Task-dependent differences between mono- and bi-articular heads of the triceps brachii muscle

We studied motor-unit recruitment and decruitment thresholds in the three heads of the human elbow extensor, the triceps brachii muscle (caput mediale and laterale, both mono-articular heads, and caput longum, the bi-articular head) by means of intramuscular electromyographic-recordings. Two experiments were performed: an isometric and a movement experiment. In the isometric experiment, subjects were asked to increase the elbow extension torque isometrically to a specific level, keep the torque at the level for 10 s, and then decrease the torque again to zero. In the movement experimental subjects moved their forearm from 90° to 110° extension against an increasing flexion torque, kept the latter position for 10 s and then moved their forearm back while the torque decreased. Results for caput longum showed that recruitment thresholds were higher than decruitment thresholds, whereas in caput mediale and laterale no difference in thresholds was found. In caput longum recruitment thresholds were found to be lower in movement conditions than in isometric conditions. The reverse effect was observed in caput mediale, whereas no difference in recruitment thresholds was found in caput laterale. Our results point to a transfer of force from mono-articular muscles in isometric conditions to bi-articular muscles in movement conditions. A similar transfer is found when recruitment and decruitment are compared. This means that the transfer is not only a property of the elbow-flexor muscles, but is a more common trait. A qualitative analysis of firing frequencies at recruitment and at decruitment in both conditions supports our findings.     

Relationship between shot put performance and triceps brachii fiber type composition and power production

It is commonly accepted that shot put performance is mainly determined by the ability of the lower body to produce power. The purpose of the present study was to investigate the relationship between shot put performance and triceps brachii muscle fiber type composition and strength capacity. Thirteen male physical education students were selected to participate in the study based upon their shot put performance after 5 weeks of shot put technique instruction. At the completion of this technique-instruction period, they performed the following tests: shot put with a 6-kg shot, isokinetic torque measurements of the elbow extensors at 0, 0.52, 1.04, 1.57, 2.09, 3.14, and 4.19 rad·s⁻¹, maximal strength (1 RM) and explosive-throwing bench-press tests, one-arm seated shot put with 1-, 2-, 3-, 4-, 5- and 6-kg shot. Whole-body and dominant upper-arm bioimpedance measurements were used to estimate whole-body and upper-arm muscle mass. Muscle biopsy samples from the long head of the dominant triceps brachii were obtained and analyzed for fiber type composition with ATPase histochemistry. Shot put performance was significantly correlated with type II fiber area (r=0.70, P<0.01), one-arm seated shot put (range r=0.60 to r=0.79, P<0.05), elbow extensors' isokinetic torque (range r=0.65 to r=0.78, P<0.05), bench-press tests (r>0.86, P<0.01) and estimated arm muscle cross-sectional area (r=0.68, P<0.05). These results suggest that fiber type composition and the functional capacity of triceps brachii muscle (e.g., isokinetic torque) explain a part of shot put performance. The magnitude of the correlation coefficients between shot put and the upper-body power tests suggests that other body parts (e.g., lower extremities) may play a significant role in this event.     

Myoelectric response of the human triceps brachii to displacement-controlled oscillations of the forearm

Summary The dynamic relations between the surface myoelectric activity in tonically contracting triceps brachii and the forearm rotation (proportional to triceps stretch) were measured by imposing small, sinusoidal, displacement-controlled perturbations on the forearm position. Three normal, adult, male subjects participated in these experiments. The amplitude of the forearm rotation, the driving frequency, and the tonic contraction level were all carefully regulated. The mean rectified triceps EMG (the output) showed a strong harmonic at the driving frequency, and the frequency-response characteristics were computed directly by comparing the amplitude and phase of this harmonic to that of the forearm flexion angle (the input). The (electrical) reflex gain is defined as the amplitude ratio of output to input. The system response was measured from 2 to 18 Hz, at two tonic contraction levels and two forearm rotation amplitudes, about a mean position of 90° forearm flexion. The results show clearly that the system response is nonlinear: the reflex gain decreases with forearm rotation amplitude. (This gain also increases with tonic contraction level for sufficiently low values of the latter variable.) The measured frequency-response characteristics of the system can be modeled approximately as a second order linear lead filter with a single time delay, followed by a saturating nonlinearity. Both model independent estimates and least-squares model fitting, yielded values of the time delay of the order of 25 ms, suggesting that a segmental mechanism mediates reflex activity. Simplified calculations and limited measurements are presented to show that a nonlinear system of the type we have identified with constant displacement driving may appear linear under constant torque driving. Our directly-measured frequency-response characteristics differ from those reported by investigators employing random, rather than periodic, driving; possible reasons for these apparent discrepancies are discussed.     

Mechanomyogram and force relationship during voluntary isometric ramp contractions of the biceps brachii muscle

The aim of the present study was to examine the non-stationary mechanomyogram (MMG) during voluntary isometric ramp contractions of the biceps brachii muscles using the short-time Fourier transform, and to obtain more detailed information on the motor unit (MU) activation strategy underlying in the continuous MMG/force relationship. The subjects were asked to exert ramp contractions from 5% to 80% of the maximal voluntary contraction (MVC) at a constant rate of 10% MVC/s. The root mean squared (RMS) amplitude of the MMG began to increase slowly at low levels of force, then there was a slight reduction between 12% and 20% MVC. After that, a progressive increase was followed by a decrease beyond 60% MVC. As to the mean power frequency (MPF), a relatively rapid increase up to 30% MVC was followed by a period of slow increment between 30% and 50% MVC. Then temporary reduction at around 50% MVC and a further rapid increase above 60% MVC was observed. The interaction between amplitude and MPF of the MMG in relation to the MU activation strategy is discussed for five force regions defined on the basis of their inflection points in the RMS-amplitude/force and MPF/force relationships. It was found that the MMG during ramp contractions enables deeper insights into the MU activation strategy than those determined during traditional separate contractions. In addition, this contraction protocol is useful not only to ensure higher force resolution in the MMG/force relationship, but also to markedly shorten the time taken for data acquisition and to reduce the risk of fatigue. Accepted: 31 August 2000     

Spatiotemporal surface EMG characteristics from rat triceps brachii muscle during treadmill locomotion indicate selective recruitment of functionally distinct muscle regions

Multichannel surface EMG recordings of a multiheaded skeletal muscle during cyclic locomotion combined with cineradiography were analysed in a chronic experiment. The resulting detailed two-dimensional activation pattern from the long and lateral triceps brachii heads of the rat during treadmill locomotion were combined with gait characteristics and fibre typing of the muscle. Shortly before ground contact of the forelimb, maximum muscle activity was found in the proximal part of the long head of the muscle. During the stance phase maximum activity was observed in the proximal part of the lateral head. The frequency dependent behaviour of cross-covariance functions over both muscle heads confirmed this selective shift in activation. In the lateral triceps brachii head of the investigated rats, exclusively type II fibres were found. In the long head the frequency of type I fibres was the highest in the deep muscle layers, proximally more than distally, whereas type II fibres were dominant in more superficial muscle layers. A combination of physiological and histological findings supports an anticipating mechanism whereby fine-tuning of the vertical foot down manoeuvre is mainly achieved by the (type I fibre dominated) proximal deep compartment of the biarticular long triceps brachii head and force generation is predominantly executed by the monoarticular lateral triceps brachii head.     

Simulation linking EMG power spectra to recruitment and rate coding.

For muscles, a graded increase in motor unit size, with the number of fibres in the motor units forming an arithmetic progression, leads to a model consistent with experimental findings. This model has been combined with electrical signal characteristics of activated muscle fibres and the signal attenuation due to the spread of the electrical signal to bipolar surface electrodes. For the biceps brachii this electromechanical model links motor unit recruitment, rate coding and frequency shifts in the surface EMG power spectra. Derived data suggests that motor unit degeneration results in an abnormally large high frequency EMG component.     

Electrophysiological studies of muscles in the human upper limb: The biceps brachii

The biceps brachii (BB) belongs to elbow flexors. However, because the BB acts not only as the flexor, but also as a forearm supinator in humans, its activities are much different from those of the other flexors (the brachialis and brachioradialis: BR and BRR, respectively). The present paper describes unique characteristics of the BB that emerged from recent studies using electrophysiological techniques (i.e. electromyography (EMG), electrical neuromuscular stimulation (ENS), Hoffmann (H)-reflex and post-stimulus time-histogram (PSTH)) in normal human subjects. The EMG studies have shown reciprocal contractions between the BB and the other flexors during forearm pronation/supination movements. Comparisons of EMG activities of the flexors between the pronated and supinated positions of the forearm have indicated clear differences of contraction properties between the BB and the other flexors. The ENS studies have shown that reciprocal activation between the BB and BRR following stimulation can produce a motion of supination with maintenance of flexion. This finding supports the theory that the reciprocal contractions should occur to keep constant force in flexion for supporting weight below the elbow. Studies using H-reflex and PSTH techniques have shown neural connections, which are spinal reflex arcs modulating motoneuron excitabilities with excitatory and inhibitory inputs from low-threshold muscle afferent fibers, among muscles in the human upper limb. These studies have demonstrated inhibitory neural connections between the BB and BRR. Moreover it has been shown that the connections concerning the BB differ from those concerning the BRR. Several reports have shown differences in connections between humans and animals. Therefore, the BB in humans has been compared with that in animals.     

Intertubercular sulcus of the humerus: biometry and morphology of 100 dry bones

Summary The dimensions and morphology of a critical zone of the intertubercular sulcus (ITS) of the humerus were specified for 100 dry bones. The upper part of the ITS was narrow and at an angle with the lower part. The tendon of the long head of the biceps brachii muscle (TLBB) was particularly worn down during its course through this critical zone. The critical zone extended from the first perceptible proximal depression to the distal extremity of the lesser tubercle. It showed a proximal part of 12.4 mm (standard deviation = 2.23) and a distal part of 15.1 mm (s.d.=3.37). The two parts made an angle of 142° (s.d.=7.87). Its total length was 28.5 mm (s.d.=4.74), depth 2.44 mm (s.d.=0.49), width 6.33 mm (s.d.=0.84). The supratubercular ridge was the only bony variation, found in 45 % of the bones of this series. Bony abnormalities were of three types: (1) calcifications in 39% of the bones, (2) lateral spurs in 32% and medial spurs in 23%, (3) degenerative changes in 9% of the bones. Biometric considerations showed that the TLBB inside the ITS was unstable. The supratubercular ridge could increase this instability and thus favor disease of the tendon. Bony abnormalities which may be associated with TLBB lesions, can be seen in radiographs.

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Sillon intertuberculaire de l'humérus: biométrie et morphologie de 100 os secs

Résumé Les dimensions et la morphologie d'une zone critique du sillon intertuberculaire (SIT) ont été déterminées sur 100 os secs. À son niveau proximal le SIT est étroit et présente une angulation. Le tendon du chef long du m. biceps brachial (TLBB) est particulièrement sollicité au cours de son passage à travers cette zone critique. La zone critique s'étend depuis la première dépression proximale perceptible, jusqu'au niveau de l'extrêmité distale du tubercule mineur. Elle présentait und partie proximale de 12,4 mm de long (écart type=2,23) et une partie distale de 15,1 mm de long (e.t.=3,37). Les deux parties formaient un angle de 142° en moyenne (e.t.=7,87). Sa longueur totale était de 28,5 mm (e.t.=4,74); sa profondeur moyenne, de 2,44 mm (e.t.=0,49); sa largeur moyenne, de 6,33 mm (e.t.=0,84). La crête supratuberculaire était la seule variation osseuse trouvée sur 45 % des os de cette série. Les anomalies osseuses rencontrées étaient de trois types: (1) calcifications sur 39% des os, (2) éperons latéraux sur 32% des os et éperons médiaux sur 23% des os, (3) altérations dégénératives sur 9% des os. Les données biométriques montraient que le TLBB est instable par rapport au SIT. La crête supra-tuberculaire pourrait augmenter l'instabilité du TLBB et favoriser ainsi les affections du TLBB. Les anomalies morphologiques du SIT susceptibles d'être associées à une pathologie du TLBB peuvent être mises en évidence par des clichés radiographiques.