Measuring the force production of the hormogonia of Mastigocladus laminosus

The cyanobacterium Mastigocladus laminosus forms hormogonia, which glide slowly away from the parent colony by extruding slime out of nozzles. Using video microscopy, we observed hormogonia embedded in and moving through 1-4% agar solutions with an average velocity of 0.5 microm/s. Agar is non-Newtonian and is subject to shear-thinning so that its viscosity greatly increases at low shear rates. We measured the viscosity of these agar solutions at the very low shear rates appropriate for gliding hormogonia and found it to vary from 1 to 52 million centipoise. Then, by applying a Newtonian drag coefficient for a 100-microm-long, cigar-shaped hormogonium, we found that it produced a force of several million pN. A typical hormogonium has 10-100 thousand 9-nm-wide slime extrusion nozzles. Wolgemuth et al. have proposed hydration-driven swelling of the polyelectrolyte slime ejected from these nozzles as the force production mechanism, and our experiment found a large nozzle force that was consistent with this hypothesis. Average single nozzle force depended on viscosity, being large when the viscosity was high: 71 pN in 3% and 126 pN in 4% agar.     

Effect of fatigue on force production and force application technique during repeated sprints

We investigated the changes in the technical ability of force application/orientation against the ground vs. the physical capability of total force production after a multiple-set repeated sprints series. Twelve male physical education students familiar with sprint running performed four sets of five 6-s sprints (24s of passive rest between sprints, 3min between sets). Sprints were performed from a standing start on an instrumented treadmill, allowing the computation of vertical (F(V)), net horizontal (F(H)) and total (F(Tot)) ground reaction forces for each step. Furthermore, the ratio of forces was calculated as RF=F(H)F(Tot)(-1), and the index of force application technique (D(RF)) representing the decrement in RF with increase in speed was computed as the slope of the linear RF-speed relationship. Changes between pre- (first two sprints) and post-fatigue (last two sprints) were tested using paired t-tests. Performance decreased significantly (e.g. top speed decreased by 15.7±5.4%; P<0.001), and all the mechanical variables tested significantly changed. F(H) showed the largest decrease, compared to F(V) and F(Tot). D(RF) significantly decreased (P<0.001, effect size=1.20), and the individual magnitudes of change of D(RF) were significantly more important than those of F(Tot) (19.2±20.9 vs. 5.81±5.76%, respectively; P<0.01). During a multiple-set repeated sprint series, both the total force production capability and the technical ability to apply force effectively against the ground are altered, the latter to a larger extent than the former.     

Movement of myosin fragments in vitro: domains involved in force production

We have used the Nitella-based movement assay to localize the site of force production in myosin. Methods were developed to use nonfilamentous myosin or proteolytic fragments of myosin in place of the thick filaments used in the original assay. In the experiments described here, the tail of myosin or its subfragments is anchored via antibodies to the surface of small particles. Nonfilamentous myosin or its subfragments move along Nitella actin cables at speeds similar to those obtained with filamentous myosin. We generated short HMM, a myosin fragment containing the heads and only 400 A of the tail. Although short HMM lacks the "hinge" region proposed by Harrington to be the site of force generation, and is incapable of forming thick filaments, it moves along actin at speeds above 1 micron/sec. Therefore, neither a thick filament nor the carboxy-terminal 1100 A of the tail is required for movement along actin. The results indicate that force production occurs in or near the myosin heads.     

Growth cone behavior and production of traction force

The growth cone must push its substrate rearward via some traction force in order to propel itself forward. To determine which growth cone behaviors produce traction force, we observed chick sensory growth cones under conditions in which force production was accommodated by movement of obstacles in the environment, namely, neurites of other sensory neurons or glass fibers. The movements of these obstacles occurred via three, different, stereotyped growth cone behaviors: (a) filopodial contractions, (b) smooth rearward movement on the dorsal surface of the growth cone, and (c) interactions with ruffling lamellipodia. More than 70% of the obstacle movements were caused by filopodial contractions in which the obstacle attached at the extreme distal end of a filopodium and moved only as the filopodium changed its extension. Filopodial contractions were characterized by frequent changes of obstacle velocity and direction. Contraction of a single filopodium is estimated to exert 50-90 microdyn of force, which can account for the pull exerted by chick sensory growth cones. Importantly, all five cases of growth cones growing over the top of obstacle neurites (i.e., geometry that mimics the usual growth cone/substrate interaction), were of the filopodial contraction type. Some 25% of obstacle movements occurred by a smooth backward movement along the top surface of growth cones. Both the appearance and rate of movements were similar to that reported for retrograde flow of cortical actin near the dorsal growth cone surface. Although these retrograde flow movements also exerted enough force to account for growth cone pulling, we did not observe such movements on ventral growth cone surfaces. Occasionally obstacles were moved by interaction with ruffling lamellipodia. However, we obtained no evidence for attachment of the obstacles to ruffling lamellipodia or for directed obstacle movements by this mechanism. These data suggest that chick sensory growth cones move forward by contractile activity of filopodia, i.e., isometric contraction on a rigid substrate. Our data argue against retrograde flow of actin producing traction force.     

Anticipatory responses of catecholamines on muscle force production.

Few data exist on the temporal relationship between catecholamines and muscle force production in vivo. The purpose of this study was to examine the influence of preexercise arousal on sympathoadrenal neurohormones on muscular force expression during resistance exercise. Ten resistance-trained men completed two experimental conditions separated by 7 days: 1) acute heavy resistance exercise protocol (AHREP; 6 x 10 repetitions parallel squats, 80% 1 repetition maximum) and 2) control (Cont; rest). Peak force (F(peak)) was recorded during a maximal isometric squat preceding each set and mean force (F(mean)) was measured during each set. Serial venous blood samples were collected before the AHREP and immediately preceding each set. Blood collection times were matched during Cont. Preexercise epinephrine (Epi), norepinephrine (NE), and dopamine (DA) increased (P or= 0.05) in muscular performance (F(peak), F(mean)) during AHREP and that five subjects (F(reducers)) had significant reductions in F(peak) and F(mean). Integrated area under the curve for Epi, NE, and F(peak) were greater (P < 0.02) for F(maintainers) than F(reducers). In conclusion, an anticipatory rise in catecholamines existed, which may be essential for optimal force production at the onset of exercise.     

"Psyching-up" enhances force production during the bench press exercise

We investigated the effect of "psyching-up" on force production during the bench press. Twelve men (mean age +/- SD: 27.4 +/- 11.2 years) and 8 women (20.9 +/- 2.5 years) with strength-training experience performed 5 bench press repetitions on a modified Biodex isokinetic dynamometer during 3 interventions. The interventions were counterbalanced and included a free-choice psych-up, a cognitive distraction, and an attention-placebo. Peak force recorded after psyching-up (mean +/- SD: 764 +/- 269 N.m) was significantly different from both distraction (703 +/- 282 N.m, p = 0.003) and attention-placebo (708 +/- 248 N.m, p = 0.01). The mean percentage increase in peak force from distraction to psyching-up was 11.8% (6 to 18%, 95% confidence interval [CI]) and 8.1% from placebo to psyching-up (3 to 13%, 95% CI). The results of the present study indicate that psyching-up may increase force production during the bench press exercise in participants with at least 1 year strength-training experience.     

Radial force distribution changes associated with tangential force production in cylindrical grasping, and the importance of anatomical registration

Radial force (F(r)) distributions describe grip force coordination about a cylindrical object. Recent studies have employed only explicit F(r) tasks, and have not normalized for anatomical variance when considering F(r) distributions. The goals of the present study were (i) to explore F(r) during tangential force production tasks, and (ii) to examine the extent to which anatomical registration (i.e. spatial normalization of anatomically analogous structures) could improve signal detectability in F(r) data. Twelve subjects grasped a vertically oriented cylindrical handle (diameter=6 cm) and matched target upward tangential forces of 10, 20, and 30 N. F(r) data were measured using a flexible pressure mat with an angular resolution of 4.8°, and were registered using piecewise-linear interpolation between five manually identified points-of-interest. Results indicate that F(r) was primarily limited to three contact regions: the distal thumb, the distal fingers, and the fingers' metatacarpal heads, and that, while increases in tangential force caused significant increases in F(r) for these regions, they did not significantly affect the F(r) distribution across the hand. Registration was found to substantially reduce between-subject variability, as indicated by both accentuated F(r) trends, and amplification of the test statistic. These results imply that, while subjects focus F(r) primarily on three anatomical regions during cylindrical grasp, inter-subject anatomical differences introduce a variability that, if not corrected for via registration, may compromise one's ability to draw anatomically relevant conclusions from grasping force data.     

Coordinated force production in multi-finger tasks: finger interaction and neural network modeling

During maximal voluntary contraction (MVC) with several fingers, the following three phenomena are observed: (1) the total force produced by all the involved fingers is shared among the fingers in a specific manner (sharing); (2) the force produced by a given finger in a multi-finger task is smaller than the force generated by this finger in a single-finger task (force deficit); (3) the fingers that are not required to produce any force by instruction are involuntary activated (enslaving). We studied involuntary force production by individual fingers (enslaving effects, EE) during tasks when (an)other finger(s) of the hand generated maximal voluntary pressing force in isometric conditions. The subjects (n = 10) were instructed to press as hard as possible on the force sensors with one, two, three and four fingers acting in parallel in all possible combinations. The EE were (A) large, the slave fingers always producing a force ranging from 10.9% to 54.7% of the maximal force produced by the finger in the single-finger task; (B) nearly symmetrical; (C) larger for the neighboring fingers; and (D) non-additive. In most cases, the EE from two or three fingers were smaller than the EE from at least one finger (this phenomenon was coined occlusion). The occlusion cannot be explained only by anatomical musculo-tendinous connections. Therefore, neural factors contribute substantially to the EE. A neural network model that accounts for all the three effects has been developed. The model consists of three layers: the input layer that models a central neural drive; the hidden layer modeling transformation of the central drive into an input signal to the muscles serving several fingers simultaneously (multi-digit muscles); and the output layer representing finger force output. The output of the hidden layer is set inversely proportional to the number of fingers involved. In addition, direct connections between the input and output layers represent signals to the hand muscles serving individual fingers (uni-digit muscles). The network was validated using three different training sets. Single digit muscles contributed from 25% to 50% of the total finger force. The master matrix and the enslaving matrix were computed; they characterize the ability of a given finger to enslave other fingers and its ability to be enslaved. Overall, the neural network modeling suggests that no direct correspondence exists between neural command to an individual finger and finger force. To produce a desired finger force, a command sent to an intended finger should be scaled in accordance with the commands sent to the other fingers. Received: 17 October 1997 / Accepted in revised form: 12 May 1998     

Mechanisms of force and power production in unsteady ricochetal brachiation

Brachiators travel by swinging beneath handholds, and it is not obvious how these animals manage to accelerate and decelerate in a horizontal direction, especially when moving rapidly. Most previous analyses focused on brachiation in highly constrained laboratory conditions that induced steady-state locomotion. Emerging understanding of brachiation suggests that much of gibbon locomotory behavior and morphology must be considered within the context of the complexities of the natural environment: the forest canopy is three-dimensional, with high variation in handhold availability and properties. The goal of this paper is to quantify the active mechanisms by which gibbons can dynamically control their velocity.Force production and kinematics were analyzed from a white-handed gibbon Hylabates lar during ricochetal brachiation. Both the mechanisms of force production and power input may be inferred for accelerating and decelerating brachiation by combining force data with kinematics. Examples of steady-state, accelerating, and decelerating ricochetal brachiation are highlighted.Gibbons are able to produce net horizontal impulses by releasing early (resulting in a loss of potential energy, but an accelerating horizontal impulse) or delaying release (associated with an increase in potential energy, and a decelerating horizontal impulse).Torque about the shoulder, leg-lifting (or dropping), and elbow flexing (or straightening) are discussed as potential mechanisms for controlling energy within the brachiating system. Of these possibilities, leg-lifting and arm-flexing were observed as mechanisms of adding mechanical energy. Net energy loss, and substantial torques about the shoulder, were not observed.     

Matrix analyses of interaction among fingers in static force production tasks

The fingers on a hand show interactions in force production tasks. The interfinger connection matrices (IFMs) quantify these interactions (Li et al. 2002; Zatsiorsky et al. 2002b; Danion et al. 2003). The goal of the present study was to explore the differences in the IFMs of individual subjects and, in particular, to establish a procedure that may be used in the future for diagnostic purposes. Subjects (n=20) pressed downward maximally with ten different combinations of the four fingers, index (I), middle (M), ring (R), and little (L): I, M, R, L, IM, MR, RL, IMR, MRL, and IMRL. Voluntary activation of a subset of the four fingers was accompanied by an involuntary force production by fingers that were not intentionally activated (enslaving). Interfinger connection matrices were computed for each subject by the artificial neural network. The similarities/dissimilarities (proximities) between the individual matrices were determined. This procedure was performed twice: (a) for nonnormalized IFMs whose elements represented the amount of force (in newtons) exerted by a finger i in response to a unit command to a finger j; and (b) for normalized IFMs, after dividing the elements of each IFM by the total force produced by the four fingers acting together (the elements of the matrix are in percents). The 20×20 matrix of the proximities was subjected to multidimensional scaling (MDS) to reduce the number of dimensions and identify the major ones. To interpret the meaning of the computed dimensions, they were regressed on a set of finger force parameters described in the text. For the nonnormalized IFMs an interpretable dimension was the strength of the subjects. For the normalized IFMs two dimensions were interpreted: (a) the location of the point of resultant force application along the mediolateral axis that is defined by the pattern of force sharing among the fingers and (b) the total contribution of the enslaved forces into the total finger force. We speculate that the similarity of typical everyday tasks across the population promotes the similarity of the IMFs reflecting optimal hand functioning over these tasks. AcknowledgementsThis study was partly supported by NIH grants NS-35032, AR-048563 and AG-18751. The support from the Whittaker Foundation to Dr. Z.M. Li is also acknowledged.     

Duration of static stretching influences muscle force production in hamstring muscles

The purpose of the present study was to investigate whether duration of static stretching could affect the maximal voluntary contraction (MVC). Volunteer male subjects (n = 10) underwent 2 different durations of static stretching of their hamstring muscles in the dominant leg: 30 and 60 seconds. No static stretching condition was used as a control condition. Before and after each stretching trial, hamstring flexibility was measured by a sit and reach test. MVC was then measured using the maximal effort of knee flexion. The hamstring flexibility was significantly increased by 30 and 60 seconds of static stretching (control: 0.5 +/- 1.1 cm; 30 seconds: 2.1 +/- 1.8 cm; 60 seconds: 3.0 +/- 1.6 cm); however, there was no significant difference between 30 and 60 seconds of static stretching conditions. The MVC was significantly lowered with 60 seconds of static stretching compared to the control and 30 seconds of the stretching conditions (control: 287.6 +/- 24.0 N; 30 seconds: 281.8 +/- 24.2 N; 60 seconds: 262.4 +/- 36.2 N). However, there was no significant difference between control and 30 seconds of static stretching conditions. Therefore, it was concluded that the short duration (30 seconds) of static stretching did not have a negative effect on the muscle force production.     

The multiple roles of titin in muscle contraction and force production

Titin is a filamentous protein spanning the half-sarcomere, with spring-like properties in the I-band region. Various structural, signaling, and mechanical functions have been associated with titin, but not all of these are fully elucidated and accepted in the scientific community. Here, I discuss the primary mechanical functions of titin, including its accepted role in passive force production, stabilization of half-sarcomeres and sarcomeres, and its controversial contribution to residual force enhancement, passive force enhancement, energetics, and work production in shortening muscle. Finally, I provide evidence that titin is a molecular spring whose stiffness changes with muscle activation and actin–myosin-based force production, suggesting a novel model of force production that, aside from actin and myosin, includes titin as a “third contractile” filament. Using this three-filament model of sarcomeres, the stability of (half-) sarcomeres, passive force enhancement, residual force enhancement, and the decrease in metabolic energy during and following eccentric contractions can be explained readily.     

Effects of repeated Achilles tendon vibration on triceps surae force production

Many studies reported benefits of whole-body vibration (WBV) on muscle force production. Therefore, WBV may be an important technique for muscle re-education. However vibrating platforms are heavy tools that cannot be easily used by all patients. Thus, we propose to apply vibrations directly to the Achilles tendon at rest with a portable vibrator. We investigated whether 14 days of such a vibration program would enhance triceps surae force production in healthy subjects. If successful, such a protocol could be utilized to prevent deleterious effects of hypo-activity. Twenty-nine healthy students participated in this study. The electrical evoked twitch and maximal voluntary contraction (MVC) in plantar-flexion, and electromyograms (EMG) were quantified before and at the end of the program. The vibration program consisted of 14 days of daily vibration applied at rest (duration: 1 h; frequency: 50 Hz). After the program, there was an increase in MVC associated with greater EMG of the TS. No sign of hypertrophy were found on the twitch parameters and the EMG–torque relationships. Repeated vibrations of the Achilles tendon lead to an increase in plantar-flexor activation and thus to greater force developed in voluntary conditions whilst the contractile properties assessed by the twitch are not modified. This program could be beneficial to persons with hypo-activity who are not candidates for WBV.     

Muscle force production during bent-knee,bent-hip walking in humans

Researchers have long debated the locomotor posture used by the earliest bipeds. While many agree that by 3–4 Ma (millions of years ago), hominins walked with an extended-limb human style of bipedalism, researchers are still divided over whether the earliest bipeds walked like modern humans, or walked with a more bent-knee, bent-hip (BKBH) ape-like form of locomotion. Since more flexed postures are associated with higher energy costs, reconstructing early bipedal mechanics has implications for the selection pressures that led to upright walking. The purpose of this study is to determine how modern human anatomy functions in BKBH walking to clarify the links between morphology and energy costs in different mechanical regimes. Using inverse dynamics, we calculated muscle force production at the major limb joints in humans walking in two modes, both with extended limbs and BKBH. We found that in BKBH walking, humans must produce large muscle forces at the knee to support body weight, leading to higher estimated energy costs. However, muscle forces at the hip remained similar in BKBH and extended limb walking, suggesting that anatomical adaptations for hip extension in humans do not necessarily diminish the effective mechanical advantage at the hip in more flexed postures. We conclude that the key adaptations for economical walking, regardless of joint posture, seem to center on maintaining low muscle forces at the hip, primarily by keeping low external moments at the hip. We explore the implications of these results for interpreting locomotor energetics in early hominins, including australopithecines and Ardipithecus ramidus.