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Previous studies reported a significant association between postural control and lower-limb strength of several muscle groups, however, they were focused especially on knee muscles and ankle plantar and dorsal flexors. The aim of the present study is to examine the correlation between the muscle strength of ankle invertors, evertors, plantar flexors, and dorsal flexors and the control of bipedal stance in young and older adults. Thirty one young (aged 22.8 ± 2.6 years) and thirty one older adults (aged 70.5 ± 7.2 years) voluntarily participated in this study. Ankle muscle strength was evaluated by an isokinetic dynamometer. Normalized peak torque and work were averaged for four repetitions and for both lower limbs. The control of bipedal stance was evaluated by the sample entropy derived from an accelerometer placed on the lumbar spine while the subject stood on a foam pad with eyes open. Results showed significant age-related differences in ankle muscle strength and sample entropy in medial-lateral direction. More interestingly, the correlation between ankle muscle strength and the sample entropy was significantly different between young and older adults. Indeed, no significant correlation was observed in the younger adults. Conversely, in the older adults, the work of the ankle evertors positively correlated with sample entropy in the medial-lateral direction during bipedal stance (r = 0.36), whereas the peak torque and work of the dorsal flexors were significantly correlated with sample entropy in the anterior-posterior direction during bipedal stance (r = 0.44 for both variables). In the young adults, results suggest that, standing on foam with eyes open is a relatively easy postural task that does not require the full ankle muscle strength capacity. Taken together, the present findings suggest that older adults have a different association between ankle muscle strength and the sample entropy during bipedal stance.

For quantifying muscle strength in clinical and research practice, establishing the reliability of measurements, specifically to the procedures used, is essential for credible findings. The objective was to establish the reliability of isokinetic measurement of ankle plantar and dorsal flexors (PF/DF) and invertors and evertors (INV/EV) on an IsoMed 2000 dynamometer. Twenty healthy subjects (10 males, 10 females, mean age: 23.1 ± 3.1 years) completed an isokinetic measurement session. The intraclass correlation coefficient (ICC) and standard error of measurement were assessed for peak torque and work of ankle PF/DF (concentric and eccentric) and INV/EV (concentric) for the preferred and nonpreferred limb. Standardized isokinetic measurements of reciprocal PF/DF and INV/EV muscle actions were associated with ICC ranging from 0.77 to 0.98 for the majority of observed parameters. The exception was work in the eccentric mode in the ankle DF and peak torque in the concentric mode in the ankle INV on the preferred limb, where ICC ranged from 0.64 to 0.71. The IsoMed 2000 isokinetic dynamometer can be reliably employed in future studies for reciprocal ankle PF/DF and INV/EV assessment in healthy adult subjects after implementation of a familiarization session.

Balance and lower limb strength deficits are associated with a high incidence of falls in older adults. This study investigated the association between balance control during and after stair descent onto a compliant surface and lower limb strength. Thirty-five women and 14 men participated in this study. Stair descent time, mean center of pressure velocity in anteroposterior and mediolateral direction during stair descent (CoP VAP and CoP VML), and CoP velocity in the first 5 s of restabilization phase (CoP V5) were evaluated. Bilateral strength of the knee flexors and extensors, and ankle plantar and dorsal flexors was evaluated. Spearman correlation analysis with Bonferroni correction yielded a significant association between the strength of the knee flexors on the trailing limb and stair descent time in women (r = 0.502, p = 0.002, R2 = 0.246). The same analysis in men revealed a significant association between the strength of the knee flexors on the trailing limb and CoP VAP (r = −0.820, p < 0.001, R2 = 0.280) and CoP VML (r = −0.697, p = 0.006, R2 = 0.359). The strength of the ankle plantar flexors on the trailing limb was significantly associated with stair descent time (r = 0.684, p = 0.007, R2 = 0.429) and CoP VAP (r = −0.723, p = 0.003, R2 = 0.408) in men. Stair descent balance control is associated with knee flexion strength on trailing limb in women, and with ankle plantar flexion and knee flexion strength on the same limb in men.

Ageing-related deterioration of the lower limb muscle strength could highly influence the functional performance of elderly adults, particularly females, which are more prone to injurious falls. Precise analyses on the effects of ageing on individual lower-limbs' muscle groups and their relationship with feeble functional performance could provide us with information that aid in designing preventive interventions. To this end, this study was designated to investigate the impacts of advancing age on lower limb muscles strength and consequently sit-to-stand and functional reaching performance among 34 community-dwellingelderly females (eighteen 60-69 years old and sixteen 70-79 years old). Results illustrated a significant decrease in produced relative peak torque (RPT) and power (RPP) values of ankle invertors, plantarflexors and knee extensor among 70-79 group, and their 5RSTS execution time significantly increased in comparison with 60-69 group (p<0.05). Outcomes also demonstrated that RPT and RPP values of ankle invertors and evertors have significant correlations with lateral reaching tests in both groups. Ankle plantarflexors, knee flexors and extensors are also shown a strong contribution in the decrement of 5RSTS duration in both groups, while ankle plantar flexors and knee extensors had also a significant correlation with FRT performance among them. The ankle and knee joints muscle strength, which has an undeniable contribution to human locomotion and balance status, underwent sharp attenuation by advancing age, consequently resulting in changing position impairments among elderly females. Findings of the current study might help in designing the interventional training programs, which could help community-dwelling elderly females in the prevention of fall-related risks of injuries.

EMD as a factor influencing the speed of muscle force production, measured during squat jump, can be used as a relevant substitution for isometric or isokinetic assessment. The purpose of this study was to examine the reliability of measurement of EMD in knee muscles (biceps femoris, semitendinosus, rectus femoris) during a vertical jump. Thirteen volunteers unfamiliar with the squat jump participated in this study and were measured 2 times, with rest periods of 7 days between measurements. EMD was calculated using a force platform and electromyography. Intraclass correlation coefficients ranged (ICC) from 0.69 to 0.78, indicating high reliability for the rectus femoris (RF) and semitendinosus (ST) and moderate reliability for the biceps femoris (BF). Absolute reliability expressed by standard error of the measure (SEM) ranged for RF 28.11, BF 23.18 and ST 21.20, better used as a coefficient of variation (CV of SEM) for RF 22.81%, BF 25.19% and ST 19.72%. These findings provide researchers and clinicians with another method for relatively fast measurement of EMD using portable devices.