Mette Kreutzfeldt Zebis

Early and late rate of force development: differential adaptive responses to resistance training?

The objective of this study is to investigate the potentially opposing influence of qualitative and quantitative muscular adaptations in response to high‐intensity resistance training on contractile rate of force development (RFD) in the early (<100 ms) and later phases (>200 ms) of rising muscle force. Fifteen healthy young males participated in a 14‐week resistance training intervention for the lower body and 10 matched subjects participated as controls. Maximal muscle strength (MVC) and RFD were measured during maximal voluntary isometric contraction of the quadriceps femoris muscle. Muscle biopsies were obtained from the vastus lateralis. The main findings were that RFD in the late phase of rising muscle force increased in response to resistance training whereas early RFD remained unchanged and early relative RFD (i.e., RFD/MVC) decreased. Quantitatively, muscle fiber cross‐sectional area and MVC increased whereas, qualitatively, the relative proportion of type IIX muscle fibers decreased. Multiple regression analysis showed that while increased MVC positively influenced both early and late RFD, decreased‐type IIX negatively influenced early RFD only. In conclusion, early and late RFD responded differently to high‐intensity resistance training due to differential influences of qualitative and quantitative muscular adaptations on early and later phases of rising muscle force.

Acute fatigue impairs neuromuscular activity of anterior cruciate ligament‐agonist muscles in female team handball players

In sports, like team handball, fatigue has been associated with an increased risk of anterior cruciate ligament (ACL) injury. While effects of fatigue on muscle function are commonly assessed during maximal isometric voluntary contraction (MVC), such measurements may not relate to the muscle function during match play. The purpose of this study was to investigate the effect of muscle fatigue induced by a simulated handball match on neuromuscular strategy during a functional sidecutting movement, associated with the incidence of ACL injury. Fourteen female team handball players were tested for neuromuscular activity [electromyography (EMG)] during a sidecutting maneuver on a force plate, pre and post a simulated handball match. MVC was obtained during maximal isometric quadriceps and hamstring contraction. The simulated handball match consisted of exercises mimicking handball match activity. Whereas the simulated handball match induced a decrease in MVC strength for both the quadriceps and hamstring muscles (P<0.05), a selective decrease in hamstring neuromuscular activity was seen during sidecutting (P<0.05). This study shows impaired ACL‐agonist muscle (i.e. hamstring) activity during sidecutting in response to acute fatigue induced by handball match play. Thus, screening procedures should involve functional movements to reveal specific fatigue‐induced deficits in ACL‐agonist muscle activation during high‐risk phases of match play.

Evaluation of elastic bands for lower extremity resistance training in adults with and without musculo‐skeletal pain

Therapists commonly use elastic bands in resistance exercises during rehabilitation of smaller muscles, such as in the shoulder. However, the effectiveness has not yet been investigated for larger muscle groups. This study investigates muscle activity during lower extremity exercises. Electromyographic (EMG) activity of 10 muscles was measured in 24 women and 18 men during lunges with elastic resistance, lunges with dumbbells, and unilateral leg press in machine using 10 repetition maximum loadings, and normalized to maximal voluntary isometric contraction EMG. Lunges with dumbbells and leg press showed higher activity than lunges with elastic resistance for the vasti and rectus femoris (P < 0.01), whereas lunges with elastic resistance showed higher activity of gluteus maximus, hamstrings, and erector spinae (P < 0.01). Gender, age, and pain in the knees and hip did not influence these findings. However, pain in the lower back decreased muscular activity of the gluteus maximus and vastus medialis (P < 0.01). Lunges with elastic resistance induce high levels of muscle activity in all the large muscle groups at the hip, knee, and back. Importantly, the efficiency of these exercises was equally high regardless of gender, age, and pain in the knees and hip, whereas pain in the lower back led to altered activation strategies.

Loading intensity of jumping exercises in post-menopausal women: Implications for osteogenic training

Post‐menopausal women frequently exhibit low bone mineral density, and therefore, evidence‐based exercises that induce osteogenic loading and prevent osteoporosis are often essential. The purpose of this study was to investigate the loading intensity of 3 different jumping exercises in post‐menopausal women. Fourteen post‐menopausal women participated in this study and completed a series of countermovement jumps, drop jumps, and hard landings. A full‐body kinematic and kinetic analysis was performed to estimate the load intensity. Peak hip extensor moment and rate of moment change were significantly greater (P < .05; η2 = 0.483‐0.693) in the first landing of the drop jump than the countermovement jump and hard landing. Hip stiffness approached significance (P = .067), while peak vertical ground reaction force, vertical ground reaction force loading rate, and vertical ground reaction force index (peak*loading rate) were significantly greater (P < .01; η2 = 0.259‐0.864) during the hard landing. The drop jump and hard landing appear to generate the greatest loads at the highest rates and therefore are likely to have the largest osteogenic impact. Thus, future rehabilitation programs aimed at enhancing osteogenesis in post‐menopausal women are encouraged to include these easily implemented jumping exercises.