The influence of smoothness and speed of stand-to-sit movement on joint kinematics, kinetics, and muscle activation patterns.

Background: Stand-to-sit (StandTS) is an important daily activity widely used in rehabilitation settings to improve strength, postural stability, and mobility. Modifications in movement smoothness and speed significantly influence the kinematics, kinetics, and muscle activation patterns of the movement. Understanding the impact of StandTS speed and smoothness on movement control can provide valuable insights for designing effective and personalized rehabilitation training programs. Research question: How do the smoothness and speed of StandTS movement affect joint kinematics, kinetics, muscle activation patterns, and postural stability during StandTS? Methods: Twelve healthy younger adults participated in this study. There were two StandTS conditions. In the reference condition, participants stood in an upright position with their feet positioned shoulder-width apart on the force plate. Upon receiving a visual cue, participants performed StandTS at their preferred speed. In the smooth condition, participants were instructed to perform StandTS as smoothly as possible, aiming to minimize contact pressure on the seat. Lower leg kinetics, kinematics, and coordination patterns of muscle activation during StandTS were measured: (1) angular displacement of the trunk, knee, and hip flexion; (2) knee and hip extensor eccentric work; (3) muscle synergy pattern derived from electromyography (EMG) activity of the leg muscles; and (4) postural sway in the anterior-posterior (A-P), medio-lateral (M-L), and vertical directions. Results: Compared to the reference condition, the smooth condition demonstrated greater eccentric knee extensor flexion and increased joint work in both the knee and hip joints. Analysis of specific muscle synergy from EMG activity revealed a significant increase in the relative contribution of hip joint muscles during the smooth condition. Additionally, a negative correlation was observed between knee extensor and vertical postural sway, as well as hip extensor work and M-L postural sway. Conclusion: Smooth StandTS facilitates enhanced knee eccentric control and increased joint work at both the hip and knee joints, along with increased involvement of hip joint muscles to effectively manage falling momentum during StandTS. Furthermore, the increased contributions of knee and hip joint work reduced postural sway in the vertical and M-L directions, respectively. These findings provide valuable insights for the development of targeted StandTS rehabilitation training.

Age-related differences in lower limb muscle activation patterns and balance control strategies while walking over a compliant surface.

Substantial evidence demonstrates that falls in older adults are leading causes of fatal and non-fatal injuries and lead to negative impacts on the quality of life in the aging population. Most falls in older fallers result from unrecoverable limb collapse during falling momentum control in the single limb support (SLS) phase. To understand why older adults are more likely to fall than younger adults, we investigated age-related differences in knee extensor eccentric control, lower limb muscle activation patterns, and their relation to balance control. Ten older and ten younger healthy adults were compared during balance control while walking on a compliant surface. There was a positive correlation between knee extensor eccentric work in the perturbed leg and the swinging leg's speed and margin of stability. In comparison to younger adults, older adults demonstrated (1) less eccentric work, reduced eccentric electromyography burst duration in the perturbed leg, (2) higher postural sway during SLS, and (3) impaired swinging leg balance control. The group-specific muscle synergy showed that older adults had a prominent ankle muscle activation, while younger adults exhibited a more prominent hip muscle activation. These findings provide insight into targeted balance rehabilitation directions to improve postural stability and reduce falls in older adults.

Effects of Visual Input Absence on Balance Recovery Responses to Lateral Standing Surface Perturbations in Older and Younger Adults.

Although the ability to recover balance in the lateral direction has important implications with regard to fall risk in older adults, the effect of visual input on balance recovery in response to lateral perturbation and the effect of age are not well studied. We investigated the effect of visual input on balance recovery response to unpredictable lateral surface perturbations and its age-related changes. Ten younger and 10 older healthy adults were compared during balance recovery trials performed with the eyes open and eyes closed (EC). Compared with younger adults, older adults showed increased electromyography (EMG) peak amplitude of the soleus and gluteus medius, reduced EMG burst duration of the gluteus maximus and medius, and increased body sway (SD of the body's center of mass acceleration) in EC. In addition, older adults exhibited a smaller % increase (EC-eyes open) of the ankle eversion angle, hip abduction torque, EMG burst duration of the fibularis longus, and a greater % increase of body sway. All kinematics, kinetics, and EMG variables were greater in EC compared with eyes open in both groups. In conclusion, the absence of visual input negatively affects the balance recovery mechanism more in older adults compared with younger adults.

Age-related differences in stepping stability following a sudden gait perturbation are associated with lower limb eccentric control of the perturbed limb.

INTRODUCTION: Falls are a leading cause of severe injuries and a major threat to quality of life in older adults. Elderly fallers demonstrate insufficient eccentric quadriceps control during the weight acceptance phase of initial single limb stance. However, the functional role of eccentric control of the perturbed (leading) leg during walking balance recovery and its age-related differences have not yet been studied; thus we investigated age-related differences in eccentric control at the knee of the perturbed leg and its influence on the postural sway and stability of the trailing leg during balance recovery following unexpected surface drop perturbations. METHODS: Ten younger and ten older healthy adults were compared during balance recovery following an 8 cm unexpected surface drop perturbation at gait initiation. Outcomes related to perturbed leg included 1) eccentric knee extensor work; 2) electromyography (EMG) peak amplitude, peak latency, and eccentric EMG burst duration of the rectus femoris (RF); and 3) knee flexion angle during the single limb support. Outcomes related to stability of the trailing leg included 4) margin of stability (MoS) at first compensatory step touchdown after the perturbation. 5) Postural sway (standard deviation of center of mass acceleration) was measured in the anterior-posterior (A-P), medio-lateral (M-L), vertical directions during the single limb support. RESULTS: Compared to younger adults, older adults demonstrated lower eccentric knee extensor work (p = 0.034), shorter RF EMG burst duration (p < 0.01), delayed RF EMG peak latency (p = 0.01), smaller knee flexion angle (p = 0.01) and MoS (p = 0.04), and higher postural sway (M-L (p = 0.02), vertical (p < 0.01)). There was a positive correlation between eccentric work and MoS (p = 0.03) and a negative correlation between M-L postural sway and 1) RF eccentric EMG burst duration (p = 0.04), and 2) eccentric work (p = 0.01). CONCLUSIONS: Older adults demonstrated deficits in eccentric knee extensor control in the perturbed leg during single limb support, which contributed to reduced stability of the trailing leg compensatory step and greater postural sway during balance recovery. This finding provides insight into mechanisms of fall recovery from an unexpected unilateral postural perturbation and directions for lower limb strengthening exercises for aging populations.

Perturbation-Induced Protective Arm Responses: Effect of Age, Perturbation-Intensity, and Relationship with Stepping Stability: A Pilot Study.

During balance recovery from slip perturbations, forward flexion (elevation) of the arms serves to counterbalance the posteriorly displaced center of mass (CoM). We aimed to investigate whether aging affects modulation of arm responses to various intensities of unpredictable slip perturbations and whether arm responses are related to compensatory stepping stability. Ten healthy young adults and ten healthy older adults participated. Participants were asked to react naturally to three randomly administered levels of slip-like surface perturbations (intensity 1 (7.75 m/s2), intensity 2 (12.00 m/s2) and intensity 3 (16.75 m/s2), which occurred by means of forward acceleration of the treadmill belt while standing. Kinematic data were collected using a motion capture system. Outcomes included arm elevation displacement, velocity, and margin of stability (MoS) of compensatory stepping. The results reveal no modulation of arm elevation velocity in older adults from perturbation intensity 1 to 2, whereas younger adults demonstrated progressive increases from intensity 1 to 2 to 3. At intensity 3, older adults demonstrated reduced maximal arm elevation velocity compared to younger adults (p = 0.02). The results in both groups combined reveal a positive correlation between maximal arm elevation velocity and first compensatory step MoS at intensity 3 (p = 0.01). Together, these findings indicate age-related decreases in arm response modulation and the association of arm elevation response with protective stepping stability, suggesting that fall prevention interventions may benefit from an emphasis on arm elevation velocity control in response to greater perturbation intensities.

Effects of initial foot position on postural responses to lateral standing surface perturbations in younger and older adults.

BACKGROUND: An age-related decline in standing balance control in the medio-lateral direction is associated with increased risk of falls. A potential approach to improve postural stability is to change initial foot position (IFP). RESEARCH QUESTIONS: In response to a lateral surface perturbation, how are lower extremity muscle activation levels different and what are the effects of different IFPs on muscle activation patterns and postural stability in younger versus older adults? METHODS: Ten younger and ten older healthy adults participated in this study. Three IFPs were tested [Reference (REF): feet were placed parallel, shoulder-width apart; Toes-out with heels together (TOHT): heels together with toes pointing outward; Modified Semi-Tandem (M-ST): the heel of the anterior foot was placed by the big toe of the posterior foot]. Unexpected lateral translations of the standing surface were applied. Electromyographic (EMG) activity of the lower extremity muscles, standard deviation (SD) of the body's CoM acceleration (SD of CoMAccel), and center of pressure (CoP) sway area were compared across IFPs and age. RESULTS: Activation levels of the muscles serving the ankle and gluteus medius were greater than for the knee joint muscles and gluteus maximus in the loaded leg across all IFPs in both groups. TOHT showed greater EMG peak amplitude of the soleus and fibularis longus compared to REF, and had smaller SD of CoMAccel and CoP sway area than M-ST. Compared to younger adults, older adults demonstrated lower EMG peak amplitude and delayed peak timing of the fibularis longus and greater SD of CoMAccel and CoP sway area in all IFPs during balance recovery. SIGNIFICANCE: During standing balance recovery, ankle muscles and gluteus medius are important active responders to unexpected lateral surface perturbations and a toes-out IFP could be a viable option to enhance ankle muscle activation that diminishes with age to improve postural stability.

Trunk kinematics and muscle activation patterns during stand-to-sit movement and the relationship with postural stability in aging.

BACKGROUND: Stand-to-sit (StandTS) movement is an important functional activity that can be challenging for older adults due to age-related changes in neuromotor control. Although trunk flexion, eccentric contraction of the rectus femoris (RF), and coordination of RF and biceps femoris (BF) muscles are important to the StandTS task, the effects of aging on these and related outcomes are not well studied. RESEARCH QUESTION: What are the age-related differences in trunk flexion, lower extremity muscle activation patterns, and postural stability during a StandTS task and what is the relationship between these variables? METHODS: Ten younger and ten older healthy adults performed three StandTS trials at self-selected speeds. Outcomes included peak amplitude, peak timing, burst duration, and onset latency of electromyography (EMG) activity of the RF and BF muscles, trunk flexion angle and angular velocity, whole body center of mass (CoM) displacement, center of pressure (CoP) velocity, and ground reaction force (GRF). RESULTS: There were no age-related differences in weight-bearing symmetry, StandTS and trunk flexion angular velocity, or BF activity. In both groups, EMG peak timing of RF was preceded by BF. Compared to younger adults, older adults demonstrated shorter RF EMG burst duration, reduced trunk flexion, and reduced stability as indicated by the longer duration in which CoM was maintained beyond the posterior limit of base of support (BoS), greater mean anterior-posterior CoP velocity and larger standard deviation of CoM vertical acceleration during StandTS with smaller vertical GRF immediately prior to StandTS termination. Trunk flexion angle and RF EMG burst duration correlated with stability as measured by the duration in which the CoM stayed within the BoS. SIGNIFICANCE: Decreased trunk flexion and impaired eccentric control of the RF are associated with StandTS instability in aging and suggest the importance of including StandTS training as a part of a comprehensive balance intervention.

Bone Mineral Density Differences Across Female Olympic Lifters, Power Lifters, and Soccer Players.

ABSTRACT: Jeon, W, Harrison, JM, Stanforth, PR, and Griffin, L. Bone mineral density differences across female Olympic lifters, power lifters, and soccer players. J Strength Cond Res 35(3): 638-643, 2021-Athletic training improves bone mineral density (BMD) through repeated mechanical loading. The location, intensity, and direction of applied mechanical pressure play an important role in determining BMD, making some sports more advantageous at improving BMD at specific regions. Thirty-seven (10 power lifters [PL], 8 Olympic lifters [OL], 8 soccer players [SP], and 11 recreationally active [RA]) women participated in a cross-sectional study. We measured lumbar spine (L1-L4), femoral neck, total-body BMD, and overall body composition (total fat mass, lean mass, percent body fat) with dual-energy x-ray absorptiometry. All athletic groups had greater total BMD than RA (p = 0.01 [PL]; p < 0.001 [OL]; p = 0.01 [SP]). Olympic lifters had the highest total BMD than all other athletic groups. Olympic lifters had the significantly greater total BMD than PL (p = 0.018), but there was no difference in total BMD between PL and SP. As compared with RA, OL showed greater BMD at both the total lumbar spine (p = 0.002) and the femoral neck (p = 0.007), whereas PL showed greater BMD only for the total lumbar spine (p = 0.019) and SP showed greater BMD only for the femoral neck (p = 0.002). Olympic-style lifting includes both high-impact and odd-impact loading modalities that are associated with the highest BMD at both the lumbar spine and femoral neck.

Effects of different initial foot positions on kinematics, muscle activation patterns, and postural control during a sit-to-stand in younger and older adults.

BACKGROUND: Performing a sit-to-stand (STS) can be a challenging task for older adults because of age-related declines in neuromuscular strength and coordination. We investigated the effects of different initial foot positions (IFPs) on kinematics, muscle activation patterns, and balance control during a STS in younger and older adults. METHODS: Ten younger and ten older healthy adults participated in this study. Four symmetric IFPs were studied: (1) reference (REF), (2) toes-out with heels together (TOHT), (3) toes-out (TO), and (4) Wide. Lower-extremity muscle activation patterns and kinetic and kinematic data in the sagittal and frontal planes were measured. RESULTS: The trunk forward-tilt angle and hip extension torque during uprising were smaller in TO and Wide for both age groups. Postural sway and center of pressure sway area were smallest in TO after completion of uprising with no difference between age groups. Adductor longus and gluteus medius activity was greater in TO than in the other IFPs, and older adults activated these muscles to a greater degree than younger adults. CONCLUSION: Smaller trunk flexion angles with greater activation of the hip abductor and adductor muscles in TO contributed to improving postural stability during the STS. SIGNIFICANCE: STS training with a toes-out foot position could be an effective rehabilitation strategy for older adults to strengthen hip muscles that control medio-lateral balance required for balance during a STS.

Muscle activity and balance control during sit-to-stand across symmetric and asymmetric initial foot positions in healthy adults.

BACKGROUND: Rising from a sit to a stand has biomechanical factors that are dependent on initial foot position. Little is known about the effect of initial foot position on leg muscle activation patterns during a sit-to-stand and balance maintenance of stance after a sit-to-stand. RESEARCH QUESTION: What are the effects of different symmetric and asymmetric initial foot positions on leg muscle activation patterns and balance during and after a sit-to-stand? METHODS: Three symmetric (neutral; both ankles positioned under the knees at a 90° flexion; one-third; and two-thirds foot length posterior to neutral) and three asymmetric (neutral non-dominant leg with one-third back dominant leg, neutral non-dominant with two-thirds back dominant leg, and one-third back non-dominant leg with two-thirds back dominant leg) initial foot positions were tested. EMG of the lower extremity muscles and sagittal plane kinematic data were measured along with balance assessments in the anterior-posterior and medial-lateral axes. RESULTS: In the symmetric initial foot positions, a faster forward velocity of the body's center of mass was required for more anterior initial foot positions. Even though the hip extensors activated earlier to decelerate the forward velocity of the body's center of mass before rising, the greater forward velocity caused postural sway following completion of upright stance. In the asymmetric initial foot positions, the posterior leg supported more weight during the sit-to-stand, resulting in balance perturbations in the posterior leg. In the one-third back non-dominant leg with two-thirds back dominant leg asymmetric initial foot position, however, the weight-bearing symmetry was not different from the symmetric initial foot positions during the sit-to-stand. Postural stability after completion of uprising was also improved in this asymmetric initial foot position, showing greater but delayed onset of the tibialis anterior in the anterior leg during the momentum transfer phase. SIGNIFICANCE: With a neutral symmetric initial foot position, earlier onset of the hip extensors during eccentric lengthening contributed to decelerating the forward velocity of the body's center of mass for balance control during a sit-to-stand. With asymmetric initial foot positions, the weight distribution during a sit-to-stand can be increased by positioning both feet posterior to neutral foot position. Performing a sit-to-stand with this asymmetric initial foot position can improve postural stability after uprising. Thus, this foot position could be used in designing rehabilitation interventions for clinical populations and the frail elderly.

Effects of pulse duration on muscle fatigue during electrical stimulation inducing moderate-level contraction.

INTRODUCTION: Neuromuscular electrical stimulation (NMES) is used to prevent muscle atrophy. However, the effect of pulse duration modulation for reducing muscle fatigue and pain is unknown. METHODS: Two 2-minute stimulation protocols were applied to the knee extensors of 10 healthy individuals. In 1 session, a long pulse duration (1,000 µs) and a low current amplitude (LL), set to evoke 25% maximal voluntary contraction at 30 Hz, were applied. The other session was identical except that a short pulse duration (200 µs) and a high current amplitude (SH) were used. RESULTS: Muscle fatigue was lower for LL than for SH (P < 0.01). Force recovery rate was higher for LL than for SH (P < 0.05). Pain scores were also lower for LL than for SH (P < 0.05). DISCUSSION: The use of 1-ms pulse durations reduces fatigue and pain during NMES for moderate-level contractions compared with 200-µs durations. Muscle Nerve 57: 642-649, 2018.