The influence of the crimp and slope grip position on the finger pulley system

In this study the influence of the grip position (crimp grip vs. slope grip position) on the pulley system of the finger was investigated. For this purpose 21 cadaver finger (11 hands, 10 donors) were fixed into an isokinetic loading device. Nine fingers were loaded in the slope grip position and 12 fingers in the crimp grip position. The forces in the flexor tendons and at the fingertip were recorded. A rupture of the A4 pulley occurred most often in the crimp grip position (50%) but did not occur in the slope grip position, in which alternative events were the most common (67%). The forces in the deep flexor tendon (FDP) (slope grip: 371 N, crimp grip: 348 N) and at the fingertip (slope grip: 105 N, crimp grip: 161 N) were not significantly different between the 2 finger positions, but the forces acting on the pulleys were higher in the crimp grip position (A2 pulley: 287 N, A4 pulley: 226 N) than in the slope grip position (A2 pulley: 121 N, A4 pulley: 103 N). The crimp grip position may be the main cause for A4 pulley ruptures but the slope grip position may be hazardous for other injuries as the forces recorded in the flexor tendons and at the fingertip were comparable at the occurrence of a terminal event.     

Analysis of musculoskeletal loading in an index finger during tapping

Since musculoskeletal disorders of the upper extremities are believed to be associated with repetitive excessive muscle force production in the hands, understanding the time-dependent muscle forces during key tapping is essential for exploring the mechanisms of disease initiation and development. In the current study, we have simulated the time-dependent dynamic loading in the muscle/tendons in an index finger during tapping. The index finger model is developed using a commercial software package AnyBody, and it contains seven muscle/tendons that connect the three phalangeal finger sections. Our simulations indicate that the ratios of the maximal forces in flexor digitorum superficialis (FS) and flexor digitorum profundus (FP) tendons to the maximal force at the fingertip are 0.95 and 2.9, respectively, which agree well with recently published experimental data. The time sequence of the finger muscle activation predicted in the current study is consistent with the EMG data in the literature. The proposed model will be useful for bioengineers and ergonomic designers to improve keyboard design minimizing musculoskeletal loadings in the fingers.     

Friction between human finger flexor tendons and pulleys at high loads

A method was developed to indirectly measure friction between the flexor tendons and pulleys of the middle and ring finger in vivo. An isokinetic movement device to determine maximum force of wrist flexion, interphalangeal joint flexion (rolling in and out) and isolated proximal interphalangeal (PIP) joint flexion was built. Eccentric and concentric maximum force of these three different movements where gliding of the flexor tendon sheath was involved differently (least in wrist flexion) was measured and compared. Fifty-one hands in 26 male subjects were evaluated. The greatest difference between eccentric and concentric maximum force (29.9%) was found in flexion of the PIP joint. Differences in the rolling in and out movement (26.8%) and in wrist flexion (14.5%) were significantly smaller. The force of friction between flexor tendons and pulleys can be determined by the greater difference between eccentric and concentric maximum force provided by the same muscles in overcoming an external force during flexion of the interphalangeal joints and suggests the presence of a non-muscular force, such as friction. It constitutes of 9% of the eccentric flexion force in the PIP joint and therefore questions the low friction hypothesis at high loads.     

In vivo forces generated by finger flexor muscles do not depend on the rate of fingertip loading during an isometric task

Risk factors for activity-related tendon disorders of the hand include applied force, duration, and rate of loading. Understanding the relationship between external loading conditions and internal tendon forces can elucidate their role in injury and rehabilitation. The goal of this investigation is to determine whether the rate of force applied at the fingertip affects in vivo forces in the flexor digitorum profundus (FDP) tendon and the flexor digitorum superficialis (FDS) tendon during an isometric task. Tendon forces, recorded with buckle force transducers, and fingertip forces were simultaneously measured during open carpal tunnel surgery as subjects (N=15) increased their fingertip force from 0 to 15N in 1, 3, and 10s. The rates of 1.5, 5, and 15N/s did not significantly affect FDP or FDS tendon to fingertip force ratios. For the same applied fingertip force, the FDP tendon generated more force than the FDS. The mean FDP to fingertip ratio was 2.4+/-0.7 while the FDS to tip ratio averaged 1.5+/-1.0 (p<0.01). The fine motor control needed to generate isometric force ramps at these specific loading rates probably required similar high activation levels of multiple finger muscles in order to stabilize the finger and control joint torques at the force rates studied. Therefore, for this task, no additional increase in muscle force was observed at higher rates. These findings suggest that for high precision, isometric pinch maneuvers under static finger conditions, tendon forces are independent of loading rate.     

Differential control during maximal concentric and eccentric loading revealed by characteristics of the electromyogram

Maximal eccentric loading has been associated with higher levels of spindle afferent activity but lower levels of integrated EMG as compared to maximal concentric loading. Elbow flexor EMG was recorded from 17 subjects during concentric (CONC) and eccentric (ECC) elbow flexion at 70° s⁻¹ using a Kin-Com dynamometer. We hypothesized that peak EMG amplitude would be more sensitive to fluctuations in facilitation by the spindle primary afferents via the segmental stretch reflex pathway, and that the mean EMG would be more reflective of the ongoing level of muscle activation. A ratio of peak to mean EMG (P/M EMG ratio) was predicted to be larger during maximal eccentric loading than maximal concentric loading. The peak EMG (P<0.013) and the P/M EMG ratio (P<0.001) were significantly greater during the ECC condition than the CONC condition. In a subgroup of three subjects who underwent 3 weeks of eccentrically biased weight training, EMG, peak torque and torque variability were assessed before and after training. P/M EMG ratio decreased, while peak torque and torque variability increased following the training. Differences in the P/M EMG ratio appear to reflect differences in the way eccentric and concentric muscle actions are controlled and do not simply represent less control during the eccentric task.     

Finger joint force minimization in pianists using optimization techniques

A numerical optimization procedure was used to determine finger positions that minimize and maximize finger tendon and joint force objective functions during piano play. A biomechanical finger model for sagittal plane motion, based on finger anatomy, was used to investigate finger tendon tensions and joint reaction forces for finger positions used in playing the piano. For commonly used piano key strike positions, flexor and intrinsic muscle tendon tensions ranged from 0.7 to 3.2 times the fingertip key strike force, while resultant inter-joint compressive forces ranged from 2 to 7 times the magnitude of the fingertip force. In general, use of a curved finger position, with a large metacarpophalangeal joint flexion angle and a small proximal interphalangeal joint flexion angle, reduces flexor tendon tension and resultant finger joint force.     

Estimation of finger muscle tendon tensions and pulley forces during specific sport-climbing grip techniques

The present work displayed the first quantitative data of forces acting on tendons and pulleys during specific sport-climbing grip techniques. A three-dimensional static biomechanical model was used to estimate finger muscle tendon and pulley forces during the "slope" and the "crimp" grip. In the slope grip the finger joints are flexed, and in the crimp grip the distal interphalangeal (DIP) joint is hyperextended while the other joints are flexed. The tendons of the flexor digitorum profundus and superficialis (FDP and FDS), the extensor digitorum communis (EDC), the ulnar and radial interosseus (UI and RI), the lumbrical muscle (LU) and two annular pulleys (A2 and A4) were considered in the model. For the crimp grip in equilibrium conditions, a passive moment for the DIP joint was taken into account in the biomechanical model. This moment was quantified by relating the FDP intramuscular electromyogram (EMG) to the DIP joint external moment. Its intensity was estimated at a quarter of the external moment. The involvement of this parameter in the moment equilibrium equation for the DIP joint is thus essential. The FDP-to-FDS tendon-force ratio was 1.75:1 in the crimp grip and 0.88:1 in the slope grip. This result showed that the FDP was the prime finger flexor in the crimp grip, whereas the tendon tensions were equally distributed between the FDP and FDS tendons in the slope grip. The forces acting on the pulleys were 36 times lower for A2 in the slope grip than in the crimp grip, while the forces acting on A4 were 4 times lower. This current work provides both an experimental procedure and a biomechanical model that allows estimation of tendon tensions and pulley forces crucial for the knowledge about finger injuries in sport climbing.     

Balancing a force on the fingertip of a two-dimensional finger model without intrinsic muscles

A slightly flexed human middle finger can balance an external force on the fingertip. Internal stabilization is also possible, which means that the externally unloaded finger can be kept stiff. We want to analyse whether in these situations the intrinsic hand muscles are needed. Distances from tendons to flexion axes are taken from the literature and are substituted in the moment equilibrium equations of a two-dimensional finger model. Diagrams illustrate the statically indeterminate problem of solving tendon forces. The possibilities for equilibrium without intrinsics appear to depend mainly on four tendon-to-joint distances. These distances determine to which of two groups a finger belongs: (1) one in which intrinsics are not necessary for internal stabilization nor for balancing a force on the fingertip in any direction in the sagittal plane; (2) one in which, without intrinsics, internal stabilization is impossible and only dorso-distally directed forces on the fingertip can be balanced.     

Eccentric exercise increases EMG amplitude and force fluctuations during submaximal contractions of elbow flexor muscles.

The purpose of this study was to determine the effect of eccentric exercise on the ability to exert steady submaximal forces with muscles that cross the elbow joint. Eight subjects performed two tasks requiring isometric contraction of the right elbow flexors: a maximum voluntary contraction (MVC) and a constant-force task at four submaximal target forces (5, 20, 35, 50% MVC) while electromyography (EMG) was recorded from elbow flexor and extensor muscles. These tasks were performed before, after, and 24 h after a period of eccentric (fatigue and muscle damage) or concentric exercise (fatigue only). MVC force declined after eccentric exercise (45% decline) and remained depressed 24 h later (24%), whereas the reduced force after concentric exercise (22%) fully recovered the following day. EMG amplitude during the submaximal contractions increased in all elbow flexor muscles after eccentric exercise, with the greatest change in the biceps brachii at low forces (3-4 times larger at 5 and 20% MVC) and in the brachialis muscle at moderate forces (2 times larger at 35 and 50% MVC). Eccentric exercise resulted in a twofold increase in coactivation of the triceps brachii muscle during all submaximal contractions. Force fluctuations were larger after eccentric exercise, particularly at low forces (3-4 times larger at 5% MVC, 2 times larger at 50% MVC), with a twofold increase in physiological tremor at 8-12 Hz. These data indicate that eccentric exercise results in impaired motor control and altered neural drive to elbow flexor muscles, particularly at low forces, suggesting altered motor unit activation after eccentric exercise.     

Biomechanical properties of the crimp grip position in rock climbers

Rock climbers are often using the unique crimp grip position to hold small ledges. Thereby the proximal interphalangeal (PIP) joints are flexed about 90 degrees and the distal interphalangeal joints are hyperextended maximally. During this position of the finger joints bowstringing of the flexor tendon is applying very high load to the flexor tendon pulleys and can cause injuries and overuse syndromes. The objective of this study was to investigate bowstringing and forces during crimp grip position. Two devices were built to measure the force and the distance of bowstringing and one device to measure forces at the fingertip. All measurements of 16 fingers of four subjects were made in vivo. The largest amount of bowstringing was caused by the flexor digitorum profundus tendon in the crimp grip position being less using slope grip position (PIP joint extended). During a warm-up, the distance of bowstringing over the distal edge of the A2 pulley increased by 0.6mm (30%) and was loaded about 3 times the force applied at the fingertip during crimp grip position. Load up to 116N was measured over the A2 pulley. Increase of force in one finger holds by the quadriga effect was shown using crimp and slope grip position.     

Muscle activation and coactivation during five-time-sit-to-stand movement in patients undergoing total knee arthroplasty

Quadriceps weakness is prevalent with knee osteoarthritis (OA) and after total knee arthroplasty (TKA). To compensate for quadriceps dysfunction, patients often alter movement strategies. Little is known about muscle coordination during sit-to-stand (concentric) and stand-to-sit (eccentric) movements in the acute postoperative period. This investigation characterized the distribution of muscle activation between the concentric and eccentric phases during a five-time-sit-to-stand (FTSTS) movement in late stage OA and one month after TKA. Patients and healthy participants performed a FTSTS while recording bilateral ground reaction forces (GRFs) and electromyography (EMG). Concentric and eccentric ensemble averages of the GRF and EMG were calculated for the concentric and eccentric phases. Coactivation indices, integrated EMG, and GRF were calculated for each limb and phase. Patients demonstrated higher eccentric coactivation than the healthy group. Postoperative loading was higher in the nonsurgical limb. Postoperative quadriceps activity was lower in the concentric phase and higher in the eccentric phase than the healthy group. Higher coactivation in the patients resulted from sustained distribution of quadriceps activity throughout the eccentric phase. This indicated an inability to coordinate muscle firing when rapidly lowering to a chair and occurred despite unloading of the surgical limb. Although these patterns may serve as a protective strategy, they may also impede recovery of muscle function after TKA.     

Modeling of time-dependent force response of fingertip to dynamic loading

An extended exposure to repeated loading on fingertip has been associated to many vascular, sensorineural, and musculoskeletal disorders in the fingers, such as carpal tunnel syndrome, hand-arm vibration syndrome, and flexor tenosynovitis. A better understanding of the pathomechanics of these sensorineural and vascular diseases in fingers requires a formulation of a biomechanical model of the fingertips and analyses to predict the mechanical responses of the soft tissues to dynamic loading. In the present study, a model based on finite element techniques has been developed to simulate the mechanical responses of the fingertips to dynamic loading. The proposed model is two-dimensional and incorporates the essential anatomical structures of a finger: skin, subcutaneous tissue, bone, and nail. The skin tissue is assumed to be hyperelastic and viscoelastic. The subcutaneous tissue was considered to be a nonlinear, biphasic material composed of a hyperelastic solid and an invicid fluid, while its hydraulic permeability was considered to be deformation dependent. Two series of numerical tests were performed using the proposed finger tip model to: (a) simulate the responses of the fingertip to repeated loading, where the contact plate was assumed to be fixed, and the bone within the fingertip was subjected to a prescribed sinusoidal displacement in vertical direction; (b) simulate the force response of the fingertip in a single keystroke, where the keyboard was composed of a hard plastic keycap, a rigid support block, and a nonlinear spring. The time-dependent behavior of the fingertip under dynamic loading was derived. The model predictions of the time-histories of force response of the fingertip and the phenomenon of fingertip separation from the contacting plate during cyclic loading agree well with the reported experimental observations.     

The rigidity of repaired flexor tendons increases following ex vivo cyclic loading

Transected flexor tendons are typically treated by suture repair followed by rehabilitation that generates repetitive tendon loading. Recent results in an in vivo canine model indicate that during the first 10 days after injury and repair, there is an increase in the rigidity of the tendon repair site. Our objective was to determine whether or not ex vivo cyclic loading of repaired flexor tendons causes a similar increase in repair-site rigidity. We simulated 10 days of rehabilitation by applying 6000 loading cycles to repaired canine flexor tendons ex vivo at force levels generated during passive motion rehabilitation; we then evaluated their tensile mechanical properties. High-force (peak force, 17 N) cyclic loading increased repair-site rigidity by 100% and decreased repair-site strain by 50%, whereas low-force (5 N) loading did not change the properties of the repair site. This mechanical conditioning effect may explain, in part, the changes in tensile properties observed after only 10 days of healing in vivo. Mechanical conditioning of repaired flexor tendons by repetitive forces applied during rehabilitation may lead to increases in repair-site rigidity and decreases in strain, thereby altering the mechanical loading environment of tissues and cells at the repair site.     

The effects of the antagonist muscle force on intersegmental loading during isokinetic efforts of the knee extensors

Hamstrings activation when acting as antagonists is considered very important for knee joint stability. However, the effect of hamstring antagonist activity on knee joint loading in vivo is not clear. Therefore, the purpose of this study was to examine the differences in antagonistic muscle force and their effect on agonist muscle and intersegmental forces during isokinetic eccentric and concentric efforts of the knee extensors. Ten males performed maximum isokinetic eccentric and concentric efforts of the knee extensors at 30 degrees s(-1). The muscular and tibiofemoral joint forces were then estimated using a two-dimensional model with and without including the antagonist muscle forces. The antagonist moment was predicted using an IEMG-moment model. The predicted antagonist force reached a maximum of 2.55 times body weight (BW) and 1.16 BW under concentric and eccentric conditions respectively. Paired t-tests indicated that these were significantly different (p<0.05). A one-way analysis of variance indicated that when antagonist forces are included in the calculations the patella tendon, compressive and posterior shear joint forces are significantly higher compared to those calculated without including the antagonist forces. The anterior shear force was not affected by antagonist activity. The antagonists produce considerable force throughout the range of motion and affect the joint forces exerted at the knee joint. Further, it appears that the antagonist effect depends on the type of muscle action examined as it is higher during concentric compared to eccentric efforts of the knee extensors.     

Development of a dynamic index finger and thumb model to study impairment

Modeling of the human hand provides insight for explaining deficits and planning treatment following injury. Creation of a dynamic model, however, is complicated by the actions of multi-articular tendons and their complex interactions with other soft tissues in the hand. This study explores the creation of a musculoskeletal model, including the thumb and index finger, to explore the effects of muscle activation deficits. The OpenSim model utilizes physiological axes of rotation at all joints, passive joint torques, and appropriate moment arms. The model was validated through comparison with kinematic and kinetic experimental data. Simulated fingertip forces resulting from modeled musculotendon loading largely fell within one standard deviation of experimental ranges for most index finger and thumb muscles, although agreement in the sagittal plane was generally better than for the coronal plane. Input of experimentally obtained electromyography data produced the expected simulated finger and thumb motion. Use of the model to predict the effects of activation deficits on pinch force production revealed that the intrinsic muscles, especially first dorsal interosseous (FDI) and adductor pollicis (ADP), had a substantial impact on the resulting fingertip force. Reducing FDI activation, such as might occur following stroke, altered fingertip force direction by up to 83° for production of a dorsal fingertip force; reducing ADP activation reduced force production in the thumb by up to 62%. This validated model can provide a means for evaluating clinical interventions.     

Wrist and digital joint motion produce unique flexor tendon force and excursion in the canine forelimb

The force and excursion within the canine digital flexor tendons were measured during passive joint manipulations that simulate those used during rehabilitation after flexor tendon repair and during active muscle contraction, simulating the active rehabilitation protocol. Tendon force was measured using a small buckle placed upon the tendon while excursion was measured using a suture marker and video analysis method. Passive finger motion imposed with the wrist flexed resulted in dramatically lower tendon force (approximately 5 N) compared to passive motion imposed with the wrist extended (approximately 17 N). Lower excursions were seen at the level of the proximal interphalangeal joint with the wrist flexed (approximately 1.5 mm) while high excursion was observed when the wrist was extended or when synergistic finger and wrist motion were imposed (approximately 3.5 mm). Bivariate discriminant analysis of both force and excursion data revealed a natural clustering of the data into three general mechanical paradigms. With the wrist extended and with either one finger or four fingers manipulated, tendons experienced high loads of approximately 1500 g and high excursions of approximately 3.5 mm. In contrast, the same manipulations performed with the wrist flexed resulted in low tendon forces (4-8 N) and low tendon excursions of approximately 1.5 mm. Synergistic wrist and finger manipulation provided the third paradigm where tendon force was relatively low (approximately 4 N) but excursion was as high as those seen in the groups which were manipulated with the wrist extended. Active muscle contraction produced a modest tendon excursion (approximately 1 mm) and high or low tendon force with the wrist extended or flexed, respectively. These data provide the basis for experimentally testable hypotheses with regard to the factors that most significantly affect functional recovery after digital flexor tendon injury and define the normal mechanical operating characteristics of these tendons.     

Effects of movement speed and joint position on knee flexor torque in healthy and post-surgical subjects

Coactivation of knee flexors during knee extension assists in joint stability by exerting an opposing torque to the anterior tibial displacement induced by the quadriceps. This opposing torque is believed to be generated by eccentric muscle actions that stiffen the knee, thereby attenuating strain to joint ligaments, particularly the anterior cruciate ligament (ACL). However, as the lengths of knee muscles vary with changes in joint position, the magnitude of flexor/extensor muscle force coupling may likewise vary, possibly affecting the capacity for active knee stabilization. The purpose of this study was to assess the effect of changes in movement speed and joint position on eccentric/concentric muscle action relationships in the knees of uninjured (UNI) and post-ACL-surgery (INJ) subjects (n = 14). All subjects were tested for maximum eccentric and concentric torque of the contralateral knee flexors and extensor muscles at four isokinetic speeds (15 degrees-60 degrees x s(-1)) and four joint position intervals (20 degrees-60 degrees of knee flexion). Eccentric flexor torque was normalized to the percentage of concentric flexor torque generated at each joint position interval for each speed tested (flexor E-C ratio). In order to estimate the capacity of the knee flexors to resist active knee extension, the eccentric-flexor/concentric-extensor ratios were also computed for each joint position interval and speed (flexor/extensor E-C ratio). The results revealed that eccentric torque surpassed concentric torque by 3%-144% across movement speeds and joint position intervals. The magnitude of the flexor E-C ratio and flexor/extensor E-C increased significantly with speed in both groups of subjects (P < 0.05) and tended to rise with muscle length as the knee was extended; peak values were generated at the most extended joint position (20 degrees-30 degrees). Although torque development patterns were symmetrical between the contralateral limbs in both groups, between-group comparisons revealed significantly higher flexor/extensor E-C ratios for the INJ group compared to the UNI group (P < 0.05), particularly at the fastest speed tested (60 degrees x s(-1)). The results indicate that joint position and movement speed influence the eccentric/concentric relationships of knee flexors and extensors. The INJ subjects appeared to accommodate to surgery by developing the eccentric function of their ACL and normal knee flexors, particularly at higher speeds and at more extended knee joint positions. This may assist in the dynamic stabilization of the knee at positions where ACL grafts have been reported to be most vulnerable to strain.     

Effect of object width on muscle and joint forces during thumb-index finger grasping

The objective of this study was to identify the impact of modifying the object width on muscle and joint forces while gripping objects. The experimental protocol consisted to maintain horizontally five objects of different widths (3.5, 4.5, 5.5, 6.5, and 7.5 cm) with a thumb-index finger grip. Subjects were required to grasp spontaneously the object without any instruction regarding the grip force (GF) to apply. A biomechanical model of thumb-index finger pinch was developed to estimate muscle and joint forces. This model included electromyography, fingertip force, and kinematics data as inputs. The finger joint postures and the GF varied across the object widths. The estimated muscle forces also varied significantly according to the object width. Interestingly, we observed that the muscle force/GF ratios of major flexor muscles remain particularly stable with respect to the width whereas other muscle ratios differed largely. This may argue for a control strategy in which the actions of flexors were preserved in spite of change in joint postures. The estimated joint forces tended to increase with object width and increased in the distal-proximal sense. Overall, these results are of importance for the ergonomic design of handheld objects and for clinical applications.     

Motor unit activation patterns during isometric, concentric and eccentric actions at different force levels.

Motor unit activation patterns were studied during four different force levels of concentric and eccentric actions. Eight male subjects performed concentric and eccentric forearm flexions with the movement range from 100 degrees to 60 degrees in concentric and from 100 degrees to 140 degrees elbow angle in eccentric actions. The movements were started either from zero preactivation or with isometric preactivation of the force levels of 20, 40, 60 and 80% MVC. The subjects were then instructed to maintain the corresponding relative force levels during the dynamic actions. Intramuscular and surface EMG was recorded from biceps brachii muscle. Altogether 28 motoneuron pools were analyzed using the intramuscular spike-amplitude frequency (ISAF) analysis technique of Moritani et al. The mean spike amplitude was lower and the mean spike frequency higher in the isometric preactivation phase than in the consequent concentric and eccentric actions. When the movements started with isometric preactivation the mean spike amplitude increased significantly (P<0.001) up to 80% in isometric and concentric actions but in eccentric actions the increase continued only up to 60% (P<0.01). The mean spike frequency in isometric preactivation and in concentric action with preactivation was lower only at the 20% force level (P<0.01) as compared to the other force levels while in eccentric action with preactivation the increase between the force levels was significant (P<0.01) up to 60%. When the movement was started without preactivation the mean spike amplitude at 20% and at 40% force level was higher (P<0.01) in eccentric action than in concentric actions. It was concluded that the recruitment threshold may be lower in dynamic as compared to isometric actions. The recruitment of fast motor units may continue to higher force levels in isometric and in concentric as in eccentric actions which, on the other hand, seems to achieve the higher forces by increasing the firing rate of the active units. At the lower force levels mean spike amplitude was higher in eccentric than in concentric actions which might indicate selective activation of fast motor units. This was, however, the case only when the movements were started without isometric preactivation.     

Computer keyswitch force–displacement characteristics affect muscle activity patterns during index finger tapping

This study examined the effect of computer keyboard keyswitch design on muscle activity patterns during finger tapping. In a repeated-measures laboratory experiment, six participants tapped with their index fingers on five isolated keyswitch designs with varying force–displacement characteristics that provided pairwise comparisons for the design factors of (1) activation force (0.31 N vs. 0.59 N; 0.55 N vs. 0.93 N), (2) key travel (2.5 mm vs. 3.5 mm), and (3) shape of the force–displacement curve as realized through buckling-spring vs. rubber-dome switch designs. A load cell underneath the keyswitch measured vertical fingertip forces, and intramuscular fine wire EMG electrodes measured muscle activity patterns of two intrinsic (first lumbricalis, first dorsal interossei) and three extrinsic (flexor digitorum superficialis, flexor digitorum profundus, and extensor digitorum communis) index finger muscles. The amplitude of muscle activity for the first dorsal interossei increased 25.9% with larger activation forces, but not for the extrinsic muscles. The amplitude of muscle activity for the first lumbricalis and the duration of muscle activities for the first dorsal interossei and both extrinsic flexor muscles decreased up to 40.4% with longer key travel. The amplitude of muscle activity in the first dorsal interossei increased 36.6% and the duration of muscle activity for all muscles, except flexor digitorum profundus, decreased up to 49.1% with the buckling-spring design relative to the rubber-dome design. These findings suggest that simply changing the force–displacement characteristics of a keyswitch changes the dynamic loading of the muscles, especially in the intrinsic muscles, during keyboard work.