Loss of maximal explosive power of lower limbs after 2 weeks of disuse and incomplete recovery after retraining in older adults

Abstract

Key points: Disuse in older adults can critically decrease lower limb muscle power, leading to compromised mobility and overall quality of life. We studied how muscle power and its determinants (muscle mass, single muscle fibre properties and motor control) adapted to 2 weeks of disuse and subsequent 2 weeks of physical training in young and older people. Disuse decreased lower limb muscle power in both groups; however, different adaptations in single muscle fibre properties and co-contraction of leg muscles were observed between young and older individuals. Six physical training sessions performed after disuse promoted the recovery of muscle mass and power. However, they were not sufficient to restore muscle power to pre-disuse values in older individuals, suggesting that further countermeasures are required to counteract the disuse-induced loss of muscle power in older adults.

Abstract: Disuse-induced loss of muscle power can be detrimental in older individuals, seriously impairing functional capacity. In this study, we examined the changes in maximal explosive power (MEP) of lower limbs induced by a 14-day disuse (bed-rest, BR) and a subsequent 14-day retraining, to assess whether the impact of disuse was greater in older than in young men, and to analyse the causes of such adaptations. Sixteen older adults (Old: 55-65 years) and seven Young (18-30 years) individuals participated in this study. In a subgroup of eight Old subjects, countermeasures based on cognitive training and protein supplementation were applied. MEP was measured with an explosive ergometer, muscle mass was determined by magnetic resonance, motor control was studied by EMG, and single muscle fibres were analysed in vastus lateralis biopsy samples. MEP was ∼33% lower in Old than in Young individuals, and remained significantly lower (-19%) when normalized by muscle volume. BR significantly affected MEP in Old (-15%) but not in Young. Retraining tended to increase MEP; however, this intervention was not sufficient to restore pre-BR values in Old. Ankle co-contraction increased after BR in Old only, and remained elevated after retraining (+30%). Significant atrophy occurred in slow fibres in Old, and in fast fibres in Young. After retraining, the recovery of muscle fibre thickness was partial. The proposed countermeasures were not sufficient to affect muscle mass and power. The greater impact of disuse and smaller retraining-induced recovery observed in Old highlight the importance of designing suitable rehabilitation protocols for older individuals.

Keywords: ageing; disuse; explosive muscle power; myosin isoforms; single muscle fiber.

Figure 1. Effects of bed rest and physical retraining on maximal explosive power of the lower limbs in older adults enrolled in the Control group, Interventions group and Young subjects

Values are mean ± SE. Maximal explosive power (MEP, A) and MEP normalized per unit of quadriceps femoris muscle volume (Specific MEP, B) exerted before bed rest (Pre‐BR), after bed rest (Post‐BR) and after physical retraining (R + 14). Control group (Old_Ctrl, ●), Interventions group (Old_Int, ♦) and Young subjects (○). Differences in MEP and specific MEP were tested using general linear model and following post hoc analysis with Bonferroni corrections. ^*Difference between periods in the two Old groups; ^†difference between the two Old groups and Young.

Figure 2. Effects of bed rest and physical retraining on maximal voluntary isometric contractions of the right quadriceps femoris muscle in older adults enrolled in the Control group, Interventions group and Young subjects

Values are mean ± SE. Force exerted during maximal voluntary isometric contractions (MVC, A), and MVC normalized per unit of quadriceps femoris muscle volume (Specific MVC, B). Control group (Old_Ctrl, ●), Interventions group (Old_Int, ♦) and Young subjects (○). Pre‐BR: before bed rest; Post‐BR: after bed rest; R + 14: after physical retraining. Differences in force were tested using general linear model and following post hoc analysis with Bonferroni corrections. ^*Difference between periods in the two Old groups; ^†difference between the two Old groups and Young.

Figure 3. Effects of bed rest and physical retraining on single fibre parameters

Cross sectional area (CSA: A, E, I), isometric force during maximal activation (F _o: B, F, L), specific force or tension (P _o: C, G, M) and single fibre shortening velocity (V _o: D, H, N) in slow or type 1 fibres (A–D), fast 2A (E–H) and fast 2A2X (I–N) in the older adult Control group (Old_Ctrl, ●), Interventions group (Old_Int, ♦) and in Young subjects (○). Values are mean ± SE. Differences in CSA, F _o, P _o and V _o were tested using general linear mixed model and are indicated in the lower part of each panel for Old and the upper part for Young. ^*Difference between periods; ^†difference between the two Old groups and Young.

Figure 4. Effects of bed rest and following physical retraining on the level of co‐contraction at the knee and ankle joint in older adults enrolled in the Control group, Interventions group and in Young subjects

Values are mean ± SE. Control group (Old_Ctrl, ●), Interventions group (Old_Int, ♦) and Young subjects (○). Level of co‐contraction between biceps femoris and vastus lateralis (Knee, A) and between tibialis anterior and gastrocnemius medialis (Ankle, B) recorded during explosive efforts of the lower limbs before bed rest (Pre), after bed rest (Post‐BR) and after physical retraining (R + 14). Differences in the level of Knee and Ankle co‐contraction were tested using general linear model and following post hoc analysis with Bonferroni corrections. ^*Difference between periods in the two Old groups.