters than to changes in strain trajectories, with subtle changes in activation phase and duty cycle significantly affecting predicted mechanical power output.

Muscle coordination is key to the power output and mechanical efficiency of limb movements.

The purpose of this study was to determine which features of muscle mechanics and muscle coordination affect the power output from a limb during locomotion. Eight subjects were tested while cycling at maximum exertion for 25 min on a stationary dynamometer. Cadence and load were varied to span a range of power outputs and myoelectric activity was measured from 10 muscles in the leg. Cycle-by-cycle variations in muscle coordination, cadence and power output were observed and the EMG intensity across all muscles was used as an estimate of the metabolic cost for each cycle. Data for the cycles at greatest power output were separated into three groups: maximum power, 80% power but lower EMG intensity and 80% power and higher EMG intensity. Torque-angular velocity relations were determined for the ankle and knee joints. During cycling at maximum power output the ankle joint was not extending at the velocity necessary for maximum power output; thus, maximum limb power occurs when some of the individual muscles cannot be generating maximum power output. Increases in EMG intensity occurred with no increase in power output from the limb: these corresponded to decreases in the efficiency and changes in coordination. Increases in power were achieved that were not matched by equivalent increases in EMG intensity, but did occur with changes in coordination. It is proposed that the power output from the limb is limited by the coordination pattern of the muscles rather than the maximum power output from any one muscle itself.

Comparative anatomy and muscle architecture of selected hind limb muscles in the Quarter Horse and Arab.

The Quarter Horse (bred for acceleration) and the Arab (bred for endurance) are situated at either end of the equine athletic spectrum. Studies into the form and function of the leg muscles in human sprint and endurance runners have demonstrated that differences exist in their muscle architecture. It is not known whether similar differences exist in the horse. Six Quarter Horse and six Arab fresh hind limb cadavers were dissected to gain information on the muscle mass and architecture of the following muscles: gluteus medius; biceps femoris; semitendinosus; vastus lateralis; gastrocnemius; tibialis cranialis and extensor digitorum longus. Specifically, muscle mass, fascicle length and pennation angle were quantified and physiological cross-sectional area (PCSA) and maximum isometric force were estimated. The hind limb muscles of the Quarter Horse were of a significantly greater mass, but had similar fascicle lengths and pennation angles when compared with those of the Arab; this resulted in the Quarter Horse hind limb muscles having greater PCSAs and hence greater isometric force potential. This study suggests that Quarter Horses as a breed inherently possess large strong hind limb muscles, with the potential to accelerate their body mass more rapidly than those of the Arab.

Regionalisation of the muscle fascicle architecture in the equine longissimus dorsi muscle.

REASON FOR PERFORMING STUDY: The longissimus dorsi is the largest muscle in the equine back and plays an important role in locomotor ability and performance in the horse. In vivo studies suggest that the mechanical function varies between different muscle segments, in part determined by anatomy. It is possible therefore that variations in function reflect variations in the anatomy of the longissimus dorsi along its length. OBJECTIVES: To identify if there are regional variations in muscle architecture of the longissimus dorsi. METHODS: Computed tomography scans were obtained from 8 cadaver backs to identify the cross-sectional area and volume of the muscle. The 3D orientations of the muscle fascicles were quantified in situ by their direction cosines using a magnetic digitisation system. RESULTS: Mean +/- s.e. volume of the muscle was 2.34 +/- 0.18 l for each side and mean length of the vertebral column from T4 to L6 dorsal spinous processes was 56.57 +/- 2.0 cm. There were significant differences in the cross-sectional area and the moment arm lengths between muscle segments (P<0.05). In the lumbar region, the mediolateral moment arms were 2-3 times greater than the dorsoventral moment arms. There were significant variations in the direction cosines (n = 7129) between muscle segments and between different regions within each segment (P<0.05). POTENTIAL RELEVANCE: These observations indicate that the muscle fascicle architecture predisposes the longissimus dorsi to different functions both along its length and between different regions within each segment. Detailed 3D anatomical measures of the structure with in vivo measures of function (back motion and muscle activity) will be investigated in further studies.

Kinematic analysis of video-captured falls experienced by older adults in long-term care.

Falls cause 95% of hip and wrist fractures and 60% of head injuries in older adults. Risk for such injuries depends in part on velocity at contact, and the time available during the fall to generate protective responses. However, we have no information on the impact velocities and durations of falls in older adults. We addressed this barrier through kinematic analysis of 25 real-life falls (experienced by 23 individuals of mean age 80 years (SD=9.8)) captured on video in two long-term facilities. All 25 falls involved impact to the pelvis, 12 involved head impact, and 21 involved hand impact. We determined time-varying positions by digitizing each video, using direct linear transformations calibrated for each fall, and impact velocities through differentiation. The vertical impact velocity averaged 2.14 m/s (SD=0.63) for the pelvis, 2.91 m/s (SD=0.86) for the head, and 2.87 m/s (SD=1.60) for the hand. These values are 38%, 28%, and 4% lower, respectively, than predictions from an inverted pendulum model. Furthermore, the average pelvis impact velocity was 16% lower than values reported previously for young individuals in laboratory falling experiments. The average fall duration was 1271 ms (SD=648) from the initiation of imbalance to pelvis impact, and 583 ms (SD=255) from the start of descent to pelvis impact. These first measures of the kinematics of falls in older adults can inform the design and testing of fall injury prevention interventions (e.g., hip protectors, helmets, and flooring).

Patterns of muscle coordination during stepping responses post-stroke.

This study compared self-induced stepping reactions of seventeen participants after stroke and seventeen controls. Surface electromyographic (EMG) signals were recorded bilaterally from the soleus (SOL), tibialis anterior (TA), biceps femoris (BF) and rectus femoris (RF) muscles. Principal component analysis (PCA) was used to reduce the data into muscle activation patterns and examine group differences (paretic, non-paretic, control leg). The first principal component (PC1) explained 46.7% of the EMG signal of the stepping leg. Two PCs revealed distinct activation features for the stepping paretic leg: earlier TA onset at step initiation and earlier BF and SOL onset at mid-step. For the stance leg, PC1 explained 44.4% of the EMG signal and significant differences were found in the non-paretic leg compared to paretic (p < 0.001) and control (p < 0.001). In PC1, at step onset the BF and SOL EMG and the RF and TA EMG were increased over the latter half of the step. No PC loadings were distinct for the paretic leg during stance, however differences were found in the non-paretic leg: earlier TA burst and increased BF and SOL EMG at step initiation. The results suggest impairments in the paretic leg when stepping and compensatory strategies in the non-paretic stance leg.

Modification of soft tissue vibrations in the leg by muscular activity.

Vibration characteristics were recorded for the soft tissues of the triceps surae, tibialis anterior, and quadriceps muscles. The frequency and damping of free vibrations in these tissues were measured while isometric and isotonic contractions of the leg were performed. Soft tissue vibration frequency and damping increased with both the force produced by and the shortening velocity of the underlying muscle. Both frequency and damping were greater in a direction normal to the skin surface than in a direction parallel to the major axis of each leg segment. Vibration characteristics further changed with the muscle length and between the individuals tested. The range of the measured vibration frequencies coincided with typical frequencies of impact forces during running. However, observations suggest that soft tissue vibrations are minimal during running. These results support the strategy that increases in muscular activity may be used by some individuals to move the frequency and damping characteristics of the soft tissues away from those of the impact force and thus minimize vibrations during walking and running.

Muscle activity in the leg is tuned in response to ground reaction forces.

During walking and running, the human body reacts to its external environment. One such response is to the impact forces that occur at heel strike. This study tested previous speculation that the levels of muscle activity in the lower extremities are adjusted in response to the loading rate of the impact forces. A pendulum apparatus was used to deliver repetitive impacts to the heels of 20 subjects. Impact forces were of similar magnitude to those experienced during running, but the loading rate was varied by 13% using different materials in the subjects' shoes. Myoelectric patterns were measured in the tibialis anterior, medial gastrocnemius, vastus medialis, and biceps femoris muscles. Wavelet analysis was used to resolve intensity of the myoelectric patterns into time and frequency space. Substantial and significant differences in the myoelectric activity occurred between the impact conditions for the 50 ms before and the 50 ms after impact, reaching 3 ms in timing, 16% in wavelet number, and 154% in the intensity of the muscle activity.

Soft-tissue vibrations in the quadriceps measured with skin mounted transducers.

The purpose of this study was to develop a method to characterize the frequency and damping of vibrations in the soft tissues of the leg. Vibrations were measured from a surface-mounted accelerometer attached to the skin overlying the quadriceps muscles. The free vibrations in this soft tissue were recorded after impact whilst the muscle was performing isometric contractions at 0, 50, and 100% maximum voluntary force and with the knee held at 20, 40, and 60 degrees angles of flexion. The acceleration signals indicated that the soft tissue oscillated as under-damped vibrations. The frequency and damping coefficients for these vibrations were estimated from a model of sinusoidal oscillations with an exponential decay. This technique resolved the vibration coefficients to 2 and 7% of the mean values for frequency and damping, respectively.

Variations in motor unit recruitment patterns occur within and between muscles in the running rat (Rattus norvegicus).

Motor units are generally considered to follow a set, orderly pattern of recruitment within each muscle with activation occurring in the slowest through to the fastest units. A growing body of evidence, however, suggests that recruitment patterns may not always follow such an orderly sequence. Here we investigate whether motor unit recruitment patterns vary within and between the ankle extensor muscles of the rat running at 40 cm s(-1) on a level treadmill. In the past it has been difficult to quantify motor unit recruitment patterns during locomotion; however, recent application of wavelet analysis techniques has made such detailed analysis of motor unit recruitment possible. Here we present methods for quantifying the interplay of fast and slow motor unit recruitment based on their myoelectric signals. Myoelectric data were collected from soleus, plantaris and medial gastrocnemius muscles representing populations of slow, mixed and fast fibres, respectively, and providing a good opportunity to relate myoelectric frequency content to motor unit recruitment patterns. Following wavelet transformation, principal component analysis quantified signal intensity and relative frequency content. Significant differences in signal frequency content occurred between different time points within a stride (P<0.001). We optimised high- and low-frequency wavelets to the major signals from the fast and slow motor units. The goodness-of-fit of the optimised wavelets to the signal intensity was high for all three muscles (r2>0.98). The low-frequency band had a significantly better fit to signals from the soleus muscle (P<0.001), while the high-frequency band had a significantly better fit to the medial gastrocnemius (P<0.001).

Biomechanics of fast-start swimming in fish.

Fast-starts are high acceleration manoeuvres used by fish. Fast-start escape responses can be initiated by a nervous reflex mediated by a Mauthner neurone and result in the simultaneous activation of muscle along one side of the body. Such starts have traditionally been termed 'C' starts where the fish initially bends to a tight C shape and then subsequently gains propulsion away from the stimulus. However, close examination reveals that propulsion occurs even during the initial phase of body bending. In order to generate this forward propulsion the fish must flex with a rearward travelling wave of body bending, and power generated by the muscles in the central region of the trunk must be delivered to the water through the caudal region of the body. Despite simultaneous muscle activation along the length of the fish, a wave of body bending can result from the interaction of the muscle torque acting to bend the fish and the inertia of the body and water acting to resist this bending. The Mauthner neurone causes the muscle along one side of the fish to be activated immediately prior to shortening and so there is no difference in timing between muscle activation and onset of shortening for this initial contraction along the whole length of the body. Although the fish flexes to tighter curvatures towards its caudal region, the white muscle strain is reduced in this region due to the effect of the geometry of the body and muscle. The myotomal muscle generates greater mass-specific power outputs in the central region of the fish, and greater stresses towards the caudal region and this difference in muscle function can be entirely explained by the change in muscle strain. This change in muscle function enables the high muscle powers to be delivered to the water through the caudal region of the fish where it will cause the fast-start acceleration.

Impact forces and muscle tuning: a new paradigm.

Impact forces and muscle tuning: a new paradigm. Exerc. Sport Sci. Rev., Vol. 29, No. 1, pp 37-41, 2001. We propose that repetitive impact forces during physical activities are not important from an injury perspective but are the reason for changes in myoelectric activity (muscle tuning) to minimize soft tissue vibrations. Changes in myoelectric activity (intensity, frequency, timing), comfort, and performance provide supporting evidence for this new paradigm.

Muscle power output limits fast-start performance in fish.

Fast-starts associated with escape responses were filmed at the median habitat temperatures of six teleost fish: Notothenia coriiceps and Notothenia rossii (Antarctica), Myoxocephalus scorpius (North Sea), Scorpaena notata and Serranus cabrilla (Mediterranean) and Paracirrhites forsteri (Indo-West-Pacific Ocean). Methods are presented for estimating the spine positions for silhouettes of swimming fish. These methods were used to validate techniques for calculating kinematics and muscle dynamics during fast-starts. The starts from all species show common patterns, with waves of body curvature travelling from head to tail and increasing in amplitude. Cross-validation with sonomicrometry studies allowed gearing ratios between the red and white muscle to be calculated. Gearing ratios must decrease towards the tail with a corresponding change in muscle geometry, resulting in similar white muscle fibre strains in all the myotomes during the start. A work-loop technique was used to measure mean muscle power output at similar strain and shortening durations to those found in vivo. The fast Sc. notata myotomal fibres produced a mean muscle-mass-specific power of 142.7 W kg-1 at 20 degrees C. Velocity, acceleration and hydrodynamic power output increased both with the travelling rate of the wave of body curvature and with the habitat temperature. At all temperatures, the predicted mean muscle-mass-specific power outputs, as calculated from swimming sequences, were similar to the muscle power outputs measured from work-loop experiments.

The biomechanics and evolutionary significance of thermal acclimation in the common carp Cyprinus carpio.

The effects of thermal acclimation were investigated in the common carp Cyprinus carpio L. Acclimation and acute temperature effects were tested during ontogeny from larval [9.5 mm total length (L)] to juvenile (69.0 mm L) stages and between 8 and 21 degrees C. The myosin heavy chain (MHC) composition, myofibrillar Mg(2+)-Ca(2+)-ATPase activity, and muscle strains showed significant thermal acclimation effects. MHCs were only expressed in an acclimation temperature-dependent fashion in fish longer than 37 mm. During fast starts, the temperature had a significant effect on the white muscle strain (33% increase and 50% decrease with increasing acclimation and acute temperature, respectively) and contraction duration (25% decrease with increasing acute temperature). Increases in hydrodynamic efficiency (0.19 to 0.38) and hydrodynamic power requirements (Q(10) = 3.2) occurred with increasing acute temperature (10 to 20 degrees C). Competing hypotheses about the evolutionary significance of the temperature acclimation response were tested. Acclimation extended the temperature range for fast-start behavior, but no improvements in performance at the whole animal level were found between 8 and 21 degrees C.

Surface EMG shows distinct populations of muscle activity when measured during sustained sub-maximal exercise.

Muscle activity patterns were measured from the rectus femoris, biceps femoris, tibialis anterior and gastrocnemius medialis muscles using electromyography (EMG). Recordings were made from six runners while they ran for 30 mm around a track at a constant, self-selected pace. They were made from three sites along each muscle belly, and for ten consecutive steps on each 450 m lap completed. The intensity of the myoelectric signals was resolved into components in time and frequency space using wavelet analysis. Distinct populations of high- (170-220 Hz) and low-frequency (40-60 Hz) components could be seen in the frequency spectra. There was no significant effect of the electrode position on the rates of change of EMG intensity for any of the muscles, and there was no significant difference in these rates between a 150-ms window at the end of the swing phase and a 150-ms window during the stride phase of running. There were significant differences in the way in which each subject responded to the task, and the way in which the FMG intensities changed at the different frequency bands. There was a significant reduction in EMG intensity at low frequencies and a significant increase at high frequencies, and these changes were ubiquitous for all four muscles tested. The frequencies that showed the greatest changes coincided with the frequencies where distinct populations of activity occurred within the signal. These changes in muscle activity are different from those seen for maximal fatiguing contractions. However, they suggest that the patterns of muscle recruitment may change during sustained sub-maximal exercise.

An integrative study of the temperature dependence of whole animal and muscle performance during jumping and swimming in the frog Rana temporaria.

The aims of this study were: (1) to analyze individual variation in frog locomotor performance, (2) to compare the thermal sensitivity of jumping and swimming, and (3) to contrast whole animal versus muscle fiber performance at different temperatures. The jumping and swimming performance of Rana temporaria was analyzed at 5, 10, 15 and 20 degrees C. Muscle fiber bundles were isolated from lateral gastrocnemius and subjected to the length and activation patterns thought to occur in vivo. As temperature increased, locomotor performance in R. temporaria improved with a Q10 of 1.2 for both jump take-off velocity and mean swimming velocity. The slope of the relationship between performance and temperature (TE) was similar for both locomotor parameters and was described by the equation z-scores of locomotor performance = 0.127 x TE - 1.585. Although some frogs performed better than others relative performance was affected by locomotor type and temperature. Locomotor performance improved with temperature as the power required during take-off and the mean muscle power output increased with Q10 values of 1.7 and 1.6 respectively. The mean muscle power output during take-off was only 34% of the calculated requirements for the whole animal, suggesting the involvement of elastic strain energy storage mechanisms.