Histochemical characteristics of the muscle fibers of the biceps and triceps brachii muscles in human ontogeny

By means of morphometrical and histochemical methods for revealing myosin ATPase and SDG activity development of various types of muscle fibers (MF) has been studied in the postmortem material, using m. biceps and m. triceps brachii in human ontogenesis. The flexors and extensors have features in common in the dynamics of the MF maturation, and some distinctive peculiarities. The appearance of histochemical distinctions between the MF takes place on the 5th-6th months of the intrauterine development. Morphofunctional specialization begins with formation of tonic fibers. During the 1st-2nd years phasic fibers form. A relative amount of fast MF in both muscles increases at the age of 11-12 years. The dynamics of final specialization of the MF is connected with stages of sexual maturation. The first stage of the sexual maturation (about 14 years of age) is connected with decrease in the relative amount of the MF of glycolytic type of energy supply and corresponding increase in the number of oxidative type structures. From 15-17 years of age a final differentiation begins, it is connected with an intensive transversal growth of all the MF and distinguish of thick glycolytic MF. The m. biceps brachii has a relatively greater amount of oxidative fibers, and the m. triceps brachii, glycolytic ones. The transversal section area of the MF in the m. triceps brachii exceeds that of the m. biceps brachii, beginning from the 7th month of the intrauterine development up to 14 years of age. The investigation performed does not reveal any anticipating development either in the flexors or in the extensors. The differentiating processes in the m. biceps and m. triceps brachii occur nearly simultaneously.     

The functional anatomy of the shoulder in the European starling (Sturnus vulgaris)

The excursions of wing elements and the activity of eleven shoulder muscles were studied by cineradiography and electromyography in European starlings (Sturnus vulgaris) flying in a wind tunnel at speeds of 9–20 m s^?1. At the beginning of downstroke the humerus is elevated 80–90° above horizontal, and both elbow and wrist are extended to 90° or less. During downstroke, protraction of the humerus (55°) remains constant; elbow and wrist are maximally extended (120° and 160°, respectively) as the humerus passes through a horizontal orientation. During the downstroke-upstroke transition humeral depression ceases (at about 20° below horizontal) and the humerus begins to retract. However, depression of the distal wing continues by rotation of the humerus and adduction of the carpometacarpus. Humeral retraction (to within about 30° of the body axis) is completed early in upstroke, accompanied by flexion of the elbow and carpometacarpus. Thereafter the humerus begins to protract as elevation continues. At mid-upstroke a rapid counterrotation of the humerus reorients the ventral surface of the wing to face laterad; extension of the elbow and carpometacarpus are initiated sequentially. The upstroke-downstroke transition is characterized by further extension of the elbow and carpometacarpus, and the completion of humeral protraction. Patterns of electromyographic activity primarily coincide with the transitional phases of the wingbeat cycle rather than being confined to downstroke or upstroke. Thus, the major downstroke muscles (pectoralis, coracobrachialis caudalis, sternocoracoideus, subscapularis, and humerotriceps) are activated in late upstroke to decelerate, extend, and reaccelerate the wing for the subsequent downstroke; electromyographic activity ends well before the downstroke is completed. Similarly, the upstroke muscles (supracoracoideus, deltoideus major) are activated in late downstroke to decelerate and then reaccelerate the wing into the upstroke; these muscles are deactivated by mid-upstroke. Only two muscles (scapulohumeralis caudalis, scapulotriceps) exhibit electromyographic activity exclusively during the downstroke. Starlings exhibit a functional partitioning of the two heads of the triceps (the humerotriceps acts with the pectoralis group, and does not overlap with the scapulotriceps). The biphasic pattern of the biceps brachii appears to correspond to this partitioning.     

三种啮齿类动物前肢挖掘效率分析

以甘肃鼢鼠(Myospalax cansus)、棕色田鼠(Lasiopodomys mandarinus)和小鼠(Mus musculus)为对象,对其尺骨、桡骨和肱三头肌结构进行了比较,并通过力学模型,对这三种生活类型鼠类前肢的挖掘效率进行分析。结果显示,甘肃鼢鼠肘关节位置大幅度前移,尺骨鹰嘴特化突出,形成更加省力的骨学杠杆基础,其中甘肃鼢鼠的鹰嘴尺骨比例达0.40;棕色田鼠和小鼠的鹰嘴尺骨比例分别约为0.19和0.18。此外,甘肃鼢鼠提供挖掘动力的肱三头肌近体端长头覆盖整个肩胛骨下缘,外侧头和内侧头覆盖桡神经沟到肱骨肘关节髁附近区域,远体端扁腱附着于尺骨鹰嘴,整块肌肉非常发达,棕色田鼠和小鼠均无此特化现象。说明甘肃鼢鼠前肢结构更加适应地下掘土生活,其挖掘效率远大于棕色田鼠和小鼠。     

Innervation zone location of the biceps brachii,a comparison between genders and correlation with anthropometric measurements

Avoiding the innervation zone (IZ) is important when collecting surface electromyographic data. The purposes of this study were threefold: (1) to examine the precision of two different techniques for expressing IZ location for the biceps brachii, (2) to compare these locations between men and women, and (3) to determine if IZ movement with changes in elbow joint angle is related to different anthropometric measures. Twenty-four subjects (mean ± SD ages = 21.8 ± 3.5 yr) performed isometric contractions of the right forearm flexors at each of three separate elbow joint angles (90°, 120°, and 150° between the arm and forearm). During each contraction, the location of the IZ for the biceps brachii was visually identified using a linear electrode array. These IZ locations were expressed in both absolute (i.e. as a distance (mm) from the acromion process) and relative (i.e. as a percentage of humerus length) terms. The results suggested that the estimations of IZ location were more precise when expressed in relative versus absolute terms, and were generally different for men and women. The shift in IZ location with changes in elbow joint angle was not, owever, related to height, weight, or humerus length.     

Effect of the upper limbs muscles activity on the mechanical energy gain in pole vaulting

The shoulder muscles are highly solicited in pole vaulting and may afford energy gain. The objective of this study was to determine the bilateral muscle activity of the upper-limbs to explain the actions performed by the vaulter to bend the pole and store elastic energy. Seven experienced athletes performed 5-10 vaults which were recorded using two video cameras (50Hz). The mechanical energy of the centre of gravity (CG) was computed, while surface electromyographic (EMG) profiles were recorded from 5 muscles bilateral: deltoideus, infraspinatus, biceps brachii, triceps, and latissimus dorsi muscles. The level of intensity from EMG profile was retained in four sub phases between take-off (TO1) and complete pole straightening (PS). The athletes had a mean mechanical energy gain of 22% throughout the pole vault, while the intensities of deltoideus, biceps brachii, and latissimus dorsi muscles were sub phases-dependent (p<0.05). Stabilizing the glenohumeral joint (increase of deltoideus and biceps brachii activity) and applying a pole bending torque (increase of latissimus dorsi activity) required specific muscle activation. The gain in mechanical energy of the vaulter could be linked to an increase in muscle activation, especially from latissimusdorsi muscles.     

Expiratory activity in transferred intercostal nerves in brachial plexus injury patients

Involuntary activity of transferred intercostal motor units was examined in patients with brachial plexus injury. Since the internal intercostal nerves were detached from the thorax to reinnervate the musculus biceps brachii, it was possible to record pure intercostal motor activity in humans. Respiratory activity was seen in the latter part of the expiratory phase, thus dividing the phase into two substages (E1 and E2) by the onset of the activity. CO2 rebreathing prolonged the duration of the intercostal motor activity and increased the tidal activity as determined from the integration curve. There was a close linear correlation between these two variables. These observations indicate that expiratory activity and its duration are actively controlled in humans.     

EMG and MMG of agonist and antagonist muscles as a function of age and joint angle.

The aim of this study was to determine the effect of elbow joint position on electromyographic (EMG) and mechanomyographic (MMG) activities of agonist and antagonist muscles in young and old women. Surface EMG and MMG were recorded from the triceps and biceps brachii, and brachioradialis muscles during isometric elbow extensions in young and old women. The measurements were carried out at an optimal joint angle (A(o)), as well as at smaller (A(s) = A(o) - 30 degrees ) and larger (A(l) = A(o) + 30 degrees ) angles. The normalized to force EMG amplitude (RMS-EMG/F) was smaller in old women compared to young in all muscles. The RMS-EMG/F of the triceps brachii muscle was not affected by muscle length while that of the biceps brachii and brachioradialis muscles increased at shortest muscle length in both groups. The normalized to force MMG amplitude (RMS-MMG/F) was smaller in old than in young in the triceps brachii muscle only. There was an increase in RMS-MMG/F with triceps brachii and biceps brachii muscle shortening in both groups, and in the brachioradialis muscle -- in young only. Compared to young, older women exhibited a bigger force fluctuation during maximum voluntary contraction, but these did not contribute significantly to the RMS-MMG. Skinfold thickness accounted for the RMS-EMG/F and RMS-MMG/F differences seen between old and young women in the biceps brachii muscle only. It is concluded that, the EMG and MMG response to muscles length change in agonist and antagonist muscles is generally similar in old and young women but the optimal angle shifts toward a bigger value in older women.     

Contribution of the biceps brachii and pronator teres muscles to the efforts of pronation or supination. I. Statistical work (author's transl)

The electrical activity of the biceps brachii and pronator teres muscles is studied through the prono-supination of the forearm in some isometrical conditions (static work) with different loads and joint positions. If the pronator teres is always being active in pronation, this activity is a function of the load and of the wrist and elbow positions. The same phenomena can be observed for the biceps brachii but when in supination. From the curvilinear relationships between the integrated electrical activity and the load--observed on both muscles--some torque-angle relationships can be established for the biceps brachii which show that a bifunctional muscle seems to be characterized by a very and unique force-length relationship.     

The effects of isokinetic contraction velocity on concentric and eccentric strength of the biceps brachii

The purpose of this investigation was to determine the influence of contraction velocity on the eccentric (ECC) and concentric (CON) torque production of the biceps brachii. After performing warm-up procedures, each male subject (n = 11) completed 3 sets of 5 maximal bilateral CON and ECC isokinetic contractions of the biceps at speeds of 90, 180, and 300 degrees x s(-1) on a Biodex System 3 dynamometer. The men received a 3-minute rest between sets and the order of exercises was randomized. Peak torque (Nm) values were obtained for CON and ECC contractions at each speed. Peak torque scores (ECC vs. CON) were compared using a t-test at each speed. A repeated measures analysis of variance was used to determine differences between speeds. ECC peak torque scores were greater than CON peak torque scores at each given speed: 90 degrees x s(-1), p = 0.0001; 180 degrees x s(-1), p = 0.0001; and 300 degrees x s(-1), p = 0.0001. No differences were found between the ECC peak torque scores (p = 0.62) at any of the speeds. Differences were found among the CON scores (p = 0.004). Post hoc analysis revealed differences between 90 degrees x s(-1) (114.61 +/- 23) and 300 degrees x s(-1) (94.17 +/- 18). These data suggest that ECC contractions of the biceps brachii were somewhat resistant to a force decrement as the result of an increase in velocity, whereas CON muscular actions of the biceps brachii were unable to maintain force as velocity increased.     

EMG and MMG activities of agonist and antagonist muscles in Parkinson’s disease patients during absolute submaximal load holding

The purpose of the study was (1) to assess changes in electromyographical (EMG) and mechanomyographical (MMG) signals of the biceps and triceps brachii muscles during absolute submaximal load holding in Parkinson’s disease patients tested during their medication “ON-phase” and in age-matched controls, and (2) to check whether mechanomyography can be useful in evaluation of neuromuscular system activity in Parkinson’s disease patients.The data analysis was performed on nine females with Parkinson’s disease and six healthy, age-matched females. The EMG and MMG signals were recorded from the short head of the biceps brachii (BB) and the lateral head of the triceps brachii (TB) muscles.It was concluded that compared to the controls, the Parkinson’s disease patients exhibited higher amplitude in the biceps brachii muscle and lower median frequency of the MMG signal in the both tested muscles. However, no differences in the EMG amplitude and an increase of the EMG median frequency in the triceps brachii muscle of the Parkinson’s disease group were observed. The MMG was not affected by physiological postural tremor and can depict differences between parkinsonians and controls, which may suggest that it is valuable tool for neuromuscular assessment for this condition.     

Anatomy and function of the bursa subacromialis.

The ligamentum coracohumerale, as the anterior limit of the bursa subacromialis, runs into the shoulder joint capsule, connecting--broadly based--above the sulcus intertubercularis. It does not, however, originate--as described previously--from the base of the processus coracoideus (viewed frontally), but rather medially from the base, i.e. on the side of the fossa supraspinatus. The sliding mechanism starting at 50 degrees affects the parietal sheet of the bursa subacromialis, whereby the laterally situated section slides under the medially situated section. With an abduction of up to 50 degrees, the lateral section congests in front of the corner of the acromion, sliding from there under the acromion, so that the--hitherto--medial section is located above the section now sliding away beneath it. This sliding mechanism continues on up to 100 degrees. At this point the parietal sheet of the bursa subacromialis lies as follows: The section that had been situated laterally at the beginning of the sliding mechanism now lies caudally to the section that had lain medially at the outset. The bursa subacromialis does not slide fully into the fossa supraspinatus, as in all of the cases observed, it is firmly connected, together with the fascia subdeltoidea, to the corner of the acromion. The visceral sheet does not change in the course of the sliding mechanism as described, as it is connected to the fascia of the supraspinatus muscle--with the exception of a medial stretch of 16 mm. A further finding deals with the course of the muscles of the caput breve of the biceps brachii. Individual muscle fibre components do not connect to the processus coracoideus, but rather run before the tip of the processus coracoideus into the ligamentum coracoacromiale, radiating not only into the ligamentum coracoacromiale, but also--certain components--into the shoulder joint capsule. Through this, the 'aponeurosis tendinis brachii' forms a tendon roof in front of the processus coracoideus that extends to the structures running along the head of the humerus. The muscle fibre components of the caput breve of the m. biceps brachii radiating into the shoulder joint capsule, together with the muscles of the rotator cuff and the ligamentum coracohumerale, keep the shoulder joint capsule tense, thus preventing constriction symptoms.     

Mechanical implications of chimpanzee positional behavior.

Mechanical hypotheses concerning the function of chimpanzee anatomical specializations are examined in light of recent positional behavior data. Arm-hanging was the only common chimpanzee positional behavior that required full abduction of the humerus, and vertical climbing was the only distinctive chimpanzee positional behavior that required forceful retraction of the humerus and flexion of the elbow. Some elements of the chimpanzee anatomy, including an abductible humerus, a broad thorax, a cone-shaped torso, and a long, narrow scapula, are hypothesized to be a coadapted functional complex that reduces muscle action and structural fatigue during arm-hanging. Large muscles that retract the humerus (latissimus dorsi and probably sternocostal pectoralis major and posterior deltoid) and flex the elbow (biceps brachii, probably brachialis and brachioradialis) are argued to be adaptations to vertical climbing alone. A large ulnar excursion of the manus and long, curved metacarpals and phalanges are interpreted as adaptations to gripping vertical weight-bearing structures during vertical climbing and arm-hanging. A short torso, an iliac origin of the latissimus dorsi, and large muscles for arm-raising (caudal serratus, teres minor, cranial trapezius, and probably anterior deltoid and clavicular pectoralis major) are interpreted as adaptations to both climbing and unimanual suspension.     

Facilitation of triceps brachii muscle contraction by tendon vibration after chronic cervical spinal cord injury.

One way to improve the weak triceps brachii voluntary forces of people with chronic cervical spinal cord injury may be to excite the paralyzed or submaximally activated fraction of muscle. Here we examined whether elbow extensor force was enhanced by vibration (80 Hz) of the triceps or biceps brachii tendons at rest and during maximum isometric voluntary contractions (MVCs) of the elbow extensors performed by spinal cord-injured subjects. The mean +/- SE elbow extensor MVC force was 22 +/- 17.5 N (range: 0-23% control force, n = 11 muscles). Supramaximal radial nerve stimuli delivered during elbow extensor MVCs evoked force in six muscles that could be stimulated selectively, suggesting potential for force improvement. Biceps vibration at rest always evoked a tonic vibration reflex in biceps, but extension force did not improve with biceps vibration during triceps MVCs. Triceps vibration induced a tonic vibration reflex at rest in one-half of the triceps muscles tested. Elbow extensor MVC force (when >1% of control force) was enhanced by vibration of the triceps tendon in one-half of the muscles. Thus triceps, but not biceps, brachii tendon vibration increases the contraction strength of some partially paralyzed triceps brachii muscles.     

Matrix metalloproteinase activities and their relationship with collagen remodelling in tendon pathology.

Our aim was to correlate the activity of matrix metalloproteinases (MMPs) with denaturation and the turnover of collagen in normal and pathological human tendons. MMPs were extracted from ruptured supraspinatus tendons (n=10), macroscopically normal ("control") supraspinatus tendons (n=29) and normal short head of biceps brachii tendons (n=24). Enzyme activity was measured using fluorogenic substrates selective for MMP-1, MMP-3 and enzymes with gelatinolytic activity (MMP-2, MMP-9 and MMP-13). Collagen denaturation was determined by alpha-chymotrypsin digestion. Protein turnover was determined by measuring the percentage of D-aspartic acid (% D-Asp). Zymography was conducted to identity specific gelatinases. MMP-1 activity was higher in ruptured supraspinatus compared to control supraspinatus and normal biceps brachii tendons (70.9, 26.4 and 11.5 fmol/mg tendon, respectively; P<0.001). Gelatinolytic and MMP-3 activities were lower in normal biceps brachii and ruptured supraspinatus compared to control supraspinatus (gelatinase: 0.18, 0.23 and 0.82 RFU/s/mg tendon respectively; P<0.001; MMP-3: 9.0, 8.6 and 55 fmol/mg tendon, respectively; P<0.001). Most gelatinase activity was shown to be MMP-2 by zymography. Denatured collagen was increased in ruptured supraspinatus compared to control supraspinatus (20.4% and 9.9%, respectively; P<0.001). The % D-Asp content increased linearly with age in normal biceps brachii but not in control supraspinatus and was significantly lower in ruptured supraspinatus compared to age-matched control tendons (0.33 and 1.09% D-Asp, respectively; P<0.01). We conclude that the short head of biceps brachii tendons show little protein turnover, whereas control supraspinatus tendons show relatively high turnover mediated by the activity of MMP-2, MMP-3 and MMP-1. This activity is thought to represent a repair or maintenance function that may be associated with an underlying degenerative process caused by a history of repeated injury and/or mechanical strain. After tendon rupture, there was increased activity of MMP-1, reduced activity of MMP-2 and MMP-3, increased turnover and further deterioration in the quality of the collagen network. Tendon degeneration is shown to be an active, cell-mediated process that may result from a failure to regulate specific MMP activities in response to repeated injury or mechanical strain.     

Human muscle hardness assessment during incremental isometric contraction using transient elastography

The aim of this study was to investigate the relationship between biceps brachii hardness using the transient elastography technique, and its activity level by quantifying the surface electromyographic signal (sEMG). Ten healthy subjects volunteered for this protocol. To assess the maximal biceps brachii myoelectric activity (sEMG-RMSm), subjects had to achieve their maximal voluntary contraction trial during an elbow flexion effort. They were then asked to perform an isometric biceps sEMG-RMS ramp trial in elbow flexion from 0% to 50% of their sEMG-RMSm in 120 s. A low-frequency pulse was sent every 5 s during all trials by an innovative shear elasticity probe previously placed over the belly of the biceps brachii allowing the calculation of a transverse shear modulus. The main results of this study were (i) the finding of a systematic linear relationship between the biceps brachii transverse shear moduli and the corresponding sEMG-RMS values. This was not the case when plotting transverse shear modulus versus the elbow flexion torque production. Therefore, the computation of a hardness index from the slope of individual transverse shear modulus-sEMG-RMS linear relationship was enabled; (ii) It was also found that the higher is the rest shear modulus, the lower is the hardness index, indicating that the transverse shear modulus change during contraction depends on its level at rest. Therefore, this non-invasive technique could be useful in the medical field to explore deep muscles which are unreachable by classical testing methods. It could also be applied for the follow-up of neuromuscular diseases inducing stiffness changes such as in Duchenne muscular dystrophy.     

Physiotherapy vs. capsular shift and physiotherapy in multidirectional shoulder joint instability

PurposeThe aim of the study was to compare the kinematic parameters and the on–off pattern of the muscles of patients with multidirectional instability (MDI) treated by physiotherapy or by capsular shift and postoperative physiotherapy before and after treatment during elevation in the scapular plane.ScopeThe study was carried out on 32 patients with MDI of the shoulder treated with physiotherapy, 19 patients with MDI of the shoulder treated by capsular shift and postoperative physiotherapy, and 25 healthy subjects. The motion of skeletal elements was modeled by the range of humeral elevation, scapulothoracic angle and glenohumeral angle, scapulothoracic (ST) and glenohumeral (GH) rhythms, and relative displacement between the rotation centers of the humerus and scapula. The muscle pattern was modeled by the on–off pattern of muscles around the shoulder, which summarizes the activity duration of the investigated muscles.ResultsThe different ST and GH rhythms and the increased relative displacement between the rotation centers of the scapula and the humerus were observed in MDI patients. The physiotherapy strengthened the rotator cuff, biceps brachii, triceps brachii, deltoid muscles, and increase the neuromuscular control of the shoulder joints. Capsular shift and physiotherapy enabled bilinear ST and GH rhythms and the normal relative displacement between the rotation centers of the scapula and humerus to be restored. After surgery and physiotherapy, the duration of muscular activity was almost normal.ConclusionThe significant alteration in shoulder kinematics observed in MDI patients cannot be restored by physiotherapy only. After the capsular shift and postoperative physiotherapy angulation at 60° of ST and GH rhythms, the relative displacement between the rotation centers of the scapula and humerus and the duration of muscular activity were restored.     

Evaluation of upper limb muscle fatigue based on surface electromyography

Fatigue is believed to be a major contributory factor to occupational injuries in machine operators. The development of accurate and usable techniques to measure operator fatigue is therefore important. In this study, we used a novel method based on surface electromyography (sEMG) of the biceps brachii and the Borg scale to evaluate local muscle fatigue in the upper limb after isometric muscle action. Thirteen young males performed isometric actions with the upper limb at different force levels. sEMG activities of the biceps brachii were recorded during the actions. Borg scales were used to evaluate the subjective sensation of local fatigue of the biceps brachii after the actions. sEMG activities were analyzed using the one-third band octave method, and an equation to determine the degree of fatigue was derived based on the relationship between the variable and the Borg scale. The results showed that the relationship could be expressed by a conic curve, and could be used to evaluate muscle fatigue during machine operation.     

The role of the biceps brachii in shoulder elevation.

The biceps brachii is a bi-articular muscle affecting motion at the shoulder and elbow. While its' action at the elbow is well documented, its role in shoulder elevation is less clear. Therefore, the purpose of this project was to investigate the influence of shoulder and elbow joint angles on the shoulder elevation function of the biceps brachii. Twelve males and 18 females were tested on a Biodex dynamometer with the biceps brachii muscle selectively stimulated at a standardized level of voltage. The results indicated that both shoulder and elbow joint angles influence the shoulder joint elevation moment produced by the biceps brachii. Further analysis revealed that the elevation moment was greatest with the shoulder joint at 0 degrees and the elbow flexed 30 degrees or less. The greatest reduction in the elevation moment occurred between shoulder angles of 0 degrees and 30 degrees . The shoulder elevation moment was near zero when shoulder elevation reached or exceeded 60 degrees regardless of elbow angle. These results clarify the role of the biceps in shoulder elevation, as a dynamic stabilizer, and suggest that it is a decelerator of the arm during the throwing motion.     

Anticipatory postural adjustment in bimanual unloading: role of the motor cortex in motor learning

The role of the motor cortex was investigated during learning unusual postural adjustment. Healthy subjects held their right (postural) forearm in a horizontal position while supporting a 1-kG load via an electromagnet. The postural forearm position was perturbed by the load release triggered by other elbow voluntary movement. Repetition of the imposed unloading test resulted in a progressive reduction of the maximal forearm rotation, accompanied by the anticipatory decrease in m. biceps brachii activity (learning). Control situation consisted of the voluntary forearm loading. Using the transcranial magnetic stimulation we examined changes in the motor evoked potential of the m. biceps brahii at the beginning and at the end of learning. The evoked potential amplitude did not significantly change in process of the decrease of m. biceps brachii activity. At the end of learning, motor evoked potential / baseline electromyogram ratio increased as compared to the beginning of learning and to the control situation. The results highlight the fundamental role of the motor cortex in suppression of synergies which interfere with formation of a new coordination during motor learning.     

Bone length and muscle weight in mice subjected to genetic selection for the relative length of the tibia and radius

Mice which had undergone 5 generations of selection for high and low values of the tibia length/radius length ratio were compared with unselected controls at 10 weeks of age. The ratio responded to selection in both directions. The length of the radius was increased in the low line while the response in the high line was due to an increase in the length of the tibia and a small but statistically insignificant decrease in radius length. High and low line mice were heavier than the controls. The responses of the tibia and femur and of the radius and humerus were generally similar, suggesting the existence of an association between the genetic control of the lengths of the bones of a limb. There were also correlated responses in the weight of the tibialis anterior, biceps brachii and sternomastoid, and there is some evidence to suggest that the weight of a muscle may be influenced by the length of a bone to which it is attached.