The effect of poststroke impairments on brachialis muscle architecture as measured by ultrasound

OBJECTIVE: To evaluate the joint angle dependence of brachialis muscle architecture at rest and changes in brachialis muscle architecture during isometric voluntary contractions in people after stroke. DESIGN: The pennation angle and fascicle length of the brachialis muscle were measured in the affected and unaffected sides of people after stroke at 9 different elbow angles ranging from 10 degrees to 90 degrees at the rest condition. Measurements were also carried out at a fixed joint angle of 90 degrees while the subjects were performing isometric muscle contractions at 5 incremental levels of maximal voluntary contraction (MVC) from 20% to 100% of MVC. The data obtained from the affected and unaffected sides of the subjects were compared. SETTING: A research laboratory in a rehabilitation center. PARTICIPANTS: Seven hemiplegic adults after stroke with passive range of motion in the elbow from 10 degrees to 90 degrees and Modified Ashworth Scale score larger than 1 were recruited. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Ultrasonography was used to measure brachialis muscle pennation angle and fascicle length at the rest condition and during isometric contractions. RESULTS: The measured pennation angles and fascicle lengths were found to be joint-angle-dependent in both the affected and unaffected groups at the rest condition. Further comparisons found that the pennation angles of the affected brachialis muscle were significantly larger (P<.05) than the unaffected muscle in the most extended positions (<50 degrees ), whereas the affected fascicle lengths were significantly shorter (P<.05) than the unaffected muscle in most flexed positions (>20 degrees ). As the level of isometric voluntary contraction was increased incrementally from 20% to 100% of MVC, the results showed that pennation angle increased significantly (P<.05) while fascicle length decreased significantly (P<.01) in the unaffected muscle. However, the contraction level has a significant effect only on pennation angle (P<.05) but not on fascicle length in the affected side. In addition, the measured fascicle lengths in the unaffected group were significantly shorter than those in the affected group for isometric contractions above 40% MVC. CONCLUSIONS: Our findings suggest that the architectural parameters of the brachialis muscle in people after stroke are elbow-joint-angle-dependent at the rest condition. In the affected side, pennation angle changed the most when the muscle was in an extension position and fascicle length changed the most in a flexed position when compared with the unaffected side. Immobilization and contracture might cause a shortening of the fascicle and an increase in pennation angle in the affected side. Smaller pennation angle and fascicle length changes in the affected side during isometric contraction might be due to weakness in the muscle after the onset of stroke.     

Increase of muscle strength from isometric quadriceps exercises at different knee angles.

Isometric quadriceps exercises were performed at two different knee angles, 15 and 60 degrees respectively. The maximal torque was measured in both positions before and after training in 10 healthy females. Both legs were exercised, one at each position. The purpose was to develop practical recommendations for choice of training position. The strength increase was mainly specific according to the angle at which the knee was exercised. It was suggested that isometric exercise would preferably be performed at different knee angles to secure an optimal total strength increase. Isometric exercises improved dynamic strength at a low velocity but not at a high velocity.     

Changes in the tibialis anterior tendon moment arm from rest to maximum isometric dorsiflexion: in vivo observations in man.

OBJECTIVE: In the present study, we examined the hypothesis that the tibialis anterior tendon moment arm increases during maximum isometric dorsiflexion as compared with rest. BACKGROUND: In musculoskeletal modelling applications, moment arms from passive muscles at rest are assumed representative of those measured during isometric muscle contraction. The validity of this assumption is questionable in musculotendon actuators enclosed by retinacular systems as in tibialis anterior. DESIGN AND METHODS: Sagittal-plane magnetic resonance images of the right ankle were taken in six subjects at rest and during maximum isometric dorsiflexion at six ankle angles between dorsiflexion and plantarflexion having the body placed in the supine position and the knee flexed at 90 degrees. Instant centres of rotation in the tibio-talar joint, tibialis anterior tendon action lines and moment arms were identified in the sagittal plane at ankle angles of -15 degrees, 0 degrees,+15 degrees and +30 degrees at rest and during maximum isometric dorsiflexion. RESULTS: At any given ankle angle, the tibialis anterior tendon moment arm during maximum isometric dorsiflexion increased by 0.9-1.5 cm (P<0.01) compared with rest. This was attributed to a displacement of both tibialis anterior tendon action line by 0.8-1.2 cm (P<0.01) and all instant centres of rotation by 0.3-0.4 cm (P<0. 01) distally in relation to their corresponding resting positions. CONCLUSIONS AND IMPLICATIONS: The assumption that the tibialis anterior tendon moment arm does not change from rest to maximum isometric dorsiflexion is invalid. Erroneous tendon forces, muscle stresses and joint moments by as much as 30% would be calculated using resting tibialis anterior tendon moment arms in the moment equilibrium equation around the ankle joint during maximum isometric dorsiflexion. RELEVANCE: A substantial increase in the tibialis anterior tendon moment arm occurs from rest to maximum isometric dorsiflexion. This needs to be taken into consideration when using planimetric musculoskeletal modelling for analysing maximal static ankle dorsiflexion loads.     

Effect of scapular protraction and retraction on isometric shoulder elevation strength

OBJECTIVES: To examine the effect of scapular protraction (SP) and scapular retraction (SR) on isometric shoulder elevation strength measured in the sagittal plane and to test the hypothesis that strength would be significantly reduced when tested in the SP position relative to the neutral resting scapular position (SN). DESIGN: Prospective before-after trial. SETTING: Multidisciplinary sports medicine center. PARTICIPANTS: Ten healthy volunteers (5 men, 5 women) ages 26 to 43 years recruited from the Mayo Clinic, Rochester, MN. INTERVENTIONS: Subjects completed 3 maximal isometric shoulder elevation contractions at 90 degrees of sagittal plane elevation in the SN, SP, and SR positions. The order of scapular positions was varied to minimize fatigue effects. Mean isometric strength values were compared by using Student t tests. MAIN OUTCOME MEASURES: Isometric shoulder elevation strength for the 3 scapular positions. RESULTS: Isometric strength was significantly lower for the SP position compared with the SN position (8.5 +/- 3.4 kg vs 11.1 +/- 4.0 kg, P <.0005) and for the SR position relative to the SN position (7.8 +/- 3.3 kg vs 11.1 +/- 4.0 kg, P <.00003). Strength values did not differ between the SP and SR positions (P =.38). CONCLUSIONS: Movement of the scapula into a protracted or retracted position results in a statistically significant reduction in isometric shoulder elevation strength as measured in this study. Further research is warranted to examine the relationship between scapular position and shoulder muscle function.     

The relevance of the moment arm of shoulder muscles with respect to axial rotation of the glenohumeral joint in four positions

OBJECTIVE: This study was undertaken to determine the efficiency of the shoulder girdle muscles during axial humeral rotation based on measurements of the moment arms. DESIGN: The instantaneous muscle moment arms of 10 shoulder muscles, including the three portions of the deltoid, the rotator cuff muscles, teres major, and the thoracohumeral muscle group, were measured during four specified glenohumeral rotations. BACKGROUND: Axial humeral rotation is a commonly performed movement during activities of daily living and is a targeted motion of shoulder rehabilitation, particularly in those protocols emphasizing rotator cuff strengthening. An understanding of the function of the movers and stabilizers of the shoulder requires such basic information of muscle moment arms. METHODS: The instantaneous moment arm values of the muscles were derived from the slope of the plot of tendon excursion versus glenohumeral joint rotation angle. Motion studied included axial rotation with the humerus elevated 90 degrees in the coronal, scapular, and sagittal planes, as well as in the neutral position with the arm at the side. RESULTS: Based on the findings, with the humerus in both neutral and elevated positions, the infraspinatus is potentially the most powerful external rotator, followed by teres minor and posterior deltoid. Subscapularis and possibly pectoralis major are the most effective internal rotators in this position. CONCLUSIONS: The moment arm in providing axial humeral rotation of 10 shoulder muscles in four planes were obtained. In general, the teres minor and infraspinatus had the largest moment arms in external rotation, and the subscapularis had the largest moment arm in internal rotation. The muscle function for axial humeral rotation was found to be modified by the plane of arm elevation. RELEVANCE: The data could be used for developing exercise programs in physical therapy.     

Joint-angle-dependent neuromuscular dysfunctions at the wrist in persons after stroke

OBJECTIVE: To evaluate the joint-angle-dependent neuromuscular functions at the affected wrist in hemiplegic subjects after stroke while doing isometric maximal voluntary wrist flexion and extension across different wrist angles. DESIGN: We investigated torques during isometric maximal voluntary wrist flexions and extensions at 8 different wrist angles, ranging from -45 degrees to 60 degrees. We used the associated electromyographic activities of 2 agonist and antagonist muscle pairs related to wrist and elbow joints for the analysis of muscular coactivations. We compared the data obtained from poststroke subjects' affected and unaffected sides. SETTING: A research laboratory in a rehabilitation center. PARTICIPANTS: Eleven subjects with hemiplegia after stroke with passive range of motion (ROM) in the wrist from -45 degrees to 60 degrees. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Directly measured torques, torques after normalization during maximal isometric wrist contractions, and normalized moving average electromyographic signals of each muscle at the tested positions. RESULTS: The measured torques of the affected wrists were significantly lower than those of the unaffected wrists at all tested angles during wrist flexion and extension (P<.05). The angle-dependent patterns of the normalized torque across the tested wrist angles varied from those of the unaffected wrists (2-way analysis of variance, P<.05). There were decreases in normalized torques during both flexion and extension at the extended positions in the affected group (P<.05). Abnormal cocontractions were found in agonist and antagonist muscle pairs related to wrist and elbow joints, and between the elbow flexor and wrist extensor when subjects did the wrist contractions on the paretic side, especially at the wrist extended positions. CONCLUSIONS: Wrist muscle weakness was distributed unevenly across the selected wrist ROM on the affected side, as represented by the varied patterns of the normalized torque-angle relationship, compared with the unaffected wrists. There were reductions in the selective control of muscle coactivating synergies both single-jointly and cross-jointly in the impaired nervous system during wrist contractions; the extent of these reductions was also related to the wrist angle configuration.     

Moment arms of forearm rotators

BACKGROUND: Rotation about a longitudinal axis of the forearm has been a matter of investigation for over 100 years. However, most studies were limited to only a few muscles and to their action in specific set positions of elbow and forearm rotation. This investigation aims at determining the moment arms of muscles that contribute to pronation and supination at three different angles of elbow flexion throughout the entire range of forearm rotation. METHODS: Muscle moment arms were derived from tendon excursions that were recorded on a full-size epoxy model of the radioulnar complex. The results were verified on a fresh cadaver specimen. FINDINGS: Moment arms of all major supinators exhibit peak values in 40-50 degrees of pronation, for all three positions of the elbow. These peak values vary with elbow position, the biceps muscle showing the highest dependency with its greatest moment arm in 90 degrees of elbow flexion. The pronators show a maximum of moment arm about the neutral position, with little dependency on elbow flexion. Brachioradialis brings the pronated, or supinated forearm into the neutral position. The bow of the radius is in function comparable to the 'throw' of a crankshaft, forming a greater lever arm between the point of insertion of the muscles and the axis of rotation of the radius. INTERPRETATION: The observations drawn from this study could be of eminent value in planning rotator muscle transplantation, in understanding functional disorders after injury, and in the physical treatment of forearm rotator muscle deficiency. Reconstruction of the physiological anatomical arrangement in the treatment of injuries is strongly recommended for restoration of function.     

Effect of inherent muscle length on isometric plantar flexion torque in healthy women

This study investigated the relationship between inherent muscle length and torque production in 59 healthy women. We recorded nondominant ankle range-of-motion values for each subject. These values were partitioned into quartiles for two knee positions. Women with "loose" plantar flexor muscles comprised the first quartile, and those with "tight" plantar flexor muscles the fourth quartile. Tight- and loose-muscle groups were established for the 0-degree (fully extended) and 90-degree (flexed) knee test positions for data analysis. Torque measurements were obtained using an isokinetic testing apparatus. We asked each subject to perform a maximal isometric (static) plantar flexion contraction at each of three ankle positions: 7 degrees of dorsiflexion (angle A), 0 degrees or neutral (angle B), and 30 degrees of plantar flexion (angle C). Data analysis was performed using an analysis of variance for repeated measures. Results indicated that torque produced by the tight-muscle groups was significantly greater than the torque produced by the loose-muscle groups at both knee positions (p less than .05). Additionally, the ankle ROM data obtained suggest normative data different from those currently prevalent in the literature. Findings of this study may prove valuable in the rehabilitation of ankle injuries and could be beneficial especially to physical therapists in understanding more about normal ankle function.     

Activation of the gluteus maximus,gluteus medius and tensor fascia lata muscles during hip internal and external rotation exercises at three hip flexion postures

BackgroundHip internal and external rotation exercises are usually performed in clinical practice. However, given the synergies required to stabilize the hip in the frontal plane, it is not clear how the activation of target muscles will differ between the two exercise directions.ObjectiveI) Compare the activation of the upper and lower fibers of gluteus maximus (GMax), gluteus medius (GMed) and tensor fascia lata muscles between the hip internal and external rotation exercises; ii) Compare the maximal isometric force between hip internal and external rotation exercises and; iii) Assess the effect of varying hip flexion angles on muscle activation and maximal isometric force.Study designA cross sectional study.MethodsElectromyography and force production of twenty-one participants were measured during maximum isometric hip internal and external rotation in three postures: 0°, 45° and 90° of hip flexion.ResultsMANOVA results showed a larger activation of the GMed, tensor fascia lata and upper GMax (p < .001) for hip internal rotation compared to external rotation regardless of hip flexion angle. For the lower GMax, the same was observed when the hip was kept at 90° of flexion. Maximal isometric force during hip external rotation was greater than during hip internal rotation at 0° posture, and lower at 90° posture (p < .001).ConclusionThe gluteus and the tensor fascia lata muscles were substantially recruited during the hip internal rotation exercise, and barely recruited during the hip external rotation exercise. Hip flexion influences the myoeletric activity and isometric force production during the internal rotation exercise.     

脑卒中后胫骨前肌形态结构与肌力变化的超声评估

目的:应用超声技术测量亚急性期脑卒中患者胫骨前肌静息状态与最大等长收缩状态下肌肉的形态结构参数,探讨肌肉参数的改变与关节角度以及发力功能之间的关系。方法:选择7例亚急性期脑卒中患者与7名性别年龄相匹配的正常人,应用B型超声测量不同踝关节角度下脑卒中患者健侧、患侧和正常人一侧下肢胫骨前肌的形态结构参数,包括羽状角、肌肉厚度以及肌纤维长度,同时用便携式肌力与关节活动测试仪记录踝背屈肌力及踝关节角度,研究不同关节角度下肌肉参数及其与发力之间的关系。结果:脑卒中患者患侧胫骨前肌的羽状角和肌肉厚度较健侧减小,并且羽状角和肌纤维长度均与踝关节角度相关。肌肉收缩时患侧三个参数的改变及踝背屈肌力均较健侧以及正常人减小,而健侧和正常人无显著性差异。结论:脑卒中后患者胫骨前肌的形态结构参数发生了改变,肌肉参数的改变与踝关节角度和发力是相关的,可能导致了运动功能障碍。     

Decreased neck muscle strength is highly associated with pain in cervical dystonia patients treated with botulinum toxin injections

OBJECTIVES: To compare the isometric neck muscle strength of cervical dystonia patients treated with botulinum toxin injections with that of healthy control subjects and to evaluate the association between neck strength, neck pain, and disability in these patients. DESIGN: Clinical cross-sectional study. SETTING: Outpatient rehabilitation and neurology clinics in a Finnish hospital. PARTICIPANTS: Twenty-three patients with cervical dystonia with botulinum toxin-treated neck muscles and 23 healthy control subjects. INTERVENTIONS: Not applicable.Main outcome measures Isometric neck strength was measured by a special neck strength measurement system. Disability was measured by the Neck Disability Index, and pain and symptoms of cervical dystonia by a visual analog scale. RESULTS: Isometric neck strength in all directions measured was significantly lower (25%-44%) in the cervical dystonia patients than in the healthy controls. Neck pain levels reported during the strength tests (r range, -.36 to -.70) and neck pain experienced during the preceding week (r range, -.52 to -.63) were inversely associated with isometric strength results. The difference between sides in rotation strength was 35% in the patient group (P<.001), whereas no significant difference between sides was found in the healthy controls. Fifty-one percent of the patients reported moderate or severe disability. Pain, stiffness, and incorrect position of the head were the most prominent symptoms. CONCLUSIONS: Cervical dystonia patients with botulinum toxin-treated neck muscles showed significantly lower maximal neck strength than healthy controls. The patients also had a statistically significant difference between sides in neck rotation strength. Thus, strength measures may be useful to detect disturbance in the function of the neck muscles.     

Ultrasound Evaluation of Muscle Thickness Changes in the External Oblique, Internal Oblique,and Transversus Abdominis Muscles Considering the Influence of Posture and Muscle Contraction

[Purpose] The aim of this study was to investigate muscle thickness changes in the external oblique (EO), internal oblique (IO), and transversus abdominis (TrA) muscles between the neutral position and trunk rotation, under a state of rest without voluntary contractions, and isometric contractions to both sides with resistance of 50% of the maximum trunk rotation strength. [Subjects] The subjects of this study were 21 healthy young men. [Methods] Muscle thickness changes in the EO, IO, and TrA in each position and state were evaluated by ultrasound. The range of motion at maximum trunk rotation and the maximum strength of trunk rotation were measured using a hand-held dynamometer. [Results] In the neutral position and at 50% trunk rotation to the right side, the thicknesses of the IO and TrA significantly increased with resistance. In both states, the thicknesses of the IO and TrA significantly increased at 50% trunk rotation to the right side. [Conclusion] The muscular contractions of the IO and TrA were stronger during ipsilateral rotation than in the neutral position and with resistance than at rest. Moreover, the muscular contraction was strongest in the resistive state during ipsilateral rotation.Key words: Trunk rotation, Low back pain, Functional evaluation     

Activation of the quadriceps muscle during semisquatting with different hip and knee positions in patients with anterior knee pain.

OBJECTIVE: We measured the surface electromyographic activities of vastus medialis obliquus and vastus lateralis in 16 subjects with patellofemoral joint pain syndrome. DESIGN: Each subject performed bilateral static knee extension exercises at 60% of his or her maximal voluntary effort under different combinations of hip rotation (30 degrees of medial rotation, neutral, 45 degrees of lateral rotation) and knee flexion (20 and 40 degrees) in a standing position. The ratio of surface-integrated electromyographic signals of vastus medialis obliquus over vastus lateralis was calculated for each of the six conditions. Because of significant interaction of hip rotation and knee flexion in the two-way analysis of variance, data were analyzed separately with paired t tests for the effect of knee positions and one-way repeated measures analysis of variance for hip positions. RESULTS: At 20 degrees of knee flexion, there was no significant difference among the three hip positions, whereas at 40 degrees of knee flexion, medial rotation of the hip resulted in significantly higher vastus medialis obliquus over vastus lateralis activity ratio than lateral rotation (P < 0.05). CONCLUSIONS: There was relatively more activation of vastus medialis obliquus than vastus lateralis at 40 degrees of semisquat with the hip medially rotated by 30 degrees. This finding has clinical implications for training the vastus medialis obliquus in patients with patellofemoral joint pain syndrome.     

Normative values of agonist-antagonist shoulder strength ratios of adults aged 20 to 78 years.

OBJECTIVE: To determine normative values for isometric flexion/extension, abduction/adduction, and external/internal rotation strength ratios about the shoulder and to determine if these ratios are affected by age or gender. STUDY DESIGN: A cross-sectional study of 120 healthy volunteers (60 men, 60 women) aged 20 to 78 years. SETTING: Orthopedic research laboratory. METHODS: Flexion and extension strengths were measured isometrically using a Cybex II dynamometer at arm flexion angles of 30 degrees, 60 degrees, and 90 degrees. Abduction and adduction strengths were measured at 30 degrees, 60 degrees, and 90 degrees abduction. Internal and external rotation strengths were measured (1) with the arm abducted 15 degrees and neutral external/internal rotation and (2) with the arm abducted 90 degrees and externally rotated 30 degrees above the transverse plane. OUTCOME MEASURES: Isometric strength ratios for flexion/extension, abduction/adduction, and external/internal rotation. RESULTS: No statistically significant differences in agonist/antagonist strength ratios were found between dominant and nondominant sides or between genders. Age was associated with changes in strength ratios for measurements taken with the arm flexed or abducted 90 degrees. Posture was found to affect strength ratios. CONCLUSIONS: These data can serve as a normative reference for clinical use.     

The influence of muscle length on one-joint shoulder internal and external rotator muscle strength

Abstract

Background and purpose: Kendall suggests testing the rotator cuff muscles in their maximally shortened position, since one-joint muscles are thought to be strongest. We found little evidence to support this concept. The purpose of this study was to determine if the shoulder internal rotator (IR) and external rotator (ER) muscles are strongest when placed in their shortened length position. Methods: Fifty-three subjects participated. Glenohumeral joint internal rotation and external rotation motion was measured. Muscle strength was then tested using a hand-held dynamometer in four positions: (1) end-range ER; (2) neutral 0°; (3) glenohumeral joint mid-range and (4) end-range IR. Data were analyzed using two repeated measures ANOVA’s. Results: The results suggest that rotator muscle strength is dependent on muscle length. IR strength was weakest at end-range IR in its shortest length; ER muscle strength was weakest at end-range ER in its shortest length. Muscle strength of the IR or ER was not significantly different when comparing neutral 0° to the mid-range position and at their most lengthened position. Conclusion: The IR and ER muscles were found to be weakest when placed in a position of shortest muscle length, while the neutral 0° and mid-range positions were the strongest positions.     

Kinematic analysis of the lower cervical spine in the protracted and retracted neck flexion positions

[Purpose] The aim of this study was to analyze lower cervical spine kinematics in protracted and retracted neck flexion positions in healthy people. [Subjects and Methods] The craniovertebral angle (CVA) and intervertebral body angles of the lower cervical spine of 10 healthy individuals were analyzed using fluoroscopy in a neutral sitting with the head in the neutral (N), protracted (Pro), and retracted (Ret) positions and with the neck in full flexion with the head in the neutral (N-fx), protracted (Pro-fx), and retracted (Ret-fx) positions. [Results] There were significant differences in the CVA and intervertebral body angle at the C3–4 level, and the Ret position showed the highest values followed by the N and Pro positions. Regarding the intervertebral body angle at the C4–5 level, the Pro position showed a higher value than the N and Ret positions. At the C6–7 level, the Pro position showed the lowest value compared with the N and Ret positions. In the CVA, the Ret-fx position showed a higher value than the N-fx and Ret-fx positions. [Conclusion] The results suggest that in the neutral sitting position, protraction is an ineffective posture due to overstress of the C6–7 segment, which is placed in a hyperflexed position at this level. Instead, retraction is the recommend posture for the patient with C6–7 degeneration, which makes for a more flexed position in the upper cervical spine and a less flexed position in the lower cervical spine.Key words: Head posture, Intervertebral body angle, Lower cervical spine     

Activation of abdominal muscles during some physiotherapeutic exercises.

The aim was to evaluate the theoretical efficiency of some abdominal muscle exercises for strength training. Sit-up with rounded back (curl-up) from the supine position up to 45 degrees activates the straight and oblique abdominal muscles to 50% of the values for maximum isometric contraction measured as integrated EMG. This exercise was compared with two previously suggested exercises: curl-up with simultaneous activation of the plantar-flexors of the ankle activated the abdominal muscles to the same level and curl-up from a position with dorsally extended spine did not activate the muscles more. Curl-up with lateral rotation of the trunk gave a high activation of the oblique muscles. This exercise was compared with two completely isometric exercises suggested for patients with back pain: only somewhat lower activation was obtained during an exercise in supine position with lateral resistance to the knees, but with lateral resistance against the elevated forearms there was only little activation.     

Maximal isometric torque of knee extension as a function of muscle length in subjects of advancing age

Many neuromuscular diseases are associated with muscle weakness. Assessment of this weakness by manual muscle testing or with hand-held equipment has been criticized. Furthermore, muscle length influences peak force development. One hundred fifty-three female and 116 male asymptomatic subjects between the ages of 20 and 80 (approximately 20 per decade) were studied. Maximal strength of the quadriceps group was determined isometrically, on a specially designed bench, at 3 hip angles (45 degrees, 90 degrees, and 180 degrees of extension), which represent three rectus femoris muscle lengths. Maximal strength was observed at a hip angle of 180 degrees and did not decrease significantly from 20 to 50 years of age, although the values for the male and female subjects over 50 years were significantly decreased (approximately 15% per decade). The increase in strength as the muscle was lengthened from 45 degrees to 180 degrees hip angle was about 80% (of the force at 45 degrees) in 20-year-olds; strength decreased progressively with age to a value of approximately 50% in the 70-year-olds. This greater reduction in maximal strength at longer quadriceps muscle lengths has functional significance in rising from a chair, climbing stairs, and eventually, walking and standing.