Differential atrophy of the postero-lateral hip musculature during prolonged bedrest and the influence of exercise countermeasures

Abstract

As part of the 2nd Berlin BedRest Study (BBR2-2), we investigated the pattern of muscle atrophy of the postero-lateral hip and hamstring musculature during prolonged inactivity and the effectiveness of two exercise countermeasures. Twenty-four male subjects underwent 60 days of head-down tilt bedrest and were assigned to an inactive control (CTR), resistive vibration exercise (RVE), or resistive exercise alone (RE) group. Magnetic resonance imaging (MRI) of the hip and thigh was taken before, during, and at end of bedrest. Volume of posterolateral hip and hamstring musculature was calculated, and the rate of muscle atrophy and the effect of countermeasure exercises were examined. After 60 days of bedrest, the CTR group showed differential rates of muscle volume loss (F = 21.44; P ≤ 0.0001) with fastest losses seen in the semi-membranosus, quadratus femoris and biceps femoris long head followed by the gluteal and remaining hamstring musculature. Whole body vibration did not appear to have an additional effect above resistive exercise in preserving muscle volume. RE and RVE prevented and/or reduced muscle atrophy of the gluteal, semi-membranosus, and biceps femoris long head muscles. Some muscle volumes in the countermeasure groups displayed faster recovery times than the CTR group. Differential atrophy occurred in the postero-lateral hip musculature following a prolonged period of unloading. Short-duration high-load resistive exercise during bedrest reduced muscle atrophy in the mono-articular hip extensors and selected hamstring muscles. Future countermeasure design should consider including isolated resistive hamstring curls to target this muscle group and reduce the potential for development of muscle imbalances.

Fig. 1.

Countermeasure exercise. This figure depicts a training subject performing a bilateral squat. Bilateral squats were included in the countermeasure exercise program to target the hip musculature, in particular the gluteal and hamstring muscles, which contract concentrically during the ascension phase of the squat. Other exercises included single and bilateral heel raises and back extension maneuvers. Both the resistance exercise only (RE) and resistance exercise with whole body vibration (RVE) groups performed their exercises on the Galileo Space exercise device (Novotec Medical, Pforzheim, Germany), with the RE group performing their exercises without vibration. Subjects were positioned in head-down tilt on a moveable board with their feet placed on either side of the Galileo platform. Shoulder pads and hand grips ensured subjects remained in the desired position and enabled application of force via the vibrating platform. A pneumatic system generated the force, applied through the moveable board, against which the subject was required to resist and move (via the shoulder pads and hand grips). A sport scientist supervised all exercises to ensure correct performance, and a monitor positioned in the subjects field of view provided visual feedback regarding exercise motion and speed and the subjects actual and target exercise position.

Fig. 2.

Hip and thigh muscle image measurements. Example images from the proximal hip (A), distal hip (B) and mid thigh (C). Thirty-four to 39 images were acquired from the iliac crest to the inferior-most portion of the gluteus maximus for the hip musculature. A second sequence of 63–69 images were acquired overlapping the inferior-most portion of the gluteus maximus and extending to the knee joint line to capture the hamstring musculature. The cross-sectional area (when present) of the muscles: upper gluteus maximus (UGM), lower gluteus maximus (LGM), gluteus medius (GMED), gluteus minimus (GMIN), obturator internus (OBT_INT), obturator externus (OBT_EXT), quadratus femoris (QUAF_FEM), piriformis (PIRI), biceps femoris short head (BF_SH), biceps femoris long head (BF_LH), semi-membranosus (SEMI_M), and semi-tendinosus (SEMI_ T) were measured in each image. Note the bony landmarks: iliac crest (IC), femoral head (FH), and ischial tuberosity (IT).

Fig. 3.

Estimates of rates of volume loss (exponential rates of decay) in the postero-lateral hip muscles during bedrest. Values are means and SD estimates of the time constant k in the fitted exponential decay model e^{[k·(BRx−1)]} (where BRx is the x^th day of bedrest). More negative time constants indicate faster loss of muscle volume during bedrest. BF_LH, biceps femoris long head; BF_SH, biceps femoris short head; GMED, gluteus medius; GMIN, gluteus minimus; LGM, lower gluteus maximus; OBT_EXT, obturator externus; OBT_INT, obturator internus; PIRI, piriformis; QUAD_FEM, quadratus femoris; SEMI_M, semi-mebranosus; SEMI_T, semi-tendinosus; UGM, upper gluteus maximus. Significant difference of the percentage change in muscle volume compared with zero: *P < 0.05; †P < 0.01; ‡P < 0.001 indicate. Otherwise P > 0.05.