Age-related differences in drop-jump performance are eliminated by matching triceps surae muscle strength and Achilles tendon stiffness.

INTRODUCTION Reduced leg-extensor muscle strength and tendon stiffness in old age have been associated with changes in locomotor performance and motor task execution strategy (Karamanidis and Arampatzis, 2005; Kulmala et al., 2014). However, if leg-extensor muscle-tendon unit (MTU) mechanical properties are the only primary drivers of these alterations seen with ageing remains unclear. Therefore, we examined if matching triceps surae (TS) muscle strength and Achilles tendon (AT) stiffness eliminates potential age-related differences in drop jump (DJ) performance and motor task execution strategy in younger and middle-aged adults

Matching triceps surae muscle strength and tendon stiffness eliminates age-related differences in drop-jump performance

INTRODUCTION: Age-related changes in locomotor performance and motor task execution strategy have been associated with reduced leg-extensor muscle-tendon unit capacities in old age (Karamanidis & Arampatzis, 2005; Kulmala et al., 2014). However, it remains unclear if leg-extensor muscle strength and tendon stiffness are the primary drivers of these alterations seen with ageing, or if other factors also play a role. We aimed to determine if potential age-related differences in drop jump (DJ) performance and motor task execution strategy would be eliminated in a group of younger and middle-aged adults when triceps surae (TS) muscle strength and Achilles tendon (AT) stiffness were matched. METHODS: Ankle plantarflexion moments and AT stiffness of both legs were assessed in 12 younger (20-30y) and 12 middle-aged (50-65y) adults during isometric plantarflexion contractions using dynamometry and ultrasonography synchronously. Tendon elongation during the loading phase was assessed by visualising the myotendinous junction of the gastrocnemius medialis muscle and stiffness of the AT was determined in the linear region of the force-length relationship. There were no significant differences between the matched young and middle-aged adults in maximal ankle plantarflexion moment (young: 3.2±0.4; middle-aged: 3.1±0.5 Nm/kg) and AT stiffness (580.3±121.8 vs. 590.2±108.4 N/mm). On a second occasion, the matched participants performed a series of DJs from different starting heights (13, 23, 33 and 39 cm) onto a force plate. A two-way ANOVA (factors: age, starting height) was performed in order to detect any age or starting height effects on DJ height and motor task execution strategy. The effect of TS muscle strength and AT stiffness on DJ height was investigated by using Pearson’s product-moment correlation coefficient. RESULTS: There was no significant age effect for DJ performance, meaning that matched younger and middle-aged adults achieved similar DJ heights, independent of starting height (Fig. 1). Concerning DJ mechanics, there were significant age effects (p<.05) on ground contact time, maximum vertical ground reaction force and mechanical power, with the middle-aged adults showing higher ground contact times (Fig. 2A), but lower forces (Fig. 2B) and lower mechanical power (Fig. 2C) for all starting heights. Significant correlations were found between DJ performance and TS muscle strength and AT stiffness (.41≤r≤.81; p<.05) for all starting heights. DISCUSSION: The results of the current study demonstrate that matching younger and middle-aged adults for TS muscle strength and AT stiffness eliminates age-related differences in the performance of a jumping task, independent of starting height and hence, task demand. However, the age groups used different motor task execution strategies, as measured by ground contact time, ground reaction forces and mechanical power for all starting heights, indicating that while changes in leg extensor muscle strength and tendon stiffness may be the primary drivers of deteriorated locomotor performance in older age, they may not be major contributors to motor task execution strategy during jumping. CONCLUSION: Jumping performance appears to be unaffected when leg extensor muscle strength and tendon stiffness are maintained with age and therefore, countering the degeneration of these properties may help prevent the decline in locomotor performance seen with ageing. REFERENCES: Karamanidis, K., & Arampatzis, A. (2005). Mechanical and morphological properties of different muscle-tendon units in the lower extremity and running mechanics: effect of aging and physical activity. The Journal of Experimental Biology, 208, 3907–23. Kulmala, J.-P., Korhonen, M. T., Kuitunen, S., Suominen, H., Heinonen, A., Mikkola, A., & Avela, J. (2014). Which muscles compromise human locomotor performance with age? Journal of The Royal Society Interface, 11, 20140858

Matching Participants for Triceps Surae Muscle Strength and Tendon Stiffness Does Not Eliminate Age-Related Differences in Mechanical Power Output During Jumping

Reductions in muscular power output and performance during multi-joint motor tasks with aging have often been associated with muscle weakness. This study aimed to examine if matching younger and middle-aged adults for triceps surae (TS) muscle strength and tendon stiffness eliminates age-related differences in muscular power production during drop jump. The maximal ankle plantar flexion moment and gastrocnemius medialis tendon stiffness of 29 middle-aged (40–67 years) and 26 younger (18–30 years) healthy physically active male adults were assessed during isometric voluntary ankle plantar flexion contractions using simultaneous dynamometry and ultrasonography. The elongation of the tendon during the loading phase was assessed by digitizing the myotendinous junction of the gastrocnemius medialis muscle. Eight younger (23 ± 3 years) and eight middle-aged (54 ± 7 years) adults from the larger subject pool were matched for TS muscle strength and tendon stiffness (plantar flexion moment young: 3.1 ± 0.4 Nm/kg; middle-aged: 3.2 ± 0.5 Nm/kg; tendon stiffness: 553 ± 97 vs. 572 ± 100 N/mm) and then performed series of drop jumps from different box heights (13, 23, 33, and 39 cm) onto a force plate (sampling frequency 1000 Hz). The matched young and middle-aged adults showed similar drop jump heights for all conditions (from lowest to highest box height: 18.0 ± 3.7 vs. 19.7 ± 4.8 cm; 22.6 ± 4.2 vs. 22.9 ± 4.9 cm; 24.8 ± 3.8 vs. 23.5 ± 4.9 cm; 25.2 ± 6.2 vs. 22.7 ± 5.0 cm). However, middle-aged adults showed longer ground contact times (on average 36%), lower vertical ground reaction forces (36%) and hence lower average mechanical power (from lowest to highest box height: 2266 ± 563 vs. 1498 ± 545 W; 3563 ± 774 vs. 2222 ± 320 W; 4360 ± 658 vs. 2475 ± 528 W; 5008 ± 919 vs. 3034 ± 435 W) independent of box height. Further, leg stiffness was lower (48%) in middle-aged compared to younger adults for all jumping conditions and we found significant correlations between average mechanical power and leg stiffness (0.70 ≤ r ≤ 0.83; p < 0.01). Thus, while jumping performance appears to be unaffected when leg extensor muscle strength and tendon stiffness are maintained, the reduced muscular power output during lower limb multi-joint tasks seen with aging may be due to age-related changes in motor task execution strategy rather than due to muscle weakness

Matching participants for triceps surae mechanical properties eliminates age-related differences in drop jump performance

Introduction Age-related declines in locomotor performance have been associated with a degeneration of the triceps surae muscle-tendon unit (MTU) capacities (Kulmala et al., 2014). However, to our knowledge, no studies have compared the motor task performance of young and older adults with similar triceps surae muscle strength and Achilles tendon stiffness. In the current study, we aimed to determine if differences in drop jump height or motor task execution strategy between young and middle-aged adults exist, when triceps surae MTU capacities (muscle strength and tendon stiffness) were matched. Methods The triceps surae MTU biomechanical properties of 29 middle-aged (50 - 65 years) and 26 younger (20 - 30 years) adults were assessed during isometric voluntary ankle plantarflexion contractions of the dominant leg using a custom-made dynamometer and ultrasonography (27 Hz; MyLabTMOne, Esaote; Genua, Italy) simultaneously. The resultant joint moments were calculated by means of inverse dynamics. The elongation of the tendon during contraction was assessed by digitizing the myotendinous junction of the gastrocnemius medialis muscle with a custom-made Matlab software (Matlab 2013b, MathWorks Inc., Natick, Massachusetts, USA) while taking into account the effect of potential ankle joint angular rotation during contraction (Muramatsu et al. 2001). Tendon stiffness was determined in the linear region of the force-length relationship. Following the MTU measurements, the 12 young adults with the lowest triceps surae muscle strength and the 12 middle-aged adults with the greatest muscle strength were compared. There were no significant differences between the matched young and middle-aged adults in muscle strength (young: 3.2 ± 0.4; middle-aged: 3.1 ± 0.5 Nm/kg), tendon stiffness (580.3 ± 121.8; 590.2 ± 108.4 N/mm) or tendon energy storage capacity (217 ± 63.2; 187.5 ± 82.7 J). The matched participants then completed a series of drop jumps. The instructions given to the subjects were “jump as high as possible with as little knee flexion as possible”. In order to analyse drop jump performance and motor task execution strategy, ground contact time, average vertical ground reaction force, average mechanical power and jumping height were determined during drop jumps from different heights (13, 23, 33 and 39 cm) onto a force plate (90 x 60 cm, 1000 Hz; Kistler, Winterthur, CH). A two-way repeated measures ANOVA with age and drop height as factors was conducted in order to detect age-related differences in drop jump height and motor task execution strategy. The effect of muscle strength and tendon stiffness on drop jump height was investigated using Pearson’s product-moment correlation coefficient. Results Younger and middle-aged adults attained comparable jumping heights independent of the drop jump height (Fig. 1). There were significant age effects on ground contact time (p < .01) and average vertical ground reaction force during ground contact phase (p < .01) (Fig. 1), with the middle-aged adults showing higher ground contact times but lower forces than the younger adults, leading to a significant age effect on mechanical power (p < .05). Significant (p < .05) correlations were found between triceps surae MTU capacities and drop jump height (.41 ≤ r ≤ .81; p < .05). Discussion The results of the current study demonstrate that when triceps surae MTU capacities are matched, young and middle-aged adults show comparable performance (jump height) of a jumping task. However, the motor strategies used to achieve these similar performances differ, with the middle-aged adults demonstrating longer ground contact times, lower ground reaction forces and hence lower average mechanical power. Muscle strength and tendon stiffness appear to play an important role in jumping performance and countering the degeneration of these properties may help prevent the decline in locomotor function seen with ageing. Finally, the results suggest that neuromuscular factors other than maximum isometric strength and tendon stiffness may influence motor task execution strategy during jumping, such as the power generating capacity of the triceps surae muscle. References Kulmala, J. P., Korhonen, M. T., Kuitunen, S., Suominen, H., Heinonen, A., Mikkola, A. et al. (2014). Which muscles compromise human locomotor performance with age? Journal of The Royal Society Interface, 11, 20140858. Muramatsu, T., Muraoka, T., Takeshita, D., Kawakami, Y., Hirano, Y. & Fukunaga, T. (2001). Mechanical properties of tendon and aponeurosis of human gastrocnemius muscle in vivo. Journal of Applied Physiology (1985), 90, 1671-1678

Aging and the effects of a half marathon on Achilles tendon force-elongation relationship.

PURPOSE: We aimed to determine whether there are different changes in Achilles tendon (AT) mechanical properties in middle-aged, compared to younger runners that might indicate that tendon fatigue, induced by long-distance running, is age-dependent. METHODS: 27 middle-aged (50-67 years) and 22 younger (21-29 years) participants ran a 21 km route at their own pace (mean and SD: old: 3.1 ± 0.3 m s(-1); young: 3.6 ± 0.5 m s(-1)). We tested for changes in the AT force-elongation relationship using dynamometry and ultrasonography during isometric voluntary ankle plantarflexion ramp contractions, conducted 20-28 h pre-run, immediately pre-run, immediately post-run and 20-28 h post-run. Stride frequency and number were examined to estimate cyclic tensile loading characteristics of the tendon during running. RESULTS: Muscle strength decreased significantly (P < 0.05) in both groups immediately post-run (old: 17 %; young: 11 %) and recovered to baseline within 20-28 h post-run. AT stiffness did not change for the younger adults, whereas the middle-aged adults showed a significant (P < 0.05) decrease in AT stiffness (22 %). However, tendon stiffness recovered to baseline 20-28 h post-run. Middle-aged, compared to young adults, demonstrated significantly (P < 0.05) greater stride frequency and number, but no correlations with tendon fatigue changes were determined (R (2) ≤ 0.038). CONCLUSIONS: The results suggest that the plasticity of the AT in response to short-term mechanical loading may be age dependent and that the AT length-tension properties of middle-aged runners may be more vulnerable to change following running compared to younger athletes. However, the observed AT changes in the middle-aged runners dissipated within 20-28 h post-run, suggesting that a tendon viscoelastic recovery mechanism may occur in vivo

A novel multivariate approach for biomechanical profiling of stair negotiation.

Stair falls, especially during stair descent, are a major problem for older people. Stair fall risk has typically been assessed by quantifying mean differences between subject groups (e.g. older vs. younger individuals) for a number of biomechanical parameters individually indicative of risk, e.g., a reduced foot clearance with respect to the stair edge, which increases the chances of a trip. This approach neglects that individuals within a particular group may also exhibit other concurrent conservative strategies that could reduce the overall risk for a fall, e.g. a decreased variance in foot clearance. The purpose of the present study was to establish a multivariate approach that characterises the overall stepping behaviour of an individual. Twenty-five younger adults (age: 24.5 ± 3.3 y) and 70 older adults (age: 71.1 ± 4.1 y) descended a custom-built instrumented seven-step staircase at their self-selected pace in a step-over-step manner without using the handrails. Measured biomechanical parameters included: 1) Maximal centre of mass angular acceleration, 2) Foot clearance, 3) Proportion of foot length in contact with stair, 4) Required coefficient of friction, 5) Cadence, 6) Variance of these parameters. As a conventional analysis, a one-way ANOVA followed by Bonferroni post-hoc testing was used to identify differences between younger adults, older fallers and non-fallers. To examine differences in overall biomechanical stair descent behaviours between individuals, k-means clustering was used. The conventional grouping approach showed an effect of age and fall history on several single risk factors. The multivariate approach identified four clusters. Three clusters differed from the overall mean by showing both risky and conservative strategies on the biomechanical outcome measures, whereas the fourth cluster did not display any particularly risky or conservative strategies. In contrast to the conventional approach, the multivariate approach showed the stepping behaviours identified did not contain only older adults or previous fallers. This highlights the limited predictive power for stair fall risk of approaches based on single-parameter comparisons between predetermined groups. Establishing the predictive power of the current approach for future stair falls in older people is imperative for its implementation as a falls prevention tool

Negotiating stairs with an inconsistent riser: Implications for stepping safety

Stairs are associated with falls, especially when step dimensions are inconsistent. However, the mechanisms by which inconsistencies cause this higher risk are mostly theoretical. In this experimental study we quantified the effect of inconsistent rise heights on biomechanical measurements of stepping safety from younger (n = 26) and older adults (n = 33). In ascent, both groups decreased foot clearance (~9 mm) over the inconsistently higher step (F(1,56) = 48.4, p < 0.001). In descent, they reduced foot contact length on the higher step by 3% (F(1,56) = 9.1, p < 0.01). Reduced clearance may result in a toe-catch potentially leading to a trip, while reduced foot contact lengths increase the risk of overstepping which may also lead to a fall. These effects occurred because participants did not alter their foot trajectories, indicating they either did not detect or were not able to adjust to the inconsistent rise, increasing the likelihood of a fall. Consistent stair construction is vital, and existing inconsistencies should be identified and safety interventions developed

Stair negotiation behaviour of older individuals: Do step dimensions matter?

Stair falls are a major health problem for older people. Most studies on identification of stair fall risk factors are limited to staircases set in given step dimensions. However, it remains unknown whether the conclusions drawn would still apply if the dimensions had been changed to represent more challenging or easier step dimensions encountered in domestic and public buildings. The purpose was to investigate whether the self-selected biomechanical stepping behaviours are maintained when the dimensions of a staircase are altered. Sixty-eight older adults (>65 years) negotiated a seven-step staircase set in two step dimensions (shallow staircase: rise 15 cm, going 28 cm; steep staircase: rise 20 cm, going 25 cm). Six biomechanical outcome measures indicative of stair fall risk were measured. K-means clustering profiled the overall stair-negotiating behaviour and cluster profiles were calculated. A Cramer's V measured the degree of association in membership between clusters. The cluster profiles revealed that the biomechanically risky and conservative factors that characterized the overall behaviour in the clusters did not differ for the majority of older adults between staircases for ascent and descent. A strong association of membership between the clusters on the shallow staircase and the steep staircase was found for stair ascent (Cramer's V: 0.412, p < 0.001) and descent (Cramer's V: 0.380, p = 0.003). The findings indicate that manipulating the demand of the task would not affect the underpinning mechanism of a potential stair fall. Therefore, for most individuals, detection of stair fall risk might not require testing using a staircase with challenging step dimensions

Prediction of balance perturbations and falls on stairs in older people using a biomechanical profiling approach: A 12-month longitudinal study.

BACKGROUND: Stair falls are a major health problem for older people, but presently there are no specific screening tools for stair fall prediction. The purpose of the present study was to investigate whether stair fallers could be differentiated from non-fallers by biomechanical risk factors or physical/psychological parameters and to establish the biomechanical stepping profile posing the greatest risk for a stair fall. METHODS: Eighty-seven older adults (age: 72.1±5.2 y) negotiated an instrumented seven-step staircase and performed a range of physical/psychological tasks. K-means clustering was used to profile the overall stair negotiation behaviour with biomechanical parameters indicative of fall risk as input. Falls and events of balance perturbation (combined "hazardous events") were then monitored during a 12-month follow-up. Cox-regression analysis was performed to examine if physical/psychological parameters or biomechanical outcome measures could predict future hazardous events. Kaplan-Meier survival curves were obtained to identify the stepping strategy posing a risk for a hazardous event. RESULTS: Physical/psychological parameters did not predict hazardous events and the commonly used Fall Risk Assessment Tool (FRAT) classified only 1/17 stair fallers at risk for a fall. Single biomechanical risk factors could not predict hazardous events on stairs either. On the contrary, two particular clusters identified by the stepping profiling method in stair ascent were linked with hazardous events. CONCLUSION: This highlights the potential of the stepping profiling method to predict stair fall risk in older adults against the limited predictability of single parameter approaches currently used as screening tools

The role of muscle strength on tendon adaptability in old age.

PURPOSE: The purpose of the study was to determine: (1) the relationship between ankle plantarflexor muscle strength and Achilles tendon (AT) biomechanical properties in older female adults, and (2) whether muscle strength asymmetries between the individually dominant and non-dominant legs in the above subject group were accompanied by inter-limb AT size differences. METHODS: The maximal generated AT force, AT stiffness, AT Young's modulus, and AT cross-sectional area (CSA) along its length were determined for both legs in 30 women (65 ± 7 years) using dynamometry, ultrasonography, and magnetic resonance imaging. RESULTS: No between-leg differences in triceps surae muscle strength were identified between dominant (2798 ± 566 N) and non-dominant limb (2667 ± 512 N). The AT CSA increased gradually in the proximo-distal direction, with no differences between the legs. There was a significant correlation (P < 0.05) of maximal AT force with AT stiffness (r = 0.500) and Young's modulus (r = 0.414), but only a tendency with the mean AT CSA. However, region-specific analysis revealed a significant relationship between maximal AT force and the proximal part of the AT, indicating that this region is more likely to display morphological adaptations following an increase in muscle strength in older adults. CONCLUSIONS: These findings demonstrate that maximal force-generation capabilities play a more important role in the variation of AT stiffness and material properties than in tendon CSA, suggesting that exercise-induced increases in muscle strength in older adults may lead to changes in tendon stiffness foremost due to alterations in material rather than in its size