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A Comparison of Musculo-Articular Stiffness and Maximal Isometric Plantar Flexion and Knee Extension Force in Dancers and Untrained Individuals.

Dance can be classified as a physically demanding and complex form of art within athletics. The amount of advanced dance training required for meticulous skill development has been commonly reported, but a lack of research exists on performance characteristics of dancers, such as isometric strength and ankle muscle-tendon complex stiffness. (1) These variables may provide insight into the components that determine proficiency in the coordination and execution of various dance styles. Previous findings have indicated that elite long and triple jumpers possess greater isometric strength and musculo-articular stiffness than untrained individuals. (2) Dancers may be similar to these types of jumping athletes; however, no studies have reported specifically on isometric muscle force, rate of force development, or musculo-articular stiffness in dancers. Therefore, the current study aimed to identify whether these variables may differ between dancers and untrained individuals.

The amount of isometric peak force an athlete can generate has been historically connected to maximum muscle strength capabilities and dynamic performance. (3) In sports that involve repetitive, maximal effort, strength is a key component of elite success. (3) Numerous studies cited in a meta-analysis have demonstrated that supplemental plyometric training results consistently in greater muscle peak force. (4) The dancer, as a technical, power-athlete, is known to focus on plyometric training with an aesthetic emphasis. (5) While not directly measured in their study, the necessity of strength in multi-joint leaping patterns has been demonstrated by Wyon and coworkers, who found that sequential grand jetes increase take-off power with each leap, thus exerting more force in tandem with greater velocity. (6) The grand jete developpe consists of leaping from one foot to the other, fully extending the knee and plantar flexing the ankle into a maximum center of mass displacement, full split (or nearly so) from the preceding plie. (7) The biomechanical nature of this movement pattern may render not only high peak force but substantial rate of force development (RFD) for the take-off and extension portions of the leap as well. (8) For this reason, the considerable repetition of the grand jete developpe most dancers experience from a young age serves as a basis for the current study.

A multitude of training strategies have been shown to influence performance and significantly increase RFD measures across several athletic tasks. (9-11) Quantifying RFD is a valuable tool for professionals to utilize when assessing reactivity. (12) Greater RFD is crucial for athletes who frequently use stretch-shortening cycle (SSC) mechanisms to generate force rapidly. (13) The SSC involves active stretching and shortening of the muscle-tendon unit as a whole, thereby storing and re-utilizing elastic energy to enhance concentric performance. (14) As an element that may contribute to dynamic performance, Marques and colleagues found that greater RFD strongly correlated with jump height in trained athletes. (15) Measurements in isometric strength tests might also be reflective of RFD capabilities in athletes. (16) Dance, a plyometric-based activity, requires technical coordination and neuromuscular control to execute more advanced skills properly, but limited knowledge is available pertaining to the RFD of dancers. (17)

The inherent utilization of both the contractile component (CC) and series elastic component (SEC) of the muscle-tendon unit during SSC movements optimizes performance and may affect mechanical efficiency. (13,18) The SEC, comprised of the tendon and muscle cross-bridges, can also contribute to enhanced performance with greater stiffness. (19) Stiffness is defined as the relationship between an applied load and the elastic deformation that occurs in a structure. (20) From the dance professional's perspective, it is necessary to recognize the difference between stiffness and flexibility as tension within the contractile and collagenous properties compared with passive joint range of motion. (20) When quantifying stiffness of the ankle muscle-tendon complex with the free-oscillation method, the term musculo-articular stiffness will be used to accurately describe the measurements that encompass the skin, ligaments, and articular surfaces as well. (2) Muscle-tendon characteristics play a vital role in dynamic jumping tasks and maximal strength tests. (13,21) SSC capability, aforementioned as critical to athletic performance, has been shown to correlate significantly with higher stiffness levels due to the natural strategy of increased potential energy stored within the CC and SEC. (19,22) Thus, optimal musculo-articular stiffness should result in the greatest amount of elastic energy return and utilization. (22,23) The plyometric athlete is expected to have greater than average musculo-articular stiffness, but no empirical data exist as a result of measuring active and passive SEC stiffness with a free-oscillation method on dancers.

Therefore, the purpose of this investigation was to determine whether musculo-articular stiffness and isometric plantar flexion and knee extension peak force and RFD differ between dancers and untrained individuals. As research has shown plyometric training to result in greater isometric strength, we hypothesized that dancers would achieve greater peak force and RFD than untrained individuals. (24) A second hypothesis was that dancers would possess higher musculo-articular stiffness than untrained individuals. Lastly, it was hypothesized that there would be a strong relationship between isometric plantar flexion and knee extension variables measured across all subjects.



Eight female dancers and eight untrained individuals (N = 16; age: 20.6 [+ or -] 1.5 years; height: 162.6 [+ or -] 7.1 cm; weight: 60.2 [+ or -] 13.2 kg) were recruited who had no musculoskeletal injury, neuromuscular disease, lower limb injury within the past 6 months, and were not currently pregnant. The dancers had a minimum of 10 years of dance experience and were training at least three times a week at the collegiate level. Dance style backgrounds were concentrated in two or more of the following: jazz, pom, ballet, and modern. Untrained individuals had no competitive endurance, strength training, or dance background, nor were they currently involved in any organized sports or physical activities. All participants signed an informed consent form before participating in the study. An ACSM health screening questionnaire was administered to ensure that all participants were healthy enough for testing. The authors' University Institutional Review Board approved this study.

Study Design

All subjects visited the Neuromuscular & Biomechanics Laboratory for a single testing session. Height, weight, and lower leg anthropometric measurements were recorded for all subjects upon arrival. Subjects next warmed up on a cycle ergometer (Monark Exercise AB, Vansbro, Sweden) for 5 minutes at a self-selected resistance and cadence. A validated and reliable free-oscillation method was then utilized to obtain musculo-articular stiffness measurements. (25) Subjects then performed three maximal isometric plantar flexion (MIP) contractions and three maximal isometric knee extension (MIKE) contractions. Stiffness and MIP trials were performed on a force plate. MIKE trials were performed in a chair that secured the subjects at the chest and waist, and they were attached to an ankle strap using a strain gauge (Lafayette Instrument Company, Lafayette, Indiana, USA) to measure force generated at the knee joint.


Lower leg anthropometric measurements were taken from the right leg with an anthropometer (Lafayette Instrument Company, 01291, Lafayette, Indiana, USA). Lower leg length was defined as and measured from the lateral condyle to the lateral malleolus of the fibula. Achilles tendon to medial malleolus (AT-MM) length and Achilles tendon to lateral malleolus (AT-LM) length were then recorded. Subjects placed the right foot on a block with the shank directly over the ankle. Medial and lateral pictures were taken of the right foot 112 cm from the block with a Cannon camcorder (VIXIA HF G20, Melville, New York, USA). All of these measurements in accordance with previously published methods were used exclusively for calculation with a custom MATLab program (MathWorks, Natick, Massachusetts) of average, resultant musculo-articular stiffness. (26)

Free-Oscillation Method

Musculo-articular stiffness was assessed using a modified version of the free-oscillation technique described by Ditroilo and coworkers and Butler and associates. (20,27) Subjects stood erect with the forefeet only on a wooden block positioned hips width apart atop a force plate (Fig. 1). Tape was used to ensure consistent foot placement throughout testing. Subjects were instructed to stay rigid and maintain an ankle angle of 90[degrees], with two bars resting on their shoulders. The two bars provided the means by which a perturbation could be applied. The frequency response of the ground reaction force to each perturbation was measured under various load conditions using the force plate. Increasing loads of 5 kg were added to the bars starting with no weight up to 40 kg. A total of nine trials consisted of four perturbations applied to the bars during each load. Perturbations were consistently performed by dropping a 10 lb. medicine ball from a height of approximately 10 cm to 20 cm onto the bars on the shoulders. (28) One-minute rest intervals were provided between trials. Past findings have confirmed that applied impulses of different magnitudes affect all loads the same, (29) and stiffness is unaffected. (25) Force-time curves were obtained from the oscillations of the ankle caused by the perturbations (Fig. 2). The musculo-articular stiffness of the ankle complex is determined by first modeling the complex as a mass-spring-damper system. The dynamics of such a system are described by Fukashiro and colleagues with the following equations (26):

F(t) = [e.sup.-yt]([a.sub.c] cos [[omega].sub.1]t + [a.sub.s] sin [[omega].sub.1]t) + Mg

where [gamma] is the damping coefficient, [a.sub.c] and [a.sub.s] are constants of integration, [[omega].sub.1] is the angular frequency of oscillation, M is the mass of the system, and g is the acceleration due to gravity. The ground reaction force-time data from the first period of the oscillation for each perturbation (Fig. 2) was fit to this equation using the non-linear least squares curve fitting method in MATLAB R2015b (MathWorks, Natick, Massachusetts). The resonant frequency, [[omega].sub.0], is related to [gamma] and [[omega].sub.1] by:

[[omega].sub.1] = [square root of [[omega].sup.2.sub.0] - [[gamma].sup.2]]

Then, given [[omega].sub.0], the apparent stiffness of the system (K) is:

[[omega].sub.0] = [square root of K/M]

The apparent stiffness, K, of the ankle-foot complex does not consider the anthropometry of the foot, which affects the way in which the linear force is applied to the heel via the Achilles tendon at the point of application (i.e., at the forefoot). To compensate for this anthropometry and more accurately deduce the true musculo-articular stiffness (k), the gear ratio of the foot is used to scale the apparent stiffness according to k = [a.sub.2]K, where a = [r.sub.a]/[r.sub.b]. The forefoot length, [r.sub.a], and the Achilles tendon moment arm length, [r.sub.b], were obtained using digital images of the foot in a custom MATLab program according to a previously described method. (30) The average k value across all perturbation trials was determined for each subject, and resultant musculo-articular stiffness kN*[m.sup.-1]) was used for comparison between groups.

Maximal Isometric Contractions

Maximal isometric plantar flexion (MIP) force measurements involved subjects standing with feet hip width apart on the wooden block atop the force plate with the forefoot placement marked by tape. An immovable bar was fixed at shoulder height for each individual subject comparable to back squat bar placement. Subjects were instructed to generate force at the ankle joint over 5 seconds with legs fully extended while ankles remained at a 90[degrees] angle. Three maximal effort plantar flexion trials were completed with a 2-minute rest period provided between each trial.

Single leg maximal isometric knee extension (MIKE) force was measured with subjects sitting in a chair with the right hip, knee, and ankle fixed at 90[degrees]. Subjects' upper bodies were strapped in the chair to eliminate any possible movement aside from knee extension. An ankle joint strap was fastened in alignment with the lateral and medial malleoli and connected to a strain gauge for force measurements about the knee joint. Subjects were instructed to generate maximal force while isometrically extending the right knee over 5 seconds for three trials. Two-minute rest periods separated each trial. All subjects were verbally encouraged to push as "rapidly and forcibly as possible" for both maximal isometric strength tests.

Peak force (PF; N/kg) and rate of force development (RFD; N/s) were analyzed from both the MIP and MIKE contractions in a custom LabVIEW program (National Instruments, LabVIEW, Version 8.2, Austin, Texas, USA). Each subject's trial with the highest force was selected for further analysis. Peak force was defined as the maximal force achieved during the maximal isometric contraction and reported relative to body mass. The rate of force development was analyzed in the first 250 ms of contraction. The rate of force development was obtained by subtracting the maximal force value at 250 ms from baseline force and subsequently dividing the value by the difference in time.

Statistical Analysis

Statistical analyses were completed using SPSS software (IBM SPSS Statistics for Windows, Version 19.0., IBM Corp. Armonk, New York). A multivariate analysis of variance (ANOVA) was used to determine significance between the two groups for all descriptive and dependent variables. A Bonferroni post-hoc test was used to establish differences between groups. In order to determine relationships between elected variables, a bivariate analysis was used. Pearson correlation coefficients were used for the scaling of relationships and defined as trivial (0.0-0.1), small (0.1-0.3), moderate (0.3-0.5), large (0.5-0.7), very large (0.7-0.9), or perfect (0.9-1.0). An a priori value of 0.05 was used to identify significance for all SPSS tests.


No significant differences existed concerning descriptive characteristics between female dancers and untrained individuals (Table 1). Nor were anthropometric measurements recorded for the purpose of musculo-articular stiffness calculation significantly different between groups (Table 1).

Table 2 shows that during MIP, dancers had significantly greater RFD than untrained individuals (2040 [+ or -] 657 N/s; 443 [+ or -] 260 N/s). MIP PF was also significantly higher in dancers than untrained individuals (20.5 [+ or -] 7.9 N/kg; 9.6 [+ or -] 4.8 N/kg; Table 2). While MIKE RFD was significantly greater in dancers than in untrained individuals (765 [+ or -] 319 N/s; 319 [+ or -] 257 N/s), MIKE PF was not (5.4 [+ or -] 1.6 N/kg; 3.4 [+ or -] 2.3 N/kg; Table 2). Lastly, Figure 3 shows that musculo-articular stiffness (k) was found to be significantly greater in female dancers than untrained individuals (271.1 [+ or -] 51.0 kN*[m.sup.-1]; 213.4 [+ or -] 59.2 kN*[m.sup.-1]).

Significant relationships were observed within MIP, MIKE, and musculo-articular stiffness measurements. A very large, significant correlation was identified between MIP PF and MIP RFD (r = 0.76), as well as between MIP RFD and MIKE RFD (r = 0.78). A large, significant correlation was also present between MIP PF and MIKE PF (r = 0.60), along with MIP RFD and musculo-articular stiffness (r = 0.60).


The results of this investigation suggest that dancers possess greater musculo-articular stiffness than untrained individuals. Some of the data also suggest greater peak force and rate of force development capabilities in dancers. This indicates that involvement in dance is a potential stimulus for enhanced muscular and tendinous function similar to that observed in elite and recreational jumping athletes. (2,7,31) Peak force in plantar flexion contraction but not in knee extension was significantly greater in dancers. The investigators acknowledge that the small number of participants may have statistically affected the results in the comparison of knee extension peak force, and a post-hoc power analysis revealed a 0.49 probability of a Type II error. Conclusively, the small sample size was a limitation of the current study. Nonetheless, previous research has shown that plyometric-trained subjects achieved significantly greater isometric peak torque values in knee extension than controls. (24) It is possible that dancing may be a stimulus for higher plantar flexion peak force but not knee extension peak force due to the types of movement performed. (32) However, the current investigation still indicates a significant mechanism for muscle adaptation as a result of involvement in dance.

The rate of force development was significantly greater during both isometric muscle contractions in dancers when compared to untrained individuals. As research has repeatedly shown, RFD is an integral component and quantification of dynamic performance. (13,31) Dancers place a great amount of emphasis on the minimization of the coupling phase, or eccentric to concentric transition, required for ballet sequences and other stylistic combinations. (33) This technique has been discussed extensively, and it can be concluded that less time spent transitioning benefits athletes by dissipating less heat and re-utilizing stored elastic energy. (22) Wilson and colleagues found that SSC performance is enhanced following a flexibility-improving protocol for 8 weeks, indicated by decreased time spent in the coupling phase. (34) It is possible that a well-developed SSC, thus, reflects greater RFD capabilities. (35) The current study also determined that subjects who possessed greater RFD had greater peak force capabilities as well. Therefore, it is the investigators' contention that dance-associated plyometric skills may potentially be affected by one's force generating capacity. However, the data from the current investigation cannot allow for confirmation of this hypothesis.

A strong correlation found across all subjects between musculo-articular stiffness and MIP RFD also suggests that individuals who possess less compliance within the ankle muscletendon complex are able to exert force at a higher rate. A majority of aesthetic dance skills that require center of mass displacement and motor unit recruitment assumedly work simultaneously with maximum tendon excursion. (6,21) While muscle force output relies on neural activation, maximal plantar flexion in particular is profoundly influenced by the increasing contribution of tendinous tissues during greater fascicle-shortening velocities. (21) High frequency cyclical motions that serve as framework for most athletic movement patterns are achieved with increased musculo-articular stiffness. (36) This has been reported in the literature multiple times, as well as the large correlation between dynamic performance, strength, and musculo-articular stiffness. (19,37,38) A novel discovery from the current investigation indicates that dancers have significantly greater ankle musculo-articular stiffness than their untrained counterparts. Dancers have also demonstrated regulation of stiffness levels based on landing surfaces, suggesting the ability to adapt environmentally for ideal performance. (7) The tissue characteristics and stiffness modulation of dancers may be influenced by hyperextension of the ankle and a high volume of SSC's experienced from early adolescence. (39) Other findings, however, have reported flexibility increases that fortify SSC performance to inversely cause a decrease in musculo-articular stiffness. (34) Although plyometric training has proven to result in an increase of stored elastic energy within the tendon, Kubo and associates found that stiffness after training did not significantly rise. (14) The discovery of greater musculo-articular stiffness in dancers than untrained individuals and previous work questioning optimal stiffness necessitate further investigations that compare dancers to other plyometric populations.


The higher strength performance measurements and musculo-articular stiffness levels of dancers examined in the present study may assist in defining the dancer as an athlete. The knowledge needed to enhance performance and prevent injury in all athletes is better addressed when anatomical features and capabilities are first quantified. The performance coordination traditionally demonstrated by dancers is unequivocal, not only from an artistic but also a biomechanical standpoint. Future research should seek to measure muscle and tendon performance capabilities in dancers utilizing other movement patterns of a dynamic nature, as opposed to simply an isometric assessment of performance.


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Paige E. Rice, M.S., Herman van Werkhoven, Ph.D., Denzel J. Dejournette, B.S., Reed D. Gurchiek, B.S., John W. Mackall, B.S., and Jeffrey M. McBride, Ph.D., Neuromuscular & Biomechanics Laboratory, Department of Health & Exercise Science, Appalachian State University, Boone, North Carolina.

Correspondence: Paige E. Rice, M.S., Neuromuscular & Biomechanics Laboratory, Department of Health & Exercise Science, Appalachian State University, 086 Convocation Center, Boone, North Carolina, 28607;

Caption: Figure 1 Standing free-oscillation setup.

Caption: Figure 2 Raw data of damped oscillation collected on a force plate.

Caption: Figure 3 Average musculo-articular stiffness (k) of dancers and untrained individuals. *Significant difference between female dancers and untrained individuals (p [less than or equal to] 0.05).
Table 1 Descriptive Characteristics (*)

Group              Age               Height              Weight
(N = 16)         (years)              (cm)                (kg)

Dancers      20.6 [+ or -] 1.3  163.2 [+ or -] 5.5  58.7 [+ or -] 5.8
Untrained    20.5 [+ or -] 1.9  163.9 [+ or -] 7.8  66.0 [+ or -] 17.7

Group            LL Length        AT-MM Length       AT-LM Length
(N = 16)           (cm)              (cm)               (cm)

Dancers      34.5 [+ or -] 1.5  2.5 [+ or -] 0.2  3.0 [+ or -] 0.5
Untrained    34.0 [+ or -] 2.3  2.5 [+ or -] 0.4  3.0 [+ or -] 0.5

(*) No significant differences between groups.

Table 2 Rate of Force Development (RFD) and Peak Force (PF) during
Maximal Isometric Plantar Flexion (MIP) and Maximal Isometric
Knee Extension (MIKE) Contraction Values in Dancers and
Untrained Individuals

Variable                        Dancers           Untrained Individuals

MIP RFD (N/s)            2040 [+ or -] 657 (*)      443 [+ or -] 260
Relative MIP PF (N/kg)     20.5 [+ or -] 7.9 (*)      9.6 [+ or -] 4.8
MIKE RFD (N/s)            765 [+ or -] 319 (*)      319 [+ or -] 257
Relative MIKE PF (N/kg)     5.4 [+ or -] 1.6          3.4 [+ or -] 2.3

(*) Significant difference between female dancers and untrained
individuals (p [less than or equal to] 0.05).

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Author:Rice, Paige E.; van Werkhoven, Herman; Dejournette, Denzel J.; Gurchiek, Reed D.; Mackall, John W.;
Publication:Journal of Dance Medicine & Science
Article Type:Report
Date:Oct 1, 2017
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